Critical micelle concentrationFrom Wikipedia, the free
encyclopediaJump to: navigation, searchIn colloidal and surface
chemistry, the critical micelle concentration (CMC) is defined as
the concentration of surfactants above which micelles form and all
additional surfactants added to the system go to micelles.[1]The
CMC is an important characteristic of a surfactant. Before reaching
the CMC, the surface tension changes strongly with the
concentration of the surfactant. After reaching the CMC, the
surface tension remains relatively constant or changes with a lower
slope. The value of the CMC for a given dispersant in a given
medium depends on temperature, pressure, and (sometimes strongly)
on the presence and concentration of other surface active
substances and electrolytes. Micelles only form above critical
micelle temperature.For example, the value of CMC for sodium
dodecyl sulfate in water (no other additives or salts) at 25 C,
atmospheric pressure, is 8x103 mol/L.[2]The study of the
aggregation of lipids (amphiphiles) is known as lipid
polymorphism.Description[edit]
Top to Bottom: Increasing concentration of surfactant in water
slowly forming a layer on the surface and eventually forming
micelles at or above the CMC. Notice that the existence of micelles
does not preclude the existence of individual surfactant molecules
in solution.Upon introduction of surfactants (or any surface active
materials) into the system, they will initially partition into the
interface, reducing the system free energy by:1. lowering the
energy of the interface (calculated as area times surface tension),
and2. removing the hydrophobic parts of the surfactant from contact
with water.Subsequently, when the surface coverage by the
surfactants increases, the surface free energy (surface tension)
decreases and the surfactants start aggregating into micelles, thus
again decreasing the system's free energy by decreasing the contact
area of hydrophobic parts of the surfactant with water. Upon
reaching CMC, any further addition of surfactants will just
increase the number of micelles (in the ideal case).There are
several theoretical definitions of CMC. One well-known definition
is that CMC is the total concentration of surfactants under the
conditions:[3]if C = CMC, (d3F/dCt3) = 0F = a[micelle] +
b[monomer]: function of surfactant solutionCt: total
concentrationa, b: proportional constantsThe CMC generally depends
on the method of measuring the samples, since a and b depend on the
properties of the solution such as conductance and photochemical
characteristics. When the degree of aggregation is monodisperse,
then the CMC is not related to the method of measurement. On the
other hand, when the degree of aggregation is polydisperse, then
CMC is related to both the method of measurement and the
dispersion.The common procedure to determine the CMC from
experimental data is to look for the intersection of two straight
lines traced through plots of the measured property versus the
surfactant concentration. This visual data analysis method is
highly subjective and can lead to very different CMC values
depending on the type of representation, the quality of the data
and the chosen interval around the CMC.[4] A preferred method is
the fit of the experimental data with a model of the measured
property. Fit functions for properties such as electrical
conductivity, surface tension, NMR chemical shifts, absorption,
self-diffusion coefficients, fluorescence intensity and mean
translational diffusion coefficient of fluorescent dyes in
surfactant solutions have been presented.[5][6][7] These fit
functions are based on a model for the concentrations of monomeric
and micellised surfactants in solution, which establishes a
well-defined analytical definition of the CMC, independent from the
technique.The CMC is the concentration of surfactants in the bulk
at which micelles start forming. The word bulk is important because
surfactants partition between the bulk and interface and CMC is
independent of interface and is therefore a characteristic of the
surfactant molecule. In most situations, such as surface tension
measurements or conductivity measurements, the amount of surfactant
at the interface is negligible compared to that in the bulk and CMC
can be approximated by the total concentration.There are important
situations where interfacial areas are large and the amount of
surfactant at the interface cannot be neglected. For example if we
take a solution of a surfactant above CMC and start introducing air
bubbles at the bottom of the solution, these bubbles, as they rise
to the surface, pull out the surfactants from the bulk to the top
of the solution creating a foam column thus bringing down the
concentration in bulk to below CMC. This is one of the easiest
methods to remove surfactants from effluents (foam flotation). Thus
in foams with sufficient interfacial area there will not be any
micelles. Similar reasoning holds for emulsions.The other situation
arises in detergency. One initially starts off with concentrations
greater than CMC in water and on adding fabric with large
interfacial area and waiting for equilibrium, the surfactant
concentration goes below CMC and no micelles are left. Therefore
the solubilization plays a minor role in detergency. Removal of
oily soil occurs by modification of the contact angles and release
of oil in the form of emulsion.Zeta potential
Diagram showing the ionic concentration and potential difference
as a function of distance from the charged surface of a particle
suspended in a dispersion medium.Zeta potential is a scientific
term for electrokinetic potential[1] in colloidal dispersions. In
the colloidal chemistry literature, it is usually denoted using the
Greek letter zeta (), hence -potential. From a theoretical
viewpoint, the zeta potential is the electric potential in the
interfacial double layer (DL) at the location of the slipping plane
relative to a point in the bulk fluid away from the interface. In
other words, zeta potential is the potential difference between the
dispersion medium and the stationary layer of fluid attached to the
dispersed particle.The zeta potential is caused by the net
electrical charge contained within the region bounded by the
slipping plane, and also depends on the location of that plane.
Thus it is widely used for quantification of the magnitude of the
charge. However, zeta potential is not equal to the Stern potential
or electric surface potential in the double layer,[2] because these
are defined at different locations. Such assumptions of equality
should be applied with caution. Nevertheless, zeta potential is
often the only available path for characterization of double-layer
properties.The zeta potential is a key indicator of the stability
of colloidal dispersions. The magnitude of the zeta potential
indicates the degree of electrostatic repulsion between adjacent,
similarly charged particles in a dispersion. For molecules and
particles that are small enough, a high zeta potential will confer
stability, i.e., the solution or dispersion will resist
aggregation. When the potential is small, attractive forces may
exceed this repulsion and the dispersion may break and flocculate.
So, colloids with high zeta potential (negative or positive) are
electrically stabilized while colloids with low zeta potentials
tend to coagulate or flocculate as outlined in the table.[3][4]Zeta
potential [mV]Stability behavior of the colloid
from 0 to 5,Rapid coagulation or flocculation
from 10 to 30Incipient instability
from 30 to 40Moderate stability
from 40 to 60Good stability
more than 61Excellent stability
Measurement of zeta potential[edit]Zeta potential is not
measurable directly but it can be calculated using theoretical
models and an experimentally-determined electrophoretic mobility or
dynamic electrophoretic mobility.Electrokinetic phenomena and
electroacoustic phenomena are the usual sources of data for
calculation of zeta potential.Electrokinetic phenomena[edit]Main
article: Electrokinetic phenomenaElectrophoresis is used for
estimating zeta potential of particulates, whereas streaming
potential/current is used for porous bodies and flat surfaces. In
practice, the Zeta potential of dispersion is measured by applying
an electric field across the dispersion. Particles within the
dispersion with a zeta potential will migrate toward the electrode
of opposite charge with a velocity proportional to the magnitude of
the zeta potential.This velocity is measured using the technique of
the Laser Doppler Anemometer. The frequency shift or phase shift of
an incident laser beam caused by these moving particles is measured
as the particle mobility, and this mobility is converted to the
zeta potential by inputting the dispersant viscosity and dielectric
permittivity, and the application of the Smoluchowski theories (see
below).[5]Electrophoresis[edit]Main article:
ElectrophoresisElectrophoretic velocity is proportional to
electrophoretic mobility, which is the measurable parameter. There
are several theories that link electrophoretic mobility with zeta
potential. They are briefly described in the article on
electrophoresis and in details in many books on colloid and
interface science.[6][7][8][9] There is an IUPAC Technical
Report[10] prepared by a group of world experts on the
electrokinetic phenomena.From the instrumental viewpoint, there are
two different experimental techniques: microelectrophoresis and
electrophoretic light scattering. Microelectrophoresis has the
advantage of yielding an image of the moving particles. On the
other hand, it is complicated by electro-osmosis at the walls of
the sample cell. Electrophoretic light scattering is based on
dynamic light scattering. It allows measurement in an open cell
which eliminates the problem of electro-osmotic flow for the case
of an Uzgiris, but not a capillary cell. And, it can be used to
characterize very small particles, but at the price of the lost
ability to display images of moving particles.Both these measuring
techniques may require dilution of the sample. Sometimes this
dilution might affect properties of the sample and change zeta
potential. There is only one justified way to perform this dilution
- by using equilibrium supernatant. In this case the interfacial
equilibrium between the surface and the bulk liquid would be
maintained and zeta potential would be the same for all volume
fractions of particles in the suspension. When the diluent is known
(as is the case for a chemical formulation), additional diluent can
be prepared. If the diluent is unknown, equilibrium supernatant is
readily obtained by centrifugation.Electroacoustic
phenomena[edit]Main article: Electroacoustic phenomenaThere are two
electroacoustic effects that are widely used for characterizing
zeta potential: colloid vibration current and electric sonic
amplitude, see reference.[8] There are commercially available
instruments that exploit these effects for measuring dynamic
electrophoretic mobility, which depends on zeta
potential.Electroacoustic techniques have the advantage of being
able to perform measurements in intact samples, without dilution.
Published and well-verified theories allow such measurements at
volume fractions up to 50%, see reference. Calculation of zeta
potential from the dynamic electrophoretic mobility requires
information on the densities for particles and liquid. In addition,
for larger particles exceeding roughly 300 nm in size information
on the particle size required as well.Calculation of zeta
potential[edit]The most known and widely used theory for
calculating zeta potential from experimental data is that developed
by Marian Smoluchowski in 1903.[11] This theory was originally
developed for electrophoresis; however, an extension to
electroacoustics is now also available.[8] Smoluchowski's theory is
powerful because it is valid for dispersed particles of any shape
and any concentration. However, it has its limitations: Detailed
theoretical analysis proved that Smoluchowski's theory are valid
only for a sufficiently thin double layer, when the Debye length,
1/, is much smaller than the particle radius a:
The model of the "thin double layer" offers tremendous
simplifications not only for electrophoresis theory but for many
other electrokinetic and electroacoustic theories. This model is
valid for most aqueous systems because the Debye length is
typically only a few nanometers in water. The model breaks only for
nano-colloids in a solution with ionic strength approaching that of
pure water. Smoluchowski's theory neglects the contribution of
surface conductivity. This is expressed in modern theories as the
condition of a small Dukhin number:
The development of electrophoretic and electroacoustic theories
with a wider range of validity was a purpose of many studies during
the 20th century. There are several analytical theories that
incorporate surface conductivity and eliminate the restriction of
the small Dukhin number for both the electrokinetic and
electroacoustic applications.Early pioneering work in that
direction dates back to Overbeek[12] and Booth.[13]Modern, rigorous
electrokinetic theories that are valid for any zeta potential and
often any a, stem mostly from Soviet Ukrainian (Dukhin, Shilov and
others) and Australian (O'Brien, White, Hunter and others) schools.
Historically, the first one was Dukhin-Semenikhin theory.[14] A
similar theory was created 10 years later by O'Brien and
Hunter.[15] Assuming a thin double layer, these theories would
yield results that are very close to the numerical solution
provided by O'Brien and White.[16] There are also general
electroacoustic theories that are valid for any values of Debye
length and Dukhin
number.[8][9]http://www.funsci.com/fun3_en/exper2/exper2.htm
PRESENTATIONIn this article, we collect a series of laboratory
experiments which mainly concern surface phenomena and colloidal
systems. Due to their number, these experiments will be briefly
described. As you know, our articles do not intend to supply an
exhaustive explanation of the topics we deal with, but rather to
give rise to a curiosity toward them and to give young people
exposure to interesting categories of natural phenomena.
INTRODUCTION TO SURFACE PHENOMENAWhy do some insects succeed in
skating on water instead of sinking? Why in some cases, does the
water sprinkled on a glass surface collect into drops and in other
cases spread like a thin film? Why does water climb up a thin tube?
Why can you make bubbles with soapy water and not with tap water?
For reasons we will see later on, the surface of a substance has
special properties. These surface properties are what allow these
strange phenomena we have mentioned. Not only that, but the surface
is also the place of contact among different substances. In short,
the properties of surfaces are so special and important that there
is a branch of science, the physics of surfaces, devoted to the
study of surface phenomena.SURFACE TENSION A molecule of a liquid
attracts the molecules which surround it and in its turn it is
attracted by them (figure 2). For the molecules which are inside a
liquid, the resultant of all these forces is neutral and all them
are in equilibrium by reacting with each other. When these
molecules are on the surface, they are attracted by the molecules
below and by the lateral ones, but not toward the outside. The
resultant is a force directed inside the liquid. In its turn, the
cohesion among the molecules supplies a force tangential to the
surface. So, a fluid surface behaves like an elastic membrane which
wraps and compresses the below liquid. The surface tension
expresses the force with which the surface molecules attract each
other. A way to see the surface tension in action is to observe the
efforts of a bug to climb out of the water. On the contrary, other
insects, like the marsh treaders and the water striders, exploit
the surface tension to skate on the water without sinking. Here are
some simple experiments using surface tension:
1 -The floating needle. Carefully place a needle on the surface
of a glass of water. If the water does not completely wet it, you
will see the needle float. To avoid your fingers disturbing the
surface as you place the needle, you can make a small cradle from
wire to hold the needle as you lower it gently on to the surface of
the water. Another way to make it easier to float an object heavier
than water using only the surface tension is to first float a strip
of tissue paper and lay the needle on it. Slowly, the water will
soak the strip, which will eventually sink, while the needle will
remain on the surface.Figure 3 - Floating needle. At the bottom of
the pot you can see the sunken strip of tissue paper.
2 - Make sulfur powder sink. Sprinkle some sulfur powder over a
glass of water (You can buy sulfur in a hardware store). Sulfur is
hydrophobic enough to float on the water. Add a drop of detergent
and you will see the particles of sulfur sink. This experiment also
works with talcum powder which you probably already have in your
home.http://www.ilpi.com/genchem/demo/tension/ has a short movie on
this experiment and a description of the properties of
surfactants.3 - Launch of the needle. With some steel wire, make a
ring. Place a needle on the ring and submerge in soapy water. When
you extract the ring, two membranes will be formed: one at the left
side of the needle and the other at the right side. Now, with a
finger burst one of these membranes. The needle will be thrown away
by the surface tension of the remaining membrane, which quickly
contracts, in an effort to achieve the smallest possible surface
area.4 - The strength of the soap films. With some iron wire, make
a "U" frame and a slider, as shown by the figure 4. Plunge the
frame in soapy water. When you extract it, you will see that the
slider will be drawn toward the bottom of the frame by the surface
tension of the soap membrane. By holding the slider still with your
fingers, you can feel the force of the membrane. Figure 4 -
U-shaped frame with slider. The surface tension of the membrane
draws the slider toward left.
5 - Measuring the surface tension. In order to measure the
surface tension of a liquid, you can use an equal-arm analytical
balance. As shown by the figures 5 and 6, hang a U-shaped steel
wire under one of the two weighing pans (A). By lowering the A arm
and then by lifting it up again, make a membrane to form in the
U-shaped frame. Balance it with some masses on the weighing pan B.
At this point, break the film. The balance will go down by the B
side, therefore restore the equilibrium placing some masses on the
side A. The value of these last masses (F) corresponds to the force
with which the membrane tends to close into the liquid. The surface
tension (T) is given by the force (F) divided by the width (W) of
the membrane, divided again by two because it is necessary to keep
into account the membrane possess two surfaces. So, T = F/2W. The
value of the surface tension of the distilled water is 7,42 g/m at
20C and that of ethyl alcohol is 2,27 g/m always at 20C. We supply
to you these values because you will be allowed to compare with
them those you obtain through experimentation. If you do not
possess an analytical balance, you can build one of them. It will
not be as exact, but it will allow you to do these measures. Given
the forces which play in this experiment, the balance should have
an accuracy of a hundredth of a gram at
least.http://www.pvri.com/sp/BalBuild.htm How to build a no cost
sensitive balance (by Salvatore
Previtera)http://userpages.prexar.com/dwilliamsmaine/scale/scale.html
A Home-made Balance Scale (by Dan Williams)
6 - Other method to measure the surface tension. To measure the
surface tension of liquids, you can use a metal wire ring of the
diameter comprised between 3 and 4 cm, instead of the "U" frame we
have described. This wire should be made of platinum, anyway, as
this material is costly and not easy to find, use a stainless steel
wire which you can buy in a welding shop or in a hardware store. If
you have difficulty finding a wire of this material, use an iron
wire. Its diameter should be of 1 - 2 mm. Even in this case you
should use an analytical balance.Dip the ring just under the
surface of the liquid of which you want determine the surface
tension. Level the balance in these conditions. Add some masses on
the opposite arm until the ring detaches from the liquid. The
surface tension (T) of the liquid will be given by the detachment
force (F) you have measured divided by two times the mean
circumference (crf) of the ring: T = F/2crf. This factor 2 takes
into account the two surfaces of liquid: the internal one and the
external one to the ring (figure 8). For reasons of clarity, in the
figure the ring has been drawn with the diameter greater than the
actual diameter.http://www.tensiometry.com/STMethods.htm Other
methods to measure the surface tension.7 - With distilled water,
verify the good working order of your experimental system.8 -
Determine the surface tension of the tap water.9 - Determine the
surface tension of the tap water to which you have added a little
detergent. You will notice that small amounts of surfactants are
sufficient to lower the surface tension of the water a lot.10 -
Relationship between the weight of the drops and the surface
tension. By a dropper, slowly drop some water of the test 8 and
determine the mass of a certain number of drops (ie 30). Do the
same thing with the water of the experiment n 9. Verify if there is
a relationship between the mass of the drops and the surface
tension of the solutions. Answer: The mass of the drops is
proportional to the surface tension of the liquid: M = T/K, where K
is a constant which you can determine using distilled water at 20C
of which you know the surface tension. This constant is valuable
only for this dropper. Determine the mass of a given number of
drops is a method to measure the surface tension of a liquid. In
these tests, to obtain a better precision, calculate the mean of a
series of measures. Verify if the following relationships are
valuable: T1:M1 = T2:M2.11 - Surfactant powered boats. From a thin
wooden or cardboard sheet, cut three little "boats" like those
indicated in the figure 9. They must have an opening with a seat
for a bit of soap. Place a bit of soap in the seat of a boat and
put it in a small basin with water. You will see the boat move
quickly forward. With the opening on a side or off-center, the boat
will turn. The movement of the boat can be explained by the quick
scatter of surfactant molecules on the water surface, so this
little boat would move by reaction. Another explanation recalls
Marangoni's effect, according to which, in case of a gradient of
surface tension from one zone of a liquid to another, there will be
established a flow from the zone of low surface tension toward the
one of high surface tension. In this case, the boat will be dragged
by the movement of the water surface. This amusing experiment can
also be done using substances other than soap, provided they have
surface active properties. For example, you could place a little
drop of detergent on the carving. If you will use a bit of camphor,
your boat will sail more quickly and longer. If the stretch of
water in which the boat moves is small, like a dish or a small
basin, quickly the water surface will be covered by a layer of
surfactant molecules and the boat will stop and you will need to
change the water to restart the boat. If instead you do these
experiments in a pond, you will not have this problem. Try
different shapes of boat and of carving, try hot and cold water,
different types of soap, etc. The water will quickly soak through
the wood or especially the cardboard of your boat and will disable
it. Some boards will even sink. To save your fleet, make the little
boats waterproof with acrylic paint or flatting. When the paint
dries, you will be able to restart the
races.http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html
Surface Tension ***http://teachers.net/lessons/posts/224.html
Surface tension on coinshttp://www.online-tensiometer.com/oberfl/
Some experiments on the surface
tensionhttp://www.biologylessons.sdsu.edu/ta/classes/lab1/TG.html
Properties of Waterhttp://www.ed.gov/pubs/parents/Science/soap.html
Have you ever tried using soap to power a boat?Internet keywords:
surface tension, surface phenomena, surface tension boat, soap
boat.
WETTABILITYWhy does one fabric absorb water well while another
seems to refuse it? Why does water collect into large drops on a
greasy surface and instead form an adherent film on a clean
surface? According to the nature of the liquid and the solid, a
drop of liquid placed on a solid surface will adhere to it more or
less. To understand this phenomenon it is necessary to take into
account the fact that molecules of a liquid are subject to a
cohesive force which keeps them united to one another, but there is
also an adhesive force which is the force with which the molecules
of the liquid adhere to the surface of materials that they contact.
When the forces of adhesion are greater than the forces of
cohesion, the liquid tends to wet the surface, when instead the
forces of adhesion are less by comparison to those of cohesion, the
liquid tends to "refuse" the surface. In this people speak of
wettability between liquids and solids. For example, water wets
clean glass, but it does not wet wax.1 - Measuring the contact
angle. Place a drop of a liquid on a smooth surface of a solid.
According to the wettability of the liquid in relationship to this
solid, the drop will make a certain angle of contact with the
solid. With reference to the figure 10, if the contact angle is
lower than 90, the solid is called wettable, if the contact angle
is wider than 90, the solid is named non-wettable. A contact angle
equal to zero indicates complete wettability. To measure the
contact angle use a protractor and a ruler. Taking a picture of the
outline of the drop will make easier and more exact the
measurement.
2 - Prominent drops, flat drops. Lay a water drop on a dirty
glass plate. For example a glass with a lot of fingerprints.
Measure the contact angle. Now wash the plate with water and
detergent, then rinse it with care and dry it. Make the test again
and compare the contact angle in the two cases.3 - Misted plate.
Breathe on a glass plate which has been washed, but not very well.
You will see the plate become misted, this is due to the formation
of a myriad of tiny water drops on the surface of the glass.4 -
Water film. With water and detergent, wash a plate of glass well,
then rinse it a first time with tap water and then with distilled
water and leave it to dry in a place devoid of dust. Now, breathe
on it. If the plate of glass is very clean, it will not mist
because the water will arrange on the surface as a thin and
continuous film of water. This happens because the water has
complete wettability toward a clean glass. If the cleaning method
above has not cleaned the plate well enough, wipe it with a cotton
cloth with some pure acetone in it. Use caution because acetone is
inflammable and toxic, so do this operation outdoors and with
care.By studying plants, a German scientist discovered a method to
keep surfaces clean or to clean them with less water. You have to
cover the surface with a thin layer of wax. This substance has a
very low wettability toward the water. It tends to keep clean and
it is commonly used to enhance the cleanliness and appearance of
buildings and
vehicles.http://www.fys.uio.no/~eaker/thesis/node9.html
Wettabilityhttp://www.ksvinc.com/contact_angle.htm Contact
AnglesInternet keywords: wettability, interfacial tension, IFT,
contact angleCAPILLARITY Let us stay in the field of the
wettability. Surely you have noticed that water tends to rise near
the walls of a glass container. This happens because the molecules
of this liquid have a strong tendency to adhere to the glass.
Liquids which wet the walls make concave surfaces (eg:
water/glass), those which do not wet them, make convex surfaces
(eg: mercury/glass). Inside tubes with internal diameter smaller
than 2 mm, called capillary tubes, a wettable liquid forms a
concave meniscus in its upper surface and tends to go up along the
tube (figure 11). On the contrary, a non-wettable liquid forms a
convex meniscus and its level tends to go down. The amount of
liquid attracted by the capillary rises until the forces which
attract it balance the weight of the fluid column. The rising or
the lowering of the level of the liquids into thin tubes is named
capillarity. Also the capillarity is driven by the forces of
cohesion and adhesion we have already mentioned.
1 - The rise of water along a capillary. Immerse a capillary in
a glass containing tap water and measure the height (h) of the
water column inside it.2 - Effect of the surfactants. Add a few
drops of detergent to the water and measure again. Compare the
variation in the height of the water column. You will be able to
notice that even small amounts of surfactants produce important
effects on the level reached by the water in the capillary.3 -
Effect of the diameter of the capillary. With a tube of glass and a
Bunsen burner, make a series of capillary tubes having different
diameter. Verify the relationship between the height of the water
column and the internal diameter of the capillary. (Answer: the
height of the column is described by this formula h=k/r, where h is
the height of the column, k is a constant which depends on the
surface tension of the liquid and on the contact angle between the
liquid and the wall, r is the internal radius of the capillary
tube. So, with the same liquid and material of the capillary tube,
the height of the column is in inverse proportion to the diameter
of the capillary tube. You can determine the value of k for water
using distilled water at 20C.4 - Try other liquids. Make some other
tests with liquids other than water, such as alcohol, oil, etc. and
measure the height of the liquid column. This height depends by a
number of factors such as the surface tension of the liquid, the
contact angle liquid/capillary, the radius of the capillary, the
density of the liquid, the acceleration of gravity. In fact, the
column attains the height of equilibrium between the ascensional
forces and its own weight. Oily substances tend to contaminate
inside the capillary, so when changing from one liquid to another,
clean the capillary well or replace it. The vegetable world
exploits capillarity and osmosis to bring water up to the higher
parts of plants. In this way, some trees succeed in bringing this
precious liquid up to 120 meters above the ground.5 - An emergency
plant watering system. It is summer and you are going on vacation.
You are worried about your potted plants, which risk to remain
without water. In fact, even if you have asked your neighbor to
water them, you know by experience that after the first day, he
will forget, that's just the way he is. Then, try this emergency
watering system. It bases itself on the fact that a string is able
to carry water among its fibers by capillarity. Place a tank on
some bricks and fill it with water. Place the pots round the drum.
Cut some pieces of string long enough to reach the bottom of the
tank and to be inserted into a pot. Immerse all strings in the
water to soak them well. Tie all the strings together at one end
and sink this knot to the bottom of the drum with a stone or
weight. Now, one at the time, put the free end of each string into
a different pot. Each pot has to be served by a string. Test the
system before you go on your vacation. You have to verify if it
works well, to find the suitable type of string and to proportion
the amount of water in the tank to the length of your absence. Try
strings made up of fibers of different dimension, of different
materials, even in plastic. If the string tends to become encrusted
with mineral deposits, add some vinegar to the water. Also try to
insert each string in a thin plastic tube. If the water flow is too
fast, use a thinner string. Check the effect of some drops of
detergent on the
flow.http://www.svce.ac.in/~msubbu/FM-WebBook/Unit-I/Capillarity.htm
CapillarityInternet keywords: capillary, capillarity.
SOAPS AND DETERGENTSHow do soaps and detergents work in removing
dirt? Soaps and detergents are formed by special molecules, which
have a hydrophilic head, which therefore loves to remain in water
and a hydrophobic tail, which avoids water and loves fat substances
(figure 12 A). Because of their hydrophobic tail, a part of the
molecules of detergent collects to the water surface forming a
monomolecular layer (figure 12 B), it lowers the surface tension of
the water and makes easier its penetration into the fabrics to be
cleaned. Within the water, the molecules of detergent collect
themselves in micelles and membranes, little aggregates of
molecules united by their hydrophobic tail (figure 12 B). When they
meet dirt, these molecules surround the particles and insert their
tail in them. The hydrophilic heads attract the dirt toward water
and with the agitation of the liquid they contribute to remove the
dirt from the fabric (figure 12 D). The crown of hydrophilic heads
carries the particles of dirt in the water (figure 12 D), where
they end up in suspension and then they are rinsed away. Hence, the
dirt water contains also greasy particles which have been
emulsified. For the same reason, the detergents aid the formation
of emulsions. The substances which lower the surface tension of a
liquid are called surfactants (from: surface-active agents). The
lowering of the surface tension of the water allows the formation
of soapy membranes (figure 12 C), foam and soap bubbles. Notice the
special arrangement of the surfactant molecules in these
membranes.
The phospholipids are molecules like surfactants, they also have
a hydrophilic head and this time two hydrophobic tails. These
molecules are the main components of the membranes of cells. In
fact, usually the membranes of cells are made up of two layers of
phospholipids, with the tails turned inward, in the attempt to
avoid water. As we know, the external membrane of a cell contains
all the organelles and the cytoplasm. Liposomes are empty cells
which are manufactured by some industries. They are microscopic
vesicles or containers, formed by the membrane alone. They are
widely used in the pharmaceutical and cosmetic fields because it is
possible to insert chemicals inside them. You can use liposomes to
contain hydrophobic chemicals such as greasy or oily substances so
that they can be dispersed in an aqueous medium by virtue of the
hydrophilic properties of the membrane of the
liposomes.http://cellbio.utmb.edu/cellbio/membrane_intro.htm
Membrane Structure and
Functionhttp://ntri.tamuk.edu/cell/membranes.html Architecture of
membranesInternet keywords: phospholipids membrane, cell membrane1
- Comparison of the ability of different detergents. Try the
efficacy of different detergents for glass or dishes. Soil some
microscope slides with the same type of fat. If you do not have
microscope slides, use glasses or even ceramic dishes. Clean all
the slides with a different detergent, rinse them well and dry
them. You can check the level of cleanliness by measuring the
contact angle of water drops placed on them. Another method is to
measure the reflected light by each slide in the same conditions of
illumination by means of an exposure meter: the cleaner slide
reflects less light.
SOAP BUBBLESAs long as there has been soap, making soap bubbles
has been an amusement for children. Everybody has played with soap
bubbles as a child. A straw and a glass with soapy water is all
that is needed to amuse a child for hours. One child blows bubbles
and others run after them and play with or pop them. What
astonishes the children is the spherical and perfect shape of the
bubbles, their colors, their transparency, their lightness which
competes only with that of the butterflies and fairies. By means of
thin membranes of soapy water, it is possible to do interesting
experiments and amusing games, such as to blow bubbles of different
sizes, concentric bubbles, helical bubbles, "solids" supported by
frames in metal wire, it is possible to observe and to study the
coloured interference figures on the membranes of soapy water, to
obtain membranes so thin that they lose all color and become
invisible, to obtain membranes measuring some square meters of
surface and bubbles of some cube meters of volume, so that you can
to trap a friend. And then you will learn to blow cubic bubbles...
by using a square straw, of course! No, just kidding! :)HOW DO THE
SOAP BUBBLES FORM?As Grownups, we pose questions like these: "How
do soap bubbles form? Why does soapy water produce foam while pure
water does not?". When water sprays from a tap in a small basin,
you can see bubbles form, but they burst very soon. This is due to
the fact that the surface tension of the normal water is high and
it tends to draw the water molecules into the main body of the
water, to the point where the thickness of the bubble wall is too
thin to remain intact and quickly bursts. Instead, the surface
tension of the soapy water is much lower: about a third of the pure
water, so the molecules of the bubble are less stressed and it can
last longer. Soap and detergents lower the surface tension of water
and, as we have said, they are called surfactants. As we have said
in the paragraph on the soaps and detergents, the molecules of
surfactants have a hydrophilic head and a hydrophobic tail. When
these molecules are dissolved in water, they tend to collect on the
surface with the tails outward, forming continuous layers (figure
12 B). The membranes of soapy water are made up by three layers:
the external two are formed by surfactant molecules and the
internal layer is formed by soapy water (figure 12 C). These layers
of surfactant molecules are very elastic and they deform easily
without breaking. They also slow the evaporation of the water film
and so extend the life of the bubbles.RECIPESWater is an important
ingredient to our recipes. Usually, to produce soap bubbles, people
used a mixture of tap water and soap. Unfortunately, the mineral
salts which make hard water subtract a part of soap with negative
consequences on the formation of the bubbles. In fact, soap reacts
with the calcium and magnesium salts, which are in the tap water,
forming an insoluble precipitate which subtracts surfactant
molecules from the solution. Instead, the detergents react with the
mineral salts of the water producing soluble compounds, so
detergent are less influenced by the hardness of water. If your tap
water is soft, it is OK to use for bubbles. In any case, you will
obtain the best results with distilled water.After the water, the
most important ingredient is the base surfactant. There are a lot
of surfactants which can be used as detergents and to blow bubbles.
Therefore, try some different brands of detergent until you find
the best one. Dawn and Joy brand liquid detergents for dishes
supplied good results, but try other products if you like.The
presence of water in a soapy film is important to make it last a
long time. As time goes by, a part of the water migrates by gravity
and reaches the bottom of the film or of the bubble and another
part evaporates. In this way, the membrane grows thin, weakens and
in the end bursts. To extend the life of bubbles, people add
substances which make the water more viscous, slowing its descent
toward the bottom. Other substances are added to slow the
evaporation of the water. Substances which have these effects are:
sugar, honey, glycerin, gelatin, arabic gum, viscous liquid soap.
You will have best results if you let the soapy solution rest for a
couple of days, but if you are impatient, you can use it
immediately. A cold solution makes longer lasting bubbles. For
various bubble recipes, look at the links we have put at the end of
this section on bubbles.1 - How to find the basic surfactant. To
find the main component of your recipe, the base surfactant, obtain
some dishwashing detergents, shampoo, bath soap, etc. With water,
make a solution in the ratio of 1 to 10 for each surfactant. In a
place without wind, blow a bubble of about 7 cm in diameter. Keep
it on the straw (figure 13) and measure its duration. Repeat the
test 5 times for each detergent so to obtain a more reliable mean
value. Obviously, the best detergent is the one which produces
bubbles which last longer.Figure 13 - How to keep the bubbles
during the test of duration.
2 - Adjusting the secondary ingredients. A second series of
tests will have the purpose of adjusting the recipe in its
secondary components, those destined to reduce the evaporation and
the fluidity of the water. Follow the same method as you did in
point 1.3 - Blow some bubbles. When the solution is ready, you will
be allowed to pass to the further experiments. In the meantime,
blow some bubbles and watch them fly, carried by the wind.4 - How
to make bigger bubbles. With some thick iron wire, make a ring of
about thirty cm diameter. Immerse it in bubble solution that you
have put in a small basin. Moving the ring quickly in the air, you
should be able to obtain quite large bubbles.5 - Again on the force
of the surface tension. Knot a heavy cotton thread with a slipknot
to the ring of the experiment 4. After you have wet the ring in the
soapy solution, the ring will be closed by a film. If you burst the
membrane inside the loop, you will see it take a circular shape
(figure 14). This happens because of the surface tension of the
remaining part of the soapy film.6 - A support for bubbles. To
comfortably observe bubbles, it is important they are steady. With
some iron wire, make some rings on which to put the bubbles. Leave
a stem to each ring so you can insert it into an object or you can
shape as a pedestal. To avoid bursting the bubbles you put on it,
wet the ring with bubble solution. Wood or velvet can support
bubbles for a long time without bursting them, but are harder to
fashion into a ring shape.
7 - Study the contact surface among bubbles. On a clean glass or
a rigid plastic sheet soaked with solution, place two bubbles in
contact each other. Observe the surface of contact. You will see
the smaller bubble of the two will tend to bulge into the bigger
one. This happens because of the internal pressure of the little
bubble is higher than the pressure of the large ones. This also
means that two bubbles of equal diameter have a flat contact
surface. After having made some bubbles in contact with each other,
produce some foam and observe it. Observe that sometimes the shapes
of the foam bubbles are the same as that of cells of biological
tissues, in other cases the shapes of the cells are different
because they have to increase their surface of contact or for other
reasons. Note also that the crystals of metals often have the same
shape as the foam bubbles. After all, during the solidification of
a metal, they are deformable spheres very close each other and
which cannot leave empty spaces.
Figure 15 - Membranes on a cubic frame. These membranes do not
arrange on the faces of the cube, but they are in contact each
other.Figure 16 - Membranes on a cubic frame. The cubic central
bubble has been placed with a straw.Figure 17 - Membranes in a
pyramidal frame (tetrahedron). Place a bubble in the center.Figure
18 - Membranes between two rings and having a film in common.Figure
19 - Tube-shaped membrane between two rings. It has been obtained
by breaking the film in common.
8 - Solid figures made on suitable frames. With some frames made
with metal wire, you can create flat, helical films or with many
other forms. You can also create quite complex solids (figures 15,
16, 17, 18, 19, 20). To do this you have to dip a suitable frame
into the soapy solution. When you will have withdrawn it, you will
see the membranes. Usually, people expect these films to form on
the faces of the solid, but this does not happen because they tend
to keep into contact with each other and to form figures of minimum
surface area. Remember that soapy films tend to keep the shape of
smallest energy. So, if you will make a tube-shaped membrane, do
not be surprised if its diameter will reduce in the middle.9 -
Helical films. To obtain helical films (figure 20), make a helix
with a few coils made up of iron wire (like a normal spring), place
a piece of wire along the axis of the helix and solder it to the
two extremities of the helix.10 - Regular polyhedral bubbles. What
shape frames are necessary to obtain central bubbles with the shape
of an octahedron, a dodecahedron, an icosahedron? It is possible
fabricate
them?http://www.enchantedlearning.com/math/geometry/solids/
http://wwwalu.por.ulusiada.pt/21575200/Internet keywords: regular
polyhedra
Figure 20 - Helical film.Figure 21 - Frames on metal wire to
study the soapy membranes.
The figure 21 shows some frames of metal wire which can be made
to study the soap films and to measure the surface tension of
liquids. To build them, we have used galvanized iron wire, cut in
segments which then we have soldered with tin. You can also try
plastic coating these frames by dipping them into tool handle
coating products which are sold at hardware stores."Why are soap
bubbles colored?". The membrane of the soap bubbles are formed by
three layers. The external two are both formed by a layer of
surfactant molecules with the polar head turned inward, the inner
layer is formed by soapy water (figure 12 C). The light which
crosses a soap film is in part reflected by the front surface of
the membrane and by the back one. The waves of light reflected
emerge out of phase, they sum algebraically (interference), giving
rise to variations of color. The emerging hue depends on the
thickness of the film. These colors are very fine and create
beautiful shapes formed by the zones of different color when
turbulence is present within the film. In fact, if you gently blow
on a film, you can create magnificent designs (figures 1, 22, 23,
24). Over time, due to evaporation or the descent of the water
toward the bottom, the thickness of the membrane will have become
very thin, the two reflections will fade completely and the bubble
will become black against a black background: it will not show
colors any more and will become invisible. In that condition, the
film will be also very unstable and near bursting.
Figure 22 - The interference fringes which form as the water
flows down by gravity. As the film gets thin at the top it becomes
black because its thickness is less than the wavelength of the
visible light.Figure 23 - By gently blowing on the film, you can
create beautiful turbulence zones which can be observed and
studied. Notice on the top the black zone has widened.Figure 24 -
By blowing again, the figures become more complex and rich with
details.
11 - Colors and shapes of the figures of interference on soap
films. The soap membranes are well suited to observe the colors and
the turbulences which are created by light air currents. So, by
means of a ring on iron wire, make a soap film and examine its
colors. Blow lightly on the film to observe the turbulence on its
surface (figures 23 and 24). To better see the colors of the
membrane, it is worthwhile to observe it against a black background
and illuminate it with bright white light. If you keep the frame
vertical, you will see the colors change as the film grows thinner.
Usually, shortly before bursting, a part of the film will become
black. Here some other figures of interference: figure 31, figure
32.12 - To cross a membrane without bursting it. If you touch a
film with a dry finger, the membrane will burst. If you will wet
the same finger with the soapy solution, the film will not burst
and you will be able to penetrate it.13 - Plays on the water. Make
bubbles in a small basin of water. Look for the conditions which
allow to the bubbles to bounce or to alight on the surface without
adhere to it. Place a drop of oil on the surface of the water,
which will arrange itself on the surface as a monomolecular layer,
(eg. stearic acid) and repeat the test. Also an oily hair can
deposit a thin, oily layer on the waters surface, when slowly
immersed in it.
OSMOSISIf you place two solutions of different concentration
side by side, keeping them separated only by means of a membrane,
you will see the level of the more concentrated solution increase
(figure 25). This happens because the two solutions try to attain
the same concentration by diffusion. The membrane has to be
semipermeable, that is it has to allow the passage of the solvent
but not of the solute. The molecules of the solvent have to be
smaller than those of the dissolved substance. In practice, this
condition is very frequent given that the molecules of water are
very small. It is necessary to remember that it is possible to make
solutions with other liquids also. Osmosis is the tendency of the
system to reach the same concentration in both solutions. It is a
phenomenon of great importance in biology and which is also the
basis of the function of the kidney, of the absorption of water by
plants and which is used by industries to concentrate or to purify
solutions. In fact, applying a pressure on the side of the more
concentrated solution, it is possible to reverse the process and
cause the solvent to pass to the less concentrated solution. This
is the process of the reverse osmosis. It is used also to purify
water, to concentrate solutions, etc.
In order to do experiments with osmosis, you need to obtain a
semipermeable membrane. For this purpose, you can use cellophane,
which is a thin transparent film, essentially made up of cellulose
and which is often used to pack wrap flowers and gifts. Sometimes,
florists also use a plastic which is very like cellophane, but,
instead is completely impermeable to the water and which is not
suitable for these experiments. How can you distinguish between
these two materials? Putting some water on cellophane, you will see
it soften, dilate and even the opposite side of the sheet will
become moist. This does not happen with the transparent plastic
sheet. You can obtain cellophane in a stationery shop.
Unfortunately, this material is often covered with a thin layer of
water repellent nitrocellulose which prevent the passage of the
water. This layer can be removed by immersing the cellophane in a
solvent for varnish or perhaps in acetone. Use caution because
these solvents are inflammable and toxic.Another possible source of
semipermeable membrane can also be found in certain plastic bags.
The plastic is made from starch and is used to produce
biodegradable plastic bags for recycling. In some European cities,
these plastic bags are used to collect organic wastes. When
touched, this plastic is flabby, quite elastic and near rubbery.
You can also try the membrane of a chicken egg and other membranes
you will find or you are able to fabricate.Water flows slowly
through the membrane. If you limit yourself to closing the bottom
of a tube, it will take days to see the level of the inner liquid
increase. To accelerate the flow, it is necessary to widen the
surface of exchange. It would be necessary to have special flared
tubes, which are difficult to find. Instead, you can use a small
funnel, which is much easier to obtain.1 - Diffusion by osmosis.
For the first experiment, use distilled water, some sugar, a
semipermeable membrane, a beaker, and a support for pipettes.
Obtain a flared tube of glass or transparent plastic. Or, as an
alternative, a little transparent funnel. The internal diameter of
this tube has to be at least one cm. With a rubber band or clamp,
attach a piece of membrane on the flared bottom of the tube and
then pour the concentrated solution of sugar in the tube. Insert
the tube in a beaker and put water into it until you attain the
same level of the solution in the tube. After some hours, you
should see the level of the liquid in the tube is increased (figure
25). After some time, the level will attain a maximum. If, instead
of tap water, you will use distilled water, the phenomenon will be
more evident. To render more visible the concentrated solution, you
can add a drop of ink or some watercolor. Why does the more
concentrated solution rise? As we said, there is a tendency of the
two solutions in contact via a semipermeable membrane to reach the
same concentration. The more concentrated solution absorbs solvent
from the more diluted. In these experiments, the level of the
liquid in the tube increases, but not to infinity. It goes up until
the pressure of the liquid column attains the equilibrium with the
osmotic pressure. The equilibrium pressure between a solution and
its solvent is the osmotic pressure of that solution.2 - Osmotic
pressure and density of the solution. Determine the osmotic
pressure of some solutions. Verify if it is proportional to the
amount of molecules per volume of the solution.3 - When the
dissolved particles are very small. If, instead of the sugar, you
will use salt, the osmotic pressure will result very low. This
happens because in water the salt dissociates itself into the Na+
and Cl- ions, which are smaller than the molecules of water and
they easily pass through the semipermeable membrane.4 - Osmotic
pressure and microorganisms. Place under the microscope a slide
with a small drop of water rich in protists, then add a pair of
drops of distilled water. At the beginning, the protists will swell
and you will see their vacuoles work very hard in the attempt to
expel the excess water from their cytoplasm, then you will see
their cellule explode, pouring their organelles outward. The cilia
of the mouth will continue to beat for long time, even if they are
not connected to the body any more.
INTRODUCTION TO THE COLLOIDAL SYSTEMSLet us leave the surface
phenomena to enter into the mysterious world of the colloids. A
first example of a colloid is gelatin, a strange substance: neither
liquid nor solid. It is very elastic and if deformed it returns to
its previous shape. Goofy, the friend of Mickey and Donald, learned
something about it when, in the Disney film: Mickey and the
Beanstalk, he was "walking" on a pudding of the Giant. The emulsion
of oil in water is another substance with unusual properties.
Unusual are also substances such as foams, aerosols, smokes and
fogs, not to mention the solid emulsions and foams. What do all
these curious substances have in common? That is what we will see
before long. These substances are called colloids and they are in
some ways related to the solutions and to the mixtures, even if
they do not belong to the former nor latter. To understand what
colloids are, it is necessary to know what solutions and mixtures
are.
SOLUTIONSA solution is a homogeneous mixture of two or more
substances. When placed in water, many substances dissolve and are
called soluble, others do not dissolve and are called insoluble.
Salt and sugar easily dissolve in water. If instead you put sand in
water, you can mix for as long as you want, but you will not
succeed in dissolving the sand. In fact, sand is insoluble in
water. In a solution, the material present in greater quantity is
defined solvent and that in smaller quantity solute. What does it
mean to say that a substance is soluble in another? It means that
the molecules of the solute separate each other and they disperse
among those of the solvent. Instead, the insoluble substances keep
themselves compact and their molecules do not disperse into the
solvent. As solvent, we have used the example of water because many
solids are soluble in water, but nearly every liquid can be a
solvent. And then, why we should limit ourselves to the liquids?
Let us generalize the concept of solvent and concede to all
substances, solid or liquid or gaseous the possibility to be a
solvent. At this point, even the solutes can belong to all of these
three states of matter. For example, some solid solutions are the
metal alloys such as steel (Fe+C), brass (Cu+Zn), bronze (Cu+Sn).
Finally, all gases are completely soluble among each other. Also
common are solutions of gases in liquids. For example, carbon
dioxide is added to many beverages to make them fizz. In the water
of ponds, rivers and seas, gases like oxygen, carbon dioxide and
others go into solution in a natural way. The presence of these
gases in the water make possible the life of the aquatic
organisms.The solubility of a substance is measured as the maximum
amount, in grams, which can be dissolved in 100 g of solvent. When
the solute does not dissolve any more, but a deposit is formed on
the bottom, the solution is defined saturated.CATEGORIES OF
SOLUTIONS
SOLUTESOLVENTEXAMPLE
GasGasair (nitrogen, oxygen, etc.)
LiquidGasmoist air (water vapor in air)
SolidGasatmospheric dust
GasLiquidCO2 in water (sparkling water)
LiquidLiquidwine (water + alcohol)
SolidLiquidmarine water (salt in water)
GasSolidgas in silicates (pumice stone)
LiquidSoliddental alloys (mercury in cadmium)
SolidSolidmetal alloys (steel, bronze)
1 - Saturated solution. Determine the content of salt in a
saturated solution. In order to not waste too much salt, use only a
little water.2 - To grow crystals. Determining the density of sugar
in a saturated solution is not easy because sugar continues always
to dissolve. Anyway, make a heavy sugar solution and a saturated
solution of salt in water. Put a cotton thread in each of them and
wait some days for some crystals to grow. Describe the shape of
these crystals. If you like to grow crystals, it is possible to
find packets of salts specially chosen to this purpose. Also search
the Internet with the words: growing crystals.3 - Where does sugar
go? Put a beaker on a magnetic stirrer, insert the stir bar and
fill the container with water up to the top. Slowly, add grains of
sugar so they are dissolved by the stir bar as it rotates. Note the
amount of sugar you will have put into the water before it
overflows. Do the same thing with salt and then with sand. Compare
the results and explain the different behaviors.4 - How to separate
salt from sand? Solve this problem: A day, a child who lived on the
border of the desert was sent to buy some salt. While he was coming
back and he was playing with friends of his own, the bag broke and
the sand shed on the sand. For these people the sand was important
and costly, so that child would be scold by his parents. How would
have you done to recover the precious salt, separating it from the
sand?
MIXTURESAs we have seen, by mixing sugar with water, a solution
is obtained. If instead we mix sand into water, we obtain a
mixture. Also by mixing bits of coal and iron filings we obtain a
mixture. With a pair of thin tweezers it is possible to take away
sand grains from the water or pieces of coal from the filings, but
it is not possible to take away singly molecules of sugar from the
water because they are too much small. Hence, what distinguishes a
mixture from a solution? In a mixture the particles are enough
large to be separated by mechanical means such as tweezers or
sieves, in a solution this is not possible because the particles
which form it are so small that they cannot be seen even with an
electron microscope. To separate the components of a solution it is
necessary to use physical method like distillation. So, mixtures
are formed by quite big particles, solution are formed by very
small particles.1 - A mixture. Make a mixture, for example by using
sand and wood sawdust. How could you quickly separate the two
components?2 - Sedimentation speed and size of the particles. As
indicated in the experiment on the analysis of the soil composition
in the article on the experiments on environmental education and
biology, put some water and a sample of earth in a glass or
transparent plastic jar. Close the pot and shake it until all the
earth is dissolved. Place the jar at rest and observe the different
layers of materials. On the bottom, there will be stones and
gravel, then thick sand and fine sand. Silt will require half an
hour to be deposited, clay will demand 24 hours. Very small
particles will remain in suspension, some of them will deposit very
slowly, the finest ones instead will never deposit. Some other
substances will have gone into solution. It seems the Etruscans
collected the very fine clay which deposited after some days to
obtain the black color of their earthenware.3 - To separate
particles according their grain size. If you want to separate the
thick sand from the finer sand, you can use a sieve. If you want to
clean sand from silt and clay, you can use flowing water. With a
plastic tube, make water flow into the container of the sand. The
water will carry away the smaller particles, while the larger ones
will remain in the container. This method exploits the different
sedimentation speeds to separate the particles of different grain
size. Usually, the sand destined to be put in aquariums is cleaned
to avoid water contamination. By using a sieve and with
sedimentations and cleanings, produce 100 g of thick sand, 100 g of
thin sand, 100 g of silt and 100 g of clay. Remove the water in
excess and let all components dry to obtain moist sands, soft silt
and clay. Compare the properties of these materials.4 Observe under
the microscope the finest particles. With a microscope, try to
measure the size of the particles of silt, clay and of those which
remain in suspension in water during your experiments of
sedimentation.
COLLOIDSWe have seen that in the solutions, the molecules of the
solute separate each other and disperse among those of the solvent.
In the mixtures instead, the molecules do not separate and the
particles remain compact. From the point of view of the sizes,
solutions are formed by very small particles (single molecules) and
the mixtures by quite large particles. In an intermediate position,
between mixtures and solutions, there are the colloids. They are
dispersions of small particles, but not molecule sized. What
distinguishes mixtures from colloids and from solutions is
therefore the size of the particles which form them. By convention,
a colloid is a dispersion of particles which size is comprised
between 0.2 and 0.002 m (a micrometer, or micron, = 10-6 meters).
If the particles are larger than 0.2 m, we have a mixture, if they
are smaller than 0.002 m, we have a solution. In general, the
components of a colloid are formed by small aggregates of
molecules, while the components of a solution are single molecules.
Anyway, if these molecules are large enough, as it is the case of
many macromolecules, their solution will give a colloid. So, the
criterion of distinction between colloids and solutions cannot be
the presence of single molecules, but as we were saying, the size
of the particles which form them.MIXTURESCOLLOIDSSOLUTIONS
large particles> 0.2 mmean particles0.2 - 0.002 mthin
particles< 0.002 m
According to the dispersing phase, colloids are distingued in
gaseous, liquid and solid suspensions. Gaseous suspensions, or
aerosol, are smokes and fogs. Smokes are suspensions of solid
particles in a gas. Fogs are suspensions of liquid particles in a
gas. Sols, gels, emulsions, foams are liquid suspensions. Oily
rocks, pumice stones are solid suspensions.TYPES OF COLLOIDS
DISPERSED PHASEDISPERSANT PHASENAMEEXAMPLE
SolidGasSmoke - AerosolSmoke
LiquidGasFog - AerosolFog
SolidLiquidSol, GelPaint, Gelatin
LiquidLiquidEmulsionMilk
GasLiquidFoamBeer foam
SolidSolidSolid suspensionAmethyst
LiquidSolidSolid emulsionOily rocks
GasSolidSolid foamPumice stone
The term colloid refers to substances with a glue-like
consistency, in which the dispersant phase is therefore liquid.
However, do not forget that even substances such as smokes and
aerosols, in which the dispersant phase is aeriform and which we
can also call gaseous suspensions, are colloids. Finally, even some
solid substances, in which the dispersant phase is solid and which
we can also call solid suspensions, are colloids too.Colloids have
unusual properties, for example gelatin. Colloidal systems have a
high ratio area/volume among the surface of the particles and their
volume. In other words, as in the colloids the amount of dispersed
particles is very large, their overall surface is very large too
and by consequence the interaction of the two phases is important.
For example, a cube of 1 cm a side has a surface area of 6 cm2, the
material of the same cube divided into little cubes of 0.002 m of
side, has a surface area of 3000 m2. Because of the wide surface of
contact between the two phases, often the colloids are studied with
the surface phenomena and the discipline which studies them is
called surface and colloid science.
SOLA sol is a dispersion of very thin solid particles in a
liquid. It has a liquid consistency and resembles a true solution.
An aqueous sol appears clear, very similar to common water. Anyway,
if you shine an intense beam of light across it, a part of the
light will be diffused from the particles which are in suspension.
These particles are very small, but they are still enough large to
obstruct the light and diffuse it. This phenomenon is called
Tyndall effect. You can observe it with sols, but not with true
solutions.1 - Tyndall effect. In a transparent jar, put some clayey
earth 1/4 of the volume and water until attain 3/4 of the
container. Close the jar with its cap and shake until all the earth
is "dissolved". Leave the pot to rest for a day to allow the clay
particles to settle. The liquid which is above the sediment should
have become clear. Shining an intense bundle of light through the
jar, you should see the Tyndall effect. Do the same thing with a
glass of pure water and compare the results.
GELA gel is a dispersion of very thin solid particles in a
liquid and it has a gelatinous consistency. Increasing the
concentration of the particles, a sol can pass to the state of gel.
On the contrary, by diluting a gel you will obtain a sol. So, what
makes a sol different from a gel is its fluid or gelatinous
consistency. Also the temperature can determine the passage from
sol to gel and vice versa. For example, broth gelatin is gelatinous
at room temperature, but it becomes liquid when it is heated.
Animal gelatin is a reversible gel because depending on the
temperature it can pass from gel to sol and vice versa The albumen
of eggs instead is not reversible because when heated it coagulates
and it does not come back to the state of sol. Silica gel absorbs
moisture and keeps its properties with broad concentrations of
water. Because its affinity for water it is used as dehumidifier.
When left to rest, a sol can spontaneously jell and come back to
the state of sol simply by mixing it (eg: aqueous suspensions of
kaolin).
1 - Making gelatin. Buy some dry gelatin. Dissolve it in warm
water and, with subsequent dilutions, determine what is the minimum
concentration of dry gelatin necessary to obtain a normal gelatin
at room temperature. Do not keep gelatins a long time because they
easily become cultures of bacteria. Store them in a refrigerator
and, after a day, throw them away.2 - Reversibility of the gelatin.
By means of the temperature, make some gelatin pass from the gel to
sol states and vice versa.3 - Experiments with vegetable resin.
Resins are gels and they possess useful properties. Often,
fruit-bearing plants produce gelatinous spheroids which diameter
can attain some centimeters. Conifers are important producers of
resins and often you can collect drops of resin which hang from
their trunk. You can also make an incision on a trunk to obtain
some resin. Canada Balsam is a very important resin in optics and
in microscopy. It is extracted from the Abies balsamea, a conifer
of North America and it is used to glue lenses and to make
permanent microscope slides. For their adhesive properties, resins
take part to the composition of paints. Collect resin from trees,
observe under the microscope the particles which are suspended in
it. Dissolve the resin of a fruit-bearing tree in warm water and
try to obtain a glue. Dissolve the resin of a conifer in turpentine
and assess their adhesive properties.4 - Experiments with
polysaccharides. Polysaccharides are resinous gums soluble in
water. They are used in the fabrication of cosmetics, paper and in
a lot of other applications. Some polysaccharides are edible and
are added in creams, yogurts and in other foods. You can obtain
some polysaccharides and experiment with their properties. In
particular, add to them some water and check the consistency,
viscosity and adhesiveness of the substance you will
obtain.Absolutely do not eat polysaccharides, do not inhale their
powders and do not use them in recipes for food. If eaten dry,
these substances will swell and risk obstruction of the digestive
tract. If inhaled, they will swell and risk obstruction of the
respiratory airways, causing dangerous problems in breathing. Do
not use them in food recipes, but only in experiments. Keep in mind
that some polysaccharides are not edible. When hydrated, these
substances become culture medium for bacteria, so use them for a
short time and then throw them away. An adult must be always
present during these
tests.http://saps1.plantsci.cam.ac.uk/worksheets/ssheet22.htm Some
Gum Fun (experiments with
polysaccharides).http://food.orst.edu/gums/foegeding.html
Hydrocolloids, Vegetable Gums References.
http://class.fst.ohio-state.edu/FST605/lectures/lect20.html Gums
and stabilizers (formula and other information).Internet keywords:
polysaccharides, hydrocolloids, experiments, recipes.5 - Making
photographic gelatin. Photographic gelatins have a suspension of
silver halide salts, which are sensitive to the light. When they
are still warm, these gelatins are spread on a transparent plastic
film to obtain a photographic film, or on a card to obtain paper
for photographic prints. As shown through the history of
photography, there are many methods to produce photosensitive
surfaces, and many of them do not use silver salts. In the Internet
you can find recipes to make photosensitive films and paper by many
techniques. These preparations require the use of substances and
procedures which can be dangerous. Read information on the caution
needed. Children must be guided by an adult who is expert in
chemistry.
EMULSIONSAn emulsion is a dispersion of an insoluble liquid in
another liquid. For instance, the oil is not soluble in water. If
you pour some oil in a container with water, it will float it and
keeps separate from the water. Instead, if you vigorously shake the
container, you will obtain a dispersion of small drops of oil in
water, however these drops quickly join together, so that in a
short time nearly all the oil will return as before. To make the
emulsion more stable, before shaking the container, add some
detergent. The surfactant molecules will arrange on the surface of
the oil drops with the heads outward. As these heads have an
electrical charge and as this charge is always the same, the oil
drops will repel each other and be unable to return to the
homogeneous layer as before. So, surfactants can help you to obtain
more stable emulsions. There are special surfactants for emulsions,
endowed of a higher capability to stabilize the oil drops than the
detergents. There are also emulsifying agents for alimentary use
such as lecithin and emulsifiers for industrial purposes which are
not edible. Butter is formed by small water drops suspended in fat.
Cheese and mayonnaise too are considered emulsions. A lot of creams
used both in pharmacy and in cosmetics are emulsions. Fuels
emulsified with water have been produced. Emulsified oils are used
in machine working to make it easier to cut metals with machine
tools. In fact, metal cutting can create an intense heat, which has
to be removed if you want to avoid burning the tools. The oil and
water in the cutting fluid help remove the heat and make it
possible to cut metals efficiently. Milk is another emulsion made
up by small greasy drops in an aqueous phase.1 - Stability of the
emulsions. Fill two plastic bottles halfway with water, then put 5
cc (about a spoonful) of vegetable oil in each. Only in one of
these bottles, put 0.5 cc (about 20 drops) of liquid detergent for
dishes. Close the bottles and shake them for a couple of minutes to
emulsify the oil, then place them on a table and observe them. The
drops of oil will try to reassemble and to surface. By comparing
the two emulsions, you will see that the one with detergent will be
much more stable (figure 28). In fact, even after a month, the
white color of this emulsion indicates that there is a great deal
of small oil drops in the liquid, while in the other bottle the
liquid is become nearly transparent, this is a sign that near all
the oil drops have fused together and surfaced.2 - Vinegar and
vegetable oil. Using a kitchen whisk, emulsify a teaspoon of
vinegar with 125 cc of peanut oil or olive oil. The emulsion will
result instable.
Figure 28 - The two emulsions of the experiment 1 after 24 hours
of rest. In the right bottle, some detergent has produced a more
stable emulsion.
3 - Mayonnaise. To the ingredients of the test 2, add an egg
yolk and emulsify again. The emulsion will be much more stable. Add
some salt and if you want some pepper and you will have obtained a
good mayonnaise. If you prefer, you can replace the vinegar with
lemon juice. Why is the emulsion stable with the egg yolk? This is
due to the presence of lecithin in the egg yolk. Lecithin is a
surfactant and the molecules spread on the surface of the oil drops
with the hydrophilic head outward. As these heads are electrically
charged, the oil drops will repel and their merging is prevented.
Lecithin is a phospholipid and it has a structure like that of the
phospholipids which form the membranes of cells. Another well known
lecithin and which you can find on the market is soy lecithin.
FOAMSFoam is a dispersion of a gas in a liquid (liquid foams) or
in a solid (solid foams). Among the liquid foams, we have the ones
produced by soaps and detergents, and various foods such as wine,
beer and many others. Among the solid foams we have Pumice stone,
earthenware, sponges, expanded plastics like expanded polystyrene
and expanded polyurethane. By dispersing helium in a liquid which
produced bubbles with very thin walls and which then solidified,
some researchers succeeded in fabricating a solid foam lighter than
air.1 - Foam and shape of the bubbles in contact. With a drop of
liquid detergent in a small basin of water, make a foam. Observe
the shape of the bubbles which are in contact each other. With a
microscope, observe a thin section of elder pith and compare it
with the foam.
2 - Make a solid foam. Beat egg whites and some sugar, then cook
it so to obtain its solidification: you will have obtained a
meringue, just an edible solid foam.
OTHER EXPERIMENTS WITH SURFACTANTS AND COLLOIDS1 - Who can guess
more colloids? List the colloids you have in your home or which you
know by experience: (milk, mayonnaise, resin, paint, ink, expanded
polystyrene, cell cytoplasm, blood serum, etc.).2 - A half-solid
fluid. Put in a cup four spoons of corn starch. Add some water
until you have obtained a creamy substance. While mixing, you will
notice that this substance has an odd property: if you slowly mix
it, it behaves like a liquid, but if you try to mix it fast, it
seems solid. By quickly lifting it on a side, you will be also able
to remove this cream from the cup, but you will have some
difficulties in keeping it in your hands because, even if it moves
slowly, it will escape from all sides like a liquid. Liquids which
change viscosity with the mixing speed are called dilatant fluids.
Also wet sand behaves as dilatant fluid. Sold in the US as a childs
toy under the name of Gak or Goo, you can make your own by
dissolving 1/2 cup of white glue with 1/2 cup of water, then adding
3 tablespoons of Borax, while stirring well. You will obtain a
substance which is apparently solid, but which loses its shape
within some minutes, becoming like a liquid puddle... which however
you will able to lift it as if it was a carpet.
ATOMIZER FOR AEROSOLHow do atomizers work? There are many models
of atomizers or of sprayers like those of pressurized spray paint
cans, or those provided with a small pump that you press with a
finger, those that work by mean of a rubber syringe or, for
industrial uses, by a compressor.1 - Anatomy of an atomizer.
Disassemble a trigger spray bottle. Often, these devices breaks so,
if you have one of them broken, dismantle it to try to understand
why it does not work any more and try to repair
it.http://www.howstuffworks.com/question673.htm Trigger spray
bottle.2 - Build an atomizer. To build a small atomizer, take two
thin straws and fix them as shown in figure 30. At the end of the
horizontal straw, insert a plug with a hole of one mm of diameter.
Under the vertical straw, mount a small bottle with water. Now,
blow with force in the horizontal straw. The air jet which comes
out of the hole will cause an area of low pressure above the
vertical can which will draw some water up the straw and blow it
away atomizing it. To produce an air jet, you can also use a rubber
syringe. Usually, this type of atomizers is used for perfumes, but
you can use it also to humidify the leaves of a house plant.
InstrumentsSince our research is strongly experimentally
oriented, we maintain a substantial instrumental park in our
laboratory. These instruments can be classified in three main
groups, namely light scattering, atomic force microscopy, and
surface sensitive techniques. We also operate various standard
laboratory equipment, such as, refractometers, viscometers,
titrators, pH sensors, conductivity meters, or water purification
systems.General course objectives:This introductory master course
presents colloid and surface chemistry. The course deals with
important principles and phenomena related to colloid systems and
surface chemistry. These subjects are fundamental to the
understanding and design of a range of processes like e.g.
adhesion, lubrication, cleaning, oil recovery, water and air
purification. Furthermore the subjects are essential for the
application and design of a number of chemical products like e.g.
paint, glue, detergents, cosmetics, drugs and foods. Finally, the
course offers understanding of several naturally occurring
phenomena like e.g. fog, rain drops, the capillary effect the red
sunset, the blue sky and the rainbow, and beer foam.Learning
objectives:A student who has met the objectives of the course will
be able to: evaluate and describe colloidal nano-technological and
chemical systems, processes and products use different theories to
calculate surface and interfaces tensions and use this to estimate
e.g. wetting and other system characteristics identify mechanisms
for adhesion between surfaces and materials and use different
methods to estimate this describe the most important and
fundamental theories in surface chemistry explain micellation of
surfactants, know how to measure this and calculate dependencies of
salt concentration, system temperature and surfactant chain length
compare and understand adsorption in gas-liquid and solid-liquid
surfaces and perform quantitative adsorption calculations calculate
molar mass and molecular shape of colloid particles and polymers
based on experimental data describe the interactions between
colloidal particles and identify similarities and differences for
the governing molecular forces and interactions explain the most
important parameters for the theories of colloidal interaction and
perform calculations using the theories describe the conditions for
stability of colloidal systems and discuss and compare different
mechanisms for stabilization describe mechanisms for stabilization
of emulsions and foam, and design emulsions and foam by using
various semi-empirical methodsContent:i. Common presentation of
colloidal and surface phenomena ii. Theories for calculating
surface tension (in air), liquid-liquid interfacial tensions as
well as interfacial tensions for solid surfaces iii. Fundamental
theory (Young-Laplace, Kelvin equation, Youngs equation for contact
angle and Gibbs adsorption theory) iv. Surfactants detergents:
micellation (critical micellar concentration, CMC) adsorption of
surfactants on surfaces v. Adsorption at gas-liquid, liquid-liquid
and solid-liquid surfaces. Langmuir and BET theories vi. Wetting
and adhesion mechanisms and calculations including Zisman's plot
vii. Kinetic, optical and electric properties of colloidal
particles viii. Experimental metods for characterising colloidal
particles estimation and measurement of structure, size and shape
ix. Intermolecular og interparticle forces: van der Waals and
double-layer forces (zeta potential, Debye thickness, Hamaker
constant) x. Stability of colloidal systems. DLVO theory and steric
stabilization xi. Emulsions and foam, (HLB, Bankroft-rule,
etc.)Colloids ApplicationsA colloid is typically a two phase system
consisting of a continuous phase (the dispersion medium) and
dispersed phase (the particles or emulsion droplets). The particle
size of the dispersed phase typically ranges from 1 nanometer to 1
micrometer. Examples of colloidal dispersions include solid/liquid
(suspensions), liquid/liquid (emulsions), and gas/liquid (foams). A
more complete range of colloidal dispersions is shown in the table
below.
Particle InteractionsAs particle size decreases, surface area
increases as a function of total volume. In the colloidal size
range there is much interest in particle-particle interactions.
Most colloidal commercial products are designed to remain in a
stable condition for a defined shelf life. Milk is an example where
homogenization is used to reduce droplet size to delay the onset of
phase separation (i.e., creaming with the fat rising to the
surface). Commercial suspensions may be formulated to keep
particles in suspension without sedimenting to the bottom. Examples
of phase separation mechanisms are shown below.
Colloidal StabilizationStabilization serves to protect colloids
from aggregation and/or phase separation. The two main mechanisms
for colloid stabilization involve steric and electrostatic
modifications. Electrostatic stabilization is based on the mutual
repulsion of like electrical charges. By altering the surface
chemistry to induce a charge on the surface of particles it is
possible to enhance the stability of the colloidal dispersion.
Zeta PotentialZeta potential refers to the potential in the
interfacial double layer (DL) at the location of the slipping plane
versus a point in the bulk fluid away from the interface. In other
words, zeta potential is the potential difference between the
dispersion medium and the stationary layer of fluid attached to the
dispersed particle. A classic example of colloid chemistry is to
measure zeta potential vs. pH to determine the conditions where the
zeta potential reaches zero, known as the isoelectric
point.Download the application note on Isoelectric Point
Determination (You need to be logged in).
Instrumental TechniquesScientists working to improve colloidal
stability measure particle size, zeta potential, or both. Various
techniques are now capable of measuring particle size into the
colloidal region including dynamic light scattering (DLS) and laser
diffraction. The SZ-100 nanoPartica DLS system can measure particle
size and zeta potential of colloidal dispersions and has the option
of an automatic titrator for zeta potential vs. pH studies. The
LA-960 laser diffraction particle size analyzer is the best choice
when particles above 1 micron may also be present in the particle
system. Learn how dynamic light scattering measures particle
size.Watch Webinar TE012: Introduction to Dynamic Light Scattering
(You need to be logged in)Learn how electrophoretic light
scattering measures zeta potential.Watch Webinar TE013:
Introduction to Zeta Potential Technology (You need to be logged
in)
More Information about Colloids Nanoparticles Colloidal Silica
Gold Nanoparticles Metal Nanoparticles
Colloid Analyzers
SZ-100 - Nanopartica Series InstrumentsIndustry's widest range
and highest precision measurement instrument for Nanoparticle
characterization.
LA-960 Laser Particle Size Analyzer - High Performance Laser
Diffraction AnalyzerThe LA-960 uses Mie Scattering (laser
diffraction) to measure particle size of suspensions or dry
powders. The speed and ease-of-use of this technique makes it the
popular choice for most applicationsColloidal Stability in Aqueous
Suspensions Why too much dispersant causes problems.AbstractZeta
potential in aqueous suspensions is a function of two variables:
charge at the shear plane and free salt ion concentration. [Free
here means not attached to the particle surface.] If a dispersant
is added to increase the surface charge density (increase
stability) and it's too concentrated, the contribution it makes to
the free salt ion concentration is counterproductive (promotes
instability).IntroductionColloidal suspensions are stabilized in
one of two ways. Surface charge, naturally occurring or added,
enhances electrostatic stability. Adsorption of non-polar
surfactants or polymers enhances stability through static
stabilization. Electrostatic stabilization gives rise to a mobile,
charged, colloidal particle whose electrophoretic mobility can be
measured. Zeta potential is calculated from mobility.The square of
the zeta potential is proportional to the force of electrostatic
repulsion between charged particles. Zeta potentials are,
therefore, measures of stability. Increasing the absolute zeta
potentials increases electrostatic stabilization. As the zeta
potential approaches zero, electrostatic repulsions becomes small
compared to the ever-present Van der Waals attraction. Eventually,
instability increases, that can result in aggregation followed by
sedimentation and phase separation.Electrostatic Potential
Differences: Surface Potential DefinedImagine that you had two,
infinitesimally small metal probes attached to a voltmeter. Now
imagine one probe is attached to the surface of a colloidal
particle and the other one is in the liquid in which the particle
is suspended. The reading on the meter is the electrostatic
potential difference between these two points. It is called the
surface potential o. See Figure 1 where o = +80 mV.The y-axis in
this figure also represents the solid-liquid boundary. The x-axis,
in nanometers, is the distance from the surface out into the
liquid, it being assumed there is no other particle close by. There
are two idealizations in a figure like this one. First, real solid
particles are not smooth at the atomic level. They are more like
low lying, rough hills on the atomic level. Second, the charge
density on the surface is not typically uniform, but often patchy.
The surface has lots of hydrophobic spaces characterized by no
charge and lots of hydrophilic spaces characterized by
charge.Therefore, if we could attach a tiny voltmeter probe at
specific surface locations, the surface potential would vary from
place to place. But we can neither freeze the particle motion in a
liquid nor are there probes small enough. Thus, a cartoon like this
one arises when we average spatially (vertically) over the rough
surface to define an imaginary plane to call the surface.
Figure 1: Electrostatic potential vs. distance in nanometers
from colloidal particle surface. (Courtesy of David Fairhurst)In
addition, we are averaging temporally over the rotational diffusion
time of the particle that is much faster than the time to make an
electrostatic measurement. Still, these idealizations work well and
have been the basis for using zeta potential determinations to
describe colloidal stability for more than 50 years.Before
describing the zeta potential, it is wroth nothing a few special
features of the curves in Figure 1. If nothing is specifically
adsorbed onto the surface, the corresponding anions (the
suspensions much be neutral overall), or the anions from added
salts (or surfactants) preferentially gather near the positive
surface. Thermally-driven diffusion increases the randomization of
all ions as the distance from the surface increases. The
electrostatic potential difference thus decreases. Far enough away
from the surface, if the voltmeter probes are placed in the liquid;
the electrostatic potential difference is zero since the average
charge density is constant.Depending on the sophistication of the
theory to describe what takes place close to the surface, a variety
of imaginary, but theoretically useful planes or layers are
defined. Here, the simplest is shown. It is called the Stern plane.
The electrostatic potential difference is called ?d. It represents
the average position of the counter-ions that move with the
surface.Zeta Potential DefinedAny molecule covalently bonded to the
surface move with the particle when it diffuses o