Trattamento delle sospensioni acquose

Trattamento delle sospensioniacquose

Trattamento sospensioni colloidali.

• Proprietà e classificazione colloidi.• (Filtrazione )• Aggregazione.

– Coagulazione (Coalescenza)– Flocculazione

LONG-RANGE VAN DER WAALS ATTRACTION

s0 a

s00

Hamaker constant, A,proportional tointermolecularattraction constant

Potential energyof interaction

�

! = "Aa

12s0

Attractive forceof interaction

�

Fatt

=d!

ds0

=Aa

12s0

2

ELECTROSTATIC REPULSION RETARDS AGGREGATION

van der Waals attraction

electrostatic repulsion

ELECTROSTATIC REPULSION DEPENDS ON….

1. The Stern potential, ψδ

2. The thickness of the diffuse double layer, κ-1

Electrical Repulsion

++++

++

+++++ +

- --- -

--

--

- -

- -

--

-

-

-+

++

++

+++

++

++

+++++ +

- --- -

--

--

- -

- -

--

-

-

-+

++

++

+Electrical Repulsion

strato dell'acqua legata" in quanto si muove con essa; il secondostrato, di spessore relativamente molto maggiore è detto "strato

diffuso" o "strato dell'acqua libera"

Forces between particles (surfaces)

s

Attractive forces• van der Waals forces

0�

!

kT

s

DLVO Theory

0

�

!

kT

s

Repulsive forces• electrostatic forces (aqueous or high ε media)

0�

!

kT

s

When (Φ/kT) ≥ 10, → “stability”

DerjaguinLandauVerweyOverbeek

SOMMA DI POTENZIALI DIATTRAZIONE E REPULSIONE

Controlling DLVO interactions

0

�

!

kT

s

�

!kT

=0.064"aN

AvC

# 2tanh

2 ze$%

4kT

&

' (

)

* + e

,#s ,A212a

12kTs

water, 25ºC

�

! =2e2

z2n"

##0kT=

2F2z

2C

##0RT $ 0.329z C Å-1

Stabilizing colloids against vander Waals

1. Charge Stabilization

DLVO potential (1940’s)

Derjaguin

Landau

Verwij

Overbeekpicture1968

Stabilizing colloids against vander Waals

1. Charge Stabilization

2. StericStabilization

3. Nano-halo Stabilization?

Classification of Colloidal Systems

Classification Lyophilic(Hydrophilic)

Lyophobic(Hydrophobic)

Association

DispersedPhase

Single Macro-molecule

Large collectionof ions

Collection ofsurfactantmolecules(micelle)

Interactionwith DispersionMedium

Strong Weak Strong

Formation SpontaneousReversible

Specialtechnique

Spontaneousabove cmc.

Classification of Colloidal Systems

Classification Hydrophilic Hydrophobic Association

StabilizingMechanism

HydrationElectrical

repulsion

Electricalrepulsion

Hydration/Hydrophobic

interactionsViscosity Dramatic

increaseNo change Little change;

can increase athigh conc.

Examples GelatinMethyl-cellulose

Silver IodideGold Sols

Tween 80Bile Salts

Attractive Interactions

Pair interactions are additiveV = van der Waals Attractive EnergyA

DEFINITION OF A COLLOID

Mixtures of one material (the “dispersoid”)dispersed in another as particles in thesize range of:

1nm → 10µm.

Ostwald: “die Welt der vernachlässigten Dimensionen”

Chem E 455: Surface and Colloid Science Laboratory: Spring Quarter

ColloidsColloids• Colloids are suspensions in which the suspended particles are

larger than molecules but too small to drop out of the suspensiondue to gravity.

• Particle size: 10 to 2000 Å.• There are several types of colloid:

– aerosol (gas + liquid or solid, e.g. fog and smoke),– foam (liquid + gas, e.g. whipped cream),– emulsion (liquid + liquid, e.g. milk),– sol (liquid + solid, e.g. paint),– solid foam (solid + gas, e.g. marshmallow),– solid emulsion (solid + liquid, e.g. butter),– solid sol (solid + solid, e.g. ruby glass).

Dispersed

Phase

Dispersion

mediumCommon Name Examples

Liquid Gas Aerosol Fog, fine sprays

Solid Gas Aerosol Smoke, viruses, tear "gas"

Gas Liquid Micro-foam Shaving cream, beer foam

Liquid Liquid Emulsion Milk, mayonnaise

Solid Liquid Sol Paint, ink, clay

Gas Solid Solid micro-foam Foam plastics, marshmallow

Liquid Solid Solid emulsion Butter, cheese, opal, pearl

Solid Solid Micro(nano)

composite

Injection-molded objects

LYOPHOBIC COLLOIDS

ColloidsColloids• Tyndall effect: ability of a Colloid to scatter light.

The beam of light can be seen through the colloid.

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Focus on colloids in water.• “Water loving” colloids: hydrophilic.• “Water hating” colloids: hydrophobic.• Molecules arrange themselves so that hydrophobic

portions are oriented towards each other.• If a large hydrophobic macromolecule (giant molecule)

needs to exist in water (e.g. in a cell), hydrophobicmolecules embed themselves into the macromoleculeleaving the hydrophilic ends to interact with water.

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Typical hydrophilic groups are polar (containing C-O, O-

H, N-H bonds) or charged.• Hydrophobic colloids need to be stabilized in water.• Adsorption: when something sticks to a surface we say

that it is adsorbed.• If ions are adsorbed onto the surface of a colloid, the

colloids appears hydrophilic and is stabilized in water.• Consider a small drop of oil in water.• Add to the water sodium stearate.

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Sodium stearate has a long hydrophobic tail

(CH3(CH2)16-) and a small hydrophobic head (-CO2

-Na+).• The hydrophobic tail can be absorbed into the oil

drop, leaving the hydrophilic head on the surface.• The hydrophilic heads then interact with the water

and the oil drop is stabilized in water.

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids

ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Most dirt stains on people and clothing are oil-

based. Soaps are molecules with longhydrophobic tails and hydrophilic heads thatremove dirt by stabilizing the colloid in water.

• Bile excretes substances like sodium stereate thatforms an emulsion with fats in our small intestine.

• Emulsifying agents help form an emulsion.

ColloidsColloidsRemoval of Colloidal ParticlesRemoval of Colloidal Particles• Colloid particles are too small to be separated by physical

means (e.g. filtration).• Colloid particles are coagulated (enlarged) until they can

be removed by filtration.• Methods of coagulation:

– heating (colloid particles move and are attracted toeach other when they collide);

– adding an electrolyte (neutralize the surface chargeson the colloid particles).

• Dialysis: using a semipermeable membranes separateions from colloidal particles.

INSTABILITY OF LYOPHOBIC COLLOIDS

Aggregation:

etc.

Coalescence:

Coagulation of a Colloid

trattamento di flocculazione edecantazione. Il processo di coagulazioneviene agevolato da una lenta e continua

agitazione della massa.

ELETTROFORESI

elettrodialisi