CHEM1612 - Pharmacy Week 13: Colloid Chemistry

CHEM1612 - PharmacyWeek 13: Colloid Chemistry

Dr. Siegbert SchmidSchool of Chemistry, Rm 223Phone: 9351 4196E-mail: [email protected]

Unless otherwise stated, all images in this file have been reproduced from:

Blackman, Bottle, Schmid, Mocerino and Wille,     Chemistry, John Wiley & Sons Australia, Ltd. 2008

     ISBN: 9 78047081 0866

Lecture 36 - 3

Colloids and Surface Chemistry

Particle size Classification of colloids Stability of colloids Steric interactions Blackman, Bottle, Schmid, Mocerino & Wille: Ch. 7, 22

Tyndall effect – light scattering by colloid particles

Lecture 36 - 4

What is a Colloid?Solution

homogeneous mixture, e.g.

sugar in water, single molecules

Suspension heterogeneous

mixture, e.g sand in water,

particles visible, settle out

Colloid size 1-1000 nm particles invisible, remain suspended

Lecture 36 - 5

What is a Colloid? No simple definition Intermediate between a suspension and a solution

Consists of a continuous phase and a dispersed phase. Dispersed Phase (discontinuous phase) Dispersion Medium (continuous phase)

Classified in terms of dispersed substance (s, l, g) in dispersing medium (s, l, g)

Dispersed phase At least one dimension is >1 nm and <1 micron

Thermodynamically unstable Huge total surface area

Lecture 36 - 6

Surface Effect

The surface area has increased by 1 million times but the volume is the same.This means most of the substance is now on the surface.

= 6·10-4 m2 Make sides one million times smaller: d = 10nm (1018 cubes)

The total surface area becomes 600 nm2 × 1018 = 600 m2

Lecture 36 - 7

Nano Scale

M. Dresselhaus, MIT

Lecture 36 - 8

Colloidal Dimensions

(a) kaolinite (b) Plaster of Paris, cement, asbestos (c) polymer lattices (d) network structures, e.g. porous glass, gels

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Lecture 36 - 9

Classification of ColloidsDispersed

Phase Dispersing Medium

Name of Colloidal System

Common Examples

Liquid Gas Liquid Aerosol mist, clouds, fog

Solid Gas Aerosol dust, smoke

Gas Liquid Foam suds, whipped cream

Liquid Liquid Emulsion cream, milk, mayo

Solid Liquid Sol paints, jellies, sewage

Gas Solid Solid Foam marshmallow

Liquid Solid Solid Emulsion butter, cheese

Solid Solid Solid Sol opals, some alloys

Lecture 36 - 10

ExamplesExample Class

Mist liquid aerosolMilk emulsionBlood bio-colloid (sol)Bone bio-colloid (solid sol)Asphalt emulsion (asphaltene dispersed phase and maltene

contin.)Mayonnaise emulsionToothpaste slurry/paste (solid in liquid)Smoke liquid and solid aerosolOpal solid suspension or dispersion (solid sol)Paint sol or colloidal suspensionFoams gas dispersed in liquidCement solSoap liquid emulsionSilica gel gel

Identify the following types of colloids:

Lecture 36 - 11

Natural Instability of Colloids The interaction between molecules of one substance with another

is almost always more high in energy (unfavourable) than the interaction of one substance with itself (‘like dissolves like’).

One big lump of clay in a bucket of water is thermodynamically much more stable than clay particles dispersed throughout the water.

A system will move in such a way as to eliminate unfavourable interactions, i.e, to eliminate surfaces. This is achieved when the particles stick together, rapidly growing in size, resulting in flocculation, coagulation, and sedimentation.

Much of colloid science is devoted to controlling the stability of colloidal dispersions.

Lecture 36 - 12

Flocculation

We can break the colloid stability problem into a series of steps.

particles dimers “flocs” gravity-effected separation

Lecture 36 - 13

Colloid Stability All atoms experience a short range attraction that arises from

dipole/dipole interactions of electron clouds - van der Waals attraction. These forces are between dipoles, between a permanent dipole and an induced dipole, and between two instantaneous dipoles (dispersion forces).

However we know that some colloids are stable, e.g rivers are muddy, so the clay/s and particles must be stabilised by some force.

Therefore a repulsive force is required to obtain stable colloids. This repulsion can be of different nature:

electrostatic steric

Time = t Time = t + dt

Lecture 36 - 14

Charged Surfaces In water most surfaces are electrically charged, due a number of

different mechanisms:

1. Adsorption of an ionic surfactant from solution

2. Surface ionisation, due to surface acid-base reactions,e.g. silica in a pH range

SiOH → SiO - + H+

At neutral pH most oxides have negatively charged surfaces.

3. Differential solubility of cation and anion in an insoluble salt

Lecture 36 - 15

This charge induces an electrical double layer in the vicinity of the solid, i.e. a first layer of charges of opposite sign next to the solid, where:

[counter ions] > [free ions of same charge as colloid]

Repulsion between ‘atmospheres’ of charged particles around charged colloids stabilises the colloid

Electrostatic Repulsion

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ElectricalDoublelayer

Lecture 36 - 16

Electrostatic Interactions Two like-charged surfaces repel each other within a range given by

the Debye length κD-1

. For a 1:1 electrolyte, a simplified expression for the Debye length is:

For a 1:1 electrolyte, the Debye length is KD-1= 1 nm for 0.1 M NaCl.

[salt]304.01

D

Lecture 36 - 17

Debye LengthThe Debye Length is a measure of the thickness of the diffuse layer.This table shows that the diffuse layer extends into solution by several nanometers.

[NaCl] /M -1 /nm

1.0 x 10-4 30.4

1.0 x 10-3 9.61

1.0 x 10-2 3.04

1.0 x 10-1 0.961

1.0 0.3 Increasing concentration of counter ions reduces the thickness of

the electrical double layer. Adding salt to a colloidal solution therefore destabilises it,

because the particles then can approach each other and coagulate.

Lecture 36 - 18

Atomic Force Microscopy (AFM)

AFM Tip and Cantilever

Atomic resolution imageof a mica crystal

Laser

Cantilever spring

Split Photodiode

Sample

Piezoelectric element

AFM probe: a microscopic tip is mounted at the end of a microscopic cantilever. The cantilever deflects as a consequence of forces between it and the sample.

The cantilever deflection is detected via the optical lever system, measured by the photodiode and input to the controller electronics.

AFM can be used to image surfaces with high resolution, and to measure forces with high precision.

The force F acting upon the tip is related to the cantilever deflection x by Hooke's law:

F = -k·x

where k is cantilever spring constant.

Lecture 36 - 19

Example: River + Ocean The higher concentration of positive ions in

the sea water allows the negatively charged clay particles to approach more closely before they experience a repulsive force.

Positive ions from the sea water bind to the surface of the clay particles, reducing the negative charge on them and hence the interparticle repulsion.

The action of the waves subjects the clay particles to increased shear forces, increasing the frequency of collisions.

The Nile Delta

Figure from Silberberg, “Chemistry”,

McGraw Hill, 2006.

Lecture 36 - 20

Hardy-Schulze Rule Flocculation is controlled by the valency of the counter-ion (added

electrolyte with charge opposite that of the particle surface) Fewer 3+ ions than 2+ than 1+ ions are needed to cancel out colloid

charge on negatively charged colloid more compact counter-ion cloud (the critical coagulation concentration is lower for 3+ than 2+)

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Lecture 36 - 21

Steric Interactions

If a colloid surface is coated with an adsorbed “hairy” layer of polymer, often short-range repulsive interactions are observed.

A diffuse adsorbed layer is formed at the interface, typically of the size of a polymer coil, and prevents two polymer-coated particles from coming into contact and adhering. The polymer layer must be thick enough so that van der Waals collisions are not adhesive.

The repulsion varies strongly with distance, often with dependence on 1/r8.

Lecture 36 - 22

Reason for Steric Stabilisation Polymer chains on particle surface

Bringing chains together is entropically unfavourable Increasing concentration of chains between particles induces

osmotic repulsion

Solvent flowing in

Lecture 36 - 23

Steric Stabilisation

The volume occupied by polymer chains is changed by varying Solvent Temperature

Variation: Polyelectrolytes (charged polymers) impart stabilization by a combination of electrostatics and steric effects – electrosteric stabilization. pH: charged polymers least extended at point of zero charge

Lecture 36 - 24

Destruction of ColloidsCoagulation and flocculation are the destabilisation of a colloid to form

macroscopic lumps.

Factors that induce coagulation and flocculation are: Heating: increases the velocities of the colloidal particles,

causing them to collide with enough energy that the energetic barriers are penetrated and the particles can aggregate. The particles grows to a point where they settle out.

Stirring: also increases velocities. Changing pH: can flatten/desorb electrosteric stabilisers Adding an electrolyte: neutralises the surface of the particle

allowing coagulation and settlement

Lecture 36 - 25

You should now be able to

Identify the characteristics of a colloid Classify a colloid according to the nature of the continuous and

dispersed phases Explain the electrostatic and steric stabilisation of a colloid Explain the main mechanism of coagulation of colloids, including the

role of electrolytes