CHEM Pharmacy Week 13: Colloid Chemistry Dr. Siegbert Schmid School of Chemistry, Rm 223 Phone:

Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry, John Wiley & Sons Australia, Ltd ISBN:

Lecture 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 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 nm particles invisible, remain suspended

Lecture 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 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 m 2 Make sides one million times smaller: d = 10nm (10 18 cubes) The total surface area becomes 600 nm 2 × = 600 m 2

Lecture Nano Scale M. Dresselhaus, MIT

Lecture Colloidal Dimensions (a) kaolinite (b) Plaster of Paris, cement, asbestos (c) polymer lattices (d) network structures, e.g. porous glass, gels Figure taken from “Basic Principles of Colloid Science” D.H. Everett, RSC paperbacks

Lecture Classification of Colloids

Lecture Examples ExampleClass Mistliquid aerosol Milk emulsion Blood bio-colloid (sol) Bonebio-colloid (solid sol) Asphaltemulsion (asphaltene dispersed phase and maltene contin.) Mayonnaiseemulsion Toothpasteslurry/paste (solid in liquid) Smokeliquid and solid aerosol Opalsolid suspension or dispersion (solid sol) Paintsol or colloidal suspension Foamsgas dispersed in liquid Cementsol Soapliquid emulsion Silica gelgel Identify the following types of colloids:

Lecture 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 Flocculation We can break the colloid stability problem into a series of steps. particles  dimers  “flocs”  gravity-effected separation

Lecture 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 +  t

Lecture 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 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 Electrical Double layer

Lecture Electrostatic Interactions Two like-charged surfaces repel each other within a range given by the Debye length κ D -1. For a 1:1 electrolyte, a simplified expression for the Debye length is: For a 1:1 electrolyte, the Debye length is K D -1 = 1 nm for 0.1 M NaCl.

Lecture Debye Length The 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  / nm 1.0x x x x 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 Atomic Force Microscopy (AFM) AFM Tip and Cantilever Atomic resolution image of a mica crystal 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 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 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+)

Lecture 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/r 8.

Lecture 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 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 Destruction of Colloids Coagulation 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 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