CHEM1612 - Pharmacy Week 13: Colloid Chemistry

1. CHEM1612 - PharmacyWeek 13: Colloid Chemistry Dr. SiegbertSchmid School of Chemistry, Rm 223 Phone: 9351 4196 E-mail: siegbert.schmid@sydney.edu.au

2. 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

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

4. Solution homogeneous mixture, e.g. sugar in water, single molecules Suspension heterogeneous mixture, e.g sand in water, particles visible, settle out What is a Colloid? Colloid size 1-1000 nm particles invisible, remain suspended

5. What is a Colloid? • No simple definition • Intermediate between a suspension and a solution • Consistsof 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 onedimension is >1 nm and <1 micron • Thermodynamically unstable • Huge total surface area

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. Make sides one million times smaller: d= 10nm (1018 cubes) The total surface area becomes 600 nm2 × 1018 = 600 m2 = 6·10-4 m2

7. Nano Scale M. Dresselhaus, MIT

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

9. Classification of Colloids

10. Examples Identify the following types of colloids: Example Class Mist liquid aerosol Milk emulsion Blood bio-colloid (sol) Bone bio-colloid (solid sol) Asphalt emulsion (asphaltene dispersed phase and maltenecontin.) Mayonnaise emulsion Toothpaste slurry/paste (solid in liquid) Smoke liquid and solid aerosol Opal solid suspension or dispersion (solid sol) Paint sol or colloidal suspension Foams gas dispersed in liquid Cement sol Soap liquid emulsion Silica gel gel

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 unfavourableinteractions, 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.

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

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

14. Charged Surfaces • In water most surfaces are electrically charged, due a number of different mechanisms: • Adsorption of an ionic surfactant from solution • 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. • Differential solubility of cation and anion in an insoluble salt

15. + + + + + + + + - - - - - - - - + + - - - - + + + + - - - - + + + + - - - - - - + + + + - - - - + + - - - - - - + + + + + + + + Electrostatic Repulsion • 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 Electrical Double layer

16. Electrostatic Interactions • Two like-charged surfaces repel each other within a range given by the Debye lengthκD-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.

17. [NaCl] /M -1 k /nm -4 1.0 x 10 30.4 -3 1.0 x 10 9.61 -2 3.04 1.0 x 10 -1 1.0 x 10 0.961 1.0 0.3 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. • 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.

18. Laser Split Photodiode Cantilever spring Sample Piezoelectric element Atomic Force Microscopy (AFM) • 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. AFM Tip and Cantilever 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. Atomic resolution image of a mica crystal

19. Example: River + Ocean Figure from Silberberg, “Chemistry”, McGraw Hill, 2006. • 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

20. + + + + - - - 2+ - - - - - - - - - + 2+ - - - - 3+ + + 3+ 2+ - - - - + + - - - - 2+ - - - - - - - + + - 2+ 3+ - - - - - 2+ 3+ + - - - - - - - - - + + 2+ 2+ + 3+ + 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+)

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.

22. Solvent flowing in Reason for Steric Stabilisation • Polymer chains on particle surface • Bringing chains together is entropically unfavourable • Increasing concentration of chains between particles induces osmotic repulsion

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

24. 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

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