SURFACE CHEMISTRY-2 BY- A.P. SINGH PGT-CHEM, KVS

1. SURFACE CHEMISTRY-2BY- A.P. SINGHPGT-CHEM, KVS

2. Colloids A colloid is a heterogeneous system in which one substance is dispersed(dispersed phase)in another substance called dispersion medium and size of dispersed phase is from 1nm-1000 nm. Solute and solvent are replaced by dispersed phase & dispersion medium Sols( solid in liquid),gels(liquids in solids), emulsions (liquid in liquid)

3. Property True solution Suspension Colloidal solution Nature Homogeneous Heterogeneous Appears to be homogenous but actually heterogeneous > 1000 Ao (100 nm) Between 10 Ao (1 nm) to 1000 Ao (100 nm) < 10–9 Ao  (1 nm) Particle size Sedimentation Do not settle Settle on standing Do not settle Diffusion Diffuse quickly Unable to diffuse Diffuse slowly Visibility Particles invisible Particles visible by naked eye or under microscope Particles scatter light and can be observed under ultramicroscope Filterability Pass easily through animal membrane and filter paper Unable to pass through animal membrane or filter paper Pass through filter paper but not through animal membrane Appearance Clear and transparent Opaque Translucent

4. Classification of colloids Classification is based on following criteria Physical state of dispersed phase and dispersion medium. Nature of interaction between dispersed phase and dispersion medium. Types of particles of the dispersed phase.

5. Dispersed phase Dispersion medium Type of colloid Example Solid Solid Solid sol Some coloured glasses, and gem stones Solid Liquid Sol Paints, cell fluids Solid Gas Aerosol Smoke, dust Liquid Solid Gel Cheese butter, jellies Liquid Liquid Emulsion Milk, hair cream Liquid Gas Aerosol Fog, mist, cloud, insecticide sprays Gas Solid Solid sol Pumice stone, foam rubber Gas Liquid Foam Froth, whipped cream, soap-lather Classification based on physical state of dispersed phase and dispersion medium Eight types of colloidal systems are possible.

6. Classification based on nature of interaction • Lyophobic colloids (solvent hating colloids ) • these colloids can not be prepared by simply mixing of dispersed phase into dispersion medium. • need stabilizing to preserve them. • irreversible. • For example, colloidal solutions of gold, silver, Fe(OH)3, AS2S3, etc. • Lyophilic colloids ( solvent loving) • these colloids can be prepared by simply mixing of dispersion phase into dispersion medium. • self-stabilizing • reversible sols • For example, gums, gelatin, starch, albumin in water.

7. Classification based on type of particles of the dispersed phase Multimolecular colloids on dissolution, a large number of atoms or smaller Molecules ( diameter < 1nm) of a substance aggregate together to form species having size in colloidal range. The species thus formed are called Multimolecular colloids. e.g. Sulphur sol. Macromolecular colloids: In these colloids, the molecules have sizes and dimensions comparable to colloidal articles. For example, proteins, starch, cellulose.

8. Associated colloids some substances in low concentration behaves as normal strong electrolyte but at higher concentration exhibit colloidal behavior due to formation of aggregates. The aggregated particles are called micelles and also known as associated colloids. The formation of micelles takes place only above a particular temperature called Kraft temperature (Tk) and above a particular concentration of electrolyte called Critical Micelle Concentration E.g Soaps and detergents

9. Multi-molecular colloids Macromolecular colloids Associated colloids Formed by aggregation of large number of atoms or molecules with diameters less than 1 nm Formed by aggregation of large number of ions in concentrated solution Formed by large sized molecules Lyophilic in nature Lyophobic in nature Both lyophilic and lyophobic in nature Molecular mass is intermediate High molecular mass High molecular mass Held by weak van der Waals’ forces Held by stronger van der Waals’ forces due to the long chains van der Waals’ forces increase with increase in concentration Comparison between multi-molecular, macromolecular and associated colloids

10. Preparation of Lyophobic sols Condensation methods Particles of atomic or molecular size are induced to form aggregates Oxidation method Sulphur colloids are prepared by oxidation of H2S by SO2. SO2 +2H2S → 3S (SOL)+2H2O Reduction Silver colloids are prepared by passing H2 through a saturated aqueous solution of silver oxide at 65° C. Hydrolysis Dark brown Fe(OH)3 colloidal solution is prepared by adding FeCl3 into boiling water.FeCl3 +3H2O → Fe (OH)3 (sol) + 3HCl Double decomposition Arsenious sulphide colloidal solution is prepared by passing of H2S gas into a solution of As2O3. Exchange of solvent Colloidal solution of phosphorus is prepared by addition of alcohol into a solution of phosphorous in excess water.

11. Preparation of Lyophobic sols Mechanical disintegration By vigorous mechanical agitation. Peptization: Process of converting of a freshly prepared precipitate into colloidal particles by adding suitable electrolyte is known as peptization e.g. Fe(OH)3 solution is formed from FeCl3. During peptization, the precipitate adsorbs one of the ions of the electrolyte on its surface. This causes the development of positive or negative charge on precipitates, which ultimately break up into smaller particles of the size of a colloid. Electrol-disintegration (Bredig’s arc method) In this method, electric arc is struck between electrodes of the metal immersed in the dispersion medium. The intense heat produced vapourises the metal, which then condenses to form particles of colloidal size.

12. Purification of colloids Dialysis In this process, the colloidal particles are separated from the impurities (mainly electrolytes) by the diffusion through a porous membrane such as parchment, collodion, etc. Electrodialysis This is a special type of dialysis process, which is accelerated by the application of a potential difference across the membrane. So ions migrate faster than the colloids . Ultrafiltration In this process the colloidal particles are separated by the process of filtration, through a filter paper, which is impregnated with gelatin or collodion followed by hardening in formaldehyde.

13. Properties of colloids Optical properties: Tyndall effect When a beam of light falls at right angles to the line of view through a solution, the solution appears to be luminescent and due to scattering of light the path becomes visible. This effect is known as Tyndall effect. Quite strong in lyophobic colloids while in lyophilic colloids it is quite weak.

14. Properties of colloids Brownian movement: Zig- zag movement of colloidal particles in a colloidal sol

15. Properties of colloids Charge on colloidal particles: Colloidal particles always carry an electric charge, it may be either positive or negative. e.g.

16. Theories of origin of Charge on colloidal particles The charge on the sol particles is due to one or more reasons- A) Due to electron capture by sol particles during electrodispersion of Metals. B) Preferential adsorption of ions- It is the most accepted reason. The sol particles acquire positive or negative charge by preferential adsorption of +ve or –ve ions. When two or more ions are present in the dispersion medium, preferential adsorption of the ion common to the colloidal particle usually takes place. e.g. When silver nitrate solution is added to potassium iodide solution,the precipitated silver iodide adsorbs iodide ions from the dispersion medium and negatively charged colloidal solution results. However, when KI solution is added to AgNO3 solution, positively charged sol results due to adsorption of Ag+ ions from dispersion medium.AgI/I– AgI/Ag+ Negatively charged Positively charged The electrical charges of the particles prevent them from coming together due to electrostatic repulsion. All the dispersed particles in a colloidal solution carry the same charge while the dispersion medium has equal and opposite charge.

17. Helmholtz electrical double layer & zeta potential Surface of a colloidal particle acquired a positive or a negative charge by selective adsorption of ions , this layer attracts counter ions from the medium forming a second layer, as shown below. AgI/I-,K+ AgI/Ag+,I- The combination of the two layers of opposite charges around the colloidal particle is called Helmholtz electrical double layer. The first layer of ions is firmly held (fixed layer) while the second layer is mobile (diffused layer). The potential difference between the fixed layer and the diffused layer of opposite charges is called the electrokinetic potential or zeta potential.

18. Properties of colloids Electrophoresis Movement of Colloidal particles towards opposite electrode in presence of external electric field.

19. Properties of colloids Electro-osmosis: molecules of dispersion medium are allowed to move under influence of electric field Coagulation or flocculation: Process which involves coming together of colloidal particles so as to change into large sized particles which ultimately settle as a precipitate or float on surface. It is generally brought about by addition of electrolytes. The minimum amount of an electrolyte that must be added to one litre of a colloidal solution so as to bring about complete coagulation or flocculation is called coagulation or flocculation value. Smaller is the flocculation value of an electrolyte, greater is the coagulating or precipitating power.

20. Properties of colloids The coagulation of the lyophobic sols can be carried out in the following ways: (i) By electrophoresis (ii) By mixing two oppositely charged sols: Oppositely Charged sols when mixed in almost equal proportions, neutralize their charges and get partially or completely precipitated. (iii) By addition of electrolytes: When excess of an electrolyte is added, the colloidal particles are precipitated. (iv) By persistent dialysis:

21. Properties of colloids Hardy schulze law : Coagulating power of an electrolyte increases rapidly with the increase in the valency of cation or anion. For negatively charged sol, the coagulating power of electrolytes are AlCl3 > BaCl2 > NaCl or Al3+ > Ba2+ > Na+ For positively charged, then the coagulating power of electrolytes follow the following order:

22. Gold Number & Protection of colloids Covering up of lyophobic particles by lyophilic particles is known as its protective action and such colloids are called protective colloids. Gold number is defined as amount of protective sol (in mg) that will prevent the coagulation of 10 ml of a gold solution on the addition of 1 ml of 10% NaCl solution. Smaller the gold number, higher is protective power

23. Emulsion A colloidal dispersion of one liquid in another immiscible liquid is known as an emulsion, e.g. milk, Na-soaps, vanishing cream, etc. Types of emulsions • Oil in water, where oil is the dispersed phase and water is the dispersion medium, e.g. milk. • Water in oil where water is the dispersed phase and oil is the dispersed medium, e.g. butter, cream. Emulsifying agent or emulsifier- Emulsions of oil in water are unstable ,for stabilisation of an emulsion, a third component called emulsifying agent is usually added. The emulsifying agent forms an interfacial film between suspended particles and the medium. The principal emulsifying agents for O/W emulsions are proteins, gums, natural and synthetic soaps, etc., and for W/O, heavy metal salts of fatty acids, long chain alcohols, lampblack, etc.

24. Cleaning Action of Soap • Soap contains a nonpolar carbon end that dissolves in nonpolar fats and oils, and a polar end that dissolves in water. • Dust and soap molecules form micelles that dissolve in water and are washed away. Soap forms a precipitate with ions in hard water (Ca2+, Mg2+, Fe3+)

25. Applications of colloids • Rubber plating • Sewage disposal • Smoke screen • Purification of water • Cleaning action of soap • In medicine • Formation of delta • Photography • Artificial rain

26. Thank you