Surface Chemistry


  • Emulsions: A dispersion obtained by shaking two immiscible (or) partly miscible liquids gives a liquid-liquid colloidal system known as emulsion. Its types are
    (a) Oil in Water (O/W) type: Water is dispersion medium.
    (b) Water in Oil (W/O) type: Oil is dispersion medium.
  • Emulsifiers:
    (a) For O/W type: gums, soaps, detergents, proteins, etc.
    (b) For W/O type: long chain alcohols, metal salts of fatty acids etc.
    (a) O/W type: day face creams, vanishing creams, milk etc.
    (b) W/O type: Cold creams.
  • Demulsification: Separation of emulsion into its components by freezing, boiling, centrifugation, electrophoresis, etc. is called demulsification.
  • Applications:
    (a) As medicine: Cough syrups; cod liver oil etc
    (b) As disinfectant: Dettol, lyzol, etc, produce emulsion in water.
    (c) Digestion of fats and oils in intenstine is basically digestion of emulsions of fats and oils by the attack of enzymes.
    (d) Cleansing action of soap: Grease, dirts, lipids are converted to emulsions by soap and detergents (Micelles formation) for their removal from surfaces of clothes, utensils etc.
  • Tests:
    (a) Dye test: If an oil soluble dye is shaken with emulsion and a drop is seen under microscope, the appearance of homogeneous colour shows W/O type emulsion, otherwise O/W type.
    (b) Electrical conductivity: O/W type emulsions show much higher electrical conductivity than W/O type emulsions
    (c) Spreading test: W/O type emulsions easily spread on the surface of oil.
    (d) Dilution test: O/W emulsions can be easily diluted with water eg. milk.
  • GELS: These are "liquid in solid" semi solid colloidal system with water mainly as dispersed phase. Cheese jellies, butter, silica gel, jam, curd, agar-agar, boot-polish etc. are common examples
S.No Elastic gels Non-elastic gels
1. These gels change to solid mass on dehydration and again be converted to original form by adding water and subsequent heating (or) cooling. It is called thixotropy. These gels can be converted to solid mass but cannot be converted to the original form by adding water followed by adding water followed by heating (or) cooling.
2. These can easily absorb water and swell. The phenomenon is called assimilation. These do not show the property of assimilation.
3. Common example are gelatin, agar-agar etc. Most common examples are boot-polish, silica gel etc
  • (b) Imbibition: The property of a gel to absorb a definite quantity of water and to increase its volume is called imbibition (or) swelling.
  • (c) Weeping or syneresis of gel: On cutting, a gel loses some liquid held by it. The phenomenon is called weeping or syneresis of gel.

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