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Liquid-Gas and Liquid-Liquid Interfaces (Chapter 4, pp. 64-114 in Shaw)

Air Liquid. t~10 -6 s. Liquid-Gas and Liquid-Liquid Interfaces (Chapter 4, pp. 64-114 in Shaw). Surface and Interfacial Tension : Forces in the bulk are different from forces in the interfacial region:. Net inward pull - molecules want to leave the surface area of a liquid.

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Liquid-Gas and Liquid-Liquid Interfaces (Chapter 4, pp. 64-114 in Shaw)

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  1. Air Liquid t~10-6 s Liquid-Gas and Liquid-Liquid Interfaces (Chapter 4, pp. 64-114 in Shaw) • Surface and Interfacial Tension: • Forces in the bulk are different from forces in the interfacial region: • Net inward pull - molecules want to leave the surface area of a liquid. • Surfaces tend to contract spontaneously and form the lowest possible surface-to-volume ratio (i.e. sphere if possible).

  2. Interface (or surface for liquid-gas system) – • possesses an interfacial free energy; • work must be done to increase the interfacial area; • if this is not so then the two phases will be thermodynamically miscible and there would be no interface.

  3. Surface Tension, go – • force acting at right angles to any line of unit length on the liquid surface: Force DA Liquid film inside a wire frame DW = go x DA m2 mJ mN/m

  4. Surface or Interfacial Tension – the work or energy requiredto increase the surface area of a substance isothermally and reversibly by unit amount (i.e. by 1 m2). Interface – term used for boundary between two phases (liquid-liquid, liquid-gas, solid-liquid, or solid-gas). Surface – customary term used for interface when one phase is a gas. Interfacial tensions usually lie between the individual surface tensions.

  5. Some Interfacial Tensions

  6. Forces responsible for interfacial tension: • van der Waals forces (London, Keesom and Debye variants); • hydrogen bonding (e.g. in water); and • metal bonding (e.g. in mercury). Forces are not appreciably influenced by each other and hence are additive: For water: gw = gwd + gwh For mercury: gHg = gHgd + gHgm For pure hydrocarbons: g = gd

  7. Fowkes proposed (pp. 66-67 in Shaw): gow = god + (gwd + gwh) – 2(gwd x god)1/2 (note the typographical error in eqn. 4.3 in Shaw) For n-hexane and water: 51.1 = 18.4 + 72.8 – 2 x (gwd x 18.4) 1/2 which gives: gwd = 21.8 mN/m; and gwh = 72.8 – 21.8 = 51.0 mN/m The high surface tension of water can be explained by the high hydrogen bonding contribution!

  8. Importance of Interfacial Tension: • Emulsions and Emulsification • Microemulsions • Wetting • Waterproofing (GoretexTM) • Capillary processes • Adhesion • Ovol-80, Phazyme, Maalox Plus • Car polishes (Simonize, AutofomTM) • Deodorants • Detergents (shampoo, dishwashing) • Soaps • Ore flotation • Foams Next lectures: The Kelvin Equation and Measurement of Surface Tensions

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