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: Air Liquid t~10-6 s 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).

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.

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

Surface or Interfacial Tension – the work or energy required to 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.

Some Interfacial Tensions Liquid go, mN/m gi, mN/m H2O Benzene Acetic Acid Acetone CCl4 Ethanol n-Octanol n-Hexane n-Octane Mercury 72.8 28.9 27.6 23.7 26.8 22.3 27.5 18.4 21.8 485 - 35.0 45.1 8.5 51.1 50.8 375

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

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!

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