1. Liquid-Gas and Liquid-Liquid Interfaces
(Chapter 4, pp 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).

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 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.

5. 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

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 = – 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