Liquid-gas and liquid-liquid interfaces 65The surface tension y 0 of a liquid is often defined as the force acting
at right angles to any line of unit length on the liquid surface.
However, this definition (although appropriate in the case of liquid
films, such as in foams) is somewhat misleading, since there is no
elastic skin or tangential force as such at the surface of a pure liquid,
It is more satisfactory to define surface tension and surface free
energy as the work required to increase the area of a surface
isothermally and reversibly by unit amount.
There is no fundamental distinction between the terms surface and
interface, although it is customary to describe the boundary between
two phases one of which is gaseous as a surface and the boundary
between two non-gaseous phases as an interface.
At the interface between two liquids there is again an imbalance of
intermolecular forces but of a lesser magnitude. Interfacial tensions
usually lie between the individual surface tensions of the two liquids
in question.
The above picture implies a static state of affairs. However, it must
be appreciated that an apparently quiescent liquid surface is actually
in a state of great turbulence on the molecular scale as a result of two-
way traffic between the bulk of the liquid and the surface, and
between the surface and the vapour phase^41. The average lifetime of
a molecule at the surface of a liquid is c. 10~^6 s,
Table 4.1 Surface tensions and interfacial tensions against water for liquids at 20°C
(in mN m"^1 )
Liquid y 0 -ft Liquid y 0 j-,
Water
Benzene
Acetic acid
Acetone
CC1 4
72.8
28.9
27.6
23.7
26.8_
35.0
—- 45.1
Ethanol
n-Octanol
n-Hexane
n-Octane
Mercury22.3
27.5
18.4
21.8
485._
8.5
51.1
50.8
375Additivity of intermolecular forces at interfaces
The short-range intermolecular forces which are responsible for
surface/interfacial tensions include van der Waals forces (in particular,
London dispersion forces, which are universal) and may include