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phenomena. From a practical standpoint, the most important interfacial aspects of
L/L and L/V systems are related to interfacial tension and the effects of adsorbed
surfactants on that property. To have a concept of the role of surfactants at such
interfaces, it is necessary to understand, in a qualitative way at least, the molecular
forces involved.
3.2.2. Surface and Interfacial Tensions
The interfacial tension between a pure liquid and its vapor or between two immi-
scible or partially miscible liquids reflects the difference in the forces of attraction
acting on molecules at the interface as a result of differences in the density or
chemical compositions of the two phases. It has long been accepted that the exis-
tence of condensed phases of matter, especially the liquid state, is a result of van der
Waals attractions between molecules. That is especially true for materials that do
not possess any chemical structure that could lead to the action of forces of an elec-
trostatic, dipolar, or other related specific character. For the sake of simplicity,
consider a liquid whose molecules interact only through van der Waals or disper-
sion forces. In the bulk of the phase under consideration, all molecules will be
surrounded by an essentially uniform force field, so that the net force acting on
each will be zero. Molecules located at or near an interface, on the other hand,
will experience a distorted field resulting in a net attraction for the surface mole-
cules by the bulk. The unbalanced force of attraction acting on the surface mole-
cules will cause the liquid to contract spontaneously to form, in the absence of
gravity, a spherical drop.
To visualize the concept of the surface tension of a liquid, it is convenient to
define it as a force acting tangentially to the surface at all points, the net result
of which is the apparent formation of a surface ''skin,'' which contracts to confine
the liquid into a shape of minimum interfacial area. Such a definition, while facil-
itating the understanding of the results of the phenomenon, may be misleading in
the sense that no actual tangential force is acting at the surface of a pure liquid—it
only produces the appearance of such a force. A more thermodynamically appro-
priate definition of surface tension and surface free energy is the work required to
increase the area of a surface reversibly and isothermally by a unit amount. The
interface between two immiscible liquids can be viewed similarly, except that the
presence of a second, more dense liquid phase will usually result in a less severe
imbalance in the forces acting on the molecules at the interface and consequently a
lower value for the interfacial tension.
Most commonly encountered room temperature liquids have surface tensions
against air or their vapors that lie in the range of 10-80 mN/m. Liquid metals
and other inorganic materials in the molten state will exhibit significantly higher
values as a result of the much greater and more diverse interactions occurring in
such systems. Water, the most important liquid that we will consider, lies at the
upper scale of what are considered to be normal surface tensions with a value in
the range of 72-73 mN/m at room temperature, while hydrocarbons reside at the
lower end, falling in the lower to middle 20s. The interfacial tension between water
