here the very strong metallic bonds are responsible. The interfacial tension
g 12 between two condensed phases 1 and 2 is generally smaller than the sum
of the surface tensions of both (g 1 þg 2 ); see Table 10.1. This is because the
value of the surface tension is relatively high owing to the near absence
of molecules in air, whereas at an interface between condensed
phases, molecules at the interface are subject to attractive interactions
with molecules of the other phase. For pure liquids, surface and interfacial
tension always decrease with increasing temperature; an example is in Table
10.1.
Equilibrium. From a thermodynamic point of view, interfacial
tension is an equilibrium parameter. When enlarging an interface at a high
velocity, equilibrium distribution and orientation of the molecules in the
interface cannot be directly attained, and in order to measureg, the rate of
change in interfacial area should be slow and reversible. Nevertheless, when
enlarging a liquid surface at conditions that do not allow the establishment
of equilibrium, a force can be measured, hence a surface or interfacial
tension can be derived, which differs from the equilibrium value. It may be
a transient value, but it is also possible that a constant surface tension is
measured; it then concerns a steady state. In other words, from a
mechanicalpoint of view, interfacial tension need not be an equilibrium
value.
Also asolidhas a surface tension, but when creating a new solid
surface, it may take a very long time before the molecules near the surface
have attained an equilibrium distribution and orientation. The same holds
true for the solid–liquid interface. Moreover, since a solid surface cannot be
enlarged (without grinding etc.),gcannot be measured. All the same, there is
a surface free energy and it becomes manifest in other phenomena, partly to
be discussed later. This allows making rough estimates ofg, and such values
are given in Table 10.1B.
In this chapter, we will primarily consider fluid interfaces.
10.2 ADSORPTION
The presence of solutes in a liquid may affect the surface tension. Examples
are in Table 10.1, and the matter is further illustrated in Figure 10.4. It is
seen that the solute may causegto increase or to decrease. An example of a
special case is the dependence ofgon concentration for Na-dodecyl sulfate:
a very small concentration suffices to cause a large decrease ing, whereas a
further increase in concentration has very little effect. This is due to the
solute preferentially accumulating at the surface, and after a fully packed