varying among surfactants. Additional surfactant added above the CMC
forms micelles; this means that the surfactant activity does not increase any
more, and the surface excess at any surface present reaches a plateau value.
Another variable is the hydrophile/lipophile balance (HLB value); the
higher this value, the better soluble is the surfactant in water and the less in
oil. For HLB¼7, the solubility in both phases is about equal.
Polymeric surfactants are generally (far) more surface active, but they
give lower surface pressures than most amphiphiles. At the plateau value of
the surface excess they are not very tightly packed (most amphiphiles are),
but they extend fairly far into the solution. The exchange between solution
and interface may be very slow, and the Gibbs equation does not seem to
hold. Most amphiphiles can displace polymers from the interface, if present
in sufficient concentration, since they give a lower interfacial tension. Mixed
surface layers can also be formed.
Time Effects. Surfactants that adsorb are often transported to the
interface by diffusion. For most amphiphiles this is a fast process, the times
needed ranging from a millisecond to a few minutes. For polymers, it can be
much slower. For mixtures of surfactants, changes in surface composition
and interfacial tension may take a long time. Several complications can
arise, such as very slow adsorption of poorly soluble surfactants (e.g.,
phospholipids), or a greatly enhanced adsorption rate due to convection. In
processes like foam formation, the interfacial tension at short time scales is
of importance; to obtain such values, one determines so-called dynamic
surface tensions, i.e., values obtained at rapidly expanding surfaces.
Curvature. For a curved liquid interface, the pressure at the
concave side is always higher than that at the convex side, by an amount
called the Laplace pressure; its value is greater for a smaller radius of
curvature and a larger interfacial tension. This has several consequences,
such as capillary rise of a liquid in a thin pore, if the material is wetted by the
liquid. Another consequence is that the material at the concave side (say in a
droplet) has an increased solubility in the surrounding fluid, the more so for
a smaller radius of curvature. The relation is given by the Kelvin equation,
which also holds for solid materials. The phenomenon is responsible for the
supersaturation needed for nucleation of a new phase to occur, for Ostwald
ripening (small particles in a dispersion tend to dissolve, whereas the large
ones grow), and for capillary condensation in fine pores.
Contact Angles. Where three phases are in contact with each
other, the phase boundaries meet at a given contact angle, determined by the
three interfacial tensions (Young’s equation). The contact angle determines
whether and to what extent the wetting of a surface by a liquid occurs;