Roles of the Surfactant. Emulsions and foams cannot be made
without surfactant being present. The prime function of the surfactant is the
formation of interfacial tension gradients. Thereby drainage of liquid from a
film between bubbles or drops is greatly retarded, which is essential in foam
formation and in the early stages of emulsification. Moreover, drops can
recoalesce after being formed, since two drops are often closely pressed
together during the process, and this at a stage when they are not yet fully
covered with surfactant. Colloidal repulsion between drops is generally too
weak to prevent coalescence during emulsification. However, the formation
of ag-gradient greatly slows down the outflow of liquid from the gap
between drops, thereby often preventing their coalescence. The strength of
this mechanism depends on the Gibbs elasticity of the film between drops,
which varies among surfactants and with surface load. The elasticity is
higher and the mechanism more effective if the surfactant is present in the
continuous phase rather than in the drop. This explains Bancroft’s rule: the
continuous phase becomes the one in which the surfactant is best soluble.
The surfactant also lowers the interfacial tension, thereby facilitating
droplet breakup. The effectivegvalue during breakup depends on surfactant
type and concentration and on the rate of transport to the drop surface.
Approximate equations are available for this rate, and also for the stresses
acting on a drop, the drop size resulting from breakup, and the frequency at
which the drops encounter each other.
Proteinsare not very suitable for making fine emulsions; in other
words, it takes more energy to obtain small droplets than with a small-
molecule surfactant. This is primarily due to their large molar mass. It
causes the effectivegvalue that they can produce at the O–W interface to be
fairly large. Moreover, their molar concentration is small at a given mass
concentration, causing the Gibbs elasticity to be relatively small. This means
that prevention of recoalescence is less efficient. Proteins are not suitable to
make W–O emulsions, as follows from Bancroft’s rule: they are insoluble in
oil. The adsorption layer of proteins on the droplets obtained by
emulsification is not an equilibrium layer, whereas it is for small-molecule
surfactants.
BIBLIOGRAPHY
Basic aspects of importance for the topic of the chapter are given in
J. Lucassen. Dynamic properties of free liquid films and foams. In: E. H. Lucassen-
Reynders, ed. Anionic Surfactants: Physical Chemistry of Surfactant Action.
Marcel Dekker, New York, 1981, pp. 217–265.