for polymeric surfactants, especially proteins. Interpretation of the results is
not easy. A high surface shear modulus probably can cause the interface to
act more or less as a solid in the lateral direction, thereby producing a fully
stagnant surface if liquid streams along it.
Surfactants fulfil severalfunctions, and some important ones are the
following:
.Their presence permits the formation of interfacial tension gradients,
which may be considered the most essential function. For instance, this is
all that allows the formation of foams and emulsions and provides
inherent stability to thin films.
.They lower the interfacial tension of liquid interfaces, thereby facilitating
bending of the interface, hence deformation and breakup of drops and
bubbles.
.They affect contact angles and thereby capillary phenomena such as
wetting, adhesion of particles, capillary displacement, and dispersion of
powders in a liquid.
.By adsorption onto particles they may greatly affect colloidal interaction
forces between those particles. Repulsive forces may provide long-term
stability against aggregation; attractive forces may allow the formation of
continuous networks.
BIBLIOGRAPHY
A standard text on several aspects discussed in this chapter, including methods of
measurement is
A. W. Adamson, A. P. Gast. Physical Chemistry of Surfaces, 6thed. John Wiley,
New York, 1997.
It contains little information on surfactants and very little on dynamic surface
properties. An excellent monograph is
E. H. Lucassen-Reynders, ed. Anionic Surfactants: Physical Chemistry of Surfactant
Action. Marcel Dekker, New York, 1981.
Especially see Chapters 1 (adsorption), 5 (surface dilatational phenomena) and 6
(dynamic properties of films).
A comprehensive description of many small-molecule surfactants used in foods is
given by
N. J. Krog. Food emulsifiers and their chemical and physical properties. In: K.
Larsson, S. E. Friberg, eds. Food Emulsions, 2nded. Marcel Dekker, New
York, 1990.