amphiphile (SDS); for b-lactoglobulin and SDS similar results were
obtained. At high amphiphile concentration the protein is indeed completely
displaced; at very low amphiphile concentration the interface only contains
protein. The situation is, however, more complicated. The SDS concentra-
tion has to be higher than its CMC (by a factor of about 5) to displace all
protein; and at intermediate concentrations (10–100 ppm SDS), the
interfacial tension is smaller for the mixture of surfactants than for either
surfactant alone. This points to association between protein and amphi-
phile, both in solution (thereby decreasing the activity of the amphiphile)
and in the interface (thereby increasing totalGand decreasingg).
SDS is an anionic surfactant, and most ionic amphiphiles do indeed
show attractive interactions with proteins, although this will depend on pH,
ionic strength, etc. These interactions range from strong binding of a small
number of amphiphiles (one or two molecules per protein molecule) to
formation of large mixed aggregates of amphiphiles and proteins. Such
aggregates can possibly adsorb. Other situations may occur, like two-
dimensional phase separation between amphiphile and protein, or (weak)
adsorption of protein on top of a layer of amphiphiles.
Mostnonionic surfactants(like Tweens) interact weakly with proteins,
if at all. A nonionic therefore tends merely to displace proteins, if its
concentration is high enough. At lower concentration, two-dimensional
phase separation occurs: generally, islands of the amphiphile are formed in a
network of protein. Of course, the plateau value ofg produced by the
amphiphile must be significantly smaller than that of the protein. This may
explain why most monoglycerides do not or only partly displace proteins
from an O–W interface, unless the temperature is below the chain
crystallization temperature of the monoglyceride. The latter situation allows
much closer packing of monoglyceride at the interface and hence a lower
value ofg, hence displacement of proteins. This is, e.g., observed in ice-
cream mix, where fat globules are still covered with protein, despite the
presence of, say, glycerol monopalmitate, but lose the protein at low
temperature (e.g., 5 8 C).
10.4 TIME EFFECTS
A surfactant can be transported to a certain site at an interface by
adsorption from solution and by lateral transport in the interface. In all
cases it leads to an even distribution of surfactant over the interface and to a
lowering of interfacial tension. These processes take time.
For the case of adsorption of a surfactant, its transport toward the
interface often proceeds bydiffusion. For a surfactant concentration in