ADSORPTION OF SERUM-ALBUMIN AT THE OIL-WATER INTERFACE

Proteins adsorb at the oil/water interface, forming a protein-rich interfacial layer. This phenomenon is of prime importance in the manufacture and stabilization of emulsions in the food industry and in cosmetics. However, almost nothing is known about the structure of this interfacial layer. It has been believed for decades that the polypeptide chains form loops in both solvents, according to the solubility of the amino acids which make up their sequences. This crude concept is quantitated using the current theories on the properties of polymers in solution. An adsorbed protein molecule is viewed as a series of loops located alternatively in oil or in water. The loops bathing in a solvent occupy the volume of a ''pancake'' lying flat on the interface. There is currently no theory on the properties of layers formed by polymers having such a conformation. Thus a simplified model has been developed based on a diblock polymer trapped between the interface and two planes at distances close to the size of the loops. This model, derived using scaling laws, predicts, in the semidilute regime, a power-law relationship between the interfacial pressure and the interfacial protein concentration. The value of the exponent of this law :is deduced from the Flory exponents relating the radius of a polymer coil to the ''quality'' of the solvent. Previously published data concerning bovine serum albumin show a power-law dependence of the interfacial pressure versus the interfacial concentration. The exponents of these power laws are within the theoretical range and seem to reflect correctly the experimental conditions of measurement.