THE ROLE OF ADDITIVES FOR THE BEHAVIOR OF THIN EMULSION FILMS STABILIZED BY PROTEINS

Experimental results obtained with thin aqueous films of emulsion type stabilized by bovine serum albumin (BSA) and beta-casein are presente d. The film behaviour is time dependent. The contact angle increases w ith ageing and exhibits pronounced hysteresis. With BSA one observes s low reversible aggregation on the surface (but not in the bulk) and th e protein lumps are gradually squashed by the capillary pressure as th e film thins. The findings can be explained by slow surface denaturati on, accompanied by developing attraction and partial entanglement of t he BSA molecules. These processes are promoted by oleic acid dissolved in the oil phase. Electrostatic interactions were found to be importa nt: without salt the films remain thick, whereas in the presence of 0. 15 M NaCl one obtains Newton black films whose contact angle depends u pon the molecular charge. A marked difference in the surface mobility is observed with foam and emulsion films stabilized by BSA. Lenses, co ntaining protein aggregates and liquid, when surrounded by an area whi ch has reached the black film stage, remain entrapped in foam films bu t are slowly squeezed out in emulsion films. Hydrophobization of the p rotein molecules may be responsible for this behaviour. With beta-case in, ageing effects in films are observed only at the isoelectric point . This protein strongly aggregates in the bulk, but the lumps are read ily flattened on the film interfaces. Addition of Ca2+ ions leads to a decrease in film thickness, depending on the concentration. (C) 1997 Elsevier Science B.V.