Interfacial properties of diglycerol esters and caseinate mixed films at the air-water interface

The surface pressure (pi) at equilibrium (surface pressure isotherm) and surface dynamic properties (dynamic surface pressure and surface dilatational characteristics) of diglycerol ester (diglycerol-monocaprinate, diglycerol-monolaurate, diglycerol-monostearate, and diglycerol-monooleate) and protein (sodium caseinate) as emulsifiers, at different concentrations in the aqueous phase, were measured using tensiometry and a dynamic drop tensiometer, respectively. We have observed that (1) at equilibrium the value of critical micelle concentration (CMC) decreases and the maximum surface excess (Gamma max) increases as the hydrocarbon chain increases because the hydrophobic character of the lipid also increases. The presence of a double bond in the hydrocarbon chain also increases the value of CMC and decreases those of Gamma max. Caseinate presents higher adsorption efficiency but the surface activity is between those for lipids. The surface pressure isotherm of mixed systems is dependent on the emulsifier concentration and the protein/lipid ratio in the mixture. (2) The adsorption of pure emulsifiers at the air-water interface increases with emulsifier concentration in the aqueous phase via diffusion and penetration of the emulsifier at the interface. For mixed films, the rate of adsorption depends on the concentration and composition of the mixture. Competitive or cooperative phenomena were observed during the adsorption of both emulsifiers at the interface. (3) The surface dilatational characteristics of mixed films are viscoelastic. The surface dilatational modulus reflects the amount of emulsifier adsorbed at the interface and confirms the idea that the protein-lipid interactions at the air-water interface are somewhat weak, there even being the possibility of phase separation.