We have studied the effect of monolayer structure on dilational characteristics (surface dilatational modulus and its elastic and viscous components) of monoglyceride monolayers (monoolein and monopalmitin) spread on the air−water interface, at 20 °C and at pH 5 and 7. The stress response to compression−expansion sinusoidal deformation of the interface in a modified Wilhelmy-type trough with two oscillating barriers was measured as a function of deformation amplitude (within the range of 1−20% of the initial area), frequency (within the range of 1−300 mHz), and superficial density (within the range of 1−3.5 mg/m2). The same experimental device coupled with Brewster angle microscopy makes it possible to determine the structure and morphology of the monolayer. The monolayer structure and, especially, the conditions at which the monolayer collapses determine the viscoelastic behavior of the monolayer and the linear response of the stress to area deformation. The nonlinear viscoelastic behavior of the interface has been associated with the monoglyceride monolayer collapse. It was found that the dilatational modulus is not only determined by the interactions between spread molecules (which depend on the surface density) but that the structure of the spread monolayer also plays an important role.

The adsorption kinetics at the air-water phase interface of soy globulins (β-conglycinin, glycinin, and reduced glycinin) has been studied. The adsorption kinetics was determined by surface tension measurements coupled with Brewster angle microscopy (BAM) as a function of time, protein concentration in the solution (within the range of 1-0.001%, wt/wt), and pH (2.0, 5.0, and 8.0). The ionic strength (0.05 M) and the temperature (20 °C) were maintained constant. The adsorption of soy globulins to the interface increases with the protein concentration in the solution, depending on the protein and, especially, on the pH. The adsorption decreases dramatically at pH 5.0, close to the isoelectric point of the protein. A lag period was observed at lower protein concentrations. The adsorption kinetics at the beginning of the adsorption is diffusion-controlled. However, the mechanism that controls the long-term adsorption is the penetration and unfolding of the protein. The molecular conformation of soy globulins, which depends on the pH, has an effect on the adsorption kinetics.

Prof. Garcı ´a Gonza ´lez, s/nu ´m. 41012-Seville. SpainThe static and dilatational properties of mixed emulsifiers are of interest due to their importance in relation to dispersion formation and stability. In this work, we have used different and complementary interfacial techniques (surface film balance, Brewster angle microscopy, and interfacial dilatational rheology), to analyze the static (structure, morphology, reflectivity, and miscibility) and dynamic (surface dilatational properties) characteristics of whey protein isolate (WPI) and monoglyceride (monopalmitin or monoolein) mixed films spread on the air-water interface. The static and dynamic characteristics of the mixed films depend on monolayer composition and the surface pressure. At higher surface pressures, collapsed WPI residues may be displaced from the interface by monoglyceride molecules with important repercussions on the interfacial characteristics of the mixed films. A close relationship between interfacial dilatational rheology and changes in molecular structure, interactions, miscibility, and relaxation phenomena has been established from the frequency dependence of the surface dilatational properties.

Relaxation phenomena in a whey protein isolate (WPI) and monoglycerides (monopalmitin, monolaurin, and monoolein) mixed films at the air−water interface were studied using surface film balance and Brewster angle microscopy (BAM). Relaxation in surface area at a constant surface pressure (at 20 mN/m) or at constant area at the collapse point have been analyzed according to models for desorption, collapse, and/or organization/reorganization changes. At a constant surface pressure, the organization/reorganization change of WPI molecules in monopalmitin− and monoolein−WPI mixed films is the mechanism that controls the relaxation process. At the collapse point of the mixed film, the relaxation phenomena may be due either to nucleation and growth of critical nuclei of monoglyceride or to a complex mechanism including competition between desorption and monolayer collapse. For WPI−monolaurin mixed films the relaxation phenomena are mainly due to the irreversible loss of monolaurin molecules by desorption into the bulk aqueous phase.