The effect of alpha-lactalbumin on the phase behavior of microemulsions.

摘要

We have seen that the presence of the protein α-lactalbumin has a significant effect on the phase behavior and structures that form in sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane mixtures. Nuclear magnetic resonance (NMR) was used to study the surfactant and oil contents of the microemulsion and excess aqueous phase as a function of the total surfactant, protein and salt concentrations. The protein and water content of the system were examined using ultraviolet spectroscopy (UV) and Karl Fischer titration, respectively. As the total α-lactalbumin present in the system is increased, there is an initial increase in the water and protein content of the microemulsion. As the molar ratio of α-lactalbumin to AOT passes through a critical ratio of 1:300, the partitioning of water, protein, and surfactant shifts to the excess aqueous phase. Significant amounts of isooctane are also solubilized in the aqueous phase under these conditions.; The sizes of the self-assembled complexes in both the organic and excess aqueous phase were measured, using dynamic light scattering, as a function of the total protein, surfactant and sodium chloride content. It was found that the hydrodynamic radius of the water-in-oil microemulsion droplets increased in the upper phase with increasing protein concentration at constant AOT concentration. The size of the droplets decreased upon increasing salt concentration; however, for all ionic strengths, α-lactalbumin induced an 80% increase in droplet size relative to the empty droplets. The microemulsion droplets contained multiple protein molecules, and their shape deviated from spherical.; The protein-rich aqueous phase aggregates were monodisperse with a radius of approximately 9 nm over the full range of protein and surfactant concentrations studied. The aggregates in the excess aqueous phase disassembled at high ionic strengths. At intermediate salt concentrations, a third, gel-like phase forms, and at high salt concentrations, protein and surfactant dissolve again into the aqueous and organic phases, respectively. Neither the effect of protein on droplet size nor protein partitioning to the microemulsion is affected by ionic strength, suggesting that the protein-surfactant interaction is governed by factors other than electrostatics, such as hydrophobic interactions.