Understanding the properties and stabilities of emulsions

Abstract

Our knowledge of the behaviour of emulsions and the oil-water interface at the molecular level is relatively sparse when compared to the huge importance of these systems in everyday life. This thesis describes the development of several techniques that have the potential to extend this knowledge further. Ellipsometry is exquisitely sensitive to density variations across a fluid-fluid interface, but the interpretation of the ellipsometric response from the oil-water interface, both in the absence and presence of adsorbed species, is nontrivial. The responses from the interface between simple linear alkanes and water suggest that these interfaces are more complex than the air-water interface. The results are indicative of a "drying layer" and a model based on the hard sphere repulsion of a hydrocarbon surface is developed to explain these results. The conclusions from this study on the pristine oil-water interface are used as a basis for the interpretation of the ellipsometric response of a surfactant monolayer at the oil-water interface. The results suggest that the nonionic hydrocarbon surfactant C(_10)E(_8) forms a disordered monolayer with partially-hydrated headgroups. The structure of this film is largely independent of the nature of the superphase; whether air, hexadecane or triolein. The competitive adsorption of the milk proteins ß-casein and ß -lactoglobulin with C(_10)E(_8) at the hexadecane-water interface is also studied. At low and high concentrations of C(_10)E(_8), the measured ellipticity is indicative of an unperturbed protein and surfactant film, respectively. At intermediate concentrations, the measured ellipticity suggests a mixed surfactant/protein film. Raman spectroscopies deliver considerably more data than ellipsometry, but the difficulties of delivering and collecting light from a buried interface require more sophisticated experimental engineering. An attempt is made to sandwich a 100-nm- thick oil film between water and silica in order to probe the oil-water interface by evanescent wave Raman scattering employing a visible excitation source. Unfortunately this procedure has proved unsuccessful to date. The requirement for a thin oil film can be bypassed if the signal from the adsorbed species is enhanced. The design and construction of an ultra-violet resonance Raman microspectrometer is described to achieve this aim. This instrument also has the potential to probe proteins and peptides adsorbed in lipid bilayers. The commissioning and development of a combined in-situ confocal Raman- tribometer is also presented. This instrument is capable of determining the composition of emulsions under shear within a soft elastohydrodynamic contact, as well as the morphology of the soft, deformed surface and the thickness of the lubricating film.