A study of the microstructure of four-component sucrose ester microemulsions by SAXS and NMR

Abstract

Sucrose esters form a class of surfactants with the important properties of being biodegradable, non-toxic and capable of forming temperature-insensitive microemulsions. Such microemulsions would be expected to suit a variety of food-based and pharmaceutical applications; however to date little is known about their structure and stability. In this study, the Winsor IV microemulsion systems composed of sucrose esters (SE)/1-butanol/water and oils such as n-dodecane, n-hexadecane and medium chain triglyceride (MCT), have been investigated using small angle X-ray scattering (SAXS), pulsed gradient spin echo (PGSE) NMR and viscosity measurements. The SAXS results for the sucrose monostearate (S1570) system at SE/MCT/1-butanol=1.5:1.1 clearly indicate that the periodicity d increases with increase in water content and is not sensitive to the nature of the oil. From the amphiphilicity factor, f a, and the correlation length, ξ, one can conclude that the n-dodecane-based microemulsion system is the most ordered. Microstructure investigation by PGSE NMR gives evidence of structural changes as the water content in the system increases. The oil self-diffusion remains unchanged when MCT serves as the oil phase. However, when the oil is paraffinic in nature ( n-dodecane and n-hexadecane) the self-diffusion coefficient indicates participation of the oil molecules at the interface. Surfactant self-diffusion is only weakly affected by the water content. The shorter chain oils ( n-dodecane and MCT) solubilize a maximum of 40 and 47 wt.% of water and cannot invert, while the long chain paraffinic ( n-hexadecane-based system) inverts into an O/W microemulsion. The viscosity of these microemulsions decreases with increasing water content. The absence of a yield stress in any of the samples studied, together with the linearity of the flow curves, is evidence that there are no relaxation processes in these microemulsions which show a non-Newtonian flow behavior.