Some Characteristics of Sugar Ester Nonionic Microemulsions in View of Possible Food Applications

Abstract

This study explores some characteristics of microemulsions composed of sucrose monostearate (SMS), medium-chain triglycerides (MCT), or R-(+)-limonene, alcohols, and water. The systems are homogeneous, soft, and waxy solids at room temperature but liquefy and structure into homogeneous microemulsions when heated to >40 °C. The amount of solubilized water is enhanced as a function of the alcohol/oil ratio and is inversely proportional to the alcohol chain length. Over 60 wt % water can be solubilized in systems consisting of propanol/MCT/SMS at a weight ratio of 3:1:4 (initial weight ratio). These microemulsions are unique and differ from nonionic ethoxylated-based microemulsions in that their viscosity is very low and is reduced with increasing amounts of solubilized water. The electrical conductivity increases only slightly as a function of the water content and does not show typical bicontinuous or percolated behavior. The water in the core of the microemulsion strongly binds to the headgroups of the surfactant. Only at >15 wt % solubilization of water was free or bulk water detected in the core of the microemulsions. Such unique behavior of the core water might have a possible application in systems requiring monitoring of enzymatic (lipase) reactions carried out in the microemulsions as microreactors. Keywords: Sucrose esters; W/O microemulsions; water solubilization; nonionic surfactants; phase diagrams; solid−liquid transitions; bound and free water; viscosity