Surface pressure isotherms, dilatational rheology, and brewster angle Microscopy of insoluble monolayers of sugar monoesters

Surface pressure isotherms of insoluble monolayers of high purity, enzymatically synthesized sucrose, lactose, glucose, and galactose stearate at the air - water and n-tetradecane-water interfaces were obtained. The dilatational moduli of these surfactant monolayers were also determined by oscillations of the interface at 0.1 Hz. Results have indicated similarities in the properties of sucrose and lactose stearate. Brewster angle microscopy has shown the transitions from gaseous to liquid expanded to liquid condensed phases upon compression of these monolayers at the air-water (A-W) interface. It was also shown that the interfacial phase separation is strongly dependent on the rate of deformation of the interface. Glucose and galactose stearate formed more aggregated monolayers at the A-W interface and (particularly galactose stearate) had higher dilatational moduli, compared to sucrose and lactose stearate. Brewster angle microscopy confirmed the extensive aggregation and the formation of a solid phase in the case of galactose stearate. The isotherms of glucose and galactose stearate at the oil-water (O-W) interface exhibited a first-order phase transition, suggesting a different aggregation pattern, in comparison to the aggregation at the A-W interface, where, possibly, no solid phase was formed. This suggested that more extended aggregation was taking place, where the interactions between the monosaccharide headgroups was the major attractive force.