Surfactant properties of purified polyglycerol monolaurates

A series of purified polyglycerol monolaurates (PGML), such as di-, tri-, tetra-, and pentaglycerol monolaurates, were synthesized, and their surfactant properties in aqueous solutions were examined. The surfactant properties of PGML were compared with those of n-dodecyl polyoxyethylene monoethers (C12EOn) to examine the function of the hydrophilic part of these compounds. The critical micelle concentration (CMC) values and the surface tension at CMC of PGML and C12EOn increased linearly with an increase in the number of glycerol and oxyethylene units, respectively; the slope of the increase was greater for PGML than C12EOn. The minimum surface area per molecule of PGML was smaller than that of C12EOn at the air/aqueous solution interface. The initial foam heights of the surfactants at the CIVIC increased with an increase in the number of glycerol or oxyethylene units, and the foam heights of PGML were consistently higher and more stable than those of C12EOn. Detergency depended on a reduction in interfacial tension. Triglycerol monolaurate showed the lowest interfacial tension and the highest detergency among all the surfactants tested. Overall, the PGML showed better performance in all the surfactant properties tested than C12EOn. It is noteworthy that the surfactant properties of PGML having few glycerol units (di- to tetraglycerol monolaurates) are on par with those of C12EOn having many oxyethylene units (hexa- and octaoxyethylene). These results suggest that PGML having a secondary hydroxyl group on every glycerol unit of the hydrophilic part could be more hydrophilic than C12EOn; this characteristic feature guaranteed the superior surfactant properties of PGML.