Water titration studies on microemulsions with a nonionic surfactant derived from castor oil and a series of polar oils

Abstract The hydration of a nonionic surfactant in micellar systems depends on the nature of its headgroup and the amount of mechanically trapped water. A water titration method was used to quantify the hydration of microemulsions with Kolliphor® RH 40, a nonionic surfactant. The polar oils used were isopropyl myristate (IPM), neopentyl glycol diethylhexanoate (NGDO), diisopropyl sebacate (DIS) and diisopropyl adipate (DIA) while the co-surfactants included ethanol, 2-propanol and diethylene glycol monoethyl ether (DEG). The existence of microemulsion particles was shown using dynamic light scattering (DLS). Various systems were titrated with water until phase separation occurred due to a small excess of oil in the bulk phase. The depths of oil penetration were related to their relative hydrophobicities as shown by the DLS data and UV studies. Hydration numbers were calculated from the difference between the water uptake values at the surfactant proportion of interest and those in which microemulsions had presumably not formed. The values obtained were all >2.0 (number of water molecules per surfactant polyoxyethylene group) but ≤10 in most systems. These results were in agreement with the ranges reported in the literature. Therefore, water titration is a promising technique for studying the hydration of nonionic surfactants in microemulsions.