Apparent Molal Volumes and Hydration Numbers from Viscosity Studies for Microemulsions with a Nonionic Surfactant Derived from Castor Oil and a Series of Polar Oils

Abstract The cloud point of a nonionic surfactant is related to the hydration of its polar shell. Viscosity data can be used to determine the total amount of water associated with micelles. The aim of this study was to apply this model to a more complex system comprising a series of oil-in-water microemulsions. These consisted of the nonionic surfactant, Kolliphor® RH 40, with and without polar oils. The oils used were isopropyl myristate (IPM), neopentyl glycol diethylhexanoate (NGDO), diisopropyl sebacate (DIS) and diisopropyl adipate (DIA) while the alcoholic solvents ethanol, 2-propanol and diethylene glycol monoethyl ether (DEG) were the co-surfactants. The apparent molal volumes were determined from the densities using a pycnometric method while the viscosities were measured using capillary viscometry. The partial excess volumes with IPM ranked as DEG>2-propanol>ethanol while the excess volumes varied as IPM>NGDO>DIS>DIA. These results were most likely related to the effect of the co-surfactant on the size and curvature of the micellar pseudophase and to the site of oil solubilization, respectively. The presence of oil induced increases in the intrinsic viscosities, the Huggins constants and the hydration numbers. The magnitudes of the latter two parameters were in excellent agreement with the values reported for micelles of nonionic surfactants. With DIS, the changes in the hydration numbers showed a good relationship with the excess volumes. Therefore, meaningful hydration values for microemulsions may be obtained with polar oils which are solubilized in the palisade layer rather than in the core as occurs with hydrophobic oils.