Temperature Insensitive Microemulsions

This paper is concerned with the preparation of temperature insensitive microemulsions stabilized using mixtures of the ionic surfactant sodium bis(2-ethylhexyl)sulphosuccinate (AOT) and the nonionic surfactant n-dodecyl pentaoxyethylene glycol ether (C12E5). The temperature dependence of microemulsions arises from the temperature sensitivity of the preferred monolayer curvature, and this is similar in magnitude but opposite in sign for AOT and C12E5 monolayers. Thus, as is well-known for mixed ionic/nonionic systems, monolayers containing a mixture of AOT and C12E5 show a reduced temperature sensitivity. In this paper we develop a simple phenomenological model for the solubilization, phase inversion, and oil−water tension properties of such systems. The model successfully describes systematic experimental results for solubilization, phase inversion temperatures, and tensions for a wide range of composition and temperature. In addition to predicting conditions for which the microemulsions are temperature insensitive, the model is useful as an aid in finding conditions where the temperature sensitivity and the extent of solubilization and oil−water tension can be simultaneously optimized. It is observed that AOT/C12E5 mixed systems show an increased hydrophilicity relative to that expected on a simple linear mixing rule. Using an electrostatic theory for the variation of monolayer charge and preferred curvature properties with monolayer composition, we show that the physical origin for the increased hydrophilicity lies in the increased dissociation of ionic surfactants when diluted in monolayers of nonionic surfactant.