Solvent Effects on AOT Reverse Micelles in Liquid and Compressed Alkanes Investigated by Neutron Spin−Echo Spectroscopy

Neutron Spin−Echo (NSE) spectroscopy has been employed to study the interfacial properties of reverse micelles formed with the common surfactant sodium bis-2-ethylhexyl-sulfosuccinate (AOT) in liquid alkane solvents and compressed propane. NSE spectroscopy provides a means to measure small energy transfers for incident neutrons that correspond to thermal fluctuations on the nanosecond time scale and has been applied to the study of colloidal systems. NSE offers the unique ability to perform dynamic measurements of thermally induced shape fluctuation in the AOT surfactant monolayer. This study investigates the effects of the bulk solvent properties, water content, and the addition of octanol cosurfactant on the bending elasticity of AOT reverse micelles and the reverse micelle dynamics. By altering these solvent properties, specific trends in the bending elasticity constant, k, are observed where increasing k corresponds to an increase in micelle rigidity and a decrease in intermicellar exchange rate, k ex. The observed corresponding trends in k and k ex are significant in relating the dynamics of microemulsions and their application as a reaction media. Compressed propane was also examined for the first time with a high-pressure, compressible bulk solvent where variations in temperature and pressure are used to tune the properties of the bulk phase. A decrease in the bending elasticity is observed for the d-propane/AOT/W = 8 reverse micelle system by simultaneously increasing the temperature and pressure, maintaining constant density. With isopycnic conditions, a constant translational diffusion of the reverse micelles through the bulk phase is observed, conforming to the Stokes−Einstein relationship.