Phase Stability and Miscibility in Ethanol/AOT/n-heptane Systems: Evidence of Multilayered Cylindrical and Spherical Microemulsion Morphologies.

We describe the effects of ethanol on the phase behavior of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in n-heptane. Using dynamic light scattering (DLS), molecular dynamics (MD) simulations, and nuclear magnetic resonance (H-1 NMR) spectroscopy, we investigate the aggregation behavior of AOT across a wide range of ethanol/AOT/n-heptane compositions. We conclude that reverse micelles do not form at any of the investigated concentrations. Instead, we observe the formation of other surfactant aggregate morphologies unique to this system, namely, multilayered cylindrical structures and spherical AOT-in-ethanol structures, which vary significantly with changes in ethanol concentration. We also identify mixed-solvent polarity as a driving factor for the surfactant behavior in the system. When the concentration of ethanol is 20 wt % or below, the system is inhomogeneous with varying sizes of AOT, ethanol, and AOT + ethanol aggregates, with the ethanol primarily exhibiting a cosurfactant behavior, almost exclusively binding at the surface of AOT aggregates. With increased ethanol concentration, the ethanol in the system also exhibits solvent-like behaviors in addition to the cosurfactant behaviors. Most significantly, when the ethanol concentration is raised above 35 wt %, the transition to solvent-like behavior allows AOT Na+ counterions to dissociate from the headgroups and they are dissolved in the ethanol. We use these results to construct a preliminary phase diagram for the ethanol/AOT/n-heptane system.