Dynamics of low temperature induced water shedding from AOT reverse micelles

The effects of low temperature and ionic strength on water encapsulated within reverse micelles were investigated by solution NMR. Reverse micelles composed of AOT and pentane and solutions with varying concentrations of NaCl were studied at temperatures ranging from 20 °C to -30 °C. One-dimensional 1H solution NMR spectroscopy was used to monitor the quantity and structure of encapsulated water. At low temperatures, e.g., -30 °C, reverse micelles lose water at rates that are dependent on the ionic strength of the aqueous nanopool. The final water loading (w₀ = [water]/[surfactant]) of the reverse micelles is likewise dependent on the ionic strength of the aqueous phase. Remarkably, water resonance(s) at temperatures between -20 °C and -30 °C displayed fine structure indicating the presence of multiple transient water populations. Results of this study demonstrate that reverse micelles are an excellent vehicle for studies of confined water across a broad range of conditions, including the temperature range that provides access to the supercooled state.