Aqueous solvation of amphiphilic solutes: concentration and temperature dependent study of the ultrafast polarisability relaxation dynamics

An understanding of the influence of hydrophilic and hydrophobic interactions on the dynamics of solvating water molecules is important in a diverse range of phenomena. The polarisability anisotropy relaxation dynamics of aqueous solutions of the amphiphiles TBA (t-butyl alcohol) and TMAO (trimethylamine N-oxide) have been measured as a function of concentration and temperature. TMAO is shown to have a greater effect on the picosecond relaxation dynamics of water than TBA. This result is consistent with hydrophilic interactions being mainly responsible for the slowing down the polarisability relaxation in aqueous solutions. The room temperature Raman spectral densities of the two solutions are remarkably similar to that of bulk water, an effect which is tentatively ascribed to the formation of nanoscale structure in the solutions, allowing the formation of bulk-like water pools. The temperature dependent spectral density of TMAO remains similar to that of bulk water at all temperatures, while that for TBA shows a marked decrease in the amplitude of the response usually ascribed to a water-water stretch with increasing temperature. This is discussed in terms of the temperature dependent structure of TBA aggregates in solution.