Tuning the Hydrophobic Interaction: Ultrafast Optical Kerr Effect Study of Aqueous Ionene Solutions

The molecular origin of the hydrophobic effect continues to be widely studied. Here we design an experiment to tune independently hydrophilic and hydrophobic interactions through the study of a series of aqueous ionene solutions. The dynamics of these solutions are probed using the ultrafast optical Kerr effect, which measures polarizability anisotropy relaxation. Analysis of these data yields information on both structural dynamics within the water hydrogen-bonded network and the low frequency intermolecular bending and stretching H-bond modes. In all cases the ionene solute retards the structural dynamics compared to bulk water. However, the effect is small and cannot be assigned specifically to water-hydrophobe interactions. There is no evidence for a dramatic slowdown of the water dynamics observed by the optical Kerr effect when water is in the solvation shell of a hydrophobic group. The low frequency spectrum was recorded as a function of ionene concentration. Again the effect of the solute was small, and could be assigned mainly to the effect of anion solvation.