Hydrophobic hydration in water-tert-butyl alcohol solutions by extended depolarized light scattering

Molecular dynamics and structural properties of water-tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand of GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and inter-molecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The retardation factor of hydration water with respect to the bulk,  ≈ 4, almost concentration independent, is one of the smallest found by EDLS over a wide variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective in perturbing the H-bond network of liquid water than hydrophilic and more complex molecules. At increasing TBA concentration, up to mole fractions as high as ≈ 0.03, both the average number of perturbed water molecules and the characteristic frequency of librational motions show a pronounced decrease, consistent with self-aggregation of TBA molecules.