Water hydrogen bond dynamics in aqueous solutions of amphiphiles.

The hydrogen bond dynamics of water in a series of amphiphilic solute solutions are investigated through simulations and analytic modeling with an emphasis on the interpretation of experimentally accessible two-dimensional infrared (2D IR) photon echo spectra. We evidence that for most solutes the major effect in the hydration dynamics comes from the hydrophilic groups. These groups can retard the water dynamics much more significantly than can hydrophobic groups by forming strong hydrogen bonds with water. By contrast, hydrophobic groups are shown to have a very moderate effect on water hydrogen bond breaking kinetics. We also present the first calculation of the 2D IR spectra for these solutions. While 2D IR spectroscopy is a powerful technique to probe water hydrogen bond network fluctuations, interpretations of aqueous solution spectra remain ambiguous. We show that a complementary approach through simulations and calculation of the spectra lifts the ambiguity and provides a clear connection between the simulated molecular picture and the experimental spectroscopy data. For amphiphilic solute solutions, we show that, in contrast with techniques such as NMR or ultrafast anisotropy, 2D IR spectroscopy can discriminate between waters next to the solutes hydrophobic and hydrophilic groups. We also evidence that the water dynamics slowdown due to the hydrophilic groups is dramatically enhanced in the 2D IR spectral relaxation, because these groups can induce a slow chemical exchange with the bulk, even when recognized exchange signatures are absent. Implications for the understanding of water around chemically heterogeneous systems such as protein surfaces and for the interpretation of 2D IR spectra in these cases are discussed.