Hydrogen-bond dynamics for water confined in carbon tetrachloride-acetone mixtures.

In a variety of biological scenarios water is found trapped within hydrophobic environments (e.g., ion channels). Its behavior under such conditions is not well understood and therefore is attracting enormous scientific attention. It is of particular interest to understand how the confining environment affects both the structure and dynamics of water. Within this scenario, we report molecular dynamics simulation results for water trapped in a mixture of acetone and carbon tetrachloride whose composition mimics the one employed in recently reported experiments [Gilijamse, J. J.; et al. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 3202]. We show here that the water molecules dissolved in the carbon tetrachloride-acetone mixture assemble in clusters of varying sizes, that the longevity of hydrogen bonds between confined water molecules strongly depends on the cluster size, and that hydrogen bonds last longer for small water clusters in confined water than they do in bulk water. The simulated FT-IR spectra for the confined water are shifted at longer frequencies compared to those observed for bulk liquid water. We discuss the dependence of the FT-IR spectrum on the size of the water clusters dispersed in the carbon tetrachloride-acetone matrix. We also study in detail the rotational orientation of the dispersed water molecules, and we discuss how the composition of the organic matrix affects the results. By enhancing the interpretation of the experimental data, our results contribute to developing a molecular-based understanding of the relationship between environment and water properties.