Experimental and theoretical studies on water
Cho, Chul Hee
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The properties of water near surfaces or in confined volumes are not well understood. Here, the biologically relevant system chosen for studying the behavior of water near a surface is composed of small water pools encapsulated within sodium bis (2-ethylhexyl)sulfosuccinate (AOT) reverse micelles (RM's), whose surfaces are highly hydrophihc. The time-correlated single-photon counting technique together with an ultrafast laser system was employed to measure the fluorescence lifetimes of the probe molecule ANS within various RM sizes. The resulting decay data were interpreted by using the analytical method and the non-linear least squares fitting technique in order to seek a relationship between diffiision of the probe and a fast nonradiative event. By discovering these trends, information about water properties within different regions of the aqueous core of the RM can be obtained as a function of distance from a surface. The ability of the probe to undergo the fast nonradiative process depends on a reorientational relaxation tune of the water solvent, which may become orders of magnitude slower for water near a surface. Perturbations on the translational velocity autocorrelation function of the probe, as measured by the diffusion fluxes, are very large, extending nearly to the center of the largest RM studied (radius -55 A). On the other hand, perturbations on the orientation relaxation of the solvent, as measured by the probe Hfetimes, were found to extend no more than -10-15 A from the surface of any of the RM's studied. An explanation of the density maximum in water near 4 °C can be described in terms of a dense second-neighbor structure obtained from the bending of hydrogen bonds. The analytical and computational models of water, which provide an explanation of this anomaly, are proposed. In these models, the water-water potential is altered to include more realistic interactions in the second-neighbor shell. Support for this idea is provided here by considering the exactly soluble one-dimensional Takahashi fluid model and then by an NVE molecular dynamics simulation for a realistic water model.
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