TIME-RESOLVED AND SPACE-RESOLVED STUDIES OF THE PHYSICS AND CHEMISTRYOF LIQUID WATER NEAR A BIOLOGICALLY RELEVANT INTERFACE

In this paper, a new method for analyzing certain physical and chemica l properties of liquid water near surfaces is described. ''Probe'' mol ecules are dissolved in the water system and are excited by a shea las er pulse. The ability of the probe to undergo a fast nonradiative proc ess depends on a reorientational relaxation time of the water solvent, which may become orders of magnitude slower for water neat a surface. Using time-resolved methods and a sufficiently fast probe, one can ob serve a direct dynamic competition between diffusion of the probe and the nonradiative event. Thus, in principle, it is possible to obtain b oth these rates as a function of distance from a surface. The methods can be applied to a variety of surfaces. Here, they are used to invest igate the small biologically relevant water pools in sodium bis(2-ethy lhexyl)sulfosuccinate (AOT) reverse micelles, whose surfaces are highl y hydrophilic. Perturbations on the translational velocity autocorrela tion function of the probe, as measured by the diffusion fluxes, are v ery large, extending nearly to the center of the largest micelle studi ed (radius similar to 55 Angstrom). On the other hand, perturbations o n the orientation relaxation of the solvent, as measured by the probe fluorescence lifetimes, were found to extend no more than similar to 1 0-15 Angstrom from the surface of any of the micelles studied.