Ch. Cho et al., TIME-RESOLVED AND SPACE-RESOLVED STUDIES OF THE PHYSICS AND CHEMISTRYOF LIQUID WATER NEAR A BIOLOGICALLY RELEVANT INTERFACE, Journal of physical chemistry, 99(19), 1995, pp. 7806-7812
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.