Time- and Space-Resolved Studies of the Physics and Chemistry of Liquid Water near a Biologically Relevant Interface

Abstract

In this paper, a new method for analyzing certain physical and chemical properties of liquid water near surfaces is described. "Probe" molecules are dissolved in the water system and are excited by a short laser pulse. The ability of the probe to undergo a fast nonradiative process depends on a reorientational relaxation time of the water solvent, which may become orders of magnitude slower for water near a surface. Using timeresolved methods and a sufficiently fast probe, one can observe a direct dynamic competition between diffusion of the probe and the nonradiative event. Thus, in principle, it is possible to obtain both 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 investigate the small biologically relevant water pools in sodium bis(2-ethylhexy1)sulfosuccinate (AOT) reverse micelles, whose surfaces are highly hydrophilic. 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 micelle studied (radius -55 A). On the other hand, perturbations on the orientation relaxation of the solvent, as measured by the probe fluorescence lifetimes, were found to extend no more than -10-15 A from the surface of any of the micelles studied.

Energy Gap Dependence of the Nonradiative Decay Rate Constant of 1-Anilino-8-naphthalene Sulfonate in Reverse Micelles

“…2(e). The decay time (τ) ranged from 0.25 to 14.2 ns, depending on the nature of the solvent ( 14,37 In the micellar systems, on the other hand, the fluorescence decayed faster for the micelles with a larger w 0 value. Furthermore, it is worth noting that the decay profile cannot be analyzed by a single exponential function, but by a double-or triple-exponential function as judged from the weighted residuals, the chai-squared (χ 2 ) and Durbin-Watson (D.W.) parameters for the fitting; the results are summarized in Table 2.…”

“…7,[13][14][15][16][17][18][19][20][21] As an example, Cho et al reported that ANS in reverse AOT micelles exhibited a double exponential decay: short component=0.3 -0.6 ns and long component=2.6 -5.1 ns at w 0 =4 -33.…”

“…As one of the several advantages of a reverse AOT micellar system, the size of the water pool can be controlled precisely at the nanometer level through the molar ratio of water to AOT: 7 Therefore, size effects on chemical and physical properties in nanometer dimensions can be studied by the use of reverse AOT micelles. Indeed, reverse AOT micelles have been studied by various techniques such as fluorescence spectroscopy [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] , NMR 22,23 , ESR 24,25 , IR [26][27][28] , and so on. [29][30][31][32] These studies demonstrated that polarity [8][9][10] , viscosity 11 , pH 12 , and other properties 30,32 of the water pool in the micelles were different from those in the bulk state and were dependent on the w 0 value.…”

“…Particularly, recent time-resolved fluorescence spectroscopy has provided invaluable information about the structures and dynamical behavior of water molecules inside the micelles. 7,[13][14][15][16][17][18][19][20][21] It is noteworthy that these studies are essentially based on the changes in the fluorescence properties of a probe molecule with the size of the water pool. However, we supposed that detailed discussions on the fluorescence characteristics should be based on photophysical parameters represented by the radiative and nonradiative rate constants of a probe molecule.…”

New Approaches to Liquid Interfaces through Changes in the Refractive Index and Nonlinear Susceptibility Utilizing Ultrashort Laser Pulses

“…12 Thus, they have been extensively studied as models of confined water in macromolecules, or as models of biologically relevant interfaces. 13 We found that the UTL has provided further information on the corrective behaviors of such inhomogeneous environments where several kinds of molecules interact with each other. 14,15 The second and third sections present the development of new instruments that we have newly developed.…”