Probing Dynamics of water Molecules in Mesoscopic Disordered media by NMR Dispersion and 3D Simulations in Reconstructed
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Probing dynamics of water molecules in mesoscopic disordered media by NMR dispersion and 3D simulations in reconstructed confined geometries.
P. Levitz1, J.P. Korb2, A. Van Quynh3 and R.G. Bryant3 1
CRMD-CNRS, 1B rue de la ferollerie, 45071 Orléans Cedex 2, France. LPMC-CNRS , Ecole Polytechnique, 91128 Palaiseau France. 3 Chemistry Department, University of Virginia, Charlottesville, VA 22901 USA. 2
ABSTRACT Disordered mesoporous materials with pore sizes ranging from 2 nm to some 10 nm develop large specific surface areas. These matrices can be easily filled with polar fluids And the interfacial region between the solid matrix and the pore network strongly influences the molecular dynamics of the entrapped fluid. A promising way to probe such a coupling on a large time-scale is to look at the dispersion of the nuclear spin-lattice relaxation rate of the polar liquid using field cycling NMR relaxometry technique. We have performed such an experiment on a fully hydrated porous Vycor glass, free of electron paramagnetic impurities. The proton nuclear magnetic relaxation rate (1/T1) exhibits a logarithmic dependence on Larmor frequency over the range from 0.01 to 30 MHz. A cross-over is observed below 0.1 MHz. In order to understand the relationship between geometric disorder, interfacial confinement, and nuclear magnetic relaxationdispersion (NMRD), we first compute an off-lattice reconstruction of the Vycor glass This model agrees with available experimental data (specific surface, porosity, chord length distributions, small angle scattering and tortuosity). A Brownian dynamics simulation is performed to analyze long time molecular self-diffusion and NMRD data. These later are well reproduced and appear to be connected with the translation diffusion of water near the SiO2 interface. The logarithmic character of the NMRD is specifically related to the interfacial geometry of the Vycor glass. Several other multiconnected interfacial structures such as periodic minimal surfaces do not exhibit such an evolution. Therefore, NMRD appears to be selectively sensitive to the interfacial geometry of mesoscopic disordered materials (MDM). INTRODUCTION Mesoscopic disordered media such as mesoporous materials develop large specific surface area. This interfacial region strongly influences the molecular dynamics of the embedded fluids. The confinement might influence the molecular dynamics on a length scale ranging from 1 nm up to several µm . At short times t and small distances r (t 10 µs (see text for details). At high frequencies, we find a relaxation rate, R1, slightly higher than the typical value of the T2.6.2
bulk liquid. From 10 kHz to 0.1 MHz, R1 exhibits a logarithmic behavior. Finally, a plateau is observed at very low frequencies (fcross~ 0.1 MHz), that we attribute to a “cross-over” time of 1 µs. We address below, the following questions: How to explain the log evolution ? How is it related to the Vycor interfacial geometry? Why a such low fcross value is found? To answer these questions, we have first tried to
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