High Pressure Nmr Studies of Confined Liquids
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HIGH PRESSURE NMR STUDIES OF CONFINED LIQUIDS
J. JONAS, JING ZHANG, AND SHU XU
Department of Chemistry, School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801
ABSTRACT The main goal of our novel NMR experiments on confined liquids was to determine the effects of pressure on the dynamics of liquids in the surface layer by using the twostate, fast exchange model and compare them to the pressure effects observed for bulk liquids. With this goal in mind, the deuteron NMR spin-lattice relaxation times, Tj, in liquid pyridine-d 5 , nitrobenzene-d5, and methylcyclohexape-d confined to sol-gel porous silica glasses with pore radii in the range from 18 A to 49A were measured as a function of pressure up to 5 kbar at 300 J. In another set of high resolution natural abundance 1LC NMR experiments, the kJC relaxation behavior of each carbon in 2ethylhexyl benzoate model lubricant was measured as a function of pressure in porous silica glasses. In fact, the described experimental approach which allows investigation of the effects of pressure on the dynamic behavior of surface-layer liquids, may provide a new tool in studies of model liquid lubricants at extreme conditions of pressure and temperature. In addition4 selected results of a recent study of acetonitrile-d 3 are reviewed. The 2H and 1 N spin-lattice relaxation times of acetonitrile-d 3 in porous silica glasses were measured in order to study the confinement effects on the anisotropic reorientation characterized by rotational diffusion constants, D_- and D11 , of this symmetric-top molecule. INTRODUCTION
The progress in understanding the sol-gel process for making glasses and ceramics allowed development of synthetic procedures leading to porous silica glasses with narrow pore-size distribution and specific pore radius which can be varied over a wide range. The sol-gel prepared porous silica glasses represent an excellent host material to study the behavior of liquids in confined geometries. This work represents a continuation of our efforts [1-9] to improve the understanding of the dynamic behavior of the confined liquids. There were several results of our earlier studies [4,6] which provided the main motivation for the present experiments. First of all, we have established the applicability of the two-state, fast exchange model to analyze the spin-lattice relaxation time, T 1 , data for polar molecular liquids confined to sol-gel prepared porous silica glasses with a wide range of pore diameters. It is important to point out that this allows one to determine the relaxation rate, (l/T 1 ), for the surface layer liquid. Therefore, it is of interest to find out whether the two-state, fast exchange model is also applicable for the analysis of relaxation data of confined liquids at high pressures, as this approach will allow one to investigate for the first time the effects of pressure on the reorientational dynamics of the surface layer liquid. However, so far all NMR or other spectroscopic studies on liquids in confined geometries we
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