Dynamics of Liquids in Confined Geometries
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ABSTRACT An overview of several NMR studies of liquids in confined geometries is presented. First, the NMR relaxation rates, lI/T 1 , /T1 p, and 1/T2 were measured for several molecular liquids confined to porous silica glasses with pore radii in the range from 12 A to 100 A as a function of temperature, pore size, and frequency. The experimental relaxation data were interpreted in terms of bulk, surface, and topological contributions using the following
expression: 1
1
Tj
T.
2_6
1
1
Ai(c)
R~7Ti
T.b
R
where 1/Tib and 1/Tis are the bulk and the surface layer relaxation rates, S is the thickness of the surface layer, R is the pore radius, and Ai(co) represents the pure topological effect. Second, the pressure effects on the dynamics of the confined liquid of acetonitrile-d 3 were also investigated. Third, the natural abundance of 13 C spin lattice relaxation rates for CS 2 confined to porous silica glasses provided information about confinement effects on the angular momentum behavior of this simple liquid.
INTRODUCTION In view of its importance the problem of the dynamical behavior of liquids in confined geometries has recently received considerable attention both from theoretical and experimenta points of view [1,2]. Understanding the role of confined geometry on dynamics of liquids is important for a variety of technological processes including catalysis, chromatography, oil recovery, and membrane separations. Vycor glass, zeolites, membranes, polymers, packed powders, and porous silica glasses are examples of porous media used in these studies. The progress in understanding the sol-gel process [3-5] 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. 925 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
The sol-gel prepared porous silica glasses represent an excellent host material to study the behavior of liquids in confined geometries. In continuation of our efforts [6-15] to improve the understanding of the dynamic behavior of confined liquids we shall report selected results of three recent studies. The experimental T 1 , T2 and TIp NMR relaxation data for polar liquid nitrobenzene and non-polar liquid methylcyclohexane confined to porous silica glasses allowed us to evaluate the relative contributions of surface interactions and true topological effects on the observed relaxation rates. This evaluation of surface and topological contributions [14] is based on a theoretical model [ 13] for dipolar translational diffusion of a liquid confined in model porous medium. The investigation of the pressure effects on the anisotropic reorientation [16] of confined liquid acetonitrile-d 3 was motivated by our earlier high pressure experiments [ 11] on confined pyridine-d 5 , and the study of confinement effects on the anisotropic reorientation of acetonitrile-d 3 at atmospheric pressure [15]. The main goal of these experiments was
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