Theory of Nuclear Relaxation in Confining Systems. Application to Non Wetting Liquids in Porous Silica Glasses
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THEORY OF NUCLEAR RELAXATION IN CONFINING SYSTEMS. APPLICATION TO NON WETTING LIQUIDS IN POROUS SILICA GLASSES J.-P. KORB*, A. DELVILLE**, Shu XU and J. JONAS*** *Laboratoire de Physique de la Mati~re Condensee, Ecole Polytechnique, 91128 Palaiseau, France **Centre de Recherche sur la Mati~re Divis6e, CNRS, lb rue de la F6rollerie, 45071 Orldans, Cedex 02, France ***Material Research Laboratory and Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, USA. ABSTRACT This work shows how the geometrical confinements enhances the nuclear relaxation of a non wetting liquid in a model porous systems. Application of the proposed theory is made to interpret the size and frequency dependences of the 11 relaxation of methylcyclohexane liquid in sol-gel porous silica glasses with narrow pore-size distribution.
INTRODUCTION The role of confined geometry on dynamics of liquids has recently been widely studied in view of its importance for a variety of technological processes including catalysis, chromatography, oil recovery and membrane separation. Systems of interest include porous silica glasses [1, 2], two-dimensional systems [3], molecules physisorbed on beads [4], and cross-linked polymer resins [5]. Nuclear relaxation is particularly sensitive to the confinement and has been used for the analysis of pore sizes in silica glasses [1, 2], resins [5] and rocks [6]. It is known that the confinement enhances significantly the spin relaxation rates 1/Ti (i = 1 or 2) and alters their frequency and temperature dependences [3, 4]. Generally the observation of a linear dependence of spin relaxation rates with the filling fraction of the pores proves the homogeneity of the solvent at every coverage [7]. This also supports the two-fraction fast-exchange model where one of the fluid components preferentially wets the pore surface while the other filled the pore domain. As a consequence the longitudinal or transverse nuclear relaxation rates 1/Ti of a solvent inside a pore is an average of a bulk l/Ti~b and surface 1/Ti,' contributions : l/Ti = 1/Ti.b + (ld)(1/Ti.- IT/i.b), where e is the thickness of the adsorbed layer and d the average pore size. This occurs for strongly interacting liquids for which one observes a linear relation 1/T1 - 1/d [1]. There are some exceptions for very weak interacting liquids where observations give l/T 1 -c l/d 2 [1, 2]. The question is now to explain such seemingly paradoxical relation when the size-dependent surface contribution disappears? We show that the dipolar relaxation
Mat. Rea. Soc. Symp. Proc. Vol. 290. 01993 Materials Research Society
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by translational diffusion of a non wetting liquid in a confined geometry can explain this latter confinement relation. The size and frequency dependences of the 1H nuclear relaxation of methylcyclohexane in sol-gel porous silica glasses (with narrow pore size distribution) support such interpretation.
THEORY OF DIPOLAR RELAXATION BY TRANSLATIONAL OF A NON WETTING LIQUID IN THE PORE SLIT MODEL
DIFFUSION
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