Deuterium NMR relaxation studies of liquids in confined geometries
Liu, Guoying
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Permalink
https://hdl.handle.net/2142/21494
Description
Title
Deuterium NMR relaxation studies of liquids in confined geometries
Author(s)
Liu, Guoying
Issue Date
1991
Doctoral Committee Chair(s)
Jonas, Jiri
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Physical
Language
eng
Abstract
Nuclear magnetic resonance (NMR) has been employed to study the confined geometry effects on reorientational dynamics of molecular liquids in porous media. The relative role of the pure geometrical confinement and the strength of the surface effect is investigated. The deuteron NMR spin-lattice relaxation times, T$\sb1$, of several molecular liquids in porous silica glasses are reported as function of pore size in the range from 17A to 143A over the temperature range from 260K to 310K. Molecular liquids studied include strongly interacting polar liquids such as pyridine-d$\sb5$, aniline-d$\sb5$ and nitrobenzene-d$\sb5$ whereas the saturated cyclic hydrocarbon liquids of cyclohexane-d$\sb{12}$ and cis-decalin-d$\sb{18}$ represent the weakly interacting liquids. In a first approximation, toluene-d$\sb1$ and dioxane-d$\sb8$ are chosen as examples of liquids with intermediate interactions with the silica surface. The experimental relaxation data are analyzed by using the two-state fast-exchange model which is found to be valid for the strongly interacting liquids and liquids with intermediate interactions. In terms of this model, the viscosity of the surface layer for pyridine-d$\sb5$ is about 30 times higher than that for bulk liquid pyridine. The importance of two-dimensional approach to describe motional dynamics of liquids confined to pores smaller than 30A is illustrated for liquid nitrobenzene-d$\sb5$.
Additional information on the relative role of surface interactions and the pure topological effects on the dynamics of liquids in confined geometries was obtained by using surface modified glasses in which the surface hydroxyl groups were replaced by OSi(CH$\sb3)\sb2$OC$\sb2$H$\sb5$ groups.
In an effort to understand the effect of geometrical confinement on the anisotropic reorientation and internal motion of liquid toluene in porous sol-gel silica glasses, a study of temperature dependence and the magnitude of the T$\sb1$ values for perdeuterated and selectively deuterated toluene was undertaken using $\sp2$H NMR techniques, which provide a means for investigating the nature of molecular rotation of the confined liquid. The rotational diffusion tensors for overall, internal, and anisotropic rotational diffusion have been determined. The effect of geometrical confinement results in a considerable decrease of the rotational diffusion constants and leads to a lowering of the symmetry of the anisotropic rotational diffusion tensor. (Abstract shortened with permission of author.)
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