DYNAMIC STRUCTURE OF METHYLCYCLOHEXANE AND PERFLUORO(METHYLCYCLOHEXANE) LIQUIDS IN CONFINEMENT AND IN BULK

The dynamic behaviors of methylcyclohexane (MCH) and perfluoro(methylcyclohexane) (PFMCH) Liquids confined to porous silica glasses prepared by the sol-gel process are compared. The NMR spin-lattice relaxation times, T-1, of the MCH and PFMCH Liquids in porous silica glasses are reported as a function of pore size in the range from 24 to 96 Angstrom over the temperature range from -8 to 45 degrees C. The pore-size-dependent experimental T-1 data are analyzed in terms of a general expression obtained from our previous studies, 1/T-1 = 1/T-1b + B/R + A/R(2), where T-1b is the relaxation time for bulk Liquid, IZ is the average pore radius, and A and B are two parameters which indicate the relative strength of surface and topological effects on the observed NMR relaxation rates of confined liquids. On the basis of the surface enhancement values, T-1b/T-1s, where T-1s is the relaxation time of the surface layer Liquid, we conclude that the confined PFMCH molecules have a stronger interaction with the glass surface and thus exhibit more hindered molecular motions than the confined MCH molecules. In contrast, topological confinement plays a smaller role in affecting the translational diffusion of PFMCH molecules near the glass surface. In addition to the confinement studies, we have investigated the dynamic structure of bulk PFMCH by analyzing shear viscosity data in terms of the rough-hard-sphere model of liquids in a manner similar to our previous bulk liquid MCH study.