NMR study of the molecular dynamics of ethanol and 2,2,2-trifluoroethanol liquids confined to nanopores of porous silica glasses

A dynamic nuclear magnetic resonance (NMR) study of the polar fluids ethanol (EtOH) and 2,2,2-trifluoroethanol (TFE) confined to porous silica sol-gel glasses is reported. The C-13 NMR spin-lattice relaxation times, T-1, were measured in glasses with pore radii ranging from 18.9 to 54.8 Angstrom, over a temperature range from -13.6 to 30.5 degrees C. The data were analyzed in terms of the two-state, fast exchange model, and surface layer relaxation times, T-1s, were calculated. On the basis of surface enhancement factors, T-1b/T-1s, where T-1b is the relaxation time of the bulk liquid, it was concluded that the more acidic TFE has a weaker hydrogen bond interaction with silica, due to the fact that the alcohols serve as hydrogen bond accepters. The experiment shows that EtOH and TFE have nearly identical surface layer viscosities, originating from the differences in hydrogen bonding with the silica surface. Confinement was found to have little effect on the internal rotation of terminal CF3 or CH3 groups.