Quenched molecular reorientation and angular momentum for liquids confined to nanopores of silica glasses

The results of a theoretical and experimental study of the confinement effects on molecular dynamics of nonpolar liquids in porous silica glasses prepared by the sol-gel process are discussed. The natural abundance ${}^{13}\mathrm{C}$ and ${}^{33}\mathrm{S}$ NMR spin-lattice relaxation times of liquid ${\mathrm{CS}}_{\mathrm{2}}$ are reported as a function of pore radius in the range from 15 Å to 102 Å over the temperature range of 168 K to 293 K. Since spin-rotation interactions dominate the ${}^{13}\mathrm{C}$ relaxation at higher temperatures these experiments allowed us for the first time to follow the confinement effects on angular momentum correlation times. The low-temperature ${}^{13}\mathrm{C}$ $T_1$ data and the ${}^{33}\mathrm{S}$ $T_1$ data provide information about reorientational motions. In order to interpret the experimental NMR relaxation data, a theoretical model for anisotropic molecular reorientation and angular velocity, which accounts for the motional behavior of nonpolar liquids confined to nanopores, is proposed. This model predicts an increase of the reorientational and angular momentum correlation times when the pore size is decreased. Application of this theoretical model to the interpretation of the ${\mathrm{CS}}_{\mathrm{2}}$ NMR relaxation data and to earlier relaxation results obtained for confined nonpolar cyclohexane-$d_{12}$ liquid proved successful.