NMR LONGITUDINAL RELAXATION STUDY OF THE FLUIDITY AT OCTADECYLSILICA SURFACES - ACETONITRILE AS A PROBE OF SURFACE VISCOSITY

Deuterium and nitrogen-14 longitudinal (spin-lattice) relaxation measurements are used to assess the surface fluidity under conditions typical of reversed-phase high-performance liquid chromatography. A model of two phases in rapid exchange and values of the observed H-2 and N-14 spin-lattice relaxation time (T1) in suspensions of microporous silica gel (bare and C-18 modified) and acetonitrile-d3 (CD3CN) are used to assess the rotational motion of surface-associated CD3CN. With use of bulk theories of relaxation and predicted values of the longitudinal relaxation rate of surface-bound nuclei (R1,s = 1/T1,s), the reorientational correlation time, tau-c, of surface-associated CD3CN is predicted and used to determine a surface microviscosity. The predicted value for octadecylsilica (ODS) surface viscosity of 4.6 cP, 13 times the value for neat acetonitrile, is shown to be representative of intermolecular interactions occurring in the hydrocarbon fraction of the heterogeneous ODS surface. Comparisons are made between the values for ODS surface viscosity given here and those in previous investigations.