PRESSURE DEPENDENCIES OF ROTATIONAL, TRANSLATIONAL, AND VISCOUS FRICTION COEFFICIENTS IN WATER-D(2), ACETONITRILE-D(3), ACETONITRILE, CHLOROFORM, AND BENZENE

Molecular rotational friction coefficients (zeta) were determined for neat water-d(2), neat acetonitrile-d(3), neat acetonitrile, a 15% solution of chloroform-d(1) in chloroform, and a 3% solution of benzene-d(6) in benzene by measuring H-2 and N-14 nuclear magnetic resonance spin-lattice relaxation times as a function of pressure (0.1-300 MPa). The pressure dependencies of the rotational zeta values were obtained from the single-body rotational correlation times for deuterated molecules in each liquid. The pressure dependencies were compared with those of the translational and viscous zeta values derived, respectively, from the known self-diffusion coefficients and viscosities. In such simple molecular liquids as chloroform and benzene, the translational and viscous zeta values had almost the same pressure coefficient or activation volume, whereas the rotational zeta values had considerably smaller pressure coefficients. The fractional viscosity (eta) exponent alpha in the phenomenological linear relation between zeta and eta(alpha) was 0.9 for the translational zeta in acetonitrile and 0.4-0.6 for the rotational zeta in acetonitrile (tumbling motion), chloroform, and benzene. Water was found to be exceptional because the pressure dependence of zeta depended more strongly on the modes of molecular motions. The deviation of the viscosity exponent from unity clearly indicates a breakdown of the Stokes-Einstein-Debye law with respect to pressure variations. The viscosity exponent is not universal, but specific to intermolecular interactions and therefore dependent on the liquid structure.