DYNAMICS IN REGULAR SOLUTIONS - CONCENTRATION AND VISCOSITY DEPENDENCE OF ORIENTATIONAL CORRELATION TIME OF A BENZENE MOLECULE

The conformal solution theory by Longuet-Higgins was extended to the dynamic problem through the first-order perturbation theory for the interpretation of the concentration dependence of dynamic solution properties; molecular similarity and dissimilarity are reflected by the linear ideal terms and the quadratic regular terms, respectively. To test the theory, the orientational correlation times-tau-2R were determined for benzene-d6 in the three regular solutions of benzene (B) with toluene (T), carbon tetrachloride (C), and cyclohexane (H) as a function of the concentration (mole fraction x, 0-less-than-or-equal-to x less-than-or-equal-to 1) at 25-degrees-C by measuring the spin-lattice relaxation time of the quadrupole nucleus H-2. The temperature effect was also studied in the BC solution. The tau-2R(-x) curve is linear in BT and BC whose excess enthalpies H(E) are small, but it is quadratic in BH with a larger H(E). The quadratic concentration dependence indicates the important effect of liquid structure on the orientation dynamics as well as the viscosity. The validity of the Stokes-Einstein-Debye (SED) law was tested with respect to the linear relation between tau-2R and eta/T (T the temperature, eta the solution viscosity). The SED law holds when eta is varied only by T or x under some limited conditions. The conditions for this simple relation have been clarified using the extended conformal solution theory.