On the nonlinear variation of dc conductivity with dielectric relaxation time.

The long-known observations that dc conductivity sigma(dc) of an ultraviscous liquid varies nonlinearly with the dielectric relaxation time tau, and the slope of the log sigma(dc) against log tau plot deviates from -1 are currently seen as two of the violations of the Debye-Stokes-Einstein equation. Here we provide a formalism using a zeroth order Bjerrum description for ion association to show that in addition to its variation with temperature T and pressure P, impurity ion population varies with a liquid's equilibrium dielectric permittivity. Inclusion of this electrostatic effect modifies the Debye-Stokes-Einstein equation to log(sigma(dc)tau)=constant+log alpha, where alpha is the T and P-dependent degree of ionic dissociation of an electrolytic impurity. Variation of a liquid's shear modulus with T and P would add to the nonlinearity of sigma(dc)-tau relation, as would a nonequivalence of the shear and dielectric relaxation times, proton transfer along the hydrogen bonds, or occurrence of another chemical process. This is illustrated by using the data for ultraviscous acetaminophen-aspirin liquid.