Conductivity and electrophoretic mobility of dilute ionic solutions

Two complementary continuum theories of electrokinetic transport are examined with particular emphasis on the equivalent conductance of binary electrolytes. The small ion model [R.M. Fuoss, L Onsager, J. Phys. Chem. 61 (1957) 668] and large ion model [R.W. O'Brien, L.R. White, J. Chem. Soc. Faraday Trans. 2 (74) (1978) 1607] are both discussed and the large ion model is generalized to include an ion exclusion distance and to account in a simple but approximate way for the Brownian motion of all ions present. In addition, the large ion model is modified to treat slip hydrodynamic boundary conditions in addition to the standard stick boundary condition. Both models are applied to the equivalent conductance of dilute KCl, MgC2, and LaCl3 solutions and both are able to reproduce experimental conductances to within an accuracy of several tenths of a percent. Despite fundamental differences in the small ion and large ion theories, they both work equally well in this application. In addition, both stick-large ion and slip-large ion models are equally capable of accounting for the equivalent conductances of the three electrolyte solutions. (C) 2010 Published by Elsevier Inc.