Anomalous Osmotic Flow and the Frictional Model of an Ionic Membrane

Abstract

SynopsisIn the system Zeo-Karb 315 and SrBr:, a reversal of solvent flow has been found with increasing external concentration. This is not easily predicted either from Schlogl's theory or from thermodynamics. It is desired to discover what behavior of the ionic and molecular friction coefficients can cause a reversal of flow under these conditions. Dorst, Staverman, and Caramazza's treatment has been extended to permit coefficients to be calculated from a set of available transport data, which include the osmotic flows. These coefficients may be calculated from the ratio of osmotic solvent and salt fluxes, the coion tracer-diffusion coefficient, the electrical conductance, the electroosmotic permeability, and the sorption of salt by the membrane. By assuming that the friction coefficient of coion and solvent in the membrane has the same value as in free solution a set of coefficients r i k has been obtained that are consistent with the known physical requirements of the system. They show that the friction of the solvent with the matrix and with the counterions both increase with increasing external salt concentration; whereas the friction of the counterion with the matrix decreases. The obtained rik values are also consistent with the change in counterion tracer-diffusion coefficient with concentration. These results may be interpreted to mean that the coions are transported in solution-filled pore-like interstices, whereas the counterions move through a denser, gel-like medium.