Model Calculations for Setchenow Coefficients

The calculations of Setehenow coefficients reported earlier [H. L. Friedman, C. V. Krishnan, and C. Jolicoeur, Ann. N.Y. Acad. Sci. 204, 79 ( 1973)] have been extended to aqueous solutions of various nonelectrolytes mixed with alkali or alkylammonium halides. A few data for Setchenow coefficients in methanol have also been treated. The Gurney A(ij) parameters for nonelectrolyte-ion interactions in models which fit the data are mostly negative, and more so the larger the solute molecules. A value of A(ij) near - 100 cal-mole(-1) seems to characterize hydrophobic bonding. In several systems there is evidence that some nonsolvation contribution to the u(ij) pair potential which is not explicitly accounted for in the models is important in the real systems. Quite possibly this contribution is due to dispersion forces or to the charge-polarizability interaction. On the whole, the A(ij) parameters do not seem to depend upon the charges on solute particles i and j; this is evidence that the model is fairly realistic.