Ionic association of some tetraheptylammonium and tetraoctylphosphonium salts in ethanol studied by vapour pressure osmometry

Abstract

The extent of ionic association of (hept)₄NCl, (hept)₄NBr, (hept)₄NI, (hept)₄NNO₃ and (oct)₄PNO₃ in ethanol at 40°C has been measured by vapour pressure osmometry with matched thermistors. The osmotic coefficients, activity coefficients and association constants have been calculated. Ionic association increases with the increasing size of the anion, contrary to the prediction of electrostatic theory of ion pair formation, but in qualitative agreement with the effect of ionic polarizability based on an approximate hard sphere “dielectric bubble” model. The association constants are considerably higher than those predicted by the Denison-Ramsay-Fuoss model of contact ion pairs.