ELECTROLYTE-SOLVENT
1181
INTERACTIONS
Electrolyte-Solvent Interaction.
XV.
The Isodielectric
Sy sterns Methand-Ace tonitrile-Ni trome thane
by Michael A. Coplan2and Raymond M. Fuoss Contribution N o . 1751 from the Sterling Chemistry Laboratorg, Yule Unbersity, New ffacen, Connecticut (Received December 14,1963)
Conductances of solutions of triisoamyl-n-butylammonium (TAB) iodide, picrate, and tetraphenylboride have been measured in (nearly) isodielectric mixtures of the solvents methanol, acetonitrile, and nitromethane. The anionic Walden products (calculated on the assumption that t)he conductances of the two ions of TAB tetraphenylboride are the same in all solvents) are not constant. Selective solvation by monomeric methanol or by acetonitrile in preference to nitromethane is proposed to describe the observed changes in Walden product (and Stokes radius).
To first approximation, the limiting conductance of an ion varies inversely as the viscosity of the solvent, but as is well-known, the Walden product is not constant. One source of its variation is a relaxation effect produced by the motion of ions in a polarizable medium,3~4as a consequence of which the reciprocal Walden product depends linearly on the reciprocal dielectric constant. This is a volume effect, in that it can be calculated5J by integrating over the entire medium surrounding the reference ion. If, however, specific interaction due to short-range interactions between ions and solvent dipoles occurs, then one might expect variation in the Walden product for a given ion in a series of solvents which had the same dielectric constant, but which varied in details of dipole structure. The purpose of this paper is to present examples of this situation. llixtures of acetonitrille (D = 36.0), nitromethane ( D = 36.0), and methanol (D = 82.6) are practically isodielectric. Still, the Walden products for the iodide, picrate, and tetraphenylboride ions in these mixtures varied considerably. Selective solvation by monomeric methanol or by acetonitrile is suggested as the explanation of the observed results.
Experimental Preparation and purification of the salts are described in the preceding paper,' which also gives details of the purification of the methanol. Cells, electrical
equipment, and technique have all been described. Acetonitrile was refluxed over Drierite under nitrogen and then fractionated through an 80-cm. column which was packed with glass beads (conductance, 3-4 X Sitromethane (Matheson P 1240) was treated with Analabs
