The setchenov constant of benzene in non-aqueous electrolyte solutions. Alkali-metal halides and aliphatic and aromatic salts in methanol at 298.15 K

Abstract

The salt effect of alkali-metal, aromatic and aliphatic halides on dilute benzene solutions in methanol has been determined from precise vapour-pressure measurements at 298.15 K. The salting constants obtained have been compared with the same electrolytes in water. A general levelling-off effect for all salts is observed in methanol when compared with water. In particular, benzene is salted-out by tetra-alkylammonium salts from tetramethyl-to tetraheptyl-ammonium bromide, an effect which increases slightly with size beyond tetrabutylammonium bromide. The opposite effect is observed in water. The salting-in effect observed with aromatic salts is ten times smaller than in water. These differences are interpreted in terms of structural considerations which favour the salting-in effect of benzene by hydrophobic ions in water and which are absent in methanol. The relevance of these findings to the discussion of the high partition coefficient values of aromatic molecules between cationic micelles and water is pointed out.

The McDevit–Long theory (MLT) and the scaled-particle theory (SPT) have been applied to the salting constant of benzene in methanol with different electrolytes. The SPT predicts successfully the salt effect of the alkali-metal halides, the agreement between experimental and calculated values being better than in aqueous solutions. MLT shows poor agreement with experiment for inorganic salts, as is the case in water. Both theories predict qualitatively the salting-out effect of the lower homologues of the tetra-alkylammonium salts, but fail to predict the slight increase of the salting constant with cation size. This effect is interpreted as a consequence of the increased screening of the charge on the nitrogen atom by the surrounding aliphatic molecules, which dominates over the cavity effect as the size of the cations increases.