Volumetric studies of ion solvation in propylene carbonate + N,N-dimethylformamide electrolyte solutions

Apparent molar volumes [V.sub.2,[phi]] and standard partial molar volumes [V.sup.[omicron].sub.2,[phi]] for tetraethylammonium bromide ([Et.sub.4]NBr), tetrapropylammonium bromide ([Pr.sub.4]NBr), tetrabutylammonium bromide ([Bu.sub.4]NBr), and tetrahexylammonium bromide ([Hex.sub.4]NBr) have been determined at 298.15 K from precise density measurements in solvent mixtures of propylene carbonate (PC) with N,N-dimethylformamide (DMF). Combined with our previous data for LiCl[O.sub.4] and LiBr in the same solvents, ionic molar volumes of [Li.sup.+], [Et.sub.4][N.sup.+], [Pr.sub.4][N.sup.+], [Bu.sub.4][N.sup.+], [Hex.sub.4][N.sup.+], and related anions have been deduced from the extrapolation method suggested by Conway and co-workers. It is shown that the molar volumes of these cations are quite independent of the nature of the solvent and the composition of the solvent mixtures, in contrast to those of CI[O.sub.4.sup.-] and [Br.sup.-] anions. This suggests that the Lewis-base-type solvents with similar molecular volumes have similar interactions with [Li.sup.+]. The constancy in partial molar volume for tetraalkylammonium ions provides helpful evidence for the lack of solvation of large tetraalkylammonium cations in organic solvents. These findings have been interpreted using scaled-particle theory. The results are discussed in terms of ion solvation, packing effects of solvent molecules in the solvation shell, and the electrostriction of solvents. Key words: ionic volumes, propylene carbonate, N,N-dimethylformamide, solvent mixtures, solvation, lithium batteries.