Ion pairing of quaternary salts in solvent mixtures

Abstract

Conductances at 25.00°C are reported for the following systems: tetrabutylammonium bromide in dimethyl sulfoxide-acetone mixtures (Bu4NBr in Me2SO−Me2CO); tetraphenylphosphonium bromide (Ph4PBr) in water Me2SO, Me2CO, and in the mixtures H2O−Me2SO, Me2SO−Me2CO and Me2CO−H2O; Ph4PCl in Me2SO, Me2CO, H2O−Me2SO, and Me2SO−Me2CO; and tetrapropylammonium bromide (Pr4NBr) in Me2SO and Me2CO. The data were analyzed using the Fuoss 1978 equation which is based on the coupled equilibria: (unpaired ions)⇌(solvent-separated pairs)⇌(contact pairs). The conductimetric pairing constantKA=KR(l+Ks) is the product of two factors:KR, which describes the first (diffusion controlled) equilibrium andKs=exp(−Es/kT), which describes the second (system-specific) equilibrium. Ions with overlapping cospheres (of diameterR) are defined as paired: their center-to-center distancer lies in the rangea≤r≤R; contact pairs (r=a) are ions which have one ion of opposite charge as a nearest neighbor, all other nearest and next nearest neighbors being solvent molecules. The quantityEs is the difference in free energy between the states defined byr=R andr=a. For the Me2SO−Me2CO systems,Es is positive for solutions in Me2SO and decreases through zero to negative values as the fraction of the less polarizable acetone increases. For solutions in waterEs is also positive. On addition of Me2SO or Me2CO,Es initially increases, goes through a maximum, and then decreases to negative values as the fraction of the less polarizable component increases. The decrease is an electrostatic effect, common to all the systems. The initial increase inEs appears when the small water molecules surrounding solvent-separated pairs are replaced by organic molecules which have greater volumes than water.