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 constantK _A =K _R(l+K _s) is the product of two factors:K _R, which describes the first (diffusion controlled) equilibrium andK _s=exp(−E _s/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 quantityE _s is the difference in free energy between the states defined byr=R andr=a. For the Me₂SO−Me₂CO systems,E _s 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 waterE _s is also positive. On addition of Me₂SO or Me₂CO,E _s 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 inE _s appears when the small water molecules surrounding solvent-separated pairs are replaced by organic molecules which have greater volumes than water.