Theoretical and Experimental Study on Ion-Pair Formation and Partitioning of Organic Salts in Octanol/Water and Dichloromethane/Water Systems

Abstract

A good linear correlation (r = 0.945) of the octanol/water and dichloromethane/water log P‘ values has been found for quaternary amines with organic counterions. Monte-Carlo simulation based potential of mean force curves (PMF) for medium and large quaternary amine cations, and small and medium size anions show partially different characters in dichloromethane (DCM) solvent. The PMFs show, with one exception, both locally stable contact and solvent-separated ion-pair arrangements. The solvent-separated form is at least as stable as the contact ion-pair. Consideration of two water molecules at the interaction sites of the ionic heads remarkably lowers the activation free energy toward the formation of the contact ion arrangement. The quaternary substitution of the nitrogen prevents hydrogen bonding to the cation at its most positive site. Secondary acceptor sites can still be important, but at larger separations the water molecules always formed strong hydrogen bonds only at the −COO- and −SO3- anionic sites. Stability of the contact ion-pair is enhanced by a water-bridged hydrogen-bond network possible only at small ion separations in DCM. According to a suggested putative model for ion-pair partitioning between water and DCM, the anion enters the organic phase and drags some water molecules. The cation separated by about 8−10 Å follows the hydrated anion in the organic phase. A mixture of the contact and solvent separated ion-pairs in DCM is preferably formed if the free energy difference does not exceed 2 kcal/mol, and the activation free energy of the contact ion-pair formation is no more than about 3 kcal/mol, as calculated for some ion-pairs here.