The effect of alkyl-chain branching on the hydration and ion association of tetrabutylammonium cations

Association constants and single-ion conductivities of the bromide salts of five isomeric tetrabutylammonium ions were determined with dilute-solution conductivity measurements for aqueous solutions at 298.15 K. In addition, for two of the isomers dielectric relaxation experiments were performed. It was found that, despite similar size of all cations, the hydrodynamic radii of the branched isomers were significantly smaller than that of the un-branched Bu4N + species. It appears that the more compact branched cations exert only a minor perturbation on the H-bond network of the surrounding solvent and easily strip off their hydration shell. This effect, and the -on average- closer approach between anions and cations in ion pairs of Bu2isoBu2NBr and isoBu4NBr probably explain the rather large association constants of the latter salts.