Enthalpy coefficients of pairwise interactions between polar molecules and long-chain aliphatic ions in water at 298.15 K

Abstract

The enthalpy of dissolution of trimethyldecylammonium bromide has been determined in the water-rich region of binary mixtures with dimethylsulphoxide, N-methylpyrrolidone, NN-dimethylformamide, propan-l-ol, urea and acetonitrile. Some additional measurements are also presented for tetrabutylammonium bromide in some of these binary solvent mixtures. From these data the enthalpy coefficient of pairwise interaction h_NE has been calculated. Using previously published data and available literature values for the tetra-alkylammonium bromide salts, a discussion is presented on the enthalpy of pairwise interaction of aliphatic ions with 12 polar molecules in water.

Using the scaled particle theory, the cavity contribution to the h_NE values is calculated and shown to be large in comparison with the interaction contribution in all cases except alcohols and acetonitrile. From a comparison of h_NE values for the long-chain surfactant and for the bulky tetrabutylammonium ion, two different classes of organic molecules emerge: (1) acetonitrile, acetone, 2-methylpropan-2-ol and propan-1-ol and (2) the other polar molecules studied, which include sulpholane, propylene carbonate, dioxane and ethylene glycol. Class (1) are known to decrease the critical micelle concentration of surfactants in water through partitioning between water and the pseudo-micellar phase; class (2) increase the critical micelle concentration and do not interact with the micelles. Thus the behaviour of molecules at post-micellar concentrations may be deduced from information on the thermodynamics of pre-micellar solutions.