Using Thermodynamics to Assess the Molecular Interactions of Tetrabutylphosphonium Carboxylate-Water Mixtures

Densities, ρ, viscosities, η, and enthalpies of mixing, CH18481-IE1.gif, of binary [P_{4 4 4 4}][C_nCOO]-water mixtures (with n = 1, 2 or 7) were determined at atmospheric pressure as a function of temperature. The excess, CH18481-IE2.gif, apparent, CH18481-IE3.gif, and partial, CH18481-IE4.gif, molar volumes were deduced from experimental data, as well as fragilities, m∗, and excess Gibbs free energies of activation of viscous flow, CH18481-IE5.gif. CH18481-IE6.gif exhibited predominantly negative deviation from ideality, with a minimum at approximately CH18481-IE7.gif ∼0.8 for all three systems, indicating strong hydrogen-bonding interactions. All three binary systems were found to be fragile, with [P_{4 4 4 4}][C₇COO] showing the smallest deviations in fragility with the addition of water. CH18481-IE8.gif values of the systems were exothermic over the entire composition range, having the following trend: [P_{4 4 4 4}][C₂COO] > [P_{4 4 4 4}][C₇COO] > [P_{4 4 4 4}][C₁COO].