Thermodynamic properties (excess and partial molar volume, Gibbs free energy of activation for viscous flow, enthalpy, and entropy) of propylammonium nitrate/water mixtures

Abstract

The purpose of this study is to use thermodynamic functions to clarify the change in the solution structure and interaction of the propylammonium nitrate (PAN)/water system in response to changes in the temperature and water concentration. Specifically, the viscosity and density of the PAN/water system were measured over the entire water concentration range and over the temperature range from 278.15 to 323.15 K. The molar free Gibbs energy of activation for viscous flow (ΔGm‡,E), entropy (ΔSm‡,E), and entropy (ΔHm‡,E) and their PAN (ΔGm,PAN‡,E, TΔSm,PAN‡,E, ΔHm,PAN‡,E) and water (ΔGm,H2O‡,E, ΔHm,H2O‡,E and TΔSm,H2O‡,E) partial quantities were calculated, and we investigated the validity of the unique interpretation that water might be accommodated in the cavities of the liquid structure of PAN. In the water concentration dependence, TΔSm‡,E and ΔHm‡,E were consistently negative andTΔSm‡,E < ΔHm‡,E. Therefore, the results suggest that PAN forms a more ordered structure when mixed with water even though the attractive intermolecular interaction decreases. However, the increase of ΔHm‡,E and TΔSm‡,E in the temperature range 278–323 K was 2.35 kJ mol−1 and 1.60 kJ mol−1, respectively, indicating that ΔHm‡,E > TΔSm‡,E. That is, the attractive intermolecular interaction increases with increasing temperature, suggesting that the ordered structure changes toward disorder. In addition, at a water mole fraction of 0.1, which was the lowest concentration of water investigated in this study, the partial molar volume of water is 16% lower than that of pure water and both ΔHm,H2O‡,E and TΔSm,H2O‡,E are reduced, suggesting that the attractive interaction of water is weaker in PAN than in bulk water and that the structure of PAN is closer to that of a pure liquid in this concentration range. Water is speculated to be accommodated into the cavities of the PAN structure.