Surface and bulk characterisation of mixtures containing alkylammonium nitrates and water or ethanol: Experimental and simulated properties at 298.15 K

Abstract This work is a contribution to the knowledge of the surface and bulk behaviour of mixtures containing an ammonium based protic ionic liquid (PIL) and a molecular solvent. To achieve our goal, we have measured the surface tension and the density of the binary mixtures {ethylammonium nitrate, propylammonium nitrate or butylammonium nitrate + water or ethanol} over the whole composition range at 298.15 K. The values obtained for both magnitudes depend on the PIL alkyl chain length, the solvent nature and the solvent concentration. The surface tension of the aqueous mixtures rich in water show a deep dependence on the concentration of PIL, whereas the values of the PIL/ethanol mixtures are not influenced by the alkyl chain length of the PIL up to equimolar composition. The results obtained aid us to describe the structure of the surface. Furthermore, we have calculated the excess molar volumes from the densities and the values obtained were negative in all cases. The sign and the sequence of the curves can be explained mainly in terms of the H-bond interactions and steric hindrance, respectively. To provide a microscopic insight into the surface and the bulk of the mixtures studied, we have performed molecular dynamic simulations observing a good relationship between the simulated results and the experimental ones. Finally, we have measured the refractive index of the mixtures in order to verify the validity of the Gladstone-Dale phenomenological model to predict refractive indexes from density.