Volume Properties of Imidazolium Proline Ionic Liquids Aqueous Solution

Densities for three binary mixtures of 1-butyl-3-methylimidazolium prolineC₄mim L-Pro、1-hexyl-3-methylimidazolium prolineC₆mim L-Pro and 1-octyl-3-methylimidazolium prolineC₈mim L-Pro with water were measured at the temperature ranging from 298.15 K to 323.15 K over the entire compositions. The influence of temperature and composition on densities was studied. The excess molar volumes for mixtures and partial molar volumes at infinite dilution for ionic liquids and water were calculated and correlated using empirical equations. The results showed that the densities linearly decreased with temperature, and the relationship between densities and temperature can be correlated precisely using equation ρ=α+β(T-273.15). The densities for mixtures decreased with alkyl length of the cation at the same temperature and composition. The excess molar volumes were exclusively negative for these three mixtures because of strong hydrogen bonding and bulk stacking effect between amino acid based ionic liquids and water molecules. The relationship between excess molar volumes and composition can be correlated well using a Redlich-Kister equation. The partial molar volumes at infinite dilution for ionic liquids and water were all lower than molar volumes for pure ionic liquids and water at the same temperature. This is also due to the strong hydrogen bonding between ionic liquids and water. As the alkyl length of the cation increases, the degree of decline decreased for partial molar volumes at infinite dilution compared with molar volumes for ionic liquids and water. The interaction energy between ionic liquids and water were calculated and the results showed that the interaction energy between ionic liquids and water decreased gradually with alkyl length of the cation.