Energetics of Aqueous Solutions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethylsulfate

Abstract

The energetics of aqueous solutions of the ionic liquid (IL) 1-ethyl-3-methylimidazolium ethylsulfate in the concentration range m < 165 mol·kg(-1) was analyzed on the basis of the enthalpies of solution of the IL in water (Δ(sol)H(m)) and the enthalpies of dilution (Δ(dil)H(m)) of solutions with different IL concentrations. The data were both obtained experimentally, by calorimetry, and theoretically, by using Molecular Dynamics (MD) simulations. Particular attention was given to the low-concentration range (m < 5 mol·kg(-1)), which had not been covered in previous experimental studies of this system. The dependence of Δ(sol)H(m) from the molality of the IL observed within this m < 5 mol·kg(-1) range could be fitted to a fourth-order polynomial with an average relative deviation of ∼0.13%. This polynomial function shows a minimum of Δ(sol)H(m) at m ≈ 0.6 mol·kg(-1) (or a molar fraction x(IL) ≈ 0.01) that could be approximately captured by the MD simulations performed in this work but not through extrapolations based on previously reported experimental or simulation data. The decomposition of our MD simulation Δ(sol)H(m) results in contributions from different types of interaction (IL-IL, H(2)O-H(2)O, and IL-H(2)O), indicated that the minimum essentially results from two opposite effects: the differences between the IL-IL and H(2)O-H(2)O interactions in the solution and in the pure liquids are both positive and increase with the dilution of the IL, while the contribution of the IL-H(2)O interactions (that is only present in the solution) is negative and decreases with the IL dilution. It was also found that the observed trends in Δ(sol)H(m) are dominated by electrostatic rather than dispersion interactions.