Abstract

A central difficulty in simulating BmimBF4/water mixtures is that force fields validated separately for the pure ionic liquid and pure water do not necessarily remain thermodynamically consistent when combined. The predicted miscibility may depend strongly on the water model, ionic-liquid parameters, cross-interactions, and charge treatment. For this reason, the free energy of mixing is a more stringent validation criterion than density or transport properties alone, because it directly probes whether the model predicts a thermodynamic tendency toward mixing or demixing.1

In this work, the compatibility of the SPC/E2 water model with the Canongia Lopes3 force field for BmimBF4 was assessed from the composition dependence of the excess enthalpy and excess free energy of mixing. The latter was obtained using an alchemical thermodynamic-integration approach,4 in which intermolecular interactions were progressively removed along a λ-path: electrostatic interactions were first switched off, followed by Lennard-Jones interactions using a soft-core potential. The resulting free energies of the mixtures were then compared with the mole-fraction-weighted values of the pure components to obtain the excess free energy as a function of BmimBF4 content.

The calculated excess free energy displays a strongly non-ideal and non-monotonic composition dependence. It is slightly negative on the water-rich side, indicating favorable mixing at low BmimBF4 content, but becomes positive at intermediate ionic-liquid mole fractions, with a pronounced maximum around χBmimBF₄ ≈ 0.6-0.7. This positive region suggests that, for these compositions, the force-field combination predicts an unfavorable thermodynamic contribution to mixing, consistent with a tendency toward microscopic segregation or reduced miscibility. The return to lower or negative values at higher BmimBF4 content indicates that the balance of ion–ion, water–water, and ion–water interactions changes markedly across composition.

References

(1) Idrissi, A.; Jedlovszky, P. Thermodynamics of Mixing Primary Alkanolamines with Water. J. Phys. Chem. B 2018, 122 (23), 6251–6259. https://doi.org/10.1021/acs.jpcb.8b01052.

(2) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91 (24), 6269–6271. https://doi.org/10.1021/j100308a038.

(3) Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A. H. Modeling Ionic Liquids Using a Systematic All-Atom Force Field. J. Phys. Chem. B 2004, 108 (6), 2038–2047. https://doi.org/10.1021/jp0362133.

(4) Klimovich, P. V.; Shirts, M. R.; Mobley, D. L. Guidelines for the Analysis of Free Energy Calculations. J Comput Aided Mol Des 2015, 29 (5), 397–411. https://doi.org/10.1007/s10822-015-9840-9.