MD simulations of UCST liquid-liquid phase separations of [Hbet][Tf2N]/Water mixtures

We investigate computationally the upper critical solution temperature (UCST) phase behavior of protonated trimethylglycine (betaine) bis(trifluoromethylsulfonyl)imide, [Hbet][Tf 2 N], ionic liquid (IL) mixed with water. Literature reports on charge-fixed force-field models of ILs in the neat phase indicate that in order to reproduce experimental data, down-scaling of the ionic charges is necessary, in many cases, by a factor of 0.7–0.9. This reduction in the unit charges of the ions reflects a transfer of charge from anion to cation in the condensed phase of the IL, a phenomenon not accounted for by classical mechanics. In this paper, we find that in order to reproduce the experimentally observed transformation of a liquid-liquid phase-separated (biphasic) state to a homogeneous state by a temperature increase, the charges of the cation and anion of [Hbet][Tf 2 N] ought to be scaled-down by a factor of 0.95. This scaling factor is closer to 1, compared to typical values found in neat ILs, likely because in water mixtures the anion-to-cation charge transfer is limited. The resulting phase diagram, more specifically, the chemical compositions of the IL-rich and water-rich phases as a function of temperature, display excellent agreement with experimental data. Exact determination of the simulation’s critical temperature is not decisive, and a plausible temperature range extends from the experimentally determined up to 10 K higher. Furthermore, the viscosities of the system at very low water contents were higher by a factor of 2–6, relative to values reported in experiments. Analyses of the interactions between and within the ions indicate they are governed by electrostatic forces, except of those within the anions, in which case, hydrophobic interactions between the trifluoromethyl groups seem to dominate. In contrast to ion-ion interactions, the effective strength of the ion-water attraction increases with temperature. Given that the UCST behavior of [Hbet][Tf 2 N]/water mixture has already been exploited to desalinate seawater by forward-osmosis, the results presented here can further assist in designing and fine-tuning more efficient thermoresponsive draw solutes. • [Hbet][Tf2N]/water mixtures are modeled by molecular dynamics at different temperatures. • Reproduction of the phase diagram requires down-scaling of the ionic charges by a factor of 0.95. • Excellent agreement with experiment is obtained for the compositions of the biphasic phase. • Viscosities of modeled mixtures with low water contents are larger than experimental values.