Computation of gas solubilities in choline chloride urea and choline chloride ethylene glycol deep eutectic solvents using Monte Carlo simulations

Deep eutectic solvents (DESs) are considered as green alternatives to room temperature ionic liquids (RTILs), due to their lower-cost synthesis and more environmentally friendly nature. In this work, Monte Carlo (MC) simulations have been used to compute the solubilities of CO₂, H₂S, CH₄, CO, H₂, and N₂ in choline chloride urea (ChClU) and choline chloride ethylene glycol (ChClEg) DESs. Due to the strong intermolecular interactions of DESs, leading to high viscosities, MC simulations present significant challenges with respect to system equilibration and solute molecule insertions. The Continuous Fractional Component Monte Carlo (CFCMC) method has been used with our open-source code, Brick-CFCMC, to improve molecule insertions and equilibration of the system, and directly compute the excess chemical potential and solubility (in terms of the Henry constant) of the gas molecules in the DESs. Pure DES properties, such as density and radial distribution functions (RDFs), were well reproduced by MC simulations. The solubilities of gases were, however, underestimated by the CFCMC simulations compared to available experimental data from literature. The order of solubilities of the different gases in ChClU at 328 K was obtained as H₂S > CO₂ > CH₄ > H₂ > CO > N₂, which reasonably agrees with experimental data from literature. The OPLS force field resulted in larger average Henry constants in ChClEg, compared to the GAFF force field, implying the better suitability of the GAFF force field for the calculations. Smaller ionic charge scaling factors were shown to increase the solubilities of the gases in the DESs, but result in lower densities. The differences between the computed Henry constants from MC simulations and experimental data from literature may be caused by the unsuitability of the used force field parameters of the DESs in combination with those of the solute gases. Nonetheless, experimental data from literature is scarce (except for CO₂) and in some cases contradictory, which makes the comparison with the computational results difficult.