Dynamic and interfacial properties for dichloromethane absorption in ionic liquids: Experimental and molecular dynamic insights

In this study, a strategy for dichloromethane (DCM) capture using ionic liquids (ILs) was systematically investigated from the perspective of dynamic analysis. The diffusion coefficient, Henry's law constant (H), saturated absorption capacity, and overall volumetric liquid-side mass-transfer coefficient (K(L)a(o)) of four types of ILs were obtained. Among all the ILs studied, tetrabutylphosphonium caproate ([P-4444][C5COO]) demonstrated superior DCM absorption capabilities, and its corresponding H value (3.13 Pa m(3)/mol at 25 degrees C) was markedly lower than those of solvents documented in the literature. Furthermore, its K(L)a(o) value was determined to be 2.77 x 10(-3)/s at 25 degrees C, which was measured in a customized cylindrical absorption bottle with a height of 150 mm and an inner diameter of 16 mm. The absorption bottle, fitted with a gas inlet tube having an inner diameter of 5 mm and positioned 5 mm above its base, utilized a simple bubbling technique to introduce gas into the liquid phase, eliminating the need for gas-dispersion equipment. Nevertheless, ILs with high affinities showed relatively high initial mass-transfer rates for DCM. Based on molecular dynamics (MD) simulations, the vapor-liquid interface properties, including mass and number density profiles, molecular orientation, and surface tension, were investigated. This revealed that in the pure [P-4444][C5COO] system, the alkyl chain of the anion was oriented toward the gas phase, while the carboxyl group was oriented toward the bulk. The tilt angle of [C5COO](-) with respect to the interface was within the range of 37 degrees < theta ' < 70 degrees. For the [P-4444][C5COO]/DCM mixture, at the interface, the anion exhibits an enhanced ordered orientation, while the cation shows a more disordered orientation.