Molecular simulation of ionic liquids and deep eutectic solvents in the bulk, near surfaces and inside nanopores

Abstract

Ionic liquids (ILs) are organic salts with a melting temperature lower than the boiling point of water, while deep eutectic solvents (DESs) are usually composed of two or more components that can associate with each other through hydrogen bonds, and the resulting melting point of the mixture is lower than each of the components. These two classes of solvents share many remarkable properties, such as low volatility, non- flammability, wide liquidous range, and tunability; moreover, DESs are more inexpensive and environmentally friendlier. In addition to the fundamental scientific interest in these types of materials, our modeled systems are relevant to practical applications such as dye- sensitized solar cells and gas separations, as well as developing nanomaterials based on ILs. We first performed molecular dynamics (MD) simulations to study the structure and dynamics of DESs and ILs confined inside slit-like graphite and titania [rutile (110)] nanopores. Our results show that hydrogen bond donor (HBD) molecules of modeled DESs can form long-lived hydrogen bonds with the oxygen atoms in the rutile walls, consequently the rutile walls are coated by HBD layers that are almost depleted of ions, which exhibiting a liquid structure that departs from that of the bulk DES and with very slow dynamics. In contrast, for DESs inside graphitic pores, all species are present in the fluid layers near the carbon walls, the local liquid structure everywhere is similar to that of a bulk DES, and the overall dynamics are faster than those observed inside rutile pores of the same pore size. When the pore size is reduced, in general the hydrogen bonds formed can exist for longer time, a larger proportion of the hydrogen bonds involve the walls (in the case of a rutile pore), and the overall dynamics of the confined DES become slower. We then performed MD simulations of confined DESs or ILs in contact with gas phases containing CO2, and later a binary mixture of CO2 and CH4. When the amount of DES in the pore is varied, in the absence of DES, CO2 adsorbs to the pore walls, but increasing amounts of DES inside the pores quickly displace carbon dioxide into the gas/liquid interfaces, and into dissolution within the confined DES. As the amount of DES inside the pores increases, the diffusivity of CO2 reaches a maximum in partially filled pores and decays to the values observed in pores filled with DES, which are similar to the diffusion coefficients of CO2 in the bulk DES. The average number densities of CO2 near the confined DES (i.e., dissolved in the DES, and adsorbed at the pore walls and at the gas/liquid interface) are significantly larger than the corresponding value observed in the bulk DES. Furthermore, the solubility selectivity, diffusion selectivity and permselectivity of CO2 over CH4 inside the pore will alter with different pore materials, the properties of confined DESs or ILs, the loadings of solvents in the nanopores. Additionally, biased MD simulations have been performed to model the heterogeneous nucleation of IL [dmim+][Cl-] near a graphitic surface from its supercooled liquid phase. As nucleation is a rare event, a combination of techniques (i.e. the string method in collective variables and Markovian milestoning with Voronoi tessellations) has been used to obtain a minimum free energy path connecting the supercooled liquid and crystal phases, and determine the free energy and the rates involved in the nucleation process. It is found that when the subcooled liquid phase is in contact with a graphitic disk, the free energy barrier becomes about 42% smaller (~49 kcal/mol), and the critical nucleus formed is about 17% smaller (~3.0 nm) than the one observed for homogeneous nucleation.