Structural and Dynamical Properties of a Deep Eutectic Solvent Confined Inside a Slit Pore

We performed molecular dynamics (MD) simulations to study the structure and dynamics of the deep eutectic solvent (DES) choline iodide-glycerol (CI.G) at a molar ratio of 1:3, inside slitlike titania [rutile (110)] and graphitic nanopores of width H = 5.2 and 2.5 nm, and at a temperature T = 333 K. DESs share many of the remarkable properties of ionic liquids (ILs) while being more inexpensive; furthermore, and in addition to their fundamental scientific interest, the systems modeled here are relevant to dye-sensitized solar cells and gas separations. Our results show that glycerol can form stable hydrogen bonds with the oxygen atoms in the ruffle walls, which account for similar to 86% of the total number of hydrogen bonds involving DES species (choline,iodide, and glycerol) in the first layers (near the ruffle walls), and for similar to 24% of the total hydrogen bonds observed for the DES inside a ruffle pore of width H = 5.2 nm. As a result, in these systems, the ruffle walls are coated by glycerol layers that are almost depleted of ions, have a liquid structure that departs from that of the bulk DES, and have very slow dynamics. In contrast, for DES inside graphitic pores, all species are present in the first 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; however, the DES species in the center of both ruffle and graphitic pores have comparable mobilities. When the pore size is reduced, a larger proportion of the hydrogen bonds involve the walls (in the case of a ruffle pore), the overall dynamics of the confined DES become slower, and in general the hydrogen bonds formed are present during longer times in the simulation trajectories. These observations are in general similar to the results obtained by us for the IL [EMIM+][TFMSI-] inside the same ruffle and graphitic pores; however, both of the IL ions are present in the layers near the walls, and the ions in the center of a ruffle pore have dynamics that were 2-4 times slower than those observed for the same ions in the center of a graphitic pore.