Surface of Half-Neutralized Diamine Triflate Ionic Liquids. A Molecular Dynamics Study of Structure, Thermodynamics, and Kinetics of Water Absorption and Evaporation

Abstract

Surface properties of room temperature ionic liquids (RTILs) consisting of half neutralized diamine cations (H2N–(CH2) n –NH3 +, n = 2, 4) and triflate anions have been investigated by molecular dynamics simulations, based on an empirical atomistic force field. Planar slabs periodically repeated in 2D have been considered, and the temperature range 260 ≤ T ≤ 360 K has been covered, extending from below the melting and glass point to the equilibrium liquid range of the diamine compounds under investigation. Addition of water at 1% weight concentration allowed us to investigate the kinetics of water absorption through the RTIL surface, and to characterize the structural and dynamical properties of subsurface water. Animations of the simulation trajectory highlight the quick absorption of water molecules, progressing downhill in free energy and taking place without apparent intermediate kinetic stages. To verify and quantify these observations, a variant of the umbrella sampling algorithm has been applied to compute the variation of excess free energy upon displacing a water molecule along the normal to the surface, from the center of the slab to the vapor phase. The results provide a comprehensive picture of the thermodynamic properties underlying the kinetics of water absorption and evaporation through the surface, and they also provide the ratio of the equilibrium density of water in the vapor and liquid phase at the average concentration considered by simulations. A variety of properties such as the surface energy, the 90–10% width of the profile, the layering of different species at the interface, and the electrostatic double layer at the surface are computed and discussed, focusing on the effect of water contamination on all of them.

Noncovalent Sulfoxide–Nitrile Coupling Involving Four-Center Heteroleptic Dipole–Dipole Interactions between the Sulfinyl and Nitrile Groups

“…The association and clusterization of solvent molecules is an issue of substantial chemical importance, and over the past decade, it has been the subject of various experimental and theoretical investigations. − The thermochemical stability of the associates is particularly helpful for understanding solvent effects that are among the key factors that determine structure–reactivity relationships. Apart from the deeper insight into solvent–solute interactions and solution reactivity, , these studies contributed greatly to diverse areas of chemistry such as crystal growth, , supramolecular self-assembly, surface science, catalysis, , biochemistry, , molecular machinery, thin films construction, the fuel industry, and so on.…”

Insight into the Inflection Point of Surface Tension in Aqueous Solution of Imidazolium-Based Ionic Liquids

“…However, the decrease in density at the interface is not obvious until extremely low densities are reached. We used the method of N. C. Forero-Martinez to calculate the chemical potential of ILs μ il with the expression μ normali normall = k normalB T nobreak0em0.25em⁡ ln ( ρ ) + μ normale normalx normalc ( ρ ) …”

“…Compared to the bulk phase, research on the gas–liquid interface of ILs represents only a small fraction. However, it plays a crucial role in numerous applications, including phase catalysis, − gas sensors, gas separation, nanoparticle synthesis, and metal film preparation. Therefore, understanding the structure and properties of ILs at the gas–liquid interface is crucial.…”

Water Harvesting by Thermoresponsive Ionic Liquids: A Molecular Dynamics Study of the Water Absorption Kinetics and of the Role of Nanostructuring

“…The close relation depends on the fact that the entropy term that drives the mesoscopic or macroscopic separation of IL and water is due predominantly (but not Although unusual in a Method section, we mention here what we did not use, i.e., free energy sampling methods, 34−36 that, in principle, could be used to speed up the system equilibration and to compute free energy profiles for all species across the slab, thus providing unambiguous insight on water absorption free of simulation time limitations. 37 However, the very viscous character of [P 4444 ][DMBS] and the nanostructured state of its water solutions make this analysis very uncertain or exceedingly time-consuming.…”

“…33 Nevertheless, the burying of the water islands below the surface, taking place within ∼100 ns even at low temperature, is an important aspect of the system evolution, implying that the surface will remain unaltered during water absorption, always presenting the same hydrophobic top layer with an ionic, hydrophilic layer immediately below. It might be worth emphasizing again that the process of incorporating water subsurface is not based on single water molecules diffusing inside, as observed in other cases, 37 but is a collective effect, consisting of ions crawling on the surface of water islands and covering them. This, in turn, suggests that the subsurface incorporation is irreversible since the reverse process of ions moving away and exposing patches of water prone to evaporation appears to be highly unlikely.…”