Surface Properties of N,N-Dimethylformamide-Water Mixtures, As Seen from Computer Simulations

Molecular Dynamics Study on the Heat of Phase Transition of Chloroacetate from the Bulk to the Surface Phases

Based on the surface adsorption theory developed in our group and molecular dynamics simulations, the adsorption behaviors of methyl chloroacetate, ethyl chloroacetate, propyl chloroacetate, methyl dichloroacetate, and methyl trichloroacetate at the vapor-liquid interface have been thoroughly investigated. The surface layer thickness of these five liquids shows a significant increase as temperature increases, but it decreases significantly with the increase of intermolecular interactions of chloroacetates. It is found with our surface adsorption theory that the reversible heat of phase transition for chloroacetates from bulk phase to surface phase increases from methyl chloroacetate to propyl chloroacetate with the alkyl chain and also increases from methyl chloroacetate to methyl trichloroacetate as the number of chloro group increases. Molecular dynamics simulation is exploited to calculate the entropies of these five liquids in the surface and bulk regions at various temperatures. The variation trends of the simulated results are consistent with those of the phase transition heats determined with our surface adsorption theory.

Single Particle Dynamics at the Free Surface of Imidazolium-Based Ionic Liquids

In this work, we carry out a systematic computer simulation investigation of the single particle dynamics at the free surface of imidazolium-based room temperature ionic liquids by applying intrinsic surface analysis. Besides assessing the effect of the potential model and temperature, we focus in particular on the effect of changing the anion type, and, hence, their shape and size. Further, we also address the role of the length of the cation alkyl chains, known to protrude into the vapor phase, on the surface dynamics of the ions. We observe that the surface dynamics of ionic liquids, being dominated by strong electrostatic interactions, is about 2 orders of magnitude slower than that for common molecular liquids. Furthermore, the free energy driving force for exposing apolar chains to the vapor phase "pins" the cations at the surface layer for much longer than anions, allowing them to perform noticeable lateral diffusion at the liquid surface during their stay there. On the other hand, anions, accumulated in the second layer beneath the liquid surface, stay considerably longer here than in the surface layer. The ratio of the mean surface residence time of the cations and anions depends on the relative size of the two ions: larger size asymmetry typically corresponds to larger values of this ratio. We also find, in a clear contrast with the bulk liquid phase behavior, that anions typically diffuse faster at the liquid surface than cations. Finally, our results show that the surface dynamics of the ions is largely determined by the apolar layer of the cation alkyl chains at the liquid surface, as in the absence of such a layer, cations and anions are found to behave similarly with respect to their single particle dynamics.

Liquid-Vapor Interface of Aqueous Ethylene Glycol Solutions: A Molecular Dynamics Study

While the organic constituent in an aqueous binary solution enriches its liquid-vapor (l-v) interface, the extent of enrichment can depend nonlinearly on its mole fraction. A microscopic quantification and rationalization of this behavior are crucial to understand the dependence of properties such as surface tension and evaporation rate of the solution on its composition. Extensive all-atom molecular dynamics simulations of aqueous ethylene glycol (EG) solutions show that the composition of the solution at the l-v interface deviates the most from that in the bulk solution at an EG mole fraction of 0.3. The population of EG molecules with their central C-C dihedral in the gauche conformation was found to be higher at the l-v interface than that in the bulk solution to facilitate the orientation of its hydrophobic methyl groups toward the vapor phase. Free energy calculations reveal that in dilute EG solutions, an EG molecule is most stable at the l-v interface. The behavior of vapor pressure in aqueous EG solutions is ideal and follows Raoult's law, while in contrast, the aqueous solution of dimethyl sulfoxide does not. A rationale for the same is provided through the orientational distribution of interfacial water molecules in the respective solutions.