Solute dynamics at aqueous interfaces

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.

Active hole generation in a liquid droplet dissolving into a binary solvent

In liquid-liquid dissolution, the critical point of phase separation is determined by the temperature. When the solvent consists of multi-components, in contrast, the mole fractions in the solvent also take on the role of control parameter. In this study an ionic liquid dissolves into a binary solvent composed of ethanol and water. It is found in this system that, near the critical point, a hole is spontaneously created in the droplet of the ionic liquid. The creation of the hole is initiated by a mutual interaction between the concentrations of the ionic liquid and the binary solvent via their affinity. A spatial inhomogeneity of the interfacial tension is induced through an amplification of fluctuation in the concentration due to an instability mechanism, and causes the Marangoni effect to create the hole. The hole moves inside the droplet and consequently leads to the motion of the droplet. The present system provides not only a new type of dissolution process but also a peculiar example of active matter realized in a liquid droplet.

Reaction dynamics at liquid interfaces

The liquid interface is a narrow, highly anisotropic region, characterized by rapidly varying density, polarity, and molecular structure. I review several aspects of interfacial solvation and show how these affect reactivity at liquid/liquid interfaces. I specifically consider ion transfer, electron transfer, and SN2 reactions, showing that solvent effects on these reactions can be understood by examining the unique structure and dynamics of the liquid interface region.

Transfer of the K+ cation across a water/dichloromethane interface: a steered molecular dynamics study with implications in cation extraction

In this paper we report the characterization of the dichloromethane (DCM)/water interface in terms of density profile, width, and surface structure. The use of steered molecular dynamics (SMD) to study the transfer of the K(+) cation from the organic layer to the water layer is also described. The corresponding free energy is in semiquantitative agreement with published experimental and theoretical results. The transference of the K(+) cation from the water layer toward the DCM layer occurs with concomitant water transport as a water microdroplet that detaches itself from the water layer after ca. 16 Å of penetration into the organic layer by breaking the thin water thread that unites both. Complexation of the water microdroplet by a polyethylene-glycol type podand induces the loss of water molecules from the water microdroplet to bulk DCM and, eventually, to the water layer.

Simple Ion Transfer at Liquid|Liquid Interfaces

The main aspects related to the charge transfer reactions occurring at the interface between two immiscible electrolyte solutions (ITIES) are described. The particular topics to be discussed involve simple ion transfer. Focus is given on theoretical approaches, numerical simulations, and experimental methodologies. Concerning the theoretical procedures, different computational simulations related to simple ion transfer are reviewed. The main conclusions drawn from the most accepted models are described and analyzed in regard to their relevance for explaining different aspects of ion transfer. We describe numerical simulations implementing different approaches for solving the differential equations associated with the mass transport and charge transfer. These numerical simulations are correlated with selected experimental results; their usefulness in designing new experiments is summarized. Finally, many practical applications can be envisaged regarding the determination of physicochemical properties, electroanalysis, drug lipophilicity, and phase-transfer catalysis.