Evidence for water ridges at oil-water interfaces: implications for ion transport

Reparameterization of Polarizable Force Fields for Studying Ion Transfer across Liquid-Liquid Interfaces

We have developed a general scheme for refining classical polarizable molecular dynamics (MD) force fields that can accurately describe the molecular interactions in systems with liquid-liquid interfaces. While ab initio MD (AIMD) simulations can naturally describe molecular interactions, they are often so computationally expensive that simulating large system sizes and/or long time scales is usually infeasible. To resolve this, we parameterized efficient and accurate classical polarizable force fields that use AIMD reference data by minimizing both the relative entropy and the root mean squared deviation in atomic forces. We utilized our new multiscale models to study chloride ion transfer across the water-dichloromethane (DCM) interface with and without the tetraethylammonium cation as the phase-transfer catalyst. Our calculated free-energy barrier for the water-DCM interface is consistent with the other reported simulation results. We further analyzed the ion-transfer process by studying the hydration shell structures around the chloride ion and the ion-pair formation to better understand the mechanism. We observed that electronic polarizability is an important factor for the studied phase-transfer catalyst to lower the free-energy barrier of the ion transfer. Using the water-benzene interface system as an additional example, we show that our parameterization scheme provides a general route for modeling different liquid-liquid interface systems even when the experimental data or force field parameters are not readily available.

Classification of pores, water diffusivity and penetration characteristics of waste materials, and role of water as electron carrier in landfills: A review

Coupling of biogeochemical processes occurs between different waste components and waste layers during decomposition of wastes materials deposited in landfills by mechanisms similar to those occurring in marine sediments (i.e., sediment batteries). In landfills, moisture serves as a medium for transfer of electrons and protons under anaerobic conditions for decomposition reactions to proceed spontaneously, although some reactions occur very slowly. However, the role of moisture in landfills in view of pore sizes and pore size distributions, time dependent changes in pore volumes, heterogeneity of waste layers, and associated impacts on moisture retention and transport characteristics in landfills are not well understood. The moisture transport models developed for granular materials (e.g., soils) are not appropriate to describe the conditions at landfills due compressible and dynamic conditions in landfills. During waste decomposition processes, absorbed water and water of hydration can be transformed to free water and/or become mobilized as liquid or vapor, creating a medium for transfer of electrons and protons between waste components and waste layers. The characteristics of different municipal waste components were compiled and analyzed for pore size, surface energy, and moisture retention and penetration for electron-proton transfer for continuance of decomposition reactions in landfills over time. Categorization of pore sizes appropriate for waste components and a representative water retention curve for conditions in landfills were developed to clarify the terminology and highlight the differences between the landfill conditions and granular materials (e.g., soils) for use of appropriate terminologies. Water saturation profile and water mobility were analyzed by considering water as a transfer medium for carrying electrons and protons for sustaining long-term decomposition reactions.

Interfacial solute flux promotes emulsification at the water|oil interface

Emulsions are critical across a broad spectrum of industries. Unfortunately, emulsification requires a significant driving force for droplet dispersion. Here, we demonstrate a mechanism of spontaneous droplet formation (emulsification), where the interfacial solute flux promotes droplet formation at the liquid-liquid interface when a phase transfer agent is present. We have termed this phenomenon fluxification. For example, when HAuCl₄ is dissolved in an aqueous phase and [NBu₄][ClO₄] is dissolved in an oil phase, emulsion droplets (both water-in-oil and oil-in-water) can be observed at the interface for various oil phases (1,2-dichloroethane, dichloromethane, chloroform, and nitrobenzene). Emulsification occurs when AuCl₄^- interacts with NBu₄⁺, a well-known phase-transfer agent, and transfers into the oil phase while ClO₄^- transfers into the aqueous phase to maintain electroneutrality. The phase transfer of SCN^- and Fe(CN)₆^3- also produce droplets. We propose a microscopic mechanism of droplet formation and discuss design principles by tuning experimental parameters.

Spontaneous and Ion-Specific Formation of Inverted Bilayers at Air/Aqueous Interface

Developing better separation technologies for rare earth metals, an important aspect of a sustainable materials economy, is challenging due to their chemical similarities. Identifying molecular-scale interactions that amplify the subtle differences between the rare earths can be useful in developing new separation technologies. Here, we describe the ion-dependent monolayer to inverted bilayer transformation of extractant molecules at the air/aqueous interface. The inverted bilayers form with Lu³⁺ ions but not with Nd³⁺. By introducing Lu³⁺ ions to preformed monolayers, we extract kinetic parameters corresponding to the monolayer to inverted bilayer conversion. Temperature-dependent studies show Arrhenius behavior with an energy barrier of 40 kcal/mol. The kinetics of monolayer to inverted bilayer conversion is also affected by the character of the background anion, although anions are expected to be repelled from the interface. Our results show the outsized importance of ion-specific effects on interfacial structure and kinetics, pointing to their role in chemical separation methods.

A hydrogen bond-modulated soft nanoscale water channel for ion transport through liquid-liquid interfaces

Ion transport through interfaces is of ubiquitous importance in many fields such as electrochemistry, emulsion stabilization, phase transfer catalysis, liquid-liquid extraction and enhanced oil recovery. However, the knowledge of interfacial structures that significantly affect ion transport through liquid-liquid interfaces is still lacking due to the difficulty of observing nanoscale interfaces. We studied here the evolution of interfacial structures during ion transport through the decane-water interface under different ionic concentrations and external forces using molecular dynamics simulations. The roles of hydrogen bonds in ion transport through interfaces are revealed. We identified a soft nanoscale channel during ion transport through liquid-liquid interfaces and the decane phase under specific external force. The stability of the water channel and the ion transport velocity both increase with ionic concentration due to the layered ordering structures of the water near the channel surface. We observed that the stability and connectivity of the water channel in the decane phase are remarkably improved both by the high increase of the number of hydrogen bonds in the water channel with increasing ionic concentration, and by the conformational change in water molecules near the water channel surface. Our discovery of a soft nanoscale water channel by molecular simulations implies that there is a potential stable passage for ion transport through liquid-liquid interfaces.

Hydrogen-Bond-Driven Chemical Separations: Elucidating the Interfacial Steps of Self-Assembly in Solvent Extraction

Chemical separations, particularly liquid extractions, are pervasive in academic and industrial laboratories, yet a mechanistic understanding of the events governing their function are obscured by interfacial phenomena that are notoriously difficult to measure. In this work, we investigate the fundamental steps of ligand self-assembly as driven by changes in the interfacial H-bonding network using vibrational sum frequency generation. Our results show how the bulk pH modulates the interfacial structure of extractants at the buried oil/aqueous interface via the formation of unique H-bonding networks that order and bridge ligands to produce self-assembled aggregates. These extended H-bonded structures are key to the subsequent extraction of Co²⁺ from the aqueous phase in promoting micelle formation and subsequent ejection of the said micelle into the oil phase. The combination of static and time-resolved measurements reveals the events underlying complexities of liquid extractions at high [Co²⁺]:[ligand] ratios by showing an evolution of interfacially assembled structures that are readily tuned on a chemical basis by altering the compositions of the aqueous phase. The results of this work point to new principles to design-applied separations through the manipulation of surface charge, electrostatic screening, and the associated H-bonding networks that arise at the interface to facilitate organization and subsequent extraction.