Molecular dynamics study of phase transfer catalyst for ion transfer through water–chloroform interface

Revealing the Existence of Long-Range Liquid-Liquid Interfacial Potential in Phase-Transfer Processes

By employing fluorescence wide-field microscopy and a nanoparticle-based phase transfer catalyst (PTC), consisting of a fluorescent silica nanoparticle functionalized with trioctylpropylammonium bromide, we demonstrate that in the presence of NaOH, single nanoparticles display subdiffusive motion along the axis normal to an aqueous liquid-organic liquid interface. This is because of an extended interfacial potential with a shallow well (∼1 kBT) that stretches a few μm into the organic phase, in contrast to previous molecular dynamics studies that reported narrow interfaces on the order of ∼1 nm. Spontaneous interfacial emulsification induced by NaOH results in the propagation of water-in-oil nanoemulsions into the organic solvent that creates an equilibrium hybrid-solvent composition that solvates the PTC. A greater mobility and longer residence time of the PTC at the potential well enhance the interfacial phase transfer process and catalytic efficiency.

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.

Kinetic Mechanism of Surfactant-Based Molecular Recognition: Selective Permeability across an Oil-Water Interface Regulated by Supramolecular Aggregates

Lipids are known to play a vital role in the molecular organization of all cellular life. Molecular recognition is another fundamental biological process that is generally attributed to biological polymers, such as proteins and nucleic acids. However, there is evidence that aggregates of lipids and lipid-like molecules are also capable of selectively binding to or regulating the partitioning of other molecules. We previously demonstrated that a model two-phase octanol/water system can selectively partition Red 40 and Blue 1 dyes added to an aqueous phase, with the selectivity depending on the surfactant (e.g., cetyltrimethylammonium bromide) dissolved in the organic phase. Here, we elucidate the mechanism of molecular recognition in this system by using quantitative partitioning experiments and molecular dynamics (MD) simulations. Our results indicate that the selectivity for the red dye is thermodynamically favored at all surfactant concentrations, while selectivity for the blue dye is kinetically favored at high surfactant concentrations. The kinetic selectivity for the blue dye correlates with the presence of molecular aggregation at the oil-water interface. Coarse-grained MD simulations elucidate nanoscale supramolecular structures that can preferentially bind one small molecule rather than another at an interface, providing a selectively permeable barrier in the absence of proteins. The results suggest a new supramolecular mechanism for molecular recognition with potential applications in drug delivery, drug discovery, and biosensing.

An electrochemical perspective on the interfacial width between two immiscible liquid phases

Molecular dynamics simulations and vibrational sum-frequency spectroscopy are historically the main techniques applied to the description of the molecular structure and dynamics of the immiscible liquid/liquid interface. A molecular sharpness is estimated for oil/water interfaces, with an interfacial width that extends from hundreds of Å to 1 nm. However, electrochemical studies have elucidated a deeper liquid/liquid interface on the order of several micrometers. The breaking down of single-entity electrochemistry to simpler systems and the combination of high-resolution microscopies is confirming a larger extension of the interface. What can be the role of the electrochemist in clarifying this fundamental question? We try to give a suggestion at the end of a brief historical overview of the liquid/liquid interface studies.

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.

Recent progress in simulating microscopic ion transport mechanisms at liquid-liquid interfaces

Transport of ions through liquid-liquid interfaces is of fundamental importance to a wide variety of applications. However, since it is quite challenging for experimentalists to directly and selectively observe molecules at the interfaces, microscopic mechanisms of ion transport have been largely presumed from kinetic information. This Perspective illustrates recent examples that molecular dynamics simulations with proper free energy surfaces clarified mechanistic pictures of ion transport. The key is a proper choice of coordinates and defining/calculating free energy surfaces in multidimensional space. Once the free energy surfaces for realistic systems are available, they naturally provide new insight into the ion transport in unprecedented details, including water finger, transient ion pairing, and electron transfer.

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

Understanding ion transport across interfaces is of fundamental importance in many processes such as liquid-liquid extraction, phase transfer catalysis, enhanced oil recovery and emulsion stabilisation. However, the factors that control ion transport across interfaces are poorly known due to a lack of knowledge of structural changes at interfaces. We studied here the effects of ionic concentration and external force on the transport of ions across the decane-water interface using classical molecular dynamics simulations. The results show that the evolution of interfacial structures during ion transfer across the interface is controlled by hydrogen bonding and ionic interactions at the interface. We also identified a new mode of ion transfer across the interface at low ionic concentrations, involving a 'water ridge', rather that the classical 'water finger'. In the water ridge mode, hydrogen bonds are not broken due to low ion levels, and the water ridge induces gradual interface deformation. Whereas, at high ionic concentrations, hydrogen bonds are broken by the strong ion electrostatic repulsion, thus inducing the formation of a water finger. We also found that the variation of the Gibbs free energy during ion transfer is directly relevant to the ionic concentration. The water ridge at low ionic concentrations, which displaces more water molecules towards the decane phase, induces less free energy variation than the water finger at high ionic concentrations.

Synthesis and structural characterization of CO2-soluble oxidizers [Bu4N]BrO3 and [Bu4N]ClO3 and their dissolution in cosolvent-modified CO2 for reservoir applications

CO₂ utilization in upsteam oil and gas applications requires CO₂-soluble additives such as polymers, surfactants, and other components. Here we report the facile synthesis of CO₂-soluble oxidizers composed of judiciously selected organic cations paired with oxidizing anions. [Bu₄N]BrO₃ and [Bu₄N]ClO₃ are prepared using a double displacement synthetic strategy, whereby the crystalline product is readily obtained in high yield and structurally characterized using single-crystal X-ray diffraction. The facility of the approach is demonstrated through the preparation of several additional alkylammonium bromate compounds. Static solubility studies using a high-pressure cell with viewing windows showed that tetrabutylammonium compounds could be solubilized using cosolvent-modified CO₂. Using 4 mol% ethanol as cosolvent, >3 mM [Bu₄N]BrO₃ could be dissolved in CO₂, while ∼0.75 mM [Bu₄N]ClO₃ could be dissolved in the same solvent system. The solubility properties of [Bu₄N]BrO₃ along with its thermal stability up to ∼200 °C suggest that it is a promising oilfield oxidizer that can be utilized in subterranean CO₂ applications.

Bromate and chlorate salts were hydrophobically modified with tetrabutylammonium to yield oxidizers that are soluble in CO₂-cosolvent mixtures.

Surfactant-enhanced heterogeneity of the aqueous interface drives water extraction into organic solvents

Liquid/liquid extraction (LLE) is one of the most industrially relevant separations methods. Adsorbed surfactant is demonstrated to enhance interfacial heterogeneity and lead to water protrusions that form the basis for transport into the organic phase.

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.