Ion Transport through a Water–Organic Solvent Liquid–Liquid Interface: A Simulation Study

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.

Nanoscale Carbonate Ion-Selective Amperometric/Voltammetric Probes Based on Ion-Ionophore Recognition at the Organic/Water Interface: Hidden Pieces of the Puzzle in the Nanoscale Phase

Here, we report on the successful demonstration and application of carbonate (CO₃^2-) ion-selective amperometric/voltammetric nanoprobes based on facilitated ion transfer (IT) at the nanoscale interface between two immiscible electrolyte solutions. This electrochemical study reveals critical factors to govern CO₃^2--selective nanoprobes using broadly available Simon-type ionophores forming a covalent bond with CO₃^2-, i.e., slow dissolution of lipophilic ionophores in the organic phase, activation of hydrated ionophores, peculiar solubility of a hydrated ion-ionophore complex near the interface, and cleanness at the nanoscale interface. These factors are experimentally confirmed by nanopipet voltammetry, where a facilitated CO₃^2- IT is studied with a nanopipet filled with an organic phase containing the trifluoroacetophenone derivative CO₃^2-ionophore (CO₃^2-ionophore VII) by voltammetrically and amperometrically sensing CO₃^2- in water. Theoretical assessments of reproducible voltammetric data confirm that the dynamics of CO₃^2- ionophore VII-facilitated ITs (FITs) follows the one-step electrochemical (E) mechanism controlled by both water-finger formation/dissociation and ion-ionophore complexation/dissociation during interfacial ITs. The yielded rate constant, k⁰ = 0.048 cm/s, is very similar to the reported values of other FIT reactions using ionophores forming non-covalent bonds with ions, implying that a weak binding between CO₃^2- ion-ionophore enables us to observe FITs by fast nanopipet voltammetry regardless of the nature of bondings between the ion and ionophore. The analytical utility of CO₃^2--selective amperometric nanoprobes is further demonstrated by measuring the CO₃^2- concentration produced by metal-reducing bacteria Shewanella oneidensis MR-1 as a result of organic fuel oxidation in bacterial growth media in the presence of various interferents such as H₂PO₄^-, Cl^-, and SO₄^2-.

Tunable Multi-Phase System for Highly Chemo-Selective Oxidation of Hydroxymethyl-Furfural

Three different multiphase systems (MP 1-3) comprised of two immiscible liquids, with or without an ionic liquid (IL: methyltrioctyl ammonium chloride), were investigated for the oxidation of 5-hydroxymethyl-furfural (HMF) over 5 % Ru/C as a catalyst and air (8 bar) as an oxidant. These conditions proved versatile for an excellent control of the reaction selectivity to 4 distinct products derived from full or partial oxidation of the carbonyl and alcohol functions of HMF, and each one achieved in 87-96 % isolated yield at complete conversion. MP1 based on water and isooctane, yielded 2,5-furandicarboxylic acid (FDCA, 91 % yield). In MP2, obtained by adding the IL to MP1, the oxidation proceeded towards the formation of 5-formyl-2-furancarboxylic acid (FFCA, 87-89 % yield). MP2 also proved successful in the design of a one pot-two step oxidation/reduction sequence to prepare 5-hydroxymethyl-2-furancarboxylic acid (HMFCA, 85 % yield). In MP3, the use of an acetonitrile/cyclooctane biphase yielded 2,5-diformylfuran (DFF, 96 % yield). All the multiphase systems MP 1-3 allowed a perfect segregation of the catalyst in a single phase (either the hydrocarbon or the IL) distinct from the one containing HMF and its oxidation products. This was crucial not only for the catalyst/product separation but also for the recycle of Ru/C that was possible under all the tested conditions. Accordingly, MP-reaction were run in a semicontinuous mode without removing the catalyst from the reactor nor resorting to conventional separation and activation techniques. Negligible Ru leaching, less than 0.96 ppb, was measured in all cases.

Molecular dynamics simulations of liquid-liquid interfaces in an electric field: The water-1,2-dichloroethane interface

The polarized interface between two immiscible liquids plays a central role in many technological processes. In particular, for electroanalytical and ion extraction applications, an external electric field is typically used to selectively induce the transfer of ionic species across the interfaces. Given that it is experimentally challenging to obtain an atomistic insight into the ion transfer process and the structure of liquid-liquid interfaces, atomistic simulations have often been used to fill this knowledge gap. However, due to the long-range nature of the electrostatic interactions and the use of 3D periodic boundary conditions, the use of external electric fields in molecular dynamics simulations requires special care. Here, we show how the simulation setup affects the dielectric response of the materials and demonstrate how by a careful design of the system it is possible to obtain the correct electric field on both sides of a liquid-liquid interface when using standard 3D Ewald summation methods. In order to prove the robustness of our approach, we ran extensive molecular dynamics simulations with a rigid-ion and polarizable force field of the water/1,2-dichloroethane interface in the presence of weak external electric fields.

Interfacial structure in the liquid-liquid extraction of rare earth elements by phosphoric acid ligands: a molecular dynamics study

Solvent extraction (SX), wherein two immiscible liquids, one containing the extractant molecules and the other containing the solute to be extracted are brought in contact to effect the phase transfer of the solute, underpins metal extraction and recovery processes. The interfacial region is of utmost importance in the SX process, since besides thermodynamics, the physical and chemical heterogeneity at the interface governs the kinetics of the process. Yet, a fundamental understanding of this heterogeneity and its implications for the extraction mechanism are currently lacking. We use molecular dynamics (MD) simulations to study the liquid-liquid interface under conditions relevant to the SX of Rare Earth Elements (REEs) by a phosphoric acid ligand. Simulations revealed that the extractant molecules and varying amounts of acid and metal ions partitioned to the interface. The presence of these species had a significant effect on the interfacial thickness, hydrogen bond life times and orientations of the water molecules at the interface. Deprotonation of the ligands was essential for the adsorption of the metal ions at the interface, with these ions forming a number of different complexes at the interface involving one to three extractant molecules and four to eight water molecules. Although the interface itself was rough, no obvious 'finger-like' water protrusions penetrating the organic phase were seen in our simulations. While the results of our work help us gain fundamental insights into the sequence of events leading to the formation of a variety of interfacial complexes, they also emphasize the need to carry out a more detailed atomic level study to understand the full mechanism of extraction of REEs from the aqueous to organic phases by phosphoric acid ligands.

Effect of confinement on the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces: a molecular dynamics study

Molecular dynamics simulation was used to study the adsorption behavior of inorganic and organic ions at aqueous-cyclohexane interfaces, and that with such systems confined in a kaolinite nanopore. Four aqueous solutions were used, each containing one of the four solutes (NaCl, NaOH, CaCl₂ and Ca(OH)₂) at the concentration of 1.0 M. At the interface of each of the solutions with neat cyclohexane, there existed an ion depletion zone. The more strongly hydrated ions, such as calcium and hydroxide, were more intensely depleted from the interface as compared to sodium and chloride. Such surface exclusion led to interfacial tension increases, with greater increments for solutions containing calcium or hydroxide. Upon addition of sodium decanoate to the cyclohexane phase, they partly migrated to the aqueous-organic interface, and the remaining formed inverted micelle complexes. Also, a small fraction of the solvated cations in the aqueous phase drifted to the depletion zone to interact with the organic anions, with their affinity towards the interface still being controlled by their hydration-strength. When such systems were confined in a kaolinite nanopore, behavior of decanoate anions - which were found to be prone to strong adsorption to solid surfaces in the absence of any aqueous solution - was determined by the nature of the solvated ions in water. In these systems, the more weakly-hydrated ionic species exhibited preferential adsorption to the solid surface, while affinity of the more strongly-hydrated ones was towards remaining within the water layers or to the aqueous-organic interface. With the calcium chloride solution, almost all of the organic ions were detached from the surface and adsorbed at the aqueous-cyclohexane interface. This was caused by the release of double amount of inorganic anions by this 1 : 2 salt, the inner-sphere adsorption of majority of chloride anions to the octahedral surface, and the higher charge density of calcium cations.

Ionic hydration-induced evolution of decane–water interfacial tension

We show that ionic hydration is responsible for the non-monotonic variation of the interfacial tension with increasing ionic concentration.

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.