Solvent Structure, Dynamics, and Ion Mobility in Aqueous Solutions at 25 °C

Impact of Confinement and Zwitterionic Ligand Chemistry on Ion-Ion Selectivity of Functionalized Nanopores

Membranes incorporating zwitterionic chemistries have recently emerged as promising candidates for facilitating challenging ion-ion separations. Transport of ions in such membranes predominantly occurs in hydrated nanopores lined with zwitterionic monomers. To shed light on the physics of ion-ion selectivity underlying such materials, we conducted molecular dynamics simulations of sodium halide transport in model nanopores grafted with sulfobetaine methacrylate molecules. Our results reveal that in both functionalized and unfunctionalized nanopores smaller ions prefer to reside near the pore center, while the larger ions tend to reside near the pore walls. An enhancement in the selective transport of larger anions is observed within the unfunctionalized nanopores relative to that in salt-in-water solutions. Upon functionalization of the nanopores with zwitterions (ZIs), the disparities in the anionic distribution profiles within the pores coupled with differences in the anion-ZI interactions result in a slowdown of larger anions relative to smaller anions. Increasing the ZI grafting density exacerbates these effects, further promoting the selective transport of smaller anions. Our results suggest that selectivity toward large anions can be realized by using nanoporous membranes with ZI content that is high enough to facilitate ion/water partitioning into the pores while preserving the characteristic tendency of the unfunctionalized pores to facilitate faster transport of the larger anions. On the other hand, selectivity toward smaller anions can be achieved by targeting ZI content within the pores that is high enough to significantly slow down the transport of large anions but not high enough to hinder the partitioning of ions/water molecules into the pore due to steric effects.

Ion diffusion captures composition-dependent anomalies in water-DMSO binary mixtures

Aqueous dimethyl sulfoxide (Aq-DMSO) binary mixture exhibits many fascinating composition-dependent anomalies that are explained by using the peculiarities of the water-DMSO hydrogen bond. Ions can couple strongly to these composition-dependent anomalies to produce exotic dynamics of their own. We carry out theoretical studies using computer simulations to understand the structural and dynamical aspects of rigid monovalent cations (Li+, Na+, K+, Rb+, and Cs+) in aqueous DMSO solutions, with chloride as the counterion. We uncover a number of composition-dependent ion diffusion anomalies, which can be traced back to the interplay between the size-dependent charge density of the ion and the resulting difference in interactions of the ion with water and DMSO molecules. Size and composition dependence of the diffusion coefficients of the five ions exhibit fascinating variations that can be explained partially.

A robust thin-film droplet-induced electricity generator

The pursuit of cost-effective, high-voltage electricity generators activated by droplets represents a new frontier in hydropower technology. This study presents an economical method for crafting droplet generators using common materials such as solid polytetrafluoroethylene (PTFE) films and readily available tapes, eliminating the need for specialized cleanroom facilities. A thorough investigation into voltage-limiting factors, encompassing device capacitance and induced electrode charges, reveals specific areas with potential for optimization. A substantial enhancement in the open-circuit voltage (Voc) was achieved, reaching approximately 282.2 ± 27.9 V-an impressive increase of around 60 V compared to earlier benchmarks. One device showcased its capability to power 100 LEDs concurrently, underscoring its efficacy. Ten such devices created diverse luminous patterns with uniform light intensity for each LED, showcasing the practical potential of the approach. The methodology's cost-effectiveness results in a remarkable cost reduction compared to solution-based materials, paving the way for the widespread adoption of large-scale water droplet energy harvesting.

Electrochemical water softening technology: From fundamental research to practical application

In recent decades, the environmentally benign electrochemical softening process has been gaining widespread interest as an emerging alternative for water softening. But, in spite of decades of research, the fundamental advances in laboratory involving electrolytic cell design and treatment system development have not led to urgently needed improvements in industrially practicable electrochemical softening technique. In this review, we firstly provide the critical insights into the mechanism of the currently widely used cathode precipitation process and its inherent limitations, which seriously impede its wide implementation in industry. To relieve the above limitations, some cutting-edge electrochemically homogeneous crystallization systems have been developed, the effectiveness of which are also comprehensively summarized. In addition, the pros and cons between cathode precipitation and electrochemically homogeneous crystallization systems are systematically outlined in terms of performance and economic evaluation, potential application area, and electrolytic cell and system complexity. Finally, we discourse upon practical challenges impeding the industrial-scale deployment of electrochemical water softening technique and highlight the integration of strong engineering sense with fundamental research to realize industry-scale deployment. This review will inspire the researchers and engineers to break the bottlenecks in electrochemical water softening technology and harness this technology with the broadened industrial application area.

Anomalous Concentration Dependence of Viscosity: Hidden Role of Cross-Correlations in Aqueous Electrolyte Solutions

The viscosity of aqueous electrolyte solutions exhibits well-known composition-dependent anomalies that show certain definitive trends and universal features. The viscosity of LiCl and NaCl solutions increases with concentration in a monotonic fashion, while solutions of KCl, RbCl, and CsCl exhibit a more complex behavior. Here, the viscosity first decreases and then increases with increasing concentration, with a rather broad minimum at intermediate concentrations (ca. 1-3 m). To unearth the origin of such puzzling behavior, we carried out detailed molecular-level analyses by interrogating the exact Green-Kubo expression of viscosity in terms of the stress-stress time correlation function (SS-TCF). The total SS-TCF can be decomposed into a collection of three self- and three cross-SS-TCFs arising from the three constituent components (water, cations, and anions). Mode coupling theory (MCT) analysis for the friction on ions and the viscosity of the solution suggests the possible importance of two-particle static and time-dependent cross-correlations between water and the ions. We calculate the viscosity and other dynamical properties for all five electrolyte (LiCl, NaCl, KCl, RbCl, and CsCl) solutions over a range of concentrations, using two models of water (SPC/E and TIP4P/2005). The total viscosity derives non-negligible contributions from all of the terms. The cross-correlations are found to be surprisingly large and seen to play a hidden role in the concentration dependence. However, the importance of cross-correlations is often not discussed. Our study leads to a theoretical understanding of the microscopic origin of the observed anomalies in the composition dependence of viscosity across all five electrolytes.

Novel Computational Chemistry Infrastructure for Simulating Astatide in Water: From Basis Sets to Force Fields Using Particle Swarm Optimization

Using the example of astatine, the heaviest naturally occurring halogen whose isotope At-211 has promising medical applications, we propose a new infrastructure for large-scale computational models of heavy elements with strong relativistic effects. In particular, we focus on developing an accurate force field for At- in water based on reliable relativistic density functional theory (DFT) calculations. To ensure the reliability of such calculations, we design novel basis sets for relativistic DFT, via the particle swarm optimization algorithm to optimize the coefficients of the new basis sets and the polarization-consistent basis set idea's extension to heavy elements to eliminate the basis set error from DFT calculations. The resulting basis sets enable the well-grounded evaluation of relativistic DFT against "gold-standard" CCSD(T) results. Accounting for strong relativistic effects, including spin-orbit interaction, via our redesigned infrastructure, we elucidate a noticeable dissimilarity between At- and I- in halide-water force field parameters, radial distribution functions, diffusion coefficients, and hydration energies. This work establishes the framework for the systematic development of polarization-consistent basis sets for relativistic DFT and accurate force fields for molecular dynamics simulations to be used in large-scale models of complex molecular systems with elements from the bottom of the periodic table, including actinides and even superheavy elements.

Toward Metal Extraction from Regolith: Theoretical Investigation of the Solvation Structure and Dynamics of Metal Ions in Ionic Liquids

Lunar and Martian regoliths, containing feldspar, pyroxene, ilmenite, olivine, and aluminite minerals, are excellent sources of metals such as aluminum, sodium, magnesium, and iron. Ionic liquids (ILs), which are excellent solvents with extremely low vapor pressure and high electrochemical stability, can be potentially leveraged for extracting metals from regolith in an extra-terrestrial environment. A critical step in the solvation process, which determines the effectiveness of the IL solvent, is the formation of solvation shells around the metal cations. To determine the rigidity and stability of the solvation shells, which has a direct implication on the extraction of metals, we performed classical molecular dynamics simulations of dilute solutions comprising individual metal ions Na+, Mg2+, and Al3+ in two distinct ILs, [mppy][TFSI] and [mppy][HSO4]. Our results indicate that the compactness of the structure is directly related to the charge density of the metal cation and the relative size and symmetry of the IL anion. Potentials of the mean force of the metal cation with the solvating IL anion indicate the presence of energy minima with barriers that increase with the surface charge density of the cation. The increasing energy barrier leads to greater residence time of metal cations in the solvation shell, which was confirmed by evaluating corresponding autocorrelation functions. Overall, our calculations provide fundamental insights into key factors that influence the solvation of metals and can be useful in the screening of ILs for digestion of metal-containing minerals in lunar and Martian regoliths.

Structure and dynamics of the Li+ ion in water, methanol and acetonitrile solvents: ab initio molecular dynamics simulations

Fundamental understanding of the structure and dynamics of the Li+ ion in solution is of utmost importance in different fields of science and technology, especially in the field of ion batteries. In view of this, ab initio molecular dynamics (AIMD) simulations of the LiCl salt in water, methanol and acetonitrile were performed to elucidate structural parameters such as radial distribution function and coordination number, and dynamical properties like diffusion coefficient, limiting ion conductivity and hydrogen bond correlation function. In the present AIMD simulation, one LiCl in water is equivalent to 0.8 M, which is close to the concentration of the lithium salt used in the Li-ion battery. The first sphere of coordination number of the Li+ ion was reaffirmed to be 4. The radial distribution function for different pairs of atoms is seen to be in good agreement with the experimental results. The calculated potential of mean force indicates the stronger interaction of the Li+ ion with methanol over water followed by acetonitrile. The dynamical parameters convey quite high diffusion and limiting ionic conductivity of the Li+ ion in acetonitrile compared to that in water and methanol which has been attributed to the transport of the Li-Cl ion pair in a non-dissociated form in acetonitrile. The AIMD results were found to be in accordance with the experimental findings, i.e. the limiting ion conductivity was found to follow the order acetonitrile > methanol > water. This study shows the importance of atomistic level simulations in evaluating the structural and dynamical parameters and in implementing the results for predicting and synthesizing better next generation solvents for lithium ion batteries (LIBs).

Role of Dielectric Drag in Circumventing the Solubility-Diffusivity Trade-off in Zwitterionic Copolymer Membranes

Recent experiments have revealed that random zwitterionic amphiphilic copolymer (r-ZAC) membranes exhibit excellent Cl-/F- permselectivity circumventing the solubility-diffusivity trade-off. We conducted molecular dynamics simulations to investigate the origin of the experimental results on the transport of sodium halides in r-ZAC membranes. Our results indicate that the enhancement of Cl-/F- diffusivity selectivity in r-ZAC membranes (relative to that in bulk water) stems from the increase in dielectric drag dominating over the increase in Stokes drag, zwitterionic group-induced steric hindrance, and ion-polymer interactions. The importance of dielectric drag is further demonstrated by showing that reduction in ionic charges leads to a complete reversal of the diffusivity selectivity trends. We conclude that leveraging the impact of hydrophilic nanoconfinement on the dynamics of water can be utilized as a strategy to simultaneously augment solubility selectivity and diffusivity selectivity for separations, wherein the flux of the larger ionic species is desired over that of the smaller.

Complex coupling between surface charge and thermo-osmotic phenomena

Thermo-osmotic flows, generated at liquid-solid interfaces by thermal gradients, can be used to produce electric currents from waste heat on charged surfaces. The two key parameters controlling the thermo-osmotic current are the surface charge and the interfacial enthalpy excess due to liquid-solid interactions. While it has been shown that the contribution from water to the enthalpy excess can be crucial, how this contribution is affected by surface charge remained to be understood. Here, we start by discussing how thermo-osmotic flows and induced electric currents are related to the interfacial enthalpy excess. We then use molecular dynamics simulations to investigate the impact of surface charge on the interfacial enthalpy excess, for different distributions of the surface charge, and two different wetting conditions. We observe that surface charge has a strong impact on enthalpy excess, and that the dependence of enthalpy excess on surface charge depends largely on its spatial distribution. In contrast, wetting has a very small impact on the charge-enthalpy coupling. We rationalize the results with simple analytical models, and explore their consequences for thermo-osmotic phenomena. Overall, this work provides guidelines to search for systems providing optimal waste heat recovery performance.

Determination of urea, phosphate, and potassium in agricultural runoff waters using electrochemical impedance spectroscopy

Large-scale use of chemical fertilizers has resulted in the contamination of agricultural runoff waters by soil macronutrients NPK, whose detection is of significant interest. This work reports the determination of macronutrients in the form of urea (N), orthophosphate PO43- (P), and potassium K+ (K) in simulated agricultural runoff waters. Their solutions were prepared by extracting water-soluble constituents of soil. This 'base' solution contains high concentrations of various species, including Cl-, SO42-, NO3-, PO43-, Na+, K+, and NH4+ along with natural organic matter. Predetermined amounts of urea (4 to 22.5 ppm), PO43- (7 to 50 ppm), and potassium K+ (25 to 250 ppm) were added to the base simulated runoff water to prepare standard stock solutions. Using stainless steel working and counter electrodes, a small AC perturbation (±10 mV vs. OCP, vs. Ag/AgCl) was applied and the frequency response of the working electrode-solution interface was measured from 1 Hz to 1 MHz. The interface itself was modeled as a suitable equivalent electrical circuit, and the magnitudes of its components were fitted from experimental data using nonlinear regression. It is observed that PO43- concentration is a linear function of charge transfer resistance arising from redox reaction, K+ concentration is a quadratic function of double-layer capacitance arising from its higher mobility, and urea concentration can be correlated as a linear function of constant phase element arising from its polarization in the presence of an applied electric field. The sensor exhibits good sensitivity, repeatability, and excellent performance against interfering species. These preliminary results show significant potential for development of a real-time or on-site sensing device.

Electro-Freezing of Supercooled Water Is Induced by Hydrated Al3+ and Mg2+ Ions: Experimental and Theoretical Studies

This work reports that the octahedral hydrated Al3+ and Mg2+ ions operate within electrolytic cells as kosmotropic (long-range order-making) "ice makers" of supercooled water (SCW). 10-5 M solutions of hydrated Al3+ and Mg2+ ions each trigger, near the cathode (-20 ± 5 V), electro-freezing of SCW at -4 °C. The hydrated Al3+ ions do so with 100% efficiency, whereas the Mg2+ ions induce icing with 40% efficiency. In contrast, hydrated Na+ ions, under the same experimental conditions, do not induce icing differently than pure water. As such, our study shows that the role played by Al3+ and Mg2+ ions in water electro-freezing is impacted by two synchronous effects: (1) a geometric effect due to the octahedral packing of the coordinated water molecules around the metallic ions, and (2) the degree of polarization which these two ions induce and thereby acidify the coordinated water molecules, which in turn imparts them with an ice-like structure. Long-duration molecular dynamics (MD) simulations of the Al3+ and Mg2+ indeed reveal the formation of "ice-like" hexagons in the vicinity of these ions. Furthermore, the MD shows that these hexagons and the electric fields of the coordinate water molecules give rise to ultimate icing. As such, the MD simulations provide a rational explanation for the order-making properties of these ions during electro-freezing.

Graphene Nanopores Enhance Water Evaporation from Salt Solutions: Exploring the Effects of Ions and Concentration

With increased water stress, the development of clean water technologies is an active area of research. Evaporation-based solutions offer the advantage of low energy consumption, and recently a 10-30 fold enhancement in water evaporation flux has been observed through Å-scale graphene nanopores (Lee, W.-C., et al., ACS Nano 2022, 16(9), 15382). Herein, using molecular dynamics simulations, we examine the suitability of Å-scale graphene nanopores in enhancing water evaporation from salt solutions (LiCl, NaCl, and KCl). Cation-π interactions between ions and the surface of nanoporous graphene are found to significantly influence ion populations in the nanopore vicinity, leading to varied water evaporation fluxes from different salt solutions. The highest water evaporation flux was observed for KCl solutions, followed by NaCl and LiCl solutions, with the differences reducing at lower concentrations. Relative to the bare liquid-vapor interface, 4.54 Å nanopores exhibit the highest evaporation flux enhancements ranging from 7 to 11, with an enhancement of 10.8 obtained for 0.6 M NaCl solution, which closely resembles seawater compositions. Functionalized nanopores induce short-lived water-water hydrogen bonds and reduce surface tension at the liquid-vapor interface, thereby lowering the free energy barrier for water evaporation with a negligible effect on the ion hydration dynamics. These findings can aid in developing green technologies for desalination and separation processes with low thermal energy input.

Beyond steric selectivity of ions using ångström-scale capillaries

Ion-selective channels play a key role in physiological processes and are used in many technologies. Although biological channels can efficiently separate same-charge ions with similar hydration shells, it remains a challenge to mimic such exquisite selectivity using artificial solid-state channels. Although there are several nanoporous membranes that show high selectivity with respect to certain ions, the underlying mechanisms are based on the hydrated ion size and/or charge. There is a need to rationalize the design of artificial channels to make them capable of selecting between similar-sized same-charge ions, which, in turn, requires an understanding of why and how such selectivity can occur. Here we study ångström-scale artificial channels made by van der Waals assembly, which are comparable in size with typical ions and carry little residual charge on the channel walls. This allows us to exclude the first-order effects of steric- and Coulomb-based exclusion. We show that the studied two-dimensional ångström-scale capillaries can distinguish between same-charge ions with similar hydrated diameters. The selectivity is attributed to different positions occupied by ions within the layered structure of nanoconfined water, which depend on the ion-core size and differ for anions and cations. The revealed mechanism points at the possibilities of ion separation beyond simple steric sieving.

Colloidal superionic conductors

Nanoparticles with highly asymmetric sizes and charges that self-assemble into crystals via electrostatics may exhibit behaviors reminiscent of those of metals or superionic materials. Here, we use coarse-grained molecular simulations with underdamped Langevin dynamics to explore how a binary charged colloidal crystal reacts to an external electric field. As the field strength increases, we find transitions from insulator (ionic state), to superionic (conductive state), to laning, to complete melting (liquid state). In the superionic state, the resistivity decreases with increasing temperature, which is contrary to metals, yet the increment decreases as the electric field becomes stronger. Additionally, we verify that the dissipation of the system and the fluctuation of charge currents obey recently developed thermodynamic uncertainty relation. Our results describe charge transport mechanisms in colloidal superionic conductors.

Frequency and field-dependent response of confined electrolytes from Brownian dynamics simulations

Using Brownian dynamics simulations, we investigate the effects of confinement, adsorption on surfaces, and ion-ion interactions on the response of confined electrolyte solutions to oscillating electric fields in the direction perpendicular to the confining walls. Nonequilibrium simulations allows to characterize the transitions between linear and nonlinear regimes when varying the magnitude and frequency of the applied field, but the linear response, characterized by the frequency-dependent conductivity, is more efficiently predicted from the equilibrium current fluctuations. To that end, we (rederive and) use the Green-Kubo relation appropriate for overdamped dynamics, which differs from the standard one for Newtonian or underdamped Langevin dynamics. This expression highlights the contributions of the underlying Brownian fluctuations and of the interactions of the particles between them and with external potentials. Although already known in the literature, this relation has rarely been used to date, beyond the static limit to determine the effective diffusion coefficient or the DC conductivity. The frequency-dependent conductivity always decays from a bulk-like behavior at high frequency to a vanishing conductivity at low frequency due to the confinement of the charge carriers by the walls. We discuss the characteristic features of the crossover between the two regimes, most importantly how the crossover frequency depends on the confining distance and the salt concentration, and the fact that adsorption on the walls may lead to significant changes both at high and low frequencies. Conversely, our results illustrate the possibility to obtain information on diffusion between walls, charge relaxation, and adsorption by analyzing the frequency-dependent conductivity.

Solvent Dynamics of Aqueous Halides before and after Photoionization

Electron transfer reactions can be strongly influenced by solvent dynamics. We study the photoionization of halides in water as a model system for such reactions. There are no internal nuclear degrees of freedom in the solute, allowing the dynamics of the solvent to be uniquely identified. We simulate the equilibrium solvent dynamics for Cl^-, Br^-, I^-, and their respective neutral atoms in water, comparing quantum mechanical/molecular mechanical (QM/MM) and classical molecular dynamics (MD) methods. On the basis of the obtained configurations, we calculate the extended X-ray absorption fine structure (EXAFS) spectra rigorously based on the MD snapshots and compare them in detail with other theoretical and experimental results available in the literature. We find our EXAFS spectra based on QM/MM MD simulations in good agreement with their experimental counterparts for the ions. Classical MD simulations for the ions lead to EXAFS spectra that agree equally well with the experiment when it comes to the oscillatory period of the signal, even though they differ from the QM/MM radial distribution functions extracted from the MD. The amplitude is, however, considerably overestimated. This suggests that to judge the reliability of theoretical simulation methods or to elucidate fine details of the atomistic dynamics of the solvent based on EXAFS spectra, the amplitude as well as the oscillatory period need to be considered. If simulations fail qualitatively, as does the classical MD for the aqueous neutral halogen atoms, the resulting EXAFS will also be strongly affected in both oscillatory period and amplitude. The good reliability of QM/MM-based EXAFS simulations, together with clear qualitative differences in the EXAFS spectra found between halides and their atomic counterparts, suggests that a combined theory and experimental EXAFS approach is suitable for elucidating the nonequilibrium solvent dynamics in the photoionization of halides and possibly also for electron transfer reactions in more complex systems.

Salt Effect on Donnan Equilibrium in Montmorillonite Demonstrated with Molecular Dynamics Simulations

Donnan equilibrium governs the distribution of ions in many systems such as ion exchange membranes and biological cells in contact with an external electrolyte. Herein, Donnan equilibrium between bulk salt solution and bihydrated montmorillonite was investigated because such a system is of great importance for many nuclear waste disposal concepts. Specifically, we used molecular dynamics simulations to determine the partition coefficient of chloride, which was achieved by calculating the free-energy difference of chloride in the interlayer and the bulk using enhanced sampling methodology. Montmorillonite in equilibrium with either NaCl or CaCl₂ was examined to elucidate the general difference between 1:1 and 2:1 salts. The concentration dependence of the partition coefficient for each salt was determined using three and four concentrations for NaCl and CaCl₂, respectively. In the case of NaCl, we found that the partition coefficient increased linearly with the concentration, while for CaCl₂, the increase was proportional to the square root of the concentration. A derivation of the partition coefficient using general Donnan theory that includes excess free energy contributions beyond the electrostatic Donnan potential is also presented. For both salts, the agreement between the partition coefficient from the simulations and Donnan theory was excellent. Although Donnan theory is a continuum theory derived without any reference to atomistic details, the present results justify its application to systems with nanoscale pores.

Understanding the Chemical Shifts of Aqueous Electrolyte Species Adsorbed in Carbon Nanopores

Interfaces between aqueous electrolytes and nanoporous carbons are involved in a number of technological applications such as energy storage and capacitive deionization. Nuclear magnetic spectroscopy is a very useful tool to characterize ion adsorption in such systems thanks to its nuclei specificity and the ability to distinguish between ions in the bulk and in pores. We use complementary methods (density functional theory, molecular dynamics simulations, and a mesoscopic model) to investigate the relative importance of various effects on the chemical shifts of adsorbed species: ring currents, ion organization in pores of various sizes, specific ion-carbon interactions, and hydration. We show that ring currents and ion organization are predominant for the determination of chemical shifts in the case of Li⁺ ions and hydrogen atoms of water. For the large Rb⁺ and Cs⁺ ions, the additional effect of the hydration shell should be considered to predict chemical shifts in agreement with experiments.

Flotation surface chemistry of water-soluble salt minerals: from experimental results to new perspectives

The flotation separation of water-soluble salt minerals has to be conducted under the condition of saturation in brines which represents a challenging but exciting topic of colloid and surface chemistry. Despite several proposals on explaining the success of this industrial application for many decades, our understanding of the flotation separation is still far from complete yet, owing to the complexity of the highly selective collection of salt crystals by air bubbles in brines. Here, we thoroughly review the experimental results for halogen, oxyanion, and double salts and match them with the proposed theories on the flotation of soluble salts to identify the agreed and disagreed cases. The experimental results show that the flotation of these salts varies from collectors (surfactants applied to control the crystal hydrophobicity) to collectors and is strongly affected by the brine ion composition and pH conditions. We find some exceptional flotation results that cannot be simply explained by the crystal surface charge and wettability. Furthermore, we outline several disputes and discrepancies between the experiments and the theories when different collectors are applied. Apart from the extensive consideration of surface hydration, the presence of external ion species exhibits ubiquitous effects on the surface properties of salt crystals and the colloidal properties of collectors. We conclude that the interactions between salt ions, water molecules, collectors, and salt crystals must be considered more thoroughly, and the activity of collectors at the air-liquid interface should also be the focus. Advanced techniques such as molecular dynamics simulation, atomic force microscopy, X-ray photoelectron spectroscopy, and sum-frequency generation spectroscopy are expected to be promising research tools for future studies.

Ion-Pairs in Aluminosilicate-Alkali Synthesis Liquids Determine the Aluminum Content and Topology of Crystallizing Zeolites

Using hydrated silicate ionic liquids, phase selection and framework silicon-to-aluminum ratio during inorganic zeolite synthesis were studied as a function of batch composition. Consisting of homogeneous single phasic liquids, this synthesis concept allows careful control of crystallization parameters and evaluation of yield and sample homogeneity. Ternary phase diagrams were constructed for syntheses at 90 °C for 1 week. The results reveal a cation-dependent continuous relation between batch stoichiometry and framework aluminum content, valid across the phase boundaries of all different zeolites formed in the system. The framework aluminum content directly correlates to the type of alkali cation and gradually changes with batch alkalinity and dilution. This suggests that the observed zeolites form through a solution-mediated mechanism involving the concerted assembly of soluble cation-oligomer ion pairs. Phase selection is a consequence of the stability for a particular framework at the given aluminum content and alkali type.

Enhanced Ion Rejection in Carbon Nanotubes by a Lateral Electric Field

Reverse osmosis membranes hold great promise for dealing with global water scarcity. However, the trade-off between ion selectivity and water permeability is a serious obstacle to desalination. Herein, we introduce an effective strategy to enhance the desalination performance of the membrane. A series of molecular dynamics simulations manifest that an additional lateral electric field significantly promotes ion rejection in carbon nanotubes (CNTs) under the drive of longitudinal pressure. Specifically, with the increase in the electric field, the ion flux shows a deep linear decay, while the water flux decreases only slightly, resulting in a linear increase in ion rejection. The energy barriers of ions around the CNT inlet are obtained by calculating the potentials of mean force to explain enhanced ion rejection. The lateral electric field uniformly raises the energy barriers of ions by pushing them away from the CNT inlet, corresponding to the enhanced ion velocity in the field direction. Furthermore, with the increase in CNT diameter, there is a significant increase in the flux of both ions and water; however, the lateral electric field can also obviously enhance the ion rejection in wider CNTs. Consequently, the enhancement of ion rejection by lateral electric fields should be universal for different CNT diameters, which opens a new avenue for selective permeation and may have broad implications for desalination devices with large pore sizes.

War and Peace between Electrostatic and van der Walls Forces Regulate Translational and Rotational Diffusion

Diffusion of a Brownian particle or a molecule in a liquid solvent is caused, in the Stokes-Einstein picture, by unbalanced fluctuations of osmotic forces on different sides of the particle. When the particle carries a charge or a higher multipolar moment, this picture is amended by fluctuations of electrostatic forces producing dielectric friction. Dielectric friction slows both the translational and rotational diffusion down. While this picture is well established and is physically sound, standard theories grossly overestimate the magnitude of dielectric friction for small dipolar solutes and larger colloidal particles, such as proteins. Motivated by recent simulation studies, this perspective discusses the interplay between osmotic (van der Waals) and electrostatic forces in promoting molecular and colloidal diffusion. Much can be learned about microscopic friction mechanisms from statistical and dynamical correlations between osmotic and electrostatic forces.

Structure, Thermodynamics, and Dynamics of Thiocyanate Ion Adsorption and Transfer across the Water/Toluene Interface

Molecular dynamics simulations are used to examine in detail the structure, thermodynamics, and dynamics involve in the adsorption and transfer of the thiocyanate ion (SCN^-) across the water/toluene interface. Free energy, hydration structure, and several dynamical properties as a function of the ion location along the interface normal are calculated and contrasted with recent experiments. The free energy profile exhibits a local minimum near the interface corresponding to adsorption free energy relative to bulk water of -6 kJ/mol, in reasonable agreement with experiments. The simulations provide insight into the water surface fluctuations that are coupled to the ion transfer, demonstrating formation of water finger-like structures assisting the transfer process.

A zeolite crystallisation model confirmed by in situ observation

Probing nucleation and growth of porous crystals at a molecular level remains a cumbersome experimental endeavour due to the complexity of the synthesis media involved. In particular, the study of zeolite formation is hindered as these typically form in multiphasic synthesis media, which restricts experimental access to crystallisation processes. Zeolite formation from single phasic hydrated silicate ionic liquids (HSiL) opens new possibilities. In this work, HSiL zeolite crystallisation is investigated in situ using a specifically designed conductivity measurement set-up yielding access to crystallisation kinetics. Based on the conductivity data and final yields, a crystallisation model explaining the results based on a surface growth mechanism was derived. The excellent agreement between experiment and theory indicates zeolite crystallisation from highly ionic media proceeds via a multi-step mechanism, involving an initial reversible surface condensation of a growth unit, followed by incorporation of that unit into the growing crystal. The first step is governed by the liquid phase concentration and surface energy, while the final step shows a correlation to the mobility of the cation involved.

Ion Specificity of Confined Ion-Water Structuring and Nanoscale Surface Forces in Clays

Ion specificity and related Hofmeister effects, which are ubiquitous in aqueous systems, can have spectacular consequences in hydrated clays, where ion-specific nanoscale surface forces can determine large-scale cohesive swelling and shrinkage behaviors of soil and sediments. We have used a semiatomistic computational approach and examined sodium, calcium, and aluminum counterions confined with water between charged surfaces representative of clay materials to show that ion-water structuring in nanoscale confinement is at the origin of surface forces between clay particles which are intrinsically ion-specific. When charged surfaces strongly confine ions and water, the amplitude and oscillations of the net pressure naturally emerge from the interplay of electrostatics and steric effects, which cannot be captured by existing theories. Increasing confinement and surface charge densities promote ion-water structures that increasingly deviate from the ions' bulk hydration shells, being strongly anisotropic, persistent, and self-organizing into optimized, nearly solid-like assemblies where hardly any free water is left. Under these conditions, strongly attractive interactions can prevail between charged surfaces because of the dramatically reduced dielectric screening of water and the highly organized water-ion structures. By unravelling the ion-specific nature of these nanoscale interactions, we provide evidence that ion-specific solvation structures determined by confinement are at the origin of ion specificity in clays and potentially a broader range of confined aqueous systems.

Adsorption, Structure, and Dynamics of Short- and Long-Chain PFAS Molecules in Kaolinite: Molecular-Level Insights

The ubiquitous presence of poly- and perfluoroalkyl substances (PFAS) in different natural settings poses a serious threat to environmental and human health. Soils and sediments represent one of the important exposure pathways of PFAS for humans and animals. With increasing bioaccumulation and mobility, it is extremely important to understand the interactions of PFAS molecules with the dominant constituents of soils such as clay minerals. This study reports for the first time the fundamental molecular-level insights into the adsorption, interfacial structure, and dynamics of short- and long-chain PFAS molecules at the water-saturated mesopores of kaolinite clay using classical molecular dynamics (MD) simulations. At environmental conditions, all the PFAS molecules are exclusively adsorbed near the hydroxyl surface of the kaolinite, irrespective of the terminal functional groups and metal cations. The interfacial adsorption structures and coordination environments of PFAS are strongly dependent on the nature of the functional groups and their hydrophobic chain length. The formation of large, aggregated clusters of long-chain PFAS at the hydroxyl surface of kaolinite is responsible for their restricted dynamics in comparison to short-chain PFAS molecules. Such comprehensive knowledge of PFAS at the clay mineral interface is critical to developing novel site-specific degradation and mitigation strategies.

Ionic mobility driven by correlated van der Waals and electrostatic forces

Classical theories of dielectric friction make two critical assumptions: (i) friction due to van der Waals (vdW) forces is described by hydrodynamic drag and is independent of the ionic charge and (ii) vdW and electrostatic forces are statistically independent. Both assumptions turn out to be incorrect when tested against simulations of anions and cations with varying charge magnitude dissolved in water. Both the vdW and electrostatic components of the force variance scale linearly with the ionic charge squared. The two components are strongly anticorrelated producing simple relations for the total force variance in terms of self-variances. The inverse diffusion constant scales linearly with the charge squared. Solvation asymmetry between cations and anions extends to linear transport coefficients.

Understanding the mechanism and kinetics of the formation and breaking of ring structures during silica polymerization: a computational study

Ring structures are ubiquitous in porous materials and play a crucial role in the functioning of these materials. Understanding the ring formation and breaking mechanism is essential for designing and controlling the porosity, framework density, channels, and cage formation in porous materials. The current work attempts to understand the formation, breaking, and survival of rings using a computational approach. We have used the reaction ensemble Monte Carlo simulation technique and studied silica polymerization starting from monomers to inter-connected large silica clusters in dilute and concentrated silica systems. We calculated various properties of representative smaller and bigger rings at different stages of polymerization. We found that smaller rings form in the initial polymerization stages and larger ring sizes appear at later stages. The smaller rings have a larger residence time than the bigger rings in the silica system, and the residence time changes with the polymerization stage. Both smaller and bigger rings have a shorter residence time in the dilute system than the concentrated silica system. As a result, ring formation and breaking kinetics are faster in the dilute silica system, which causes reorganization within the silica cluster leading to a dense cluster. A slow reorganization of rings in the concentrated silica system is observed, due to which clusters retain their random, branched configuration and porous region within the cluster. We also investigated a series of ring formation and breaking steps to understand the formation mechanism of isolated and grouped rings in the studied silica systems. We found that rings form and break by all possible reactions during ring-formation and cluster-aggregation stages. In contrast, only one reaction is dominant in the initial and aging stages of polymerization. The concentration of silica affects the formation of isolated rings, whereas the kinetics of a grouped ring is not significantly altered. Detailed insights into the reaction dynamics of rings at various stages of polymerization would be helpful in the rational design of porous silica polymorphs.

Control of Structural Hydrophobicity and Cation Solvation on Interlayer Water Transport during Clay Dehydration

Swelling clay hydration/dehydration is important to many environmental and industrial processes. Experimental studies usually probe equilibrium hydration states in an averaged manner and thus cannot capture the fast water transport and structural change in interlayers during hydration/dehydration. Using molecular simulations and thermogravimetric analyses, we observe a two-stage dehydration process. The first stage is controlled by evaporation at the edges: water molecules near hydrophobic sites and the first few water molecules of the hydration shell of cations move fast to particle edges for evaporation. The second stage is controlled by slow desorption of the last 1-2 water molecules from the cations and slow transport through the interlayers. The two-stage dehydration is strongly coupled with interlayer collapse and the coordination number changes of cations, all of which depend on layer charge distribution. This mechanistic interpretation of clay dehydration can be key to the coupled chemomechanical behavior in natural/engineered barriers.

Promoting Electroosmotic Water Flow through a Carbon Nanotube by Weakening the Competition between Cations and Anions in a Lateral Electric Field

Understanding the electroosmotic flow through a nanochannel is essential to the design of novel nanofluidic devices, ranging from desalination to nanometer water pumps. Nonetheless, the competition between cation and anion in electric fields inevitably leads to a limited pumping of water, and thus weakening their competition could be a new avenue for the fundamental control of water transport. In this work, through a series of molecular dynamics simulations, we find a surprising phenomenon in which under the drive of a traditional longitudinal electric field, an additional lateral electric field can significantly weaken the competitive transport of a cation and anion through a carbon nanotube, which spontaneously leads to a massive increase in electroosmotic water flux. Specifically, with the increase in the lateral electric field, the anion flux exhibits an almost linear reduction, and the cation flux is stable and can even be enhanced. As a result, the net water flux along the cation direction increases significantly. The key to this unexpected phenomenon lies in the size and mobility difference between the cation and anion. The anion is larger and has greater mobility and is thus more susceptible to the lateral electric field, which ultimately leads to the reduction of its flux. For different ion types and CNT lengths, we can observe similar electropumping phenomenon, where the friction force induced by the lateral electric field becomes nontrivial for long CNTs. Our results provide a new route to tune the competitive transport of cations and anions and should be useful for the design of novel electroosmotic pumps.

Structures of cationic and anionic polyelectrolytes in aqueous solutions: the sign effect

In this study, we use molecular dynamics simulation to explore the structures of anionic and cationic polyelectrolytes in aqueous solutions. We first confirm the significantly stronger solvation effects of single anions compared to cations in water at the fixed ion radii, due to the reversal orientations of asymmetric dipolar H₂O molecules around the ions. Based on this, we demonstrate that the solvation discrepancy of cations/anions and electrostatic correlations of ionic species can synergistically cause the nontrivial structural difference between single anionic and cationic polyelectrolytes. The cationic polyelectrolyte shows an extended structure whereas the anionic polyelectrolyte exhibits a collapsed structure, and their structural differences decline with increasing the counterion size. Furthermore, we corroborate that multiple cationic polyelectrolytes or multiple anionic polyelectrolytes can exhibit largely differential molecular architectures in aqueous solutions. In the solvation dominant regime, the polyelectrolyte solutions exhibit uniform structures; whereas, in the electrostatic correlation dominant regime, the polyelectrolyte solutions exhibit heterogeneous structures, in which the likely charged chains microscopically aggregate through counterion condensations. Increasing the intrinsic chain rigidity causes polyelectrolyte extension and hence moderately weakens the inter-chain clustering. Our work highlights the various, unique structures and molecular architectures of polyelectrolytes in solutions caused by the multi-body correlations between polyelectrolytes, counterions and asymmetric dipolar solvent molecules, which provides insights into the fundamental understanding of ion-containing polymers.

A new approach towards the Debye length challenge for specific and label-free biological sensing based on field-effect transistors

Biologically-modified field-effect transistors (BioFETs) are promising platforms for specific and label-free biosensing due to their sub-micron footprint suitable for multiplexing in ultra-small samples, low noise levels, inherent amplification, etc. Debye screening length is a well-recognized challenge for any BioFET-based technology. The screening length is the smallest at the double layer, where the solution ion population is higher than the bulk population. One way to address the small double layer screening length is to electrostatically modify the potential drop across the solution such as to minimize the potential drop over the double layer. This will decrease the population of the double layer ions and increase the screening length. However, this is not possible with BioFETs as voltage application to the reference electrode simultaneously affects both the double layer and the BioFET conducting channel. The current study addresses the screening length challenge with the novel Meta-Nano-Channel (MNC) BioFET. The MNC BioFET, which is fabricated in a complementary-metal-oxide-silicon (CMOS) process, allows decoupling of the electrostatics of the double layer from the electrodynamics of the conducting channel. The study explores the mechanism of sensing with the MNC BioFET, and demonstrates how the double layer can be electrostatically tuned in order to optimize the screening length without affecting the conducting channel. Finally, specific and label-free sensing of 10 ng ml^-1 prostate specific antigen (PSA) is demonstrated. It is shown that by electrostatically increasing the double layer screening length, the sensing signal increases from 70 mV to 133 mV.

Review on metal dissolution characteristics and harmful metals recovery from electronic wastes by supercritical water

Supercritical water (SCW) technology can be applied as an efficient and environment-friendly method to recover toxic or complex chemical wastes. Separation and chemical reactions under supercritical conditions may be realized by changing the temperature, pressure, and other operating parameters to adjust the physical and chemical properties of water. However, salt deposition and corrosion are often encountered during the treatment of inorganic substances, which will hinder the commercial applications of SCW technology. The solubility of salt in high pressure/temperature water forms the theoretical basis for studying the recovery of metal salts in supercritical water and understanding salt deposition. Therefore, this work systematically and objectively reviews different research methods used to analyze salt solubility in high pressure/temperature water, including the experimental method, prediction theoretical modeling, and computer simulation method; the research status and existing data of this parameter are also analyzed. The purpose of this review is to provide ideas and references for follow-up research by providing a comprehensive overview of salt solubility research methods and the current situation. Suggestions for more efficient metal recovery through technology integration are also provided.

A systematic and critical review of application of molecular dynamics simulation in low salinity water injection

Low Salinity Water Injection (LSWI) has been a well-researched EOR method, with several experimental and theoretical scientific papers reported in the literature over the past few decades. Despite this, there is still an ongoing debate on dominant mechanisms behind this complex EOR process, and some issues remain elusive. Part of the complexity arises from the scale of investigation, which spans from sub-pore scale (atomic and electronic scale) to pore scale, core scale, and reservoir scale. Molecular Dynamics (MD) simulation has been used as a research tool in the past decade to investigate the nano-scale interactions among reservoir rock (e.g., calcite, silica), crude oil, and brine systems in presence of some impurities (e.g., clay minerals) and additives (e.g., nanoparticles). In this paper, fundamental concepts of MD simulation and common analyses driven by MD are briefly reviewed. Then, an overview of molecular models of the most common minerals encountered in petroleum reservoirs: quartz, calcite, and clay, with their most common types of potential function, is provided. Next, a critical review and in depth analysis of application of MD simulations in LSWI process in both sandstone and carbonate reservoirs in terms of sub-pore scale mechanisms, namely electrical double layer (EDL) expansion, multi-ion exchange (MIE), and cation hydration, is presented to scrutinize role of salinity, ionic composition, and rock surface chemistry from an atomic level. Some inconsistencies observed in the literature are also highlighted and the reasons behind them are explained. Finally, a future research guide is provided after critically discussing the challenges and potential of the MD in LSWI to shed more light on governing mechanisms behind LSWI by enhancing the reliability of MD outcomes in future researches. Such insights can be used for design of new MD researches with complementary experimental studies at core scale to capture the main mechanisms behind LSWI.

Recent advances in the 3D printing of ionic electroactive polymers and core ionomeric materials

The recent advances in the 3D printing, or additive manufacturing, of ionic electroactive polymers (EAP) and their future applications.

The influence of L-aspartic acid on calcium carbonate nucleation and growth revealed by in situ liquid phase TEM

In situ transmission electron microscopy has permitted the study of nanomaterials in liquid environments with high spatial and temporal resolutions, allowing chemical reaction visualization in real time. The aim of...

Effect of charge inversion on nanoconfined flow of multivalent ionic solutions

A comprehensive understanding of fluid dynamics of dilute electrolyte solutions in nanoconfinement is essential to develop more efficient nanofluidic devices. In nanoconduits, the electrical double layer can occupy a considerable...

Nonuniqueness of hydrodynamic dispersion revealed using fast 4D synchrotron x-ray imaging

Experimental and field studies reported a significant discrepancy between the cleanup and contamination time scales, while its cause is not yet addressed. Using high-resolution fast synchrotron x-ray computed tomography, we characterized the solute transport in a fully saturated sand packing for both contamination and cleanup processes at similar hydrodynamic conditions. The discrepancy in the time scales has been demonstrated by the nonuniqueness of hydrodynamic dispersion coefficient versus injection rate (Péclet number). Observations show that in the mixed advection-diffusion regime, the hydrodynamic dispersion coefficient of cleanup is significantly larger than that of the contamination process. This nonuniqueness has been attributed to the concentration-dependent diffusion coefficient during the cocurrent and countercurrent advection and diffusion, present in contamination and cleanup processes. The new findings enhance our fundamental understanding of transport processes and improve our capability to estimate the transport time scales of chemicals or pollution in geological and engineering systems.

Long-range DNA-water interactions

DNA functions only in aqueous environments and adopts different conformations depending on the hydration level. The dynamics of hydration water and hydrated DNA leads to rotating and oscillating dipoles that, in turn, give rise to a strong megahertz to terahertz absorption. Investigating the impact of hydration on DNA dynamics and the spectral features of water molecules influenced by DNA, however, is extremely challenging because of the strong absorption of water in the megahertz to terahertz frequency range. In response, we have employed a high-precision megahertz to terahertz dielectric spectrometer, assisted by molecular dynamics simulations, to investigate the dynamics of water molecules within the hydration shells of DNA as well as the collective vibrational motions of hydrated DNA, which are vital to DNA conformation and functionality. Our results reveal that the dynamics of water molecules in a DNA solution is heterogeneous, exhibiting a hierarchy of four distinct relaxation times ranging from ∼8 ps to 1 ns, and the hydration structure of a DNA chain can extend to as far as ∼18 Å from its surface. The low-frequency collective vibrational modes of hydrated DNA have been identified and found to be sensitive to environmental conditions including temperature and hydration level. The results reveal critical information on hydrated DNA dynamics and DNA-water interfaces, which impact the biochemical functions and reactivity of DNA.