Dissolution dynamics of NaCl nanocrystal in liquid water

NaCl aerosol filtration over filter media with intrafiber porosity: can filtrate capacity be increased by wicking into this pore volume?

This paper investigates a novel fiber-based filter media wherein a NaCl filtrate is collected and reservoired not only onto the surfaces of the fibers and within their inter-fiber voidage but also within the internal porosity of high pore volume nanoporous fibers or vapor grown carbon nanofibers (VGCF) floc used to fabricate the media. This transport process is shown to occur through a NaCl dissolution into the water-filled nanopores of the fiber and a subsequent intra-fiber wicking phenomenon. The study further elucidates two distinct NaCl accommodation mechanisms which are uniquely available to filter media containing nanoporous intrafiber porosity: (1) wicking and capillary condensation of liquid NaCl aerosols directly into the intrafiber pores at high RH, and (2) dissolution of otherwise solid NaCl aerosols deposited onto fiber surfaces (at low RH) into the interior nanopores of the fiber because these pores (when hydrophilic) are saturated with water (even at low RH). To investigate these two mechanistic regimes, various media were fabricated possessing multiscale porosity in the form of: (i) embedded flocs of VGCFs (4.108 cm3gm-1pore volume), (ii) hydrophilic and high pore volume activated carbon fibers (ACFs, 0.950 cm3gm-1) and (iii) solid graphite fibers. These media were then comparatively evaluated toward NaCl aerosol filtration at different relative humidities. Pressure drop measurements versus filtrate accumulation and SEM-EDAX VGCF demonstrated the location and transport of NaCl into the intrafiber voidage. Media containing both VGCF floc and ACF accumulated 1200% more NaCl at low RH (and a specified pressure drop) than similar media prepared from non-porous graphite fibers, with an additional 315% increase from low to high RH. A Gibbs free energy driving force model is provided to illustrate the driving forces favoring water condensation into the nanopores and solid NaCl aerosol dissolution into the water phase. Filtration efficiency and quality factor assessments for the various media are also systematically evaluated to demonstrate the observed mechanistics.

Controlled dissolution of a single ion from a salt interface

Interactions between monatomic ions and water molecules are fundamental to understanding the hydration of complex polyatomic ions and ionic process. Among the simplest and well-established ion-related reactions is dissolution of salt in water, which is an endothermic process requiring an increase in entropy. Extensive efforts have been made to date; however, most studies at single-ion level have been limited to theoretical approaches. Here, we demonstrate the salt dissolution process by manipulating a single water molecule at an under-coordinated site of a sodium chloride film. Manipulation of molecule in a controlled manner enables us to understand ion-water interaction as well as dynamics of water molecules at NaCl interfaces, which are responsible for the selective dissolution of anions. The water dipole polarizes the anion in the NaCl ionic crystal, resulting in strong anion-water interaction and weakening of the ionic bonds. Our results provide insights into a simple but important elementary step of the single-ion chemistry, which may be useful in ion-related sciences and technologies.

Anomalous Phase Behaviors of Monolayer NaCl Aqueous Solutions Induced by Effective Coulombic Interactions within Angstrom-Scale Slits

Interests in subnanofluidic devices have called for molecular dynamics (MD) simulation studies of the thermodynamic behavior of monolayer salt solution within angstrom-scale slits. However, it still remains a grand challenge to accurately describe the Coulombic interactions by incorporating the effects of charge transfer and electronic dielectric screening. Herein, by using the electronic continuum model, where the effective ion charges are fine-tuned with a scaling factor of λ, we present simulation evidence that the effective Coulombic interactions among Na⁺/Cl^- ions can strongly affect the behavior of monolayer ionic aqueous solution. Our microsecond-scale MD simulations show that only the counterions with moderate effective charges (0.3 ≤ λ ≤ 0.8) can dissolve in monolayer water, whereas the high effective charges (λ ≥ 0.85) induce ions to assemble into monolayer nanocrystals, and ions with the low effective charges (λ ≤ 0.2) exhibit gas-like nanobubble. These findings could provide deeper insights into the physical chemistry behind subnanofluidic iontronic devices.

Two-dimensional monolayer salt nanostructures can spontaneously aggregate rather than dissolve in dilute aqueous solutions

It is well known that NaCl salt crystals can easily dissolve in dilute aqueous solutions at room temperature. Herein, we reported the first computational evidence of a novel salt nucleation behavior at room temperature, i.e., the spontaneous formation of two-dimensional (2D) alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement. Microsecond-scale classical molecular dynamics (MD) simulations showed that NaCl or LiCl, initially fully dissolved in confined water, can spontaneously nucleate into 2D monolayer nanostructures with either ordered or disordered morphologies. Notably, the NaCl nanostructures exhibited a 2D crystalline square-unit pattern, whereas the LiCl nanostructures adopted non-crystalline 2D hexagonal ring and/or zigzag chain patterns. These structural patterns appeared to be quite generic, regardless of the water and ion models used in the MD simulations. The generic patterns formed by 2D monolayer NaCl and LiCl nanostructures were also confirmed by ab initio MD simulations. The formation of 2D salt structures in dilute aqueous solution at room temperature is counterintuitive. Free energy calculations indicated that the unexpected spontaneous salt nucleation behavior can be attributed to the nanoscale confinement and strongly compressed hydration shells of ions.

Aqueous solutions under nanoscale confinement exhibit interesting physicochemical properties. This work reports evidence on the spontaneous formation of two-dimensional alkali chloride crystalline/non-crystalline nanostructures in dilute aqueous solution under nanoscale confinement by computer simulations.

Ion Distribution and Hydration Structure at Solid-Liquid Interface between NaCl Crystal and Its Solution

The interface structure between NaCl crystal and its solution has been investigated at the saturated concentration of 298 K by molecular dynamics simulations. We have found that there are many fine structures at this complex interface. Near the surface of crystal, most of Na⁺ only coordinate with water molecules, while almost all Cl^- coordinate with Na⁺ in addition to water molecules. An ion coordinating with more water molecules is farther away from the epitaxial position of lattice. As approaching to the interface, the first hydration shell of ions has the tendency of being ordered, while the orientation of dipole of water molecules in the first hydration shell becomes more disordered than that in the solution. Generally, the first hydration shell of Na⁺ is less affected by nearest Cl^-, whereas the first hydration shell of Cl^- is significantly affected by nearest Na⁺.

Dissolution of Sodium Halides by Confined Water on Au(111) via Langmuir-Hinshelwood Process

Salt dissolution is generally encountered in widespread phenomena in nature. As a typical case, the dissolution of NaCl has been widely investigated in aqueous environment, while the process and mechanism of the on-surface dissolution may differ from that in solution and remain to be explored. Herein, we model a NaCl dissolution process on the Au(111) surface with confined water at room temperature (RT) and above. With the assistance of adenine molecules as water reservoir and carrier, the dissolution of NaCl is achieved above RT via the Langmuir-Hinshelwood mechanism rather than the Eley-Rideal one, along with the selective formation of stable Cl^- hydrates, which desorb from the surface in the next step. To explore the generality, such a strategy has been extended to other sodium halide systems ( e. g., NaBr and NaI), and expectedly, the dissolution of sodium halides is also achieved by forming stable Br^- and I^- hydrates via the Langmuir-Hinshelwood process.

Ion dissolution mechanism and kinetics at kink sites on NaCl surfaces

Desolvation barriers are present for solute-solvent exchange events, such as ligand binding to an enzyme active site, during protein folding, and at battery electrodes. For solution-grown crystals, desolvation at kink sites can be the rate-limiting step for growth. However, desolvation and the associated kinetic barriers are poorly understood. In this work, we use rare-event simulation techniques to investigate attachment/detachment events at kink sites of a NaCl crystal in water. We elucidate the desolvation mechanism and present an optimized reaction coordinate, which involves one solute collective variable and one solvent collective variable. The attachment/detachment pathways for Na⁺ and Cl^- are qualitatively similar, with quantitative differences that we attribute to different ion sizes and solvent coordination. The attachment barriers primarily result from kink site desolvation, while detachment barriers largely result from breaking ion-crystal bonds. We compute ion detachment rates from kink sites and compare with results from an independent study. We anticipate that the reaction coordinate and desolvation mechanism identified in this work may be applicable to other alkali halides.

Spontaneous Formation of KCl Aggregates in Biomolecular Simulations: A Force Field Issue?

Realistic all-atom simulation of biological systems requires accurate modeling of both the biomolecules and their ionic environment. Recently, ion nucleation phenomena leading to the rapid growth of KCl or NaCl clusters in the vicinity of biomolecular systems have been reported. To better understand this phenomenon, molecular dynamics simulations of KCl aqueous solutions at three (1.0, 0.25, and 0.10 M) concentrations were performed. Two popular water models (TIP3P and SPC/E) and two Lennard-Jones parameter sets (AMBER and Dang) were combined to produce a total of 80 ns of molecular dynamics trajectories. Results suggest that the use of the Dang cation Lennard-Jones parameters instead of those adopted by the AMBER force-field produces a more accurate description of the ionic solution. In the later case, formation of salt aggregates is probably indicative of an artifact resulting from misbalanced force-field parameters. Because similar results were obtained with two different water parameter sets, the simulations exclude a water model dependency in the formation of anomalous ionic clusters. Overall, the results strongly suggest that for accurate modeling of ions in biomolecular systems, great care should be taken in choosing balanced ionic parameters even when using the most popular force-fields. These results invite a reexamination of older data obtained using available force-fields and a thorough check of the quality of current parameters sets by performing simulations at finite (>0.25 M) instead of minimal salt conditions.

Determination of alkali and halide monovalent ion parameters for use in explicitly solvated biomolecular simulations

Alkali (Li(+), Na(+), K(+), Rb(+), and Cs(+)) and halide (F(-), Cl(-), Br(-), and I(-)) ions play an important role in many biological phenomena, roles that range from stabilization of biomolecular structure, to influence on biomolecular dynamics, to key physiological influence on homeostasis and signaling. To properly model ionic interaction and stability in atomistic simulations of biomolecular structure, dynamics, folding, catalysis, and function, an accurate model or representation of the monovalent ions is critically necessary. A good model needs to simultaneously reproduce many properties of ions, including their structure, dynamics, solvation, and moreover both the interactions of these ions with each other in the crystal and in solution and the interactions of ions with other molecules. At present, the best force fields for biomolecules employ a simple additive, nonpolarizable, and pairwise potential for atomic interaction. In this work, we describe our efforts to build better models of the monovalent ions within the pairwise Coulombic and 6-12 Lennard-Jones framework, where the models are tuned to balance crystal and solution properties in Ewald simulations with specific choices of well-known water models. Although it has been clearly demonstrated that truly accurate treatments of ions will require inclusion of nonadditivity and polarizability (particularly with the anions) and ultimately even a quantum mechanical treatment, our goal was to simply push the limits of the additive treatments to see if a balanced model could be created. The applied methodology is general and can be extended to other ions and to polarizable force-field models. Our starting point centered on observations from long simulations of biomolecules in salt solution with the AMBER force fields where salt crystals formed well below their solubility limit. The likely cause of the artifact in the AMBER parameters relates to the naive mixing of the Smith and Dang chloride parameters with AMBER-adapted Aqvist cation parameters. To provide a more appropriate balance, we reoptimized the parameters of the Lennard-Jones potential for the ions and specific choices of water models. To validate and optimize the parameters, we calculated hydration free energies of the solvated ions and also lattice energies (LE) and lattice constants (LC) of alkali halide salt crystals. This is the first effort that systematically scans across the Lennard-Jones space (well depth and radius) while balancing ion properties like LE and LC across all pair combinations of the alkali ions and halide ions. The optimization across the entire monovalent series avoids systematic deviations. The ion parameters developed, optimized, and characterized were targeted for use with some of the most commonly used rigid and nonpolarizable water models, specifically TIP3P, TIP4P EW, and SPC/E. In addition to well reproducing the solution and crystal properties, the new ion parameters well reproduce binding energies of the ions to water and the radii of the first hydration shells.

Ions in water: from ion clustering to crystal nucleation

The clustering and nucleation of ions in aqueous solutions results from a competition between ion hydration and association. Molecular dynamics simulations of aqueous NaCl solutions are used to investigate ion clustering with a force field adjusted to reproduce experimental properties of the pure NaCl crystal and melt, and of concentrated solutions. The simulation results point to strong sensitivity of the nucleation mechanism to small changes in the force field. We report the numerical evidence for rapid crystal nucleation near saturation or under supercritical conditions.

Atomistic nature of NaCl nucleation at the solid-liquid interface

The early stage of heterogeneous nucleation of NaCl from supersaturated NaCl aqueous solution at the water-NaCl (001) interface has been investigated by molecular dynamics simulations. The critical size of the nuclei for spontaneous growth was found to be as small as two atoms (a Na(+)-Cl(-) ion pair) at high supersaturation. Due to the presence of a relatively stable water network and the effect of the hydration force at the interface, the stable nuclei formed on the NaCl (001) are found to contain more Na(+) ions than Cl(-) ions. The different deposition characteristics of the Na(+) and Cl(-) solutes lead to a positively charged substrate and thus may introduce another driving force for nucleation besides the level of solution supersaturation. The role of water was further confirmed by comparison with NaCl epitaxy growth in the vacuum.

Solubility of KF and NaCl in water by molecular simulation

The solubility of two ionic salts, namely, KF and NaCl, in water has been calculated by Monte Carlo molecular simulation. Water has been modeled with the extended simple point charge model (SPC/E), ions with the Tosi-Fumi model and the interaction between water and ions with the Smith-Dang model. The chemical potential of the solute in the solution has been computed as the derivative of the total free energy with respect to the number of solute particles. The chemical potential of the solute in the solid phase has been calculated by thermodynamic integration to an Einstein crystal. The solubility of the salt has been calculated as the concentration at which the chemical potential of the salt in the solution becomes identical to that of the pure solid. The methodology used in this work has been tested by reproducing the results for the solubility of KF determined previously by Ferrario et al. [J. Chem. Phys. 117, 4947 (2002)]. For KF, it was found that the solubility of the model is only in qualitative agreement with experiment. The variation of the solubility with temperature for KF has also been studied. For NaCl, the potential model used predicts a solubility in good agreement with the experimental value. The same is true for the hydration chemical potential at infinite dilution. Given the practical importance of solutions of NaCl in water the model used in this work, whereas simple, can be of interest for future studies.