Salt Partitioning Research Articles

Benchmarking Forcefields for Molecular Dynamics Simulations of Polyamide-based Reverse-Osmosis Membranes

Molecular dynamics simulations offer unique insights about solvent and solute transport through the active layer of reverse-osmosis membranes at sub-nanometer to nanometer length scales. Over the last two decades, several cross-linked polyamide membranes, formed by trimesoyl chloride and m-phenylenediamine, have been simulated with different forcefields and water models. However, a clear rationale for choosing a forcefield-water combination is missing. In this work, the accuracy of eleven forcefield-water combinations is evaluated by a direct comparison of properties against experimentally synthesized polyamide membranes with similar chemical compositions. Overall, six forcefields and three water models are compared. The polyamide membranes are simulated in equilibrium in both dry and hydrated states, as well as in a non-equilibrium reverse osmosis. The best-performing forcefields predicted the experimental pure water permeability of 3D printed polyamide membranes within a 95% confidence interval. Finally, the best forcefield was used to elucidate membrane properties for a range of desalination conditions. At experimentally relevant pressure pure-water permeability was validated and dense interconnected free volume regions (pores) were observed in the membrane that connected feed- and permeate-side. The desalination studies demonstrated increased salt partitioning with feed-side pressure, however, at very high pressures partitioning decreased due to membrane compaction.

Practical limits of the quartz crystal microbalance and surface plasmon resonance for elucidating salt or ion partitioning into membrane polymers

Determining the partitioning of sodium chloride into polyamide (PA) reverse osmosis membranes used for seawater desalination is indispensable for optimizing the salt retention of the membranes. Furthermore, the recent quest for ion-selective membranes requires physico-chemical characterization techniques capable of elucidating partitioning phenomena of ions into membrane polymers in general. Quartz-crystal microbalance with dissipation monitoring (QCM-D) has been explored for this purpose as it has in theory a sensitivity in the range of pg/cm2 and is an accessible and apparently simple method to use. However, it is a technique that intrinsically lacks an internal reference, and it is affected by liquid bulk changes. As the conventional applications of QCM-D are of biophysical nature where changes in solute concentrations are in the nano to micromolar range, we investigated systematically in how far QCM-D yields reliable results for determining salt partitioning coefficients at concentrations relevant for the industrial-scale. We could clearly demonstrate that QCM-D possibly detects salt absorption only at salt concentrations higher than that in seawater (0.6 M). Furthermore, the variation in the liquid bulk response was so significant that it overlapped with a possibly perceived response due to salt absorption. Therefore, we concluded that salt partitioning coefficients derived from QCM-D data have a low confidence and, in any case, should always be reported accompanied by the raw data. We corroborated these findings with multi-parameter surface plasmon resonance (MP-SPR) which widely confirmed the trends observed with QCM-D.

Coacervate Formation in Dilute Aqueous Solutions of Inorganic Molecular Clusters with Simple Divalent Countercations.

We report a complex coacervate formed by a 2.5 nm-diameter, rigid uranyl peroxide molecular cluster (Li68K12(OH)20)[UO2(O2)OH]60, U6060-) and SrCl2 salt in dilute aqueous solutions, including its location in the phase diagram, composition, rheological features, and critical conditions for phase transitions. In this coacervate, the Sr2+ cations are a major building component, and the coacervate phase covers a substantial region of the phase diagram. This coacervate demonstrates features that differ from traditional coacervates formed by oppositely charged long-chain polyelectrolytes, especially in its formation mechanism, dehydration, enhancement of mechanical strength with increasing ionic strength, and the change of salt partition preference into the coacervate and supernatant phases with ionic strength.

Supramolecular hydrogels based on hydrogen bonding: Impact of ionic comonomers

AbstractThe polymerization of N‐acryloyl glycineamide (NAGA) in water results in the creation of robust supramolecular hydrogels, exhibiting a network structure through inter‐chain crosslinking formed by hydrogen bonds between NAGA units. The stability of these hydrogen bonds in water is based on the aggregation of NAGA units into microdomains or clusters, thereby effectively shielding the hydrogen bonds from the surrounding aqueous medium. Beyond the inherent water absorption characteristics of these hydrogels, the unique ability for the dissociation and re‐association of their physical crosslinks imparts features such as reversible swelling and shrinking as well as self‐healing and remolding capabilities. In this study, we synthesize a series of supramolecular hydrogels through copolymerization of NAGA and ionic sodium acrylate comonomers for a dual purpose: first, to regulate water absorption levels, and second, to introduce a salt partitioning effect into the hydrogels during their swelling in salt solution. Our findings indicate that the properties of these hydrogels are intricately influenced by the volume fraction of the solid network, the concentration of ionic units, and temperature. Notably, supramolecular samples with ionic units exhibit a 16% salt rejection in a single cycle of salt partitioning during their swelling in a 1 g·L−1 sodium chloride solution.

Molecular and electrostatic origins of mixed salt partitioning phenomena in uncharged poly(ethylene oxide)-based membranes

Despite the increasing interest in membranes for water purification and resource recovery from aqueous electrolyte mixtures, few studies rigorously investigate the influence of salt solution composition (e.g., mole fraction and total salt concentration) on membrane material properties such as partition coefficients. Here, we systematically investigate the influence of solution composition on the partitioning of ionic species from binary aqueous salt solutions into crosslinked poly(ethylene glycol) diacrylate membranes at fixed ionic strength. These results are compared to partition coefficients observed for pure salt solutions. We observe large systematic deviations between the pure salt and mixed salt ion partition coefficients, indicating that the single salt partition coefficients that are often reported do not describe the more complex and realistic mixed salt scenario. Free energy calculations from molecular dynamics simulations, coupled with equilibrium thermodynamic modeling, indicate that mixed salt partitioning trends result from (i) different ions exhibiting different affinities for the membrane and (ii) the constraint of global electrical neutrality within the membrane. With this physical insight, we develop a simple approach to predicting the concentration of various ions in a membrane equilibrated with a mixed salt solution which requires no adjustable parameters.

Application of the Born Model to Describe Salt Partitioning in Hydrated Polymers.

The classic Born model can be used to predict salt partitioning properties observed in hydrated polymers, but there are often significant quantitative discrepancies between these predictions and the experimental data. Here, we use an updated version of the Born model, reformulated to account for the local environment and mesh size of a hydrated polymer, to describe previously published NaCl, KCl, and LiCl partitioning properties of model cross-linked poly(ethylene glycol) diacrylate polymers. This reformulated Born model describes the influence of polymer structure (i.e., network mesh size and its relationship with water content) and external salt concentration on salt partitioning in the polymers with a significant improvement relative to the classic Born model. The updated model most effectively describes NaCl partitioning properties and provides an additional fundamental understanding of salt partitioning processes, for NaCl, KCl, and LiCl, in hydrated polymers that are of interest for a variety of environmental and biological applications.

Disrupting the Hofmeister bias in salt liquid-liquid extraction with an arylethynyl bisurea anion receptor.

Host-mediated liquid-liquid extraction is a convenient method for the separation of inorganic salts. However, selective extraction of an anion, regardless of its hydrophilicity or lipophilicity as qualitatively described by its place in the Hofmeister series, remains challenging. Herein we report the complete disruption of the Hofmeister-based ordering of anions in host-mediated extraction by a rigidified tweezer-type receptor possessing remarkably strong anion-binding affinity under the conditions examined. Experiments introduce a convenient new method for determination of anion binding using phosphorus inductively coupled plasma mass spectrometry (ICP-MS) to measure extraction of tetra-n-butylphosphonium (TBP+) salts from water into nitrobenzene, specifically examining the disrupting effect of the added arylethynyl bisurea anion receptor. In the absence of the receptor, the salt partitioning follows the expected Hofmeister-type ordering favoring the larger, less hydrated anions; the analysis yields the value -24 kJ mol-1 for the standard Gibbs energy of partitioning of TBP+ cation from water into nitrobenzene at 25 °C. Selectivity is markedly changed by the addition of receptor to the nitrobenzene and is concentration dependent, giving rise to three selectivity regimes. We then used SXLSQI liquid-liquid equilibrium analysis software developed at Oak Ridge National Laboratory to fit host-mediated extraction equilibria for TBP+ salts of Cl-, Br-, I-, and NO3- to the distribution data. While the reverse-Hofmeister 1 : 1 binding of the anions by the receptor effectively cancels the Hofmeister selectivity of the TBPX partitioning into nitrobenzene, formation of unexpected 2 : 1 receptor : anion complexes favoring Cl- and Br- dominates the selectivity at elevated receptor concentrations, producing the unusual order Br- > Cl- > NO3- > I- in anion distribution wherein a middle member of the series is selected and the most lipophilic anion is disfavored. Density functional theory calculations confirmed the likelihood of forming 2 : 1 complexes, where Cl- and Br- are encapsulated by two receptors adopting energetically competitive single or double helix structures. The calculations explain the rare non-Hofmeister preference for Br-. This example shows that anion receptors can be used to control the selectivity and efficiency of salt extraction regardless of the position of the anion in the Hofmeister series.

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Control of Lithium Salt Partitioning, Coordination, and Solvation in Vitrimer Electrolytes

Significance of Co-ion Partitioning in Salt Transport through Polyamide Reverse Osmosis Membranes.

Salt permeability of polyamide reverse osmosis (RO) membranes has been shown to increase with increasing feed salt concentration. The dependence of salt permeability on salt concentration has been attributed to the variation of salt partitioning with feed salt concentration. However, studies using various analytical techniques revealed that the salt (total ion) partitioning coefficient decreases with increasing salt concentration, in marked contrast to the observed increase in salt permeability. Herein, we thoroughly investigate the dependence of total ion and co-ion partitioning coefficients on salt concentration and solution pH. The salt partitioning is measured using a quartz crystal microbalance (QCM), while the co-ion partitioning is calculated from the measured salt partitioning using a modified Donnan theory. Our results demonstrate that the co-ion and total ion partitioning behave entirely differently with increasing salt concentrations. Specifically, the co-ion partitioning increased fourfold, while total ion partitioning decreased by 60% as the salt (NaCl) concentration increased from 100 to 800 mM. The increase in co-ion partitioning with increasing salt concentration is in accordance with the increasing trend of salt permeability in RO experiments. We further show that the dependence of salt and co-ion partitioning on salt concentration is much more pronounced at a higher solution pH. The good co-ion exclusion (GCE) model─derived from the solution-friction model─is used to calculate the salt permeability based on the co-ion partitioning coefficients. Our results show that the GCE model predicts the salt permeabilities in RO experiments relatively well, indicating that co-ion partitioning, not salt partitioning, governs salt transport through RO membranes. Our study provides an in-depth understanding of ion partitioning in polyamide RO membranes and its relationship with salt transport.

Ionic Conductivity, Salt Partitioning, and Phase Separation in High-Dielectric Contrast Polyether Blends and Block Polymer Electrolytes

Ionic Conductivity, Salt Partitioning, and Phase Separation in High-Dielectric Contrast Polyether Blends and Block Polymer Electrolytes

The effect of monomer polarizability on the stability and salt partitioning in model coacervates.

Coacervation of charged polymer chains has been a topic of major interest in both polymer and biological sciences, as it is a subset of a phenomenon called liquid-liquid phase separation (LLPS). In this process the polymer-rich phase separates from the polymer-lean supernatant while still maintaining its liquid-like properties. LLPS has been shown to play a crucial role in cellular homeostasis by driving the formation of membraneless organelles. It also has the potential to be harnessed to aid in novel therapeutical applications. Recent studies have demonstrated that there is no one simple mechanism which drives LLPS, which is instead a result of the combined effect of electrostatic, dipolar, hydrophobic, and other weak interactions. Using coarse-grained polymer simulations we investigate the relatively unexplored effects of monomer polarizability and spatially varying dielectric constant on LLPS propensity, and these factors affect the properties of the resulting condensates. In order to produce spatial variations in the dielectric constant, all our simulations include explicit solvent and counterions. We demonstrate that polarizability has only a minor effect on the bulk behaviour of the condensates but plays a major role when ion partitioning and microstructure are considered. We observe that the major contribution comes from the nature of the neutral blocks as endowing them with an induced dipole changes their character from hydrophobic to hydrophilic. We hypothesize that the results of this work can aid in guiding future studies concerned with LLPS by providing a general framework and by highlighting important factors which influence LLPS.

Salt Partitioning in Cationic Thermoresponsive Hydrogels for Model‐Seawater Desalination

AbstractCharged hydrogels partially reject salt ions during swelling in a salt solution reservoir. This feature suggests applicability for the separation of salt ions from saline water for water desalination in a membrane‐free forward osmosis process. In this work, model charged cationic and thermoresponsive hydrogels are prepared and their potential for desalination of model seawater containing both monovalent and divalent ions with concentrations of 0.11–3.5 wt% is investigated. The recovery of adsorbed water, after partial salt rejection, is achieved by heating the hydrogels. The salt rejection increases upon increase of the charge density of the hydrogels and diminishes by increase of the concentration of the salt solution. In addition, the salt rejection during deswelling of the hydrogels is in the opposite of the overall process target, adding another challenge on the efficiency of the approach. Besides the experimental results, equations based on the Donnan theory are derived to predict the salt rejection of hydrogels during swelling and deswelling processes. While the theoretical values deviate from the experimental ones, the theory predict the overall trend well. Both experimental and theoretical results confirm that this approach has considerable potential for the desalination of saline water at low concentrations such as brackish waters.

Impact of Cation–Ligand Interactions on the Permselectivity of Ligand-Functionalized Polymer Membranes in Single and Mixed Salt Systems

We conduct molecular dynamics simulations to study the effects of cation–ligand interactions on salt permeation in ligand-functionalized polymer membranes. Our results indicate that salt partitioning into the membrane increases while the salt diffusivity decreases with enhancement in the cation–ligand interaction strength. Moreover, the increase in partitioning dominates the decline in salt diffusivity, leading to a net enhancement in salt permeation at higher cation–ligand interaction strengths. Equimolar binary mixed salt systems (having a common anion) generally exhibit much more selective partitioning of salts into the membrane as compared to their single salt counterparts. However, single salt systems exhibit much higher ratio of salt diffusivities than their mixed salt analogues. The permselectivity data closely resemble the trends in selective partitioning for both single and mixed salt systems, indicating that salt diffusivity selectivity plays a weaker role than salt partitioning selectivity in dictating the selective permeation of salts across the membrane.

Cation–Ligand Interactions Dictate Salt Partitioning and Diffusivity in Ligand-Functionalized Polymer Membranes

Membranes are an attractive alternative to current thermal separations due to their scalability and energy efficiency in desalinating water. Unfortunately, many of the conventional membrane materials available today are unable to differentiate between ionic solutes, especially alkali cations, compromising their use in ion–ion separations. Inspired by the ion-specific interactions exhibited by biological ion channels, recent research efforts have focused on synthesizing and characterizing new polymeric materials that incorporate ligands into polymer networks to bias solubility and/or diffusivity of one cationic species over another. Despite these efforts, little is known about the influence of incorporating ligands into polymer membranes on solubility and diffusivity of the complexing species. In this study, we first build a qualitative model of salt partitioning, diffusivity, and permeability in generic cation-complexing ligand-functionalized polymer membranes. Next, to validate our model and hypotheses, we perform atomistic molecular dynamics simulations of a 12-crown-4-functionalized membrane in the presence of alkali halide salts at low concentration. Generally, cation complexation enhances cation solubility but decreases diffusivity. Interestingly, the reduction in diffusivity is predicted to be larger than the enhancement in solubility for materials which operate by the mechanisms proposed in our physical picture, ultimately resulting in a reduction in the permeability of the selectively complexing ion.

Thermodynamic analysis of hydrogel swelling in aqueous sodium chloride solutions

A comprehensive thermodynamic model is presented for swelling and salt partitioning of poly (n-isopropyl acrylamide) hydrogels in aqueous sodium chloride (NaCl) solutions at 298 K. Together with a Helmholtz energy expression for network elastic energy, a modified electrolyte Nonradom Two-Liquid activity coefficient model is used to represent both the long-range electrostatic interactions and the short-range van der Waals interactions present in the gel phase. The model parameters include one network parameter per hydrogel system plus two binary parameters per interaction pair in the polymer–solvent-salt systems. With semiquantitave agreement with experimental data for both hydrogel swelling and salt partitioning up to salt saturation, the analysis suggests the ion hydration between NaCl and water is the root cause for hydrogel deswelling in aqueous NaCl solution at high NaCl concentrations, i.e., NaCl weight fraction >0.03. On the other hand, the strong repulsive interaction between NaCl and hydrogel polymer is the dominant factor for salt partitioning.

Interaction-based ion selectivity exhibited by self-assembled, cross-linked zwitterionic copolymer membranes

Water filtration membranes with advanced ion selectivity are urgently needed for resource recovery and the production of clean drinking water. This work investigates the separation capabilities of cross-linked zwitterionic copolymer membranes, a self-assembled membrane system featuring subnanometer zwitterionic nanochannels. We demonstrate that selective zwitterion-anion interactions simultaneously control salt partitioning and diffusivity, with the permeabilities of NaClO4, NaI, NaBr, NaCl, NaF, and Na2SO4 spanning roughly three orders of magnitude over a wide range of feed concentrations. We model salt flux using a one-dimensional transport model based on the Maxwell-Stefan equations and show that diffusion is the dominant mode of transport for 1:1 sodium salts. Differences in zwitterion-Cl- and zwitterion-F- interactions granted these membranes with the ultrahigh Cl-/F- permselectivity (PCl-/PF- = 24), enabling high fluoride retention and high chloride passage even from saline mixtures of NaCl and NaF.

Polyelectrolyte Complex Coacervation across a Broad Range of Charge Densities

Polyelectrolyte complex coacervates of homologous (co)polyelectrolytes with a near-ideally random distribution of a charged and neutral ethylene oxide comonomer were synthesized. The unique platform provided by these building blocks enabled an investigation of the phase behavior across charge fractions 0.10 ≤ f ≤ 1.0. Experimental phase diagrams for f = 0.30–1.0 were obtained from thermogravimetric analysis of complex and supernatant phases and contrasted with molecular dynamics simulations and theoretical scaling laws. At intermediate to high f, a dependence of polymer weight fraction in the salt-free coacervate phase (wP,c) of wP,c ∼ f0.37±0.01 was extracted; this trend was in good agreement with accompanying simulation predictions. Below f = 0.50, wP,c was found to decrease more dramatically, qualitatively in line with theory and simulations predicting an exponent of 2/3 at f ≤ 0.25. Preferential salt partitioning to either coacervate or supernatant was found to be dictated by the chemistry of the constituent (co)polyelectrolytes.

Salt partitioning in ionized, thermo-responsive hydrogels: perspective to water desalination.

Charged hydrogels are capable of swelling in aqueous salt solutions, whereby part of the salt ions is repelled due to the presence of fixed charged groups inside the hydrogel. This effect creates a concentration gradient between the absorbed solution and the surrounding fluid known as salt partitioning, offering a potential for these materials to be employed to desalinate saltwater. If the charged hydrogels are thermo-sensitive as well, then the purer, absorbed solution can be recovered by shrinking the hydrogels upon temperature change. To tailor that potential in water-purification and desalination applications, the main parameters influencing the salt partitioning, the deswelling of the hydrogels, and the recovery of water must be understood. In this paper, we analyze these factors based on equations derived from the Donnan theory. In addition, hydrogels composed of N-isopropyl acrylamide and acrylic acid are synthesized, and their salt rejection efficiency in a model desalination experiment is studied. A comparison of the experimental and the theoretical results demonstrates that the charge density of the hydrogels at their equilibrium swelling and the degree of water recovery are two parameters controlling the salt rejection efficiency. These parameters are individually controlled by the content of the ionic groups and the degree of cross-linking of the gel polymer network. In addition, the prediction of the theory and the experimental results demonstrate that the salt rejection efficiency can be significantly improved if a second water recovery step is performed by a secondary increase in the temperature in the deswelling process.

Phase Behavior of Partially Charged Polyelectrolyte Solutions with Salt: A Theoretical Study

AbstractWeakly (or partially) charged polyelectrolytes represent an important subset of the polyelectrolyte family. For instance, polycarboxylate‐based superplasticizers, or PCEs, are comb‐shaped polyelectrolytes possessing anionic backbones grafted with neutral side chains. PCEs play an important role in modern concrete production. However, their behavior in salt solutions remain poorly understood. The present work builds upon a recently developed liquid‐state theory for fully charged polyelectrolyte solutions and extends it to describe the phase behavior and salt partitioning of PCEs in a 2:1 salt solution. Previous studies have shown that there can be an additional short‐range attraction, often referred to as the “calcium‐binding” interaction between calcium ions and the negatively‐charged carboxylate groups of PCEs. Such a calcium‐binding interaction and how its strength affects the phase behavior are investigated. It is found that increasing the calcium‐binding strength expands the phase‐separated region and increases the critical extra salt concentration, and it also leads to a wider phase‐separated region for salting‐out and salting‐in phenomena. The structural parameters of PCEs also affect the phase behavior. Increasing the side‐chain length shrinks the phase‐separated region, while increasing the acid‐to‐ether ratio expands the phase‐separated region. Those results may find applications in molecular design of weakly charged polyelectrolytes like PCEs.

Effect of Feed Water pH on the Partitioning of Alkali Metal Salts from Aqueous Phase into the Polyamide Active Layers of Reverse Osmosis Membranes.

The partitioning of solutes into the polyamide active layers of reverse osmosis (RO) membranes is a key membrane property determining solute permeation. Quantification of partition coefficients and their dependence on feedwater pH would contribute to the development of predictive transport models of contaminant transport through RO membranes; however, neither solute partitioning nor the effect of feed solution pH on partitioning has been thoroughly characterized in the literature. Accordingly, we characterized the partitioning of all chloride salts of alkali metals (CsCl, RbCl, KCl, NaCl, and LiCl) from the aqueous phase into the polyamide active layers of five polyamide RO membranes, including one prepared in-house and four commercial membranes. We evaluated the effect of pH on the partitioning of alkali metal salts and whether the effect of pH on salt partitioning and rejection is consistent with Donnan theory predictions. Results showed that for all membranes, the partition coefficients of all salts were less than one and did not differ substantially among RO membranes. Results also indicated that for all membranes tested, Donnan theory provided an appropriate theoretical framework to estimate the effect of pH on salt partitioning (evaluated for all chloride salts of alkali metals) and salt rejection (evaluated for NaCl). Thus, we conclude that changes in salt rejection resulting from feed solution pH are primarily driven by changes in salt partitioning with comparatively small changes in salt diffusion coefficients.