Like-charged Ions Research Articles

Cluster Formation Induced by Local Dielectric Saturation in Restricted Primitive Model Electrolytes.

Experiments using the Surface Force Apparatus (SFA) have found anomalously long-ranged charge-charge underscreening in concentrated salt solutions. Meanwhile, theory and simulations have suggested ion clustering to be a possible origin of this behavior. The popular Restricted Primitive Model of electrolyte solutions, in which the solvent is represented by a uniform relative dielectric constant, εr, is unable to resolve the anomalous underscreening seen in experiments. In this work, we modify the Restricted Primitive Model to account for local dielectric saturation within the ion hydration shell. The dielectric "constant" in our model locally decreases from the bulk value to a lower saturated value at the ionic surface. The parameters for the model are deduced so that typical salt solubilities are obtained. Our simulations for both bulk and slit geometries show that our model displays strong cluster formation and these give rise to long-ranged density correlations between charged surfaces, at distances similar to what has been observed in SFA measurements. An electrolyte model wherein the dielectric constant remains uniform does not display similar clusters, even with εr equal to the low saturated value at ion contact. Hence, the observed behaviors are not simply due to an enhanced Coulomb interaction. In the latter case, cluster growth is counteracted by long-ranged repulsions between like-charged ions within clusters; this is an effect that is considerably reduced when the dielectric response drop is local. Our results imply that long-ranged interactions in these systems are mainly due to cluster-cluster correlations, rather than large electrostatic screening lengths.

Early-age hydration of tricalcium aluminate in chloride solutions

Understanding the kinetics and mechanisms involved in early-age hydration of tricalcium aluminate (C3A) in chloride solutions holds promise for implementing seawater-mixed concrete in the marine environment, as C3A remains the most reactive component of Portland cement (PC), affecting both PC and concrete's early-age hardening and long-term durability. Herein, we conducted a series of meticulously designed ex-situ and in-situ experiments to elucidate the intricate hydration behaviors of C3A in various chloride solutions. The results reveal that C3A exhibits distinct hydration kinetics and structural evolution processes in different solutions. The rapid precipitation of alumino-ferrite-mono (AFm) and C3AH6 phases contributes to the swift development of hydration heat and storage modulus in water and NaCl solutions, with a slight acceleration observed in the later one. Conversely, the formation of C3AH6 is delayed in CaCl2 and MgCl2 solutions before 20 min, with the subsequent precipitation of Cl-AFm enhancing its later production, particularly in CaCl2 solutions. Ab-initio calculations further elucidate that the acceleration effect of Cl ions originates from the ionization and structurization of hydrated surface Ca ions. However, this positive effect is significantly offset by Cl pairing with counterions, resulting in a dramatic adverse effect of solution Ca ions originated from the negative entropy effect of structuralized water molecules and electrostatic repulsion with like-charged surface Ca ions and solvent dipoles. Our findings provide valuable insights for sustainable and durable designs on cement-based materials mixed with seawater.

Anti-Coulomb ion-ion interactions: A theoretical and computational study

The free energy of ion solvation can be decomposed into enthalpic and entropic contributions. This helps one to understand the connection between the dielectric properties and the underlying forces. We present a simple linear-response model of screened charge interactions that provides an alternative understanding of solvation barriers. Moreover, it explains the “anti-Coulomb” interactions (attraction between like-charged ions and repulsion between opposite-charged ions) observed in both simulations and experiments. We show that this is a universal behavior associated to the nonlocal response function of any dielectric or metallic systems. Published by the American Physical Society 2024

Spectroscopic Evidence for Doubly Hydrogen-Bonded Cationic Dimers in the Solid, Liquid, and Gaseous Phases of Carboxyl-Functionalized Ionic Liquids.

Intermolecular interactions determine whether matter sticks together, gases condense into liquids, or liquids freeze into solids. The most prominent example is hydrogen bonding in water, responsible for the anomalous properties in the liquid phase and polymorphism in ice. The physical properties are also exceptional for ionic liquids (ILs), wherein a delicate balance of Coulomb interactions, hydrogen bonds, and dispersion interactions results in a broad liquid range and the vaporization of ILs as ion pairs. In this study, we show that strong, local, and directional hydrogen bonds govern the structures and arrangements in the solid, liquid, and gaseous phases of carboxyl-functionalized ILs. For that purpose, we explored the H-bonded motifs by X-ray diffraction and attenuated total reflection (ATR) infrared (IR) spectroscopy in the solid state, by ATR and transmission IR spectroscopy in the liquid phase, and by cryogenic ion vibrational predissociation spectroscopy (CIVPS) in the gaseous phase at low temperature. The analysis of the CO stretching bands reveals doubly hydrogen-bonded cationic dimers (c═c), resembling the archetype H-bond motif known for carboxylic acids. The like-charge doubly hydrogen-bonded ion pairs are present in the crystal structure of the IL, survive phase transition into the liquid state, and are still present in the gaseous phase even in (2,1) complexes wherein one counterion is removed and repulsive Coulomb interaction increased. The interpretation of the vibrational spectra is supported by quantum chemical methods. These observations have implications for the fundamental nature of the hydrogen bond between ions of like charge.

On hydrogen bonding in two carboxyl-functionalised imidazolium based ionic liquids

Dynamics in ionic liquids (ILs) are mainly governed by Coulombic interactions and strong, long-lived hydrogen bonds (HBs). This molecular dynamics (MD) study focuses on carboxyl-functionalised imidazolium based ILs at a temperature of 400 K, using an explicit atom force field. HB dynamics play a crucial role in determining the arrangement of ions and the structure in the bulk phase of ILs. We investigated the abundance of the defined HBs formed between the constituent ions and their lifetimes in the bulk phase of these ILs. These HBs can form between ions bearing opposite charges or pairs of like-charged ions. HBs between pairs of cations, though less in number, are longer lived than other HBs. The HB lifetimes were observed to correspond to timescales ranging from a few picoseconds to hundreds of picoseconds. Each component is associated with a different physical process leading to their breaking and reformation. Abbreviations: IL: ionic liquid; HB: hydrogen bond; IR: infrared; VDoS: vibrational density of states; VACF: velocity autocorrelation function; MD: molecular dynamics; RDF: radial distribution function

Designing artificial ion channels with strict K+/Na+ selectivity toward next-generation electric-eel-mimetic ionic power generation.

A biological potassium channel is >1000 times more permeable to K+ than to Na+ and exhibits a giant permeation rate of ∼108 ions/s. It is a great challenge to construct artificial potassium channels with such high selectivity and ion conduction rate. Herein, we unveil a long-overlooked structural feature that underpins the ultra-high K+/Na+ selectivity. By carrying out massive molecular dynamics simulation for ion transport through carbonyl-oxygen-modified bi-layer graphene nanopores, we find that the twisted carbonyl rings enable strict potassium selectivity with a dynamic K+/Na+ selectivity ratio of 1295 and a K+ conduction rate of 3.5×107 ions/s, approaching those of the biological counterparts. Intriguingly, atomic trajectories of K+ permeation events suggest a dual-ion transport mode, i.e. two like-charged potassium ions are successively captured by the nanopores in the graphene bi-layer and are interconnected by sharing one or two interlayer water molecules. The dual-ion behavior allows rapid release of the exiting potassium ion via a soft knock-on mechanism, which has previously been found only in biological ion channels. As a proof-of-concept utilization of this discovery, we propose a novel way for ionic power generation by mixing KCl and NaCl solutions through the bi-layer graphene nanopores, termed potassium-permselectivity enabled osmotic power generation (PoPee-OPG). Theoretically, the biomimetic device achieves a very high power density of >1000W/m2 with graphene sheets of <1% porosity. This study provides a blueprint for artificial potassium channels and thus paves the way toward next-generation electric-eel-mimetic ionic power generation.

Nonaqueous Ion Pairing Exemplifies the Case for Including Electronic Polarization in Molecular Dynamics Simulations.

The inclusion of electronic polarization is of crucial importance in molecular simulations of systems containing charged moieties. When neglected, as often done in force field simulations, charge-charge interactions in solution may become severely overestimated, leading to unrealistically strong bindings of ions to biomolecules. The electronic continuum correction introduces electronic polarization in a mean-field way via scaling of charges by the reciprocal of the square root of the high-frequency dielectric constant of the solvent environment. Here, we use ab initio molecular dynamics simulations to quantify the effect of electronic polarization on pairs of like-charged ions in a model nonaqueous environment where electronic polarization is the only dielectric response. Our findings confirm the conceptual validity of this approach, underlining its applicability to complex aqueous biomolecular systems. Simultaneously, the results presented here justify the potential employment of weaker charge scaling factors in force field development.

How Like-Charge Attraction Influences the Mobility of Cations in Hydroxyl-Functionalized Ionic Liquids

Attractive interactions between ions of like charge remain an elusive concept. Observing and quantifying this type of interaction in liquids and solutions is still a major challenge. Recently, we have shown that cation-cation interactions are present in hydroxyl-functionalized ionic liquids and that they can be controlled by the shape, charge distribution and functionality of the ions. In the present study, we demonstrate that cationic cluster formation does not only change the local structures of the ionic liquids but also influences the dynamics of the cations in a characteristic way. We show that solid-state 2H NMR spectroscopy is well suited for the study of molecular motion, even if the hydrogen bonded species of interest are indistinguishable due to fast deuteron exchange. We also provide valuable information about the applicability of well-accepted relaxation models.

Cover Feature: Dissecting Noncovalent Interactions in Carboxyl‐Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge (Chem. Eur. J. 46/2022)

Cover Feature: Dissecting Noncovalent Interactions in Carboxyl‐Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge (Chem. Eur. J. 46/2022)

Dissecting Noncovalent Interactions in Carboxyl-Functionalized Ionic Liquids Exhibiting Double and Single Hydrogens Bonds Between Ions of Like Charge.

We show that the carboxyl‐functionalized ionic liquid 1‐(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC‐CH2‐py][NTf2] exhibits three types of hydrogen bonding: the expected single hydrogen bonds between cation and anion, and, surprisingly, single and double hydrogen bonds between the cations, despite the repulsive Coulomb forces between the ions of like charge. Combining X‐ray crystallography, differential scanning calorimetry, IR spectroscopy, thermodynamic methods and DFT calculations allows the analysis and characterization of all types of hydrogen bonding present in the solid, liquid and gaseous states of the ionic liquid (IL). We find doubly hydrogen bonded cationic dimers (c+=c+) in the crystalline phase. With increasing temperature, this binding motif opens in the liquid and is replaced by (c+−c+−a− species, with a remaining single cationic hydrogen bond and an additional hydrogen bond between cation and anion. We provide clear evidence that the IL evaporates as hydrogen‐bonded ion pairs (c+−a−) into the gas phase. The measured transition enthalpies allow the noncovalent interactions to be dissected and the hydrogen bond strength between ions of like charge to be determined.

Mechanisms for enhanced transport selectivity of like-charged ions in hydrophobic-polymer-modified ion-exchange membranes

Effectively removing contaminants or extracting resources from water necessitates selective separation of ions with toxicity or economic value from common ions. Existing ion exchange membranes (IEM) effectively separate ions with opposite charges, but are not optimized for separating ions of like charges. An understanding of ion separation mechanisms is imperative for developing IEMs with improved selectivity between like-charged ions. In this work, we modify a commercial anion exchange membrane (AEM) with a hydrophobic polymer, polypyrrole (PPy), and show that PPy-modified AEM exhibits enhanced transport selectivity of nitrate over both chloride and sulfate. Combining ion sorption experiments and molecular dynamics simulations, we show that the enhanced selectivity between nitrate and chloride is attributed to the decoupling of the inverse relation between the ion partition into and diffusion within the polymer phase, which improves the overall ion transport selectivity between like-charged ions.

Thermodynamically Stable Cationic Dimers in Carboxyl-Functionalized Ionic Liquids: The Paradoxical Case of "Anti-Electrostatic" Hydrogen Bonding.

We show that carboxyl-functionalized ionic liquids (ILs) form doubly hydrogen-bonded cationic dimers (c+=c+) despite the repulsive forces between ions of like charge and competing hydrogen bonds between cation and anion (c+–a−). This structural motif as known for formic acid, the archetype of double hydrogen bridges, is present in the solid state of the IL 1−(carboxymethyl)pyridinium bis(trifluoromethylsulfonyl)imide [HOOC−CH2−py][NTf2]. By means of quantum chemical calculations, we explored different hydrogen-bonded isomers of neutral (HOOC–(CH2)n–py+)2(NTf2−)2, single-charged (HOOC–(CH2)n–py+)2(NTf2−), and double-charged (HOOC– (CH2)n−py+)2 complexes for demonstrating the paradoxical case of “anti-electrostatic” hydrogen bonding (AEHB) between ions of like charge. For the pure doubly hydrogen-bonded cationic dimers (HOOC– (CH2)n−py+)2, we report robust kinetic stability for n = 1–4. At n = 5, hydrogen bonding and dispersion fully compensate for the repulsive Coulomb forces between the cations, allowing for the quantification of the two equivalent hydrogen bonds and dispersion interaction in the order of 58.5 and 11 kJmol−1, respectively. For n = 6–8, we calculated negative free energies for temperatures below 47, 80, and 114 K, respectively. Quantum cluster equilibrium (QCE) theory predicts the equilibria between cationic monomers and dimers by considering the intermolecular interaction between the species, leading to thermodynamic stability at even higher temperatures. We rationalize the H-bond characteristics of the cationic dimers by the natural bond orbital (NBO) approach, emphasizing the strong correlation between NBO-based and spectroscopic descriptors, such as NMR chemical shifts and vibrational frequencies.

Anti-Electrostatic Pi-Hole Bonding: How Covalency Conquers Coulombics.

Intermolecular bonding attraction at π-bonded centers is often described as “electrostatically driven” and given quasi-classical rationalization in terms of a “pi hole” depletion region in the electrostatic potential. However, we demonstrate here that such bonding attraction also occurs between closed-shell ions of like charge, thereby yielding locally stable complexes that sharply violate classical electrostatic expectations. Standard DFT and MP2 computational methods are employed to investigate complexation of simple pi-bonded diatomic anions (BO−, CN−) with simple atomic anions (H−, F−) or with one another. Such “anti-electrostatic” anion–anion attractions are shown to lead to robust metastable binding wells (ranging up to 20–30 kcal/mol at DFT level, or still deeper at dynamically correlated MP2 level) that are shielded by broad predissociation barriers (ranging up to 1.5 Å width) from long-range ionic dissociation. Like-charge attraction at pi-centers thereby provides additional evidence for the dominance of 3-center/4-electron (3c/4e) nD-π*AX interactions that are fully analogous to the nD-σ*AH interactions of H-bonding. Using standard keyword options of natural bond orbital (NBO) analysis, we demonstrate that both n-σ* (sigma hole) and n-π* (pi hole) interactions represent simple variants of the essential resonance-type donor-acceptor (Bürgi–Dunitz-type) attraction that apparently underlies all intermolecular association phenomena of chemical interest. We further demonstrate that “deletion” of such π*-based donor-acceptor interaction obliterates the characteristic Bürgi–Dunitz signatures of pi-hole interactions, thereby establishing the unique cause/effect relationship to short-range covalency (“charge transfer”) rather than envisioned Coulombic properties of unperturbed monomers.

Frontispiece: “Anti‐electrostatic” Halogen Bonding between Ions of Like Charge

Halogen bonding, the sister noncovalent interaction to hydrogen bonding, usually utilizes cationic or neutral halogenated Lewis acids. The application of anionic halogen bond donors seems counter-intuitive, particularly in adducts with anionic Lewis bases: the electrostatic potentials of the isolated compounds seem to predict that such complexes are not feasible. For an overview on such “anti-electrostatic” halogen bonds, see the Concept review by S. V. Rosokha, S. M. Huber et al. on page 16530.

"Anti-electrostatic" Halogen Bonding between Ions of Like Charge.

Halogen bonding occurs between molecules featuring Lewis acidic halogen substituents and Lewis bases. It is often rationalized as a predominantly electrostatic interaction and thus interactions between ions of like charge (e. g., of anionic halogen bond donors with halides) seem counter‐intuitive. Herein, we provide an overview on such complexes. First, theoretical studies are described and their findings are compared. Next, experimental evidences are presented in the form of crystal structure database analyses, recent examples of strong “anti‐electrostatic” halogen bonding in crystals, and the observation of such interactions also in solution. We then compare these complexes to select examples of “counter‐intuitive” adducts formed by other interactions, like hydrogen bonding. Finally, we comment on key differences between charge‐transfer and electrostatic polarization.

Underscreening and hidden ion structures in large scale simulations of concentrated electrolytes.

The electrostatic screening length predicted by Debye-Hückel theory decreases with increasing ionic strength, but recent experiments have found that the screening length can instead increase in concentrated electrolytes. This phenomenon, referred to as underscreening, is believed to result from ion-ion correlations and short-range forces such as excluded volume interactions among ions. We use Brownian Dynamics to simulate a version of the Restrictive Primitive Model for electrolytes over a wide range of ion concentrations, ionic strengths, and ion excluded volume radii for binary electrolytes. We measure the decay of the charge-charge correlation among ions in the bulk and compare it against scaling trends found experimentally and determined in certain weak coupling theories of ion-ion correlation. Moreover, we find that additional large scale ion structures emerge at high concentrations. In this regime, the frequency of oscillations computed from the charge-charge correlation function is not dominated by electrostatic interactions but rather by excluded volume interactions and with oscillation periods on the order of the ion diameter. We also find the nearest neighbor correlation of ions sharing the same charge transitions from negative at small concentrations to positive at high concentrations, representing the formation of small, like-charge ion clusters. We conclude that the increase in local charge density due to the formation of these clusters and the topological constraints of macroscopic charged surfaces can help explain the degree of underscreening observed experimentally.

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy-Functionalized Pyridinium Cations.

The paradigm of supramolecular chemistry relies on the delicate balance of noncovalent forces. Here we present a systematic approach for controlling the structural versatility of halide salts by the nature of hydrogen bonding interactions. We synthesized halide salts with hydroxy‐functionalized pyridinium cations [HOC n Py]+ (n=2, 3, 4) and chloride, bromide and iodide anions, which are typically used as precursor material for synthesizing ionic liquids by anion metathesis reaction. The X‐ray structures of these omnium halides show two types of hydrogen bonding: ‘intra‐ionic’ H‐bonds, wherein the anion interacts with the hydroxy group and the positively charged ring at the same cation, and ‘inter‐ionic’ H‐bonds, wherein the anion also interacts with the hydroxy group and the ring system but of different cations. We show that hydrogen bonding is controllable by the length of the hydroxyalkyl chain and the interaction strength of the anion. Some molten halide salts exhibit a third type of hydrogen bonding. IR spectra reveal elusive H‐bonds between the OH groups of cations, showing interaction between ions of like charge. They are formed despite the repulsive interaction between the like‐charged ions and compete with the favored cation‐anion H‐bonds. All types of H‐bonding are analyzed by quantum chemical methods and the natural bond orbital approach, emphasizing the importance of charge transfer in these interactions. For simple omnium salts, we evidenced three distinct types of hydrogen bonds: Three in one!

The association of like-charged ions in tunable protic pyrazolium salts

The association of like-charged ions is an elusive concept due to the lack of supporting experimental evidence. In the present work, 3,5-dimethyl-1-(p-substitutedphenyl)pyrazolium hexafluorophosphate (p-Cl (2a), p-Br (2b), p-OCH3 (2c)) tunable protic pyrazolium salts have been synthesized and characterized. The association of like-charged ions in the synthesized salts was observed, as evidenced by X-ray structure analysis of single crystals. The effect of the p-substituent on the association of like-charged ions has been investigated. DFT-D3 studies were performed to unravel the links between the experimentally obtained X-ray results for the solid phase and theoretically calculated results for an isolated system in the gas phase. Additionally, infrared analysis and molecular electrostatic potential analysis were conducted using DFT-D3 to get further information.

Hydrogen Bonds between Ions of Opposite and Like Charge in Hydroxyl-Functionalized Ionic Liquids: an Exhaustive Examination of the Interplay between Global and Local Motions and Intermolecular Hydrogen Bond Lifetimes and Kinetics.

Hydroxyl-functionalized ionic liquids (ILs) represent a new interesting class of ILs where hydrogen bonds (HBs) play an important role: here, "typical" HBs between cations and anions (ca) are competing with "atypical" HBs connecting pairs of cations (cc). We study the equilibrium and kinetics of (cc) and (ca) HBs in 1-(n-hydroxyalkyl)-pyridinium bis(trifluoromethlysulfonyl)imide [HOCnPy][NTf2] ILs by means of molecular dynamics simulations. (cc) HBs are found to be between 0.96 and 3.76 kJ mol-1 stronger than their (ca) counterparts, depending on the alkyl chain length. HB lifetimes and kinetics are analyzed by means of HB population and reactive flux correlation functions. Essentially, four different HB lifetimes have to be considered, spanning about 3 orders of magnitude, each valid in its own right and each associated with different aspects of HB breaking and HB reformation. The long-time limiting behavior of the HB population correlation function is controlled by diffusion of the ions and can be quantitatively described by analytical expressions. The short-time HB behavior is tied to the localized dynamics of the hydroxyl group exploring its local solvation environment. A minimalist kinetic two-domain model is introduced to realistically describe the time evolution of the HB population correlation function for both (ca) and (cc) HBs over 5 orders of magnitude. By employing the reactive flux method, we determine the kinetics of HB breaking, unaffected by diffusion processes. We determine both, the ultrafast upper boundary and the average rate of HB breaking, allowing recrossing-events during the transient relaxation time period. For sufficiently long alkyl chains, all those computed HB lifetimes indicate a higher kinetic stability of (cc) HBs over (ca) HBs; for short chains, it is vice-versa.

The Thiouronium Group for Ultrastrong Pairing Interactions between Polyelectrolytes.

Various charged groups may be used as a repeat unit in polyelectrolytes to provide physical interactions between oppositely charged polymers leading to phase separation. The materials formed thus are termed polyelectrolyte complexes or coacervates (PECs). The strength of pairing between positive, Pol+, and negative, Pol-, repeat units depends on the specific identity of the monomer repeat unit. In this work, the pairing strength of the thiouronium group, a cation closely related to guanidinium, is evaluated using a polythiouronium polyelectrolyte. Polymers containing guanidinium, notably polyarginine, a peptide, are known for their unusual behavior, such as the formation of like-charge ion pairs and hydrogen bonding. It is shown here that some of this behavior is carried over to polythiouroniums, which results in exceptionally strong interactions with polyanions such as polysulfonates and polycarboxylates. The resilience of the polythiouronium/Pol- interaction was evaluated using the buildup of polyelectrolyte multilayers at various salt concentrations and by breaking up preformed PECs with high concentrations of added salt. The thiouronium group even interacts strongly enough with polymeric zwitterions to enable complexation with this nominally weakly interacting, net-neutral polymer.