Law Of Matching Water Affinities Research Articles

Binary aqueous solutions of choline salts: Determination and modelling of liquid density (298.15 or 313.15 K) and Vapour Pressure Osmometry (313.15 K)

Salts composed of the cholinium cation (such as choline hydroxide and choline chloride) have become one of the main reactants for the synthesis of greener ionic liquids, but, for a proper description of the aqueous solutions of these strong electrolytes by thermodynamic modelling, reliable data on liquid density (ρ) and vapour pressure (p) are needed, which are often unavailable. So, in this work, the liquid density (ρ) of binary aqueous solutions of choline bicarbonate ([Ch][Bic]), choline (2R,3R)-bitartrate ([Ch][Bit]), choline chloride ([Ch]Cl), choline dihydrogen citrate ([Ch][H2Cit]) and choline hydroxide ([Ch]OH) were measured at 298.15 or 313.15 K and 0.1 MPa and correlated using second-degree polynomials with salt molality (m), obtaining determination coefficients (R2) higher than 0.9974. Furthermore, the osmotic coefficients (ϕ) of these binaries were determined using vapour pressure osmometry (VPO), at 313.15 K and 0.1 MPa, being satisfactorily described by the Extended Pitzer Model of Archer, which yielded low values of standard deviation (3.58<σϕ·103<12.86). Then, the mean molal activity coefficients (γ±) and excess Gibbs free energies (GE/RT) were calculated, following the order [Ch][H2Cit] > [Ch][Bit] > [Ch][Bic] > [Ch]OH > [Ch]Cl, which corresponds to the decreasing polarity of the choline salts and agrees with the empirical law of matching water affinities (LMWA).

Anions and Cations Affect Amino Acid Dissociation Equilibria via Distinct Mechanisms.

Salts reduce the pKa of weak acids by a mechanism sensitive to ion identity and concentration via charge screening of the deprotonated state. In this study, we utilize constant pH molecular dynamics simulations to understand the molecular mechanism behind the salt-dependent dissociation of aspartic acid (Asp). We calculate the pKa of Asp in the presence of a monovalent salt and investigate Hofmeister ion effects by systematically varying the ionic radii. We observe that increasing the anion size leads to a monotonic decrease in Asp pKa. Conversely, the cation size affects the pKa nonmonotonically, interpretable in the context of the law of matching water affinity. The net effect of salt on Asp acidity is governed by an interplay of solvation and competing ion interactions. The proposed mechanism is rather general and can be applicable to several problems in Hofmeister ion chemistry, such as pH effects on protein stability and soft matter interfaces.

Charge regulation indicates water expulsion from silica surface by cesium cations

HypothesisSince the discovery of the Hofmeister effect in 1888, the varied propensity of ions to proteins, DNA and other surfaces has motivated research aimed at deciphering the underlying ion specific adsorption mechanism. Experimental and numerical studies have shown that in agreement with Collins' heuristic law of matching water affinity, weakly hydrated (chaotropic) ions adsorb preferentially to hydrophobic surfaces. Here, we show that this preference is driven by expulsion of bound water molecules from the surface by the adsorbing ions. ExperimentsUsing AFM spectroscopy of the force acting between two silica surfaces, we characterize surface charge regulation by adsorbed Na+ and Cs+ ions at different salt concentrations, pH values and temperatures. These data are analyzed in the framework of a recent theory of charge regulation, relating it to change in surface entropy. FindingsUpon binding to the silica, cesium cations expel water molecules from the surface to create additional adsorption sites for more ions. Cs+ adsorption is thus driven by the release of hydrating water molecules and the resulting increased surface entropy. The model indicates that on average, the binding of three cesium cations releases enough water molecules to make room for two additional bound cations. Na+ does not exhibit such behavior.

Kosmotropes and chaotropes: Specific ion effects to tailor layer-by-layer membrane characteristics and performances

So-called specific ion effects strongly impact physicochemical phenomena in among others, morphologies of polymeric membranes. This can be correlated to the identity of the ions, rather than just to their charge or concentration. We use this effect to tune the properties of layer-by-layer membranes prepared with the polyelectrolytes polydiallyldimethylammonium chloride and polystyrene sodium sulfonate. To systematically study this, membranes were prepared using different salts (NaBr, NH4Cl, MgCl2, NaCl, LiCl, MgSO4, NaH2PO4, (NH4)2SO4, Na2SO4). The specific ion effects were evaluated thanks to the classification of salts and PEs into kosmotropes (well hydrated) and chaotropes (poorly hydrated) following the law of matching water affinities (LMWA). The impact of the type of anions and cations on the multilayer formation and membrane performances was evaluated. Membranes prepared with kosmotropic salts achieved stable performances after only 4 bilayers, and almost 6 times higher water permeabilities than membranes prepared with chaotropic salts without compromising and often even improving the membrane's size exclusion behaviour. The salts used tuned the membranes charge: Kosmotropic salts delivered negatively charged membranes with Na2SO4 and MgCl2 retentions till 97% and 10% respectively, while chaotropic salts formed more positively charged membranes with reversed behaviour with Na2SO4 and MgCl2 rejection of 20 and 89%.

Cation and buffer specific effects on the DNA-lipid interaction

Knowledge of DNA - lipid layer interactions is key for the development of biosensors, synthetic nanopores, scaffolds, and gene-delivery systems. These interactions are strongly affected by the ionic composition of the solvent. We have combined quartz crystal microbalance (QCM) and ellipsometry measurements to reveal how pH, buffers and alkali metal chloride salts affect the interaction of DNA with lipid bilayers (DOTAP/DOPC 30:70 in moles). We found that the thickness of the DNA layer adsorbed onto the lipid bilayer decreased in the order citrate > phosphate > Tris > HEPES. The effect of cations on the thickness of the DNA layer decreased in the order (K+ > Na+ > Cs+ ∼ Li+). Rationalization of the experimental results requires that adsorption, due to cation specific charge screening, is driven by the simultaneous action of two mechanisms namely, the law of matching water affinities for kosmotropes (Li+) and ion dispersion forces for chaotropes (Cs+). The outcome of these two opposing mechanisms is a “bell-shaped” specific cations sequence. Moreover, a superimposed buffer specificity, which goes beyond the simple effect of pH regulation, further modulated cation specificity. In summary, DNA-lipid bilayer interactions are maximized if citrate buffer (50 mM, pH 7.4) and KCl (100 mM) are used.

Ab initio study of hydrated cesium iodide dimer (CsI)2-/0(H2O)0-6 and the cation size effect on (MI)2-/0(H2O)0-6 (M = Li, Na, K, Cs).

The structures of microsolvated (CsI)2-/0(H2O)0-6 clusters were determined using ab initio calculations. Our studies show that one Cs atom at the apex was firstly separated from the pyramid-shaped (CsI)2- unit when the water number reaches 3, whereas CsI distances did not increase significantly from n = 0 to 6 for neutrals. Additionally, the atomic charge and reduced density gradient analyses were carried out; the results reveal that the extra electrons are almost entirely localized on terminal Cs atom and the Cs+-water interactions dominate in (CsI)2-(H2O)0-6. The water-water interactions show up at n = 5. The comparison of (CsI)2-/0(H2O)n with (MI)2-/0(H2O)n (M = Li, Na, K) shows that neutral (CsI)2 is the most difficult to be separated, which matches the law of matching water affinity. As for anions, the most difficult separation occurs in the case of small size (LiI)2- due to the effect of extra electrons, and (MI)2- with larger size cation is more likely to interact with water to form a pyramid structure.

Identification and properties of ion-pairs in the aqueous solutions of LiI and NaI by FTIR and quantum chemical calculations

Existence and identification of ion pairing in aqueous electrolyte solutions are open questions to unveil the hydration structures of ions in water. In this work, we used a combination of infrared spectroscopy and quantum chemical calculations to investigate the ion pairing and hydrating properties of the aqueous solutions of lithium and sodium iodides by taking water molecules as probes. Spectral information of water molecules staying between the cations and anions in each case, as well as those contacting with the free ions of lithium and sodium, were extracted by using the strategy of equi-molar difference spectra. Peak positions of the solvent-shared ion-pair (SIP) water molecules were found to be 3340 cm−1 and 3580 cm−1 for LiI and NaI, respectively. With increasing concentration of the salts, the SIPs of NaI appear earlier than the SIPs of LiI, in accordance with the law of matching water affinities. This work opens a window for the identification of ion pairs using FTIR and may shed light on the separation of the precious lithium from sodium and the investigation of the hydration properties of other inorganic salts.

Quantifying the Counterion-Specific Effect on Surfactant Adsorption Using Modeling, Simulation, and Experiments.

Ionic surfactants behave differently in the presence of various counterions, which plays an important role in many scientific and engineering processes. Previous work has shown that the counterion-specific surface tension can be reproduced with classical adsorption models, but the underlying origin of this effect has not been explained. In this paper, we extend our previously developed adsorption model to account for the specific counterion adsorption. This model can accurately predict the surface tension of surfactant solutions like sodium dodecyl sulfate (SDS) in the presence of the monovalent salts LiCl, NaCl, KCl, and CsCl. The predicted surface excess and surface potential are validated by corresponding sum-frequency generation (SFG) spectroscopy experiments. We also used molecular dynamic (MD) simulation to explain the origin of the counterion-specific effect for surfactant behavior. Our study shows that for SDS, binding of the counterion to both the headgroup and a few CH2 fragments close to the surfactant head contributes to the counterion-specific effect. In general, SDS behaves like a large ion, and it prefers to bind with large counterions such as Cs+, which is consistent with Collins's law of matching water affinity. Therefore, large counterions enhance the surface adsorption and lower the surface tension the most.

Specific Ion Effects and the Law of Matching Solvent Affinities: A Conceptual Density Functional Theory Approach.

We study the principles behind specific ion effects of alkali and halide ions in various protic and aprotic solvents by means of a conceptual density functional theory (DFT) approach. The results of our calculations are in good agreement with experimental data and underline the crucial role of frontier molecular orbital energies. Further analysis reveals that the electronegativities and chemical hardness values of the considered ion and solvent species provide a molecular rationale for specific ion effects and the law of matching water affinities. Based on the analytical expressions and DFT calculations, we show that solvent affinities and the occurrence of specific ion effects, among other molecular mechanisms and interactions, are mainly due to electronegativity differences between the ions and the surrounding solvent molecules.

Characterization of Specific Ion Effects on PI(4,5)P2 Clustering using Molecular Dynamics Simulations and Graph-Theoretic Analysis

Numerous cellular functions mediated by phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2) involve clustering of the lipid as well as co-localization with other lipids. Although the cation-mediated electrostatic interaction is regarded as the primary mechanism for PI(4,5)P2 clustering, the ion-specific nature of the intermolecular network formation makes it challenging to characterize the clustering. Here we use all-atom molecular dynamics (MD) simulations and graph-theoretic analysis to gain insight into the clustering. MD simulations reveal that the strong cation-headgroup interactions preferentially occur between specific pairs of cations (i.e., Ca2+, Na+, and K+) and phosphate groups (i.e., P1, P4, and P5, attached to C1, C4 and C5 carbon of the headgroup, respectively), in line with Collins's law of matching water affinities (LMWA). In the Ca2+/Na+/K+ mixture, Ca2+ preferentially binds to P4 and P5 while K+ penetrates deeper into the headgroup region toward P1; Na+ is attracted to both P1 and P4/P5. These specific ion interactions lead to the characteristic PI(4,5)P2 network connectivity and clustering patterns. The intermolecular connectivity was analyzed using basic graph-theoretic measures including degree distribution and clustering coefficient, while the clustering patterns were characterized via graph entropy measures.

Characterization of Specific Ion Effects on PI(4,5)P2 Clustering: Molecular Dynamics Simulations and Graph-Theoretic Analysis.

Numerous cellular functions mediated by phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2; PIP2) involve clustering of the lipid as well as colocalization with other lipids. Although the cation-mediated electrostatic interaction is regarded as the primary clustering mechanism, the ion-specific nature of the intermolecular network formation makes it challenging to characterize the clusters. Here we use all-atom molecular dynamics (MD) simulations of PIP2 monolayers and graph-theoretic analysis to gain insight into the phenomenon. MD simulations reveal that the intermolecular interactions preferentially occur between specific cations and phosphate groups (P1, P4, and P5) of the inositol headgroup with better-matched kosmotropic/chaotropic characters consistent with the law of matching water affinities (LMWA). Ca2+ is strongly attracted to P4/P5, while K+ preferentially binds to P1; Na+ interacts with both P4/P5 and P1. These specific interactions lead to the characteristic clustering patterns. Specificially, the size distributions and structures of PIP2 clusters generated by kosmotropic cations Ca2+ and Na+ are bimodal, with a combination of small and large clusters, while there is little clustering in the presence of only chaotropic K+; the largest clusters are obtained in systems with all three cations. The small-world network (a model with both local and long-range connections) best characterizes the clusters, followed by the random and the scale-free networks. More generally, the present results interpreted within the LMWA are consistent with the relative eukaryotic intracellular concentrations Ca2+ ≪ Na+ < Mg2+ < K+; that is, concentrations of Ca2+ and Na+ must be low to prevent damaging aggregation of lipids, DNA, RNA and phosphate-containing proteins.

Thermodynamically Consistent Force Field for Coarse-Grained Modeling of Aqueous Electrolyte Solution.

We propose a thermodynamically consistent methodology to parameterize interactions between charged particles inside the dissipative particle dynamics (DPD) formalism. We used osmotic pressure experimental data as a function of the salinity in order to optimize the interaction parameters. Results for NaCl aqueous solution show that both mean osmotic and activity coefficients of individual ions allowed the determination of Na+-water, Cl--water, and Na+-Cl- DPD repulsion parameters. A simple linear relationship between the hydration-free energies of ions and the ion-water repulsion parameters that allows the parameterization of the complete series of halide and alkaline ions is proposed. Two strategies have been used to obtain the anion-cation interaction parameters for halide and alkaline ions. In the first one, the parameters are obtained based on the numerical optimization of the anion-cation repulsion parameter with respect to the experimental osmotic pressure data (with mean average deviations <4%). Second, we propose a predictive approach based on the free-energy difference of hydration energies of anions and cations in the spirit of the law of matching water affinities [with a mean absolute relative deviation of about 13%, better than 6% if small ions (Li+ and F-) are removed].

Volcano Plots Emerge from a Sea of Nonaqueous Solvents: The Law of Matching Water Affinities Extends to All Solvents.

The properties of all electrolyte solutions, whether the solvent is aqueous or nonaqueous, are strongly dependent on the nature of the ions in solution. The consequences of these specific-ion effects are significant and manifest from biochemistry to battery technology. The “law of matching water affinities” (LMWA) has proven to be a powerful concept for understanding and predicting specific-ion effects in a wide range of systems, including the stability of proteins and colloids, solubility, the behavior of lipids, surfactants, and polyelectrolytes, and catalysis in water and ionic liquids. It provides a framework for considering how the ions of an electrolyte interact in manifestations of ion specificity and therefore represents a considerable conceptual advance on the Hofmeister or lyotropic series in understanding specific-ion effects. Underpinning the development of the law of matching water affinities were efforts to interpret the so-called “volcano plots”. Volcano plots exhibit a stark inverted “V” shape trend for a range of electrolyte dependent solution properties when plotted against the difference in solvation energies of the ions that constitute the electrolyte. Here we test the hypothesis that volcano plots are also manifest in nonaqueous solvents in order to investigate whether the LMWA can be extended to nonaqueous solvents. First we examine the standard solvation energies of electrolytes in nonaqueous solvents for evidence of volcano trends and then extend this to include the solubility and the activity/osmotic coefficients of electrolytes, in order to explore real electrolyte concentrations. We find that with respect to the solvent volcano trends are universal, which brings into question the role of solvent affinity in the manifestation of specific-ion effects. We also show that the volcano trends are maintained when the ionic radii are used in place of the absolute solvation energies as the abscissa, thus showing that ion sizes, rather than the solvent affinities, fundamentally determine the manifestation of ion specificity. This leads us to propose that specific-ion effects across all solvents including water can be understood by considering the relative sizes of the anion and cation, provided the ions are spherical or tetrahedral. This is an extension of the LMWA to all solvents in which the “water affinity” is replaced with the relative size of the anion and cation.

Precision Switching in a Discrete Supramolecular Assembly: Alkali Metal Ion‐Carboxylate Selectivities and the Cationic Hofmeister Effect

A cavitand host has been shown to switch from a dimeric assembly to a tetrameric assembly in the presence of cations. Induced by pseudo-specific cation binding attenuating the net negative charge of each host, switching was shown to be highly cation selective. Thus, the concentration of cation required to induce assembly switching ranged from 2 mM in the case of N(n-Bu)4+ to ∼80 mM in the case of Na+ . Overall cation affinity was found to be essentially the reverse of Collins' law of matching water affinities, which predicts Na+ to have the strongest affinity for carboxylate groups. Combined with previous data, these results highlight the point that cation affinity for carboxylates are in large part dictated by context.

Deconstructing Free Energies in the Law of Matching Water Affinities.

The law of matching water affinities (LMWA) is explored in classical molecular dynamics simulations of several alkali halide ion pairs, spanning the size range from small kosmotropes to large chaotropes. The ion-ion potentials of mean force (PMFs) are computed using three methods: the local molecular field theory (LMFT), the weighted histogram analysis method (WHAM), and integration of the average force. All three methods produce the same total PMF for a given ion pair. In addition, LMFT-based partitioning into van der Waals and local and far-field electrostatic free energies and assessment of the enthalpic, entropic, and ion-water components yield insights into the origins of the observed free energy profiles in water. The results highlight the importance of local electrostatic interactions in determining the shape of the PMFs, while longer-ranged interactions enhance the overall ion-ion attraction, as expected in a dielectric continuum model. The association equilibrium constants are estimated from the smooth WHAM curves and compared to available experimental conductance data. By examining the variations in the average hydration numbers of ions with ion-ion distance, a correlation of the water structure in the hydration shells with the free energy features is found.

Microwave irradiation affects ion pairing in aqueous solutions of alkali halide salts

Using the molecular dynamics simulations with separate thermostats for translational and rotational degrees of freedom, we investigate the effects of water's rotational motion on the ion pairing of ionic solutes in aqueous solutions. The situation with rotational temperature higher than the translational one, Trot>Ttrs, is mimicking the non-equilibrium effects of microwaves on model solutions of alkali halide salts. The simulations reveal that an increase in the rotational temperature at constant translational temperature exerts significant changes in the structure of the solution. The latter are reflected in increased pairing of the oppositely charged ions, which can be explained by the weaker ability of rotationally excited water to screen and separate the opposite charges. It seems that Collins' law of matching water affinities retains its validity also in the non-equilibrium situation where the rotational temperature exceeds the translational one. On the other hand, the equilibrium effect (i.e., an increase in the solution's overall temperature T≡Trot = Ttrs) favors the formation of small-small (NaCl), while it has a little effect on large-large (CsI) ion pairs. This is in accordance with water becoming less polar solvent upon a temperature increase. Furthermore, we investigated the effects of excited translational motion of water (and ions) on the ion pairing by increasing the translational temperature, while keeping the rotational one unchanged (i.e., Ttrs>Trot). Interestingly, in certain cases the faster translational motion causes an increase in correlations. The temperature variations in the like-ion association constants, Kas++ and Kas--, are also examined. Here the situation is more complex but, in most cases, a decrease in the ion pairing is observed.

Cation effects on phosphatidic acid monolayers at various pH conditions

The impact of pH and cations on phase behavior, stability, and surface morphology for dipalmitoylphosphatidic acid (DPPA) monolayers was investigated. At pH<10, DPPA monolayers on water are predominantly populated by neutral species and display the highest packing density. Cations are found to expand and stabilize the monolayer in the following order of increasing magnitude at pH 5.6: Na+>K+∼Mg2+>Ca2+. Additionally, cation complexation is tied to the pH and protonation state of DPPA, which are the primary factors controlling the monolayer surface behavior. The binding affinity of cations to the headgroup and thus deprotonation capability of the cation, ranked in the order of Ca2+>Mg2+>Na+>K+, is found to be well explained by the law of matching water affinities. Nucleation of surface 3D lipid structures is observed from Ca2+, Mg2+, and Na+, but not from K+, consistent with the lowest binding affinity of K+. Unraveling cation and pH effects on DPPA monolayers is useful in further understanding the surface properties of complex systems such as organic-coated marine aerosols where organic films are directly influenced by the pH and ionic composition of the underlying aqueous phase.

Tetraalkylammonium Ionic Liquids as Dual Solvents–Catalysts for Direct Synthesis of Sugar Fatty Acid Esters

AbstractAs nonionic surfactants derived from naturally renewable resources such as sugars and fatty acids, sugar fatty acid esters have been widely utilized in food, cosmetic and pharmaceutical industries. Our present study has demonstrated that the inexpensive and halogen‐free tetraalkylammonium salts (e.g., [Bu4N][Ac], [Et4N][Ac] and [Me4N][Ac]) can act as dual solvents–catalysts for regioselective acylation to produce glucose laurate. This non‐enzymatic synthesis can proceed under mild conditions with high specificity, and the conversions obtained were superior to that when an enzyme catalyst (lipase, EC 3.1.1.3) was added. A higher yield was obtained in the ammonium salt with a longer alkyl chain on the cation, while no product was obtained in [Bu4N][HSO4] and [Bu4P][Ac]. A reaction mechanism has been proposed, which is supported by phase‐transfer catalysis and law of matching water affinity.

Ion Pairing in Alkali Nitrate Electrolyte Solutions.

In this study, we investigate the thermodynamics of alkali nitrate salt solutions, especially the formation of contact ion pairs between alkali cation and nitrate anion. The ion-pairing propensity shows an order of LiNO3 < NaNO3 < KNO3. Such results explain the salt activity coefficients and suggest that the empirical "law of matching water affinity" is followed by these alkali nitrate salt solutions. The spatial patterns of contact ion pairs are different in the three salt solutions studied here: Li(+) forms the contact ion pair with only one oxygen of the nitrate while Na(+) and K(+) can also be shared by two oxygens of the nitrate. In reproducing the salt activity coefficient using Kirkwood-Buff theory, we find that it is essential to include electronic polarization for Li(+) which has a high charge density. The electronic continuum correction for nonpolarizable force field significantly improves the agreement between the calculated activity coefficients and their experimental values. This approach also improves the performance of the force field on salt solubility. From these two aspects, this study suggests that electronic continuum correction can be a promising approach to force-field development for ions with high charge densities.

Interactions between Hofmeister anions and the binding pocket of a protein.

This paper uses the binding pocket of human carbonic anhydrase II (HCAII, EC 4.2.1.1) as a tool to examine the properties of Hofmeister anions that determine (i) where, and how strongly, they associate with concavities on the surfaces of proteins and (ii) how, upon binding, they alter the structure of water within those concavities. Results from X-ray crystallography and isothermal titration calorimetry show that most anions associate with the binding pocket of HCAII by forming inner-sphere ion pairs with the Zn(2+) cofactor. In these ion pairs, the free energy of anion-Zn(2+) association is inversely proportional to the free energetic cost of anion dehydration; this relationship is consistent with the mechanism of ion pair formation suggested by the "law of matching water affinities". Iodide and bromide anions also associate with a hydrophobic declivity in the wall of the binding pocket. Molecular dynamics simulations suggest that anions, upon associating with Zn(2+), trigger rearrangements of water that extend up to 8 Å away from their surfaces. These findings expand the range of interactions previously thought to occur between ions and proteins by suggesting that (i) weakly hydrated anions can bind complementarily shaped hydrophobic declivities, and that (ii) ion-induced rearrangements of water within protein concavities can (in contrast with similar rearrangements in bulk water) extend well beyond the first hydration shells of the ions that trigger them. This study paints a picture of Hofmeister anions as a set of structurally varied ligands that differ in size, shape, and affinity for water and, thus, in their ability to bind to—and to alter the charge and hydration structure of—polar, nonpolar, and topographically complex concavities on the surfaces of proteins.