Ion-pair Association Research Articles

Temperature Dependence of the Stability of Ion Pair Interactions, and its Implications on the Thermostability of Proteins from Thermophiles

An understanding of the determinants of the thermal stability of thermostable proteins is expected to enable design of enzymes that can be employed in industrial biocatalytic processes carried out at high temperatures. A major factor that has been proposed to stabilize thermostable proteins is the high occurrence of salt bridges. The current study employs free energy calculations to elucidate the thermodynamics of the formation of salt bridge interactions and the temperature dependence, using acetate and methylguanidium ions as model systems. Three different orientations of the methylguanidinium approaching the carboxylate group have been considered for obtaining the free energy profiles. The association of the two ions becomes more favorable with an increase in temperature. The desolvation penalty corresponding to the association of the ion pair is the lowest at high temperatures. The occurrence of bridging water molecules between the ions ensures that the ions are not fully desolvated, and this could provide an explanation for the existence of internal water molecules in thermostable proteins reported recently. The findings provide a detailed picture of the interactions that make ion pair association at high temperatures a favorable process, and reaffirm the importance of salt bridges in the design of thermostable proteins.

Tracing Dynamics, Self-Diffusion, and Nanoscale Structural Heterogeneity of Pure and Binary Mixtures of Ionic Liquid 1-Hexyl-2,3-dimethylimidazolium Bis(fluorosulfonyl)imide with Acetonitrile: Insights from Molecular Dynamics Simulations.

All-atom molecular dynamics (MD) simulations of the ionic liquid (IL) 1-hexyl-2,3-dimethylimidazolium bis(fluorosulfonyl)imide ([C6mmim][FSI]) and its binary mixtures with acetonitrile (ACN) have been reported for the first time. The presence of ACN as a cosolvent, similar to the effect of increasing temperature, causes enhancements in the ion translational motion and fluidity of the IL, leading to significant improvement of ionic conductivity and self-diffusion, which is well explained by a microscopic structural analysis. In neat IL and a concentrated IL mixture, self-diffusion of the cation is higher than that of the corresponding anion; however, further addition of ACN into the diluted mixtures with IL molar fractions (xIL's) below 0.50 results in more weakened interactions among the nearest ACN-anion neighbors compared to those among the ACN-cation neighbors so that the number of isolated anions is more than that of isolated cations under this condition, and the anions diffuse faster than the cations, as expected on the basis of their relative sizes. The velocity autocorrelation function analysis indicates an inverse relation between xIL and the mean collision time of each species. Additionally, at a fixed xIL, both the mean collision time and velocity randomization time of ACN are shorter than those of the ions. Gradual addition of ACN changes the morphology of nanosegregated domains and tends to disrupt ionic clusters (i.e., it scatters and decomposes both the polar and nonpolar domains) compared with pure IL, whereas both the radial and spatial distribution functions show the stabilization role of ACN in the close-contact ion-pair association. On the other hand, increasing ACN causes weakening of the structural correlations of the cation-cation and anion-anion neighbors in the solutions. ACN molecules appeared as a bridge, with balanced affinities between the polar and nonpolar domains, and no indication was observed for aggregation of ACN molecules in the studied mixtures that can rationalize good miscibility with imidazolium-based ILs.

A Systematic Investigation of the Thermodynamic Properties of Aqueous Barium Sulfate up to High Temperatures and High Pressures

Standard state thermodynamic properties for completely dissociated aqueous barium sulfate, BaSO4(aq), were fixed by an ionic additivity relationship using the data from other completely dissociated electrolytes, barium chloride, sodium sulfate, and sodium chloride, from 298.15 to 598.15 K and at steam saturated pressure, psat. The solubility of barium sulfate in pure water was then predicted a priori from 273.15 to 598.15 K and pressures up to 140 MPa, and the results were compared with the literature data. These predicted solubility values for barium sulfate in pure water, when corrected for ion pair association of Ba2+(aq) ion with SO42–(aq) ion and hydrolysis of SO42–(aq) ion, are in very good agreement with the literature data to well within the uncertainties of the experimental data at all temperatures and pressures considered. Using the experimental data from this study and auxiliary data from the literature, the logarithm of the molar dissociation constants of BaSO4° associated ion pair were also c...

Conductometric Analysis of some Ionic Liquids, 1-Alkyl-3-methylimidazolium Bromide with Aspirin in Acetonitrile Solutions

In recent years, ionic liquids have been used in pharmaceutical processes. Therefore, having a deep insight into the ion association behavior of ionic liquids in the presence of a drug is of particular importance. So, in this work, the molar conductivities of the ionic liquids, 1-alkyl-3-methylimidazolium bromide, [CnMIm]Br (n = 4, 6 and 8) in various concentrations of aspirin (ASA) in acetonitrile (MeCN) solutions are determined in very diluted region of ILs, (molality less than 0.01 mol kg-1) and at 298.15 K. The obtained conductivity data were analyzed by low concentration Chemical Model (lcCM) of conductance equation. Using this model the limiting molar conductivities (L0) and ion association constants (KA) were calculated. The results show that the L0 and KA values of ionic liquid are affected by the alkyl chain length of cation and the concentration of ASA. The values of L0 and KA decrease as the alkyl chain length of cation and ASA concentration increase. The KA was also used to calculate the standard Gibbs energy ( ) of ion-pairing association. In general, 1-butyl-3-methylimidazolium bromide, [BMIM]Br, has the low values of ion-pair formation and the high negative values of and stronger interaction between [BMIM]Br and ASA.

Selective extraction of nitric and acetic acids from etching waste acid using N235 and MIBK mixtures

Abstract The selective extraction of nitric and acetic acids from a simulated etching waste acid was investigated using a mixture of the extractants Alamine 336 (N235) and methyl isobutyl ketone (MIBK). The effects of dilution of the etching waste acid, N235 and MIBK concentrations, organic/aqueous (O/A) phase ratio, extraction temperature, and contact time were systematically studied. The results demonstrate that with an extractant mixture of the composition 12.5 vol% N235 and 87.5 vol% MIBK, 75% acetic acid and 85% nitric acid were extracted. However, only 3% phosphoric acid was co-extracted in a single-stage contact under the following conditions: phosphoric acid concentration of 385 g/L in the diluted acid, O/A ratio of 1.5:1, reaction temperature of 25–35 °C and reaction time of 10 min. The McCabe-Thiele analysis predicts that over 98% acetic acid and 99% nitric acid can be extracted, with co-extraction of only 5% phosphoric acid through a three-stage counter-current operation at an O/A ratio of 3:2. Further, ∼99% HNO 3 , 99% HAC, and 99% H 3 PO 4 loaded in the organic phase were stripped using 0.25 mol/L NH 3 ·H 2 O as the stripping agent at an O/A ratio of 1:2. Therefore, it is feasible to achieve almost complete separation of nitric acid and acetic acid from the waste acid to obtain purified phosphoric acid and a binary compound fertilizer containing the nutritional elements nitrogen and phosphorus. The compositions of the extraction complexes of different acids were determined to be H 3 PO 4 ·2N235, HAC·1.5N235 and HNO 3 ·N235 for the N235 extraction and HAC·2MIBK for the MIBK extraction of HAC. The results can explain the extraction tendencies of the three acids obtained above. Analyses of the FT-IR spectra of the loaded organic phases indicate that the N235 extraction of HNO 3 occurred by the ion pair association mechanism, while the acetic acid extraction using N235 or MIBK was realized through hydrogen bonding.

Spectrophotometric Determination of Diazepam in Pharmaceutical Forms by Ion-Pairing with Ferrithiocyanide Complex

A rapid and validated spectrophotometric method has been developed for determination of Diazepam (DZP) in pure and pharmaceutical dosage forms. The method was based on ion-pair association reaction of DZP with ferrithiocyanide complex in acidic medium. The color development was monitored spectrophometrically at the maximum absorption, λmax = 506 nm. The stoichiometry of the ion-pair associate was determined, and the reaction path way was postulated. The proposed method was successfully applied to the determination of DZP in five marketed brands with three labeled dosages; 2, 5, 10 mg per tablet. The analytical results of the proposed spectrophotometric method were statistically compared with those obtained from standard HPLC procedures used as a reference method. Application of the proposed procedure shows acceptable linearity, precision, repeatability, and reproducibility. The analytical results were in good agreement with the label claims. F-and Student’s t-tests proved that no significant difference regarding both accuracy and precision between the proposed and reference method, and that this spectrophotometric method can be employed for the routine analysis of DZP in bulks as well as in the commercial formulations.

Molecular Dynamics Simulation of Na+–Cl– Ion-Pair in Water–Methanol Mixtures under Supercritical and Ambient Conditions

Constrained molecular dynamics simulations have been performed to investigate the structure and thermodynamics of Na(+)-Cl(-) ion-pair association in water-methanol mixtures under supercritical and ambient conditions in dilute solutions. From the computed potentials of mean force (PMFs) we find that contact ion pairs (CIPs) are more stable than all other associated states of the ion pairs in both ambient and supercritical conditions. Stabilities of CIPs increase with increase in the mole fraction of methanol. In supercritical conditions, major changes in PMFs occur as we go from x(methanol) = 0.00 to x(methanol) = 0.50. The stable solvent shared ion pair (SShIP) which occurs in x(methanol) = 0.00 and 0.25, vanishes when x(methanol) is 0.50 or greater. The stabilities of these ion pairs increase with increasing temperature. Local structures around the ions are studied using the radial distribution functions, density profiles, angular distribution functions, running coordination numbers and excess coordination numbers. Preferential solvation analysis shows that both Na(+) and Cl(-) ions are preferentially solvated by water. From the calculation of enthalpies and entropies, we find that Na(+)-Cl(-) ion-pair association in water-methanol binary mixtures is endothermic and driven by entropy both in ambient as well as under supercritical conditions.

Redox-Controlled Ion-Pairing Association of Anionic Surfactant to Ferrocene-Terminated Self-Assembled Monolayers.

The redox-induced pairing from aqueous solution of a homologous series of sodium n-alkyl sulfate (NaCnSO4) surfactants of 6, 8, 10, and 12 carbons with gold-tethered self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate (FcC12SAu) is investigated by cyclic voltammetry combined with surface plasmon resonance (SPR) spectroscopy. The adsorbed layer thicknesses and surface coverages are consistent with the formation of a monolayer of CnSO4(-) at the oxidized FcC12SAu SAM/aqueous solution interface. A comparison of the anodic charge density with the SPR data indicates that approximately 60% of the adsorbed surfactant anions are paired with SAM-bound ferroceniums, suggesting an interdigitated layer structure. The ion-pairing capabilities of the longer-chain NaC12SO4, NaC10SO4, and NaC8SO4 relative to the short-chain NaC6SO4 are compared using the relative ion-pair formation constants calculated from the apparent SAM redox potentials and IC50 values obtained from competitive association experiments. A longer alkyl chain increases the overall hydrophobicity of the CnSO4(-) anion, thereby increasing its ability to pair with and stabilize the ferrocenium in the nonpolar environment of the SAM. Binary mixtures of NaC12SO4 and NaC6SO4 of different compositions are used to demonstrate that the differences in ion-pairing abilities can be exploited to selectively pair and adsorb C12SO4(-).

Molecular dynamics simulations of Na+-Cl− ion-pair in supercritical methanol

Constrained molecular dynamics (MD) computer simulations are presented for the solvation of Na+-Cl− ion-pair in supercritical methanol in an isothermal–isobaric ensemble (NPT) to understand the changes in the solvation structure of the ion pair as temperatures and pressures change from ambient to supercritical conditions. Potentials of mean force (PMF) of the Na+-Cl− ion-pair are calculated as a function of temperature and pressure. Contact ion-pair (CIP) is found to be the most stable and dominant in supercritical conditions. Over the temperature and pressure ranges investigated, we observe that methanol molecules retain significant hydrogen bonding. From calculations of energies and entropies through temperature derivatives of PMFs, we have found that Na+-Cl− ion-pair association in supercritical methanol is endothermic and is driven by entropy.

Conductometric study of ionic liquids in the presence of N,N′-bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE) Schiff base in acetonitrile solutions at 298.15 K

In recent years, Schiff bases and their metal complexes in the presence of ionic liquids (ILs) have been used in synthesis of various chemical compounds. To better understanding of ionic liquid influence on these processes, it is required to obtain information about the ion association behavior of the ionic liquids. Therefore, in this work, the molar conductivities of the ILs [RMIm]X (R= propyl, pentyl, and hexyl; X= Br− and Cl−) in different concentrations of N,N′-bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE) Schiff base+acetonitrile solutions were determined at 298.15K. The obtained conductivity data were analyzed by low concentration Chemical Model (lcCM) conductivity equation to calculate the limiting molar conductivities (Λ0) and ion association constants (KA). The results show that BPIE, alkyl chain length of cation, and anion type have effect on the ionic association process of the ILs. The Λ0 and KA values decrease as the Schiff base concentration, alkyl chain length of cation, and anion size increase in the solution. The association constants were also used to calculate the standard Gibbs energy (ΔG0A) of ion-pairing association. The resulted ΔG0A values are negative and become more negative as the alkyl chain length of the cation and the size of anion increase. The large negative values of the standard Gibbs energy obtained for [HMIm]Br indicates the more spontaneity and the more feasibility of the association process for this ionic liquid.

Solvation structure and dynamics of potassium chloride ion pair in dimethyl sulfoxide–water mixtures

Structure and dynamics of potassium chloride ion pair in mixtures of dimethyl sulfoxide (DMSO) and water have been investigated using constrained molecular dynamics. We have obtained the potentials of mean force (PMFs) between the K+ and Cl− ions at 298K over a wide concentration range of these mixtures. The results obtained are compared for relative stabilities of contact ion pair (CIP), solvent assisted ion pair (SAIP) and solvent separated ion pair (SSIP) in the mixtures with varying compositions. The stability of the CIPs decreases significantly as the mole fraction of water increases. The study of running coordination numbers at the first solvation shell suggests preferential solvation of ions by water in all the compositions and there is hardly any participation by DMSO molecules in the first solvation shell of the Cl− ion. The extent of association of potassium chloride ion pair in water, DMSO and their mixtures is determined by calculating the association constants. We have calculated the diffusion constants of ions and solvent molecules in the first solvation shell around K+ and Cl− in all the compositions. The diffusion constants of the shell molecules are the smallest in the DMSO mole fraction ranging from 0.2 to 0.5.

Pairing Mechanism among Ionic Liquid Ions in Aqueous Solutions: A Molecular Dynamics Study

In this study, we carried out molecular dynamics simulations to examine the molecular mechanism for ionic liquid pair association in aqueous solutions. We chose the commonly studied imidazolium-based ionic liquid pairs. We computed potentials of mean force (PMF) for four systems: 1,3-dimethlylimidazoliumchloride, 1,3-dimethlylimidazolium iodide, 1-methly-3-octylimidazolium chloride, and 1-methly-3-octylimidazolium iodide. Our PMF studies show a stronger interaction for the ion pairs of systems involving dimethlylimidazolium as the cation species compared with that of the systems containing octylimidazolium. This result indicates a decrease in ion-pair association as the cation alkyl tail length increases. We also studied the kinetics of ion-pair dissociation using different rate theories such as the Grote-Hynes and Kramer's theories. As expected, the computed rate results significantly deviated from results obtained from transition state theory because it does not account for dynamical solvent effects. Dissociative barrier curvatures are found to be very small for the systems investigated because the transmission coefficients computed using Grote-Hynes theory and Kramer's theory are approximately equal. Our analysis of the rotational dynamics of cations revealed that the time scales for molecular reorientation are longer for cations with longer alkyl tails.

Interionic Hydration Structures of NaCl in Aqueous Solution: A Combined Study of Quantum Mechanical Cluster Calculations and QM/EFP-MD Simulations

The association process of NaCl in aqueous solution was studied by a combination of quantum mechanical calculations on NaCl(H(2)O)(n) (n = 1-6) clusters and quantum mechanical/effective fragment potential-molecular dynamics (QM/EFP-MD) simulations for NaCl in 292 EFP waters. The interionic hydration structures (IHSs) were topologically classified as "ring" (R), "half-bridge" (H), and "full-bridge" (F) types on the basis of the quantum mechanical calculations. Subsequent IHS analysis on QM/EFP-MD simulations revealed that the NaCl contact ion pair (CIP) mainly involved R type hydration structures while the solvent-separated ion pair (SSIP) was composed of two different groups of F-type hydration structures. Our IHS analysis also discovered H type hydration even at large separation interionic distances (∼7 Å), which is denoted as a dissociating ion pair (DIP). The analysis was able to reveal the most complete interionic structures and their reorganizations of the association process. A strong correlation between the IHSs and interionic distance suggests that not only the solvent reorganization but also the local IHS changes are equally important. Mechanistically, it is suggested that the conversion between ring-type and full-bridge hydration structures is the main rate-determining step of ion-pair association.

Thermodynamics and kinetics of Na+/K+-formate ion pairs association in polarizable water: A molecular dynamics study

To enhance our understanding of ion specific activity in biological systems, the potential of mean force approach was utilized to study solvent effects on the interactions between two alkali cations (Na+ and K+) with a formate anion in water. A very complex free energy landscape was observed, much more so than alkali–halide ion pairs. Furthermore, a stronger binding between the Na+-formate pair was found in comparison to the K+-formate pair in water, which is in agreement with experimental and theoretical studies [1–4]. The kinetics of ion-pair inter-conversions was studied using the transition rate theory, along with a number of theoretical approaches such as the Kramers and Grote–Hynes theories. These kinetic results were used to predict solvent effects on dynamical features of ion-pair association, in which we have found that the dynamics of K+-formate pairs is faster than Na+-formate pairs.

Ion disturbance and clustering in the NaCl water solutions

Ion clustering and the solvation properties in the NaCl solutions are explored by molecular dynamics simulations with several popular force fields. The existence of ions has a negligible disturbance to the hydrogen bond structures and rotational mobility of water beyond the first ion solvation shells, which is suggested by the local hydrogen bond structures and the rotation times of water. The potential of mean force (PMF) of ion pair in the dilute solution presents a consistent view with the populations of ion clusters in the electrolyte solutions. The aggregation level of ions is sensitive to the force field used in the simulations. The ion-ion interaction potential plays an important role in the forming of the contact ion pair. The entropy of water increases as the ion pair approaches each other and the association of ion pair is driven by the increment of water entropy according to the results from the selected force fields. The kinetic transition from the single solvent separated state to the contact ion pair is controlled by the enthalpy loss of solution.

On the pH control at supercritical water-cooled reactor operating conditions

Abstract Application of LiOH (aq) for the pH control of the coolant at supercritical water-cooled reactor (SCWR) operating conditions is examined. The pH values of dilute aqueous LiOH solutions ( c = 10 −6 –10 −2 mol/kg) are calculated based on recently reported simulation data for the association constant of LiOH at supercritical water conditions [Plugatyr, A., Carvajal-Ortiz, R. A., Svishchev, I. M., 2011. Ion-pair association constant for LiOH in supercritical water. J. Chem. Eng. Data 56(9), 3637–3642]. The results indicate that at the operating pressure of the SCWR of 25 MPa, addition of LiOH to the coolant will provide adequate pH control for water densities above 0.17 g/cm 3 ( T 3 the pH control becomes progressively more difficult as water density decreases. The results also suggest that the pH control via LiOH (aq) addition at densities below 0.086 g/cm 3 ( T > 514 °C) is, arguably, impractical as it requires operation at high concentrations of LiOH (aq) .

Investigations of the ionic self-diffusion coefficients of the trivalent lanthanide 152Eu(III) in diluted Eu(ClO4)3 solutions in 1,4-dioxane + water mixtures at 298.15 K

The measurement of ionic self-diffusion coefficient D of the trivalent lanthanide ion 152Eu(III) in asymmetrical electrolytes of Eu(ClO4)3 in hydro-organic water–dioxane mixtures (at a low dielectric constant) at 298.15 K, by the open-end capillary method gives evidence of the progressive association of ion pairs. The study of the different physicochemical properties in all the domains of composition shows the existence of three distinct behaviours. The application of association theory to such an asymmetrical electrolyte (3 : 1) with a polyvalent cation enables us to delimit the validity of the Bjerrum and Fuoss's association theories. Since there is similarity of the coordination chemistry of trivalent lanthanide and actinide ions, this study comes within the framework of previous works of the investigation of structure properties of the f-elements in solution carried out in some previous papers.

Influence of ionic charge placement on performance of poly(ethylene glycol)-based sulfonated polyurethanes

Poly(ethylene glycol) (PEG)-based sulfonated polyurethanes bearing either sulfonated soft segments (SSSPU) or sulfonated hard segments (SHSPU) were synthesized using sulfonated monomers. Differential scanning calorimetry (DSC) revealed that sulfonate anions either in the soft segments or hard segments both increased the glass transition temperatures (Tg’s) of the soft segments and suppressed their crystallization. Moreover, dynamic mechanical analysis (DMA) and tensile analysis demonstrated that SSSPU possessed a higher modulus and tensile strength relative to SHSPU. Fourier transform infrared (FTIR) spectroscopy revealed that hydrogen bonding interactions in SHSPU were suppressed compared to SSSPU and noncharged PU. This observation suggested a high level of phase-mixing for SHSPU. In addition, atomic force microscopy (AFM) phase images revealed that both SSSPU and noncharged PU formed well-defined microphase-separated morphologies, where the hard segments phase-separated into needle-like hard domains at the nanoscale. However, SHSPU showed a phase-mixed morphology, which was attributed to increased compatibility of polar PEG soft segments with sulfonated ionic hard segments and disruption of hydrogen bonds in the hard segment. The phase-mixed morphology of SHSPU was further demonstrated using small angle X-ray scattering (SAXS), which showed a featureless X-ray scattering profile. In contrast, SAXS profiles of SSSPU and noncharged PU demonstrated microphase-separated morphologies. Moreover, SSSPU also displayed a broad ionomer peak ranging in q = 1–2 nm−1, which resulted from the sodium sulfonate ion pair association in the polar PEG soft phase. Morphologies of sulfonated polyurethanes correlated well with thermal and mechanical properties.

Interdependence of Conformational and Chemical Reaction Dynamics during Ion Assembly in Polar Solvents

We have utilized time-resolved vibrational spectroscopy to study the interdependence of the conformational and chemical reaction dynamics of ion assembly in solution. We investigated the chemical interconversion dynamics of the LiNCS ion pair and the (LiNCS)(2) ion-pair dimer, as well as the spectral diffusion dynamics of these ionic assemblies. For the strongly coordinating Lewis base solvents benzonitrile, dimethyl carbonate, and ethyl acetate, we observe Li(+) coordination by both solvent molecules and NCS(-) anions, while the weak Lewis base solvent nitromethane shows no evidence for solvent coordination of Li(+) ions. The strong interaction between the ion-pair dimer structure and the Lewis base solvents leads to ion-pair dimer solvation dynamics that proceed more slowly than the ion-pair dimer dissociation. We have attributed the slow spectral diffusion dynamics to electrostatic reorganization of the solvent molecules coordinated to the Li(+) cations present in the ion-pair dimer structure and concluded that the dissociation of ion-pair dimers depends more critically on longer length scale electrostatic reorganization. This unusual inversion of the conformational and chemical reaction rates does not occur for ion-pair dimer dissociation in nitromethane or for ion pair association in any of the solvents.

Ion-Pair Association Constant for LiOH in Supercritical Water

The equilibrium ion-pair association constant for LiOH in supercritical water is determined by means of molecular dynamics (MD) simulations via the potential of mean force calculation. Simulations are performed along three supercritical isotherms of (673.15, 773.15, and 873.15) K, covering a density range from (0.05 to 0.8) g·cm–3. Over the examined temperature and density range, the obtained association constant increases with increasing temperature and decreasing density. A significant increase in the association constant is observed upon transition into the low density (ρ < ρc) supercritical region. The obtained results are compared with the available experimental data at the corresponding states. To determine the corresponding states, an accurate reference equation of state for the simulated water model is used. An analytical expression for the association constant of LiOH in aqueous solution over the examined thermodynamic range is given. The results are of practical interest for chemistry control in the supercritical water-cooled nuclear reactor heat transport system.