Ion-pair Dissociation Constants Research Articles

Field theory description of ion association in re-entrant phase separation of polyampholytes.

Phase separation of several different overall neutral polyampholyte species (with zero net charge) is studied in solution with two oppositely charged ion species that can form ion pairs through an association reaction. Hereby, a field theory description of the system, which treats polyampholyte charge sequence dependent electrostatic interactions as well as excluded volume effects, is given. Interestingly, analysis of the model using random phase approximation and field theoretic simulation consistently shows evidence of a re-entrant polyampholyte phase separation at high ion concentrations when there is an overall decrease of volume upon ion association. As an illustration of the ramifications of our theoretical framework, several polyampholyte concentration vs ion concentration phase diagrams under constant temperature conditions are presented to elucidate the dependence of phase separation behavior on the polyampholyte sequence charge pattern as well as ion pair dissociation constant, volumetric effects on ion association, solvent quality, and temperature.

Effects of electrolytes on redox potentials through ion pairing

Reduction potentials have been determined for two molecules, benzophenone (BzPh) and perylene (Per), effectively in the complete absence of electrolyte as well as in the presence of three different supporting electrolytes in the moderately polar solvent THF. A description of how this can be so, and qualifications, are described in the discussion section. The primary tool in this work, pulse radiolysis, measures electron transfer (ET) equilibria in solution to obtain differences in redox potentials. Voltammetry measures redox potentials by establishing ET equilibria at electrodes, but electrolytes are needed for current flow. Results here show that without electrolyte the redox potentials were 100–451mV more negative than those with 100mM electrolyte. These changes depended both on the molecule and the electrolyte. In THF the dominant contributor to stabilization of radical anions by electrolyte was ion pairing. An equation was derived to give changes in redox potentials when electrolyte is added in terms of ion pair dissociation constants and activity coefficients. Definite values were determined for energetics, ΔGd°, of ion pairing. Values of ΔGd° for pairs with TBA+ give some doubt that it is a “weakly-coordinating cation.” Computations with DFT methods were moderately successful at describing the ion paring energies.

Osmotic and Activity Coefficients for Binary Aqueous Solutions of 1-Butyl-3-methylimidazolium Based Amino Acid Ionic Liquids at 298.15 K and at 0.1 MPa

The osmotic coefficient data for aqueous solutions containing 1-n-butyl-3-methylimidazolium ([Bmim]) based amino acid ionic liquids (AAILs) made from glycine, l-alanine, l-valine, and l-leucine are obtained in the concentration range of ∼0.01 mol·kg–1 to ∼0.6 mol·kg–1 at 298.15 K and at 0.1 MPa using vapor pressure osmometry. Activity coefficients of both components, that is, solute and solvent (water), in mixture were estimated using experimental osmotic coefficient data and have been further used to obtain the excess and mixing Gibbs free energies for the studied systems. The concentration dependence of osmotic coefficients indicates possible ion-pair formation and hence the ion-pair dissociation constant for all the studied AAILs in aqueous solutions at 298.15 K is reported. The nonelectrolyte contribution due to ion-pair formation in the aqueous AAIL solutions was discussed through estimation of osmotic second virial coefficients for AAILs using the McMillan–Mayer theory of solutions. Application of t...

Determination of vanadyl sulfate ion-pair dissociation constant at 298.15 K by Fuoss method

Vanadium battery has fast and large capacity charge and discharge characteristics, and the concentration and stability of the battery electrolyte are the key to the electrolyte performance. In actual, with higher acidity and higher concentration of the vanadium battery electrolyte, the ion-pair is dominant. The ion-pair dissociation constant is one of the important thermodynamic data related to battery performance. The vanadyl sulfate conductivity in aqueous solution was determined using the conductance method at 298.15 K. Using the origin data fitting, the limiting molar conductance is obtained, and then by the improved Ostwald dilution law and the Davies equation, the activity coefficient is solved so as to obtain the ionic strength of the true solution. Using the Fuoss method, the dissociation constant of vanadyl sulfate ion-pair was calculated. At last, the vanadyl sulfate limiting molar conductance (Λ0) is 174.8251748 S·dm2·mol−1, and the vanadyl sulfate ion-pair dissociation constant Kd is 0.002636367, and then other thermodynamic properties can be studied.

Thermodynamic Studies of Aqueous Solutions of 2,2,2-Cryptand at 298.15 K: Enthalpy–Entropy Compensation, Partial Entropies, and Complexation with K+ Ions

The osmotic coefficient measurements for binary aqueous solutions of 2,2,2-cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8] hexacosane) in the concentration range of ~0.009 to ~0.24 mol·kg(-1) and in ternary aqueous solutions containing a fixed concentration of 2,2,2-cryptand of ~0.1 mol·kg(-1) with varying concentration of KBr (~0.06 to ~0.16 mol·kg(-1)) have been reported at 298.15 K. The diamine gets hydrolyzed in aqueous solutions and needs proper approach to obtain meaningful thermodynamic properties. The measured osmotic coefficient values are corrected for hydrolysis and are used to determine the solvent activity and mean ionic activity coefficients of solute as a function of concentration. Strong ion-pair formation is observed, and the ion-pair dissociation constant for the species [CrptH](+)[OH(-)] is reported. The excess and mixing thermodynamic properties (Gibbs free energy, enthalpy, and entropy changes) have been obtained using the activity data from this study and the heat data reported in the literature. Further, the data are utilized to compute the partial molal entropies of solvent and solute at finite as well as infinite dilution of 2,2,2-cryptand in water. The concentration dependent non-linear enthalpy-entropy compensation effect has been observed for the studied system, and the compensation temperature along with entropic parameter are reported. Using solute activity coefficient data in ternary solutions, the transfer Gibbs free energies for transfer of the cryptand from water to aqueous KBr as well as transfer of KBr from water to aqueous cryptand were obtained and utilized to obtain the salting constant (ks) and thermodynamic equilibrium constant (log K) values for the complex (2,2,2-cryptand:K(+)) at 298.15 K. The value of log K = 5.8 ± 0.1 obtained in this work is found to be in good agreement with that reported by Lehn and Sauvage. The standard molar entropy for complexation is also estimated for the 2,2,2-cryptand-KBr complex in aqueous medium.

Competitive Interactions of Two Ion-Paired Salts with a Neutral Host To Form Two Non-Ion-Paired Complexes

It is demonstrated that our reported equilibrium treatments that take into account ion-paired guest and non-ion-paired complexes can be applied to competitive complexations. Satisfactory results were obtained for a system with two cationic guests [N,N'-dimethyl-4,4'-biyridinium bis(hexafluorophosphate) (1) and dibenzylammonium hexafluorophosphate (2)] having a common counterion and a single neutral host dibenzo-24-crown-8 (3), even though for this system one exchange process is slow and the other fast on the 1H NMR time scale. The competitive complexation protocol presented here provides a convenient method for the determination of KapKipd (the product of the ion-pair dissociation constant of the guest salt and the association constant for the host with the resultant free cation) for new systems from ion-paired guests that form complexes that are not ion paired.

Intermolecular Interactions in Lactone-Based Electrolytes

The viscosities (η), conductivities (σ), and enthalpies of dissolution of (LiTF) and (LiTFSI) are measured in γ-butyrolactone (BL), γ-valerolactone (VL), and mixtures of ethylene carbonate (EC) with tetrahydrofuran (THF) and diglyme (DG): EC:THF (20:80, in moles) and EC: DG (45:55, in moles). From the variations of η with the salt concentration, the B and D coefficients in the extended Jones-Dole equation are determined at different temperatures. The concentration dependence of the conductivity is analyzed on the basis of a model involving (i) a chemical equilibrium between free ions and ion pairs, (ii) the use of the Walden product to correct molar conductivities for viscosity variations, and (iii) the calculation of ion activity coefficients by the cube root law derived from the quasi-lattice model. According to this model, the limiting molar conductivities and ion pair dissociation constants are inferred. The solvation enthalpies of the salts are deduced from the heat of dissolution of LiTF and LiTFSI in BL, VL, and the EC:DG mixture. The formation of solvent-separated ion pairs in lactones and contact ion pairs in the EC mixture is deduced from the analysis of the experimental data as the result of a competition between ion-ion and ion-solvent interactions. The dissociation coefficient of ion pairs determined by Raman spectroscopy agrees well with those deduced from conductivity measurements. From the variation of the area of the band at 676 cm−1 with salt concentration, a coordination number of 4 molecules is found for the solvation of the by BL molecules. © 2002 The Electrochemical Society. All rights reserved.

Ion Pair Basicity of Some Amines in THF: Implications for Ion Pair Acidity Scales1

Amines form proton-transfer ion pairs of the type BH+ In- with acidic indicator hydrocarbons InH in THF. Relative equilibrium constants measured for a number of amines in THF differ substantially from ionic pKa values for BH+ in acetonitrile or DMSO. The results show that ion pair dissociation constants of the protonated amine ion pairs in THF vary by several orders of magnitude and point out that at the present time amines cannot be placed quantitatively on any of the ion pair acidity scales currently in use for neutral acids in THF.

Conductivity of (C4H9)4N BF4 in Liquid and Supercritical Hydrofluorocarbons

The conductivity of tetrabutylammonium tetrafluoroborate in 1,1,1,2-tetrafluoroethane (HFC 134a) in the liquid state at 30 °C and in difluoromethane (HFC32) in the liquid, near critical and supercritical states is presented as a function of concentration and pressure. The data are fitted to a Fuoss−Kraus model and the ion pair and triple ion dissociation constants are analyzed as a function of temperature and pressure. It is shown that the ionic equilibria are controlled by the relative permittivity of the media irrespective of the state of the fluid. The change in ion pair and triple ion dissociation constants with pressure are used to discuss the solvation of charged and neutral species in liquid and supercritical fluids.

The determination of crown-cation complexation behavior in dioxane/water mixtures by conductometric studies

Using conductance measurements, an attempt has been made to gain detailed information about the specific molecular interactions of crown compounds with metal ions in a 1,4-dioxane/water binary system. Analyses of the transport data of dioxane/water mixtures yielded the mobility of the crown compound action complexes and the ion-pair dissociation constant of the crown compound–electrolyte complex. Binary mixed aqueous solvents are frequently employed in broad areas of chemistry. Their applicability ranges from synthetic and mechanistic studies in organic chemistry to biophysical chemistry, with emphasis on molecular interactions in biologically significant structures. Stability constants of crown compound complexes are determined by various methods, such as potentiometry (with ion selective electrodes), polarography, voltammetry, spectrophotometry, nuclear magnetic resonance, calorimetry and solubility. In this study the conductometry measurements have been carried out with high precision at optimal concentrations in dioxane/water systems. Structures of crown–cation complexes in dioxane/water mixtures are estimated from the conductance parameters (κ, Λ and α) as well as the complex formation constant, Ke = (ΛMAm − Λ) / (Λ − ΛMaLbAm) [L]. The conductance behavior of Na+, K+ chlorides and Na+ perchlorates with 18-crown-6, 15-crown-5, and 12-crown-4 have been studied in various dioxane/water systems (50%, 80%, and 85%) at 25°C. The all experimental studies have been made by the ratio 1 : 1 of the metal-ion and the crown ether, Ke. For the calculations, Excel. 5.0 was used as an application program. Copyright © 1998 John Wiley & Sons, Ltd.

Experimental determination of aqueous sodium-acetate dissociation constants at temperatures from 20 to 240°C

Dissociation constants of sodium acetate ion pair (NaCH 3COO 0) were determined at the liquid–vapor saturation pressure by Raman spectroscopy at temperatures from 20 to 240°C and by potentiometry at temperatures from 25 to 172°C. Large differences were found between these two experimentally determined data sets; dissociation constants generated from Raman spectroscopic data were found to be ∼1.5 orders of magnitude higher than corresponding constants obtained using potentiometric measurements. Values generated from Raman spectroscopic data are consistent with the high temperature NaCH 3COO 0 dissociation constants reported by [Oscarson, J.L., Gillespie, S.E., Christensen, J.J., Izatt, R.M., Brown, P.R., 1988. Thermodynamic quantities for the interaction of H + and Na + with C 2H 3O 2 − and Cl − in aqueous solution from 275 to 320°C. J. Soln. Chem., Vol. 17, pp. 865-885], whereas those obtained by potentiometry are consistent with the low temperature NaCH 3COO 0 dissociation constants measured by [Archer, D.W., Monk, C.B., 1964. Ion-association constants of some acetates by pH (glass electrode) measurements. J. Chem. Soc., pp. 3117–3122.] and [Daniele, P.G., De Robertis, A., De Stefano, C., Sammartano, S., Rigano, C., 1985. On the possibility of determining the thermodynamic parameters for the formation of weak complexes using a simple model for the dependence on the ionic strength of activity coefficients: Na +, K +, and Ca 2+ complexes of low molecular weight ligands in aqueous solution. J. Chem. Soc. Dalton, pp. 2353–2361.], as well as the predictions of [Shock, E.L., Koretsky C.M., 1993. Metal–organic complexes in geochemical processes: Calculation of standard partial molal thermodynamic properties of aqueous acetate complexes at high pressures and temperatures. Geochim. Cosmochim. Acta, Vol. 57, pp. 4899–4922.] and [Bénézeth, P., Castet, S., Dandurand, J.L., Gout, R., Schott, J., 1994. Experimental study of aluminum acetate complexing between 60 and 200°C. Geochim. Cosmochim. Acta, Vol. 58, pp. 4561–4571]. The origin of these differences likely stems in the species being sampled by different experimental techniques; Raman spectroscopy only identifies inner sphere sodium-acetate complexes as associated species whereas potentiometric measurements identify both inner and outer sphere sodium-acetate complexes as associated species. Regression of sodium acetate ion pair dissociation constants obtained from Raman spectroscopy and potentiometry enable characterization of true dissociation constants for both inner and outer sphere complexes. These latter parameters are used to assess the formation of these complexes at temperatures and solution compositions typical of natural fluids.

Electrochemical and thermodynamic studies of the ion-pair formation of chloropentamminecobalt(III) ion in ethyl alcohol-water media containing different dicarboxylate ligands

The ion-pair dissociation constants, K D, of the ion-pair formed between chloropentamminecobalt(III) ion (CpX 2+) and a variety of dicarboxylate ligands, have been determined from EMF measurements of a cell composed of glass and calomel electrodes. Measurements were made in water and in aqueous binary mixtures of ethyl alcohol, over a wide range of solvent composition (0–60 wt% ethyl alcohol), at six different temperatures (ranging from 30 to 55°C at intervals of 5°C). The thermodynamic parameters of association Δ G ass 0, Δ H ass 0 and Δ S ass 0 have been calculated and discussed. Δ H ass 0–Δ S ass 0, Δ S ass 0–Δ S 1(or 2) 0, Δ G ass 0– G 1(or 2) 0 and Δ H ass 0–Δ H 1(or 2) 0 correlations among different solvent media and different dicarboxylate ligands were examined (where 1 and 2 denote the first and the second dissociation reactions of the studied dicarboxylic acids). The p K D value has been correlated with the dielectric constant of the medium according to Born's equation.

Interfacial Properties on the Synthesis of Ether−Ester Compounds by Liquid−Liquid Phase-Transfer Catalysis

Interfacial tensions between water/n-hexane and water/toluene in the synthesis of ether-ester compounds by phase-transfer catalysis were measured. These two-phase systems contained phase-transfer catalysts, an aqueous-phase reactant, and/or alkaline. It was shown that the measured data could be well described by the Gibbs adsorption equation coupled with the Langmuir monolayer isotherm. In addition, the interfacial parameters such as the surface excess and adsorption constant determined in one-compound systems allowed one to predict the surface pressure of the present practical liquid-liquid phase-transfer-catalyzed systems. Other interfacial properties of species such as the molecular area and ion-pair dissociation constant were also evaluated.

The influence of common cation BH+on the products of reactions between C-acids and strong guanidine bases in acetonitrile solvent

The electronic spectra of the products of reactions between C-acids (4-nitrophenylcyanoalkanes, 4-nitrophenylnitroalkanes and trinitrotoluene) and 1,1,3,3-tetramethylguanidine (TMG), pentamethylguanidine (PMG) and MTBD bases (B) have been recorded in B/BH+ buffers in acetonitrile at 25 °C. Large BH+ concentrations had little or no effect on the spectra of the nitrile or trinitrotoluene anions, while the spectra of nitroalkane anions were shifted to the blue by some 50–75 nm. These shifts were ascribed to the formation of hydrogen-bonded ion pairs. In the case of TMG or PMG reactions two types of ion pairs are formed due to homoconjugation of BH+ cations, which is why an isosbestic point is not observed, while MTBD reactions are free from such complications. The spectra of some ion pair products and ion pair dissociation constants were calculated.

Cryptate acidity scales. Solvent polarity effect on ion‐pair and free ion acidity of organic compounds

Abstract[2.1.1] Cryptated lithium salts of a wide range of CH‐acids were studied spectrophotometrically and conductimetrically in solvents of low polarity. Such salts exist in media of different dielectric constants in the form of cryptand‐separated ion pairs which are identical in their spectral and conductance properties with common solvent‐separated ion pairs. No formation of higher ionic clusters was observed for the studied ions at concentrations below 10−3 mol−1 even in the benzene. The relative equilibrium acidities in benzene, N‐methylmorpholine and tetrahydrofuran for a number of CH‐acids are reported. Solvent polarity does not influence the cryptate ion‐pair acidity of the investigated compounds. The relative free ion acidities were calculated for some of the studied acids in different solvents by combination of the ion‐pair acidity values with the conductimetric data on dissociation constants of the cryptate ion pairs. A linear relationship between the free ion acidity and the reciprocal of the dielectric constant was found. An extrapolation method is suggested for the calculation of gas‐phase acidity from the data obtained in solution.

Conductance study of ion-pairing of dibenzo-24-crown-8 complexes with sodium monoalkyl benzeneazophosphonates in acetonitrile

Conductance study of ion-pairing of dibenzo-24-crown-8 complexes with a series of sodium monoalklyl benzeneazophosphonates in acetonitrile has been described. The values of the ion-pair dissociation constants, K D, and the limiting conductances, Λ O, of the complexes are reported. The results obtained are compared and discussed relative to the corresponding complexes of the 18-membered crown ether.

A Raman spectroscopic study of the effect of ion-pairing on the structure of the triiodide and tribromide ions

The changes in the Raman spectra of aqueous solutions of I 2 in HI (1:1 mole ratio) with changing concentration, have been shown to be a result of ion-pairing: H + + I − 3 = H +I − 3. The anomalous high frequency stretching band of the solute at 172 cm −1 for these solutions arises mainly from stretching of the stronger II bond in the H +I − 3 ion-pair and not from vibrational modes of I 2 or I − 5. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known I − 3 ion-pair and not from vibrational modes of I 2 or I − 5. Ion-pair dissociation constants estimated from the Fuoss equation, combined with the known I − 3 and I − 5 formation constants, account for the intensity changes of the stretching bands. The spectra of solutions of Br 2/HBr at 1:1 mole ratio may be interpreted in the same way but other Br species are present. The spectra of the isolated X − 3 ions in solution exhibit a shoulder to high frequency of the symmetric stretching band, ν 1. In the spectrum of the I − 3 ion, this peak is assigned to one of the two frequencies, resulting from Fermi resonance between ν 1 and 2ν 2 but, in the case of the Br − 3 ion, this peak may be due to ν 3, which becomes Raman active as a result of disymmetric solvation. The consequences of such ion-pairing for the nature of I 2 dissolved in polymers are discussed.

Conductance study of ion-pairing of alkali tetrafluoroborates and hexafluorophosphates in acetonitrile containing some substituted macrocyclic polyethers

The conductance of sodium and potassium tetrafluoroborates and hexafluorophosphates in acetonitrile solutions at 25°C in the presence of bis(4-tert-butylbenzo)-18-crown-6(mtbDBC), bis(3,5-di-tert-butylbenzo)-18-crown-6 (dtbDBC) and bis(4-hexadecylbenzo)-18-crown-6 (hdDBC) has been investigated. The analysis of data has been performed by means of the Fuoss—Hsia conductance equation. Values of the limiting conductance, Λ 0, and ion-pair dissociation constants, K D , are reported. The results obtained are compared with those obtained for the conductance of sodium and potassium tetrafluoroborates and hexafluorophosphates in pure acetonitrile. Addition of macrocyclic polyethers in the salt solutions gives a decrease in the conductivity in the order dtbDBC < mtbDBC < hdDBC. All systems were found to be associated, and the trends in association are discussed in terms of the polyether interactions with alkali cation which are controlled by their size, the steric hindrance effects caused by the alkyl substituents in the polyether molecule, size and charge density of the anion, and the interaction forces of the solvent molecules on the ionic species present in solution.

Spectrophotometric Determination of Dissociation Constant of Picrates in 4-Methyl-2-pentanone

Solvent extraction of lithium, sodium, potassium and tetrabutylammonium(tba+) picrates in aqueous solutions into 4-methyl-2-pentanone(MIBK) was determined at 298K. The absorption spectrum of the MIBK solutions was measured. When the salt concentration was low, the distribution ratio of picrate was constant but it increased by an increase in the concentration. This was explained in terms that the salts were extracted as ion pairs and dissociated nearly completely in the organic phase in the lower concentration range but the dissociation became incomplete as the concentration increased. The extraction as well as the dissociation in the organic phase of the three alkaline picrates were nearly similar to each other. However, the extraction of tba+ picrate was much better and the ion pairs dissociated a little more than the alkali picrates. The absorption spectrum of MIBK solutions of tba+ picrate was similar to that of its aqueous solutions and the spectrum did not change even in the higher concentration range where a partial association of the ions was assumed from the solvent extraction data. The spectrum of MIBK solutions of alkali metal picrates agreed with that of tba+ salt when the concentration was low but the peak shifted to the lower wavelength when the concentration increased. This shift was explained in terms of incomplete dissociation of the ion pairs and the dissociation constants were determined by a dual-wavelength method. The dissociation constants of the alkali picrate ion pairs in MIBK obtained from the spectrophotometric data well agreed with those obtained from the solvent extraction data.

Solution Properties of Tetraalkylammonium Halide and Alkali-Metal Halide Ion Pairs. The Relationship Between Dissociation Constants and Solubility Products

An empirical relation describing the effect of solvent on the dissociation constant ( Kd ) of ion pairs is described. An equation of the form pKd = apKso + bD-1 + c ( Kso and D stand for the solubility product of a given salt and for the electric permittivity of a solvent, respectively) has been tested with 13 sets of experimental data for salts containing tetraalkylammonium and alkali-metal cations. A successful correlation was obtained in 100% of the cases considered.