Ionic association in aqueous solutions of strong electrolytes

A more accurate calculation of relaxation effects obtained with the standard Debye-Hückel-Onsager model has been presented recently and is here applied to several aqueous 1:1 electrolytes. The variation of the standard deviation between calculated and observed equivalent conductivities withK _A leads to an ill-defined minimum; but, where data over a wide concentration range are available, the minimum corresponds to values of the contact distancea which approximate to estimates from ionic dimensions. It is therefore proposed that, although preciseK _A values from conductance cannot be determined, the most probable values are those associated with realistic estimates ofa. When data cover a limited concentration range, minimum standard deviations are often indeterminate or vary greatly for duplicate runs. It is shown that reasonable values ofK _A can be obtained from such data if comparison is made at estimated values ofa.Notation The symbols not defined in the text are the following b e ²/kTa for 1:1 electrolytes - e electronic charge - k Boltzmann gas constant - T absolute temperature - dielectric constant of solvent - –(3/2y)(_e0/₀)     

Divalent sulfate ion pairs in aqueous solutions at pressures up to 2000 atm

Electrical conductance measurements have been made at various temperatures and at concentrations from ca. 0.00005 to ca. 0.01M in aqueous solutions of MgSO₄, CaSO₄, CoSO₄ and NiSO₄ at pressures up to 2000 atm. The data have been analyzed with the Fernandez-Prini modification of the Fuoss-Hsia conductance equation to obtain the molal dissociation constants K _m at each pressure. The results for V^°, obtained from the pressure dependence ofK _m,are compared with other experimental data and with values calculated from acoustically derived multistate dissociation models.     

Electrical conductances of tetrabutylammonium bromide and tetrapentylammonium bromide in 2-ethoxyethanol + water mixtures at 308.15, 313.15, 318.15 and 323.15 K

The electrical conductances of the solutions of tetrabutylammonium bromide (Bu₄NBr), and tetrapentylammonium bromide (Pen₄NBr) in 2-ethoxyethanol (1) + water (2) mixed solvent media containing 0.25, 50 and 0.75 mass fractions of 2-ethoxyethanol (w ₁) have been reported at 308.15, 313.15, 318.15 and 323.15 K. The conductance data have been analyzed by the 1978 Fuoss conductance–concentration equation in terms of the limiting molar conductance (Λ⁰), the association constant (K _A) and the association diameter (R). These two electrolytes are found to exist essentially as free ions in the solvent mixtures with w ₁ = 0.25 and 0.50 over the entire temperature range; however, slight ionic association was observed in the mixed solvent medium richest in 2-ethoxyethanol. The electrostatic ion–solvent interaction is found to be very weak for the tetraalkylammonium ions in the aqueous 2-ethoxyethanol mixtures investigated.     

Treating the polyelectrolytes as polymers with a draining effect. I. The statistical segment length

The statistical segment length of polyelectrolytes, A, in aqueous salt solutions is derived from the unperturbed dimensions parameter, K_θ, that is obtained from the graphical representations based on the Stockmayer–Fixman–Burchard and Dondos–Benoît equations. In order to obtain A from K_θ we use values of the Flory's parameter, Φ, which are given from an empirical equation established for the polymers presenting a draining effect, such as the wormlike polymers. With these values of Φ, the obtained values of A for different polyelectrolytes in aqueous salt solutions of different ionic strength are found very close to the values obtained from other more complicated methods. This result shows that the polyelectrolytes can be considered as polymers presenting a draining effect. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4225–4229, 2004     

Conductance of Some 1:1 Electrolytes in N,N-Dimethylacetamide at 25°C

Precise conductance measurements of solutions of lithium chloride, lithium bromide, lithium iodide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluoroarsenate, tetrabutylammonium bromide, and tetrabutylammonium tetraphenylborate in N,N-dimethylacetamide are reported at 25°C in the concentration range 0.005–0.015 mol-dm^–3. The conductance data have been analyzed by the 1978 Fuoss conductance equation in terms of the limiting molar conductance (⁰), the association constant (K _a), and the association diameter (R). The limiting ionic conductances have been estimated from an appropriate division of the limiting molar conductivity of the reference electrolyte Bu₄NBPh₄. Slight ionic association was found for all these salts in this solvent medium. The results further indicate significant solvation of Li⁺ion, while the other ions are found to be unsolvated in N,N-dimethylacetamide.     

Polynuclear Branched Tetrazole Systems. 2. New 2-(5-Tetrazolyl)ethylpodands and Their NH-Acidity

Nine new polynuclear 2-(5-tetrazolyl)ethyl podands have been obtained by the azidation of the corresponding nitriles. Using Bjerrum distribution functions, the values of pK _a ¹, pK _a ², pK _a ³, and pK _a ⁴ have been determined by a potentiometric method for 14 polynuclear tetrazoles in aqueous and aqueous methanolic solution. The found values lie in the range from 3.5 to 7.5 pH units. The overall rules and the sequence of the ionization of the spatially separated tetrazole fragments in these podand systems are discussed.     

Ionization of aqueous toluic acids: Conductance and thermodynamics

Conductivities of aqueous solutions ofortho-, meta-, andpara-toluic acids have been measured for the concentration range 0.1–2 millimolar and at 5° intervals from 5 to 100°C. At each temperature pK _a(m) andA ₀ have been calculated using the paired ion model recently described by Fuoss. Thermodynamic parameters have been calculated for the ionization of each acid, and Walden products for the anions. Results are discussed in terms of contributions to acidity by enthalpy and entropy changes as well as by hydration of the various solute species.     

Solution Properties of Ternary D-Glucose + 1-Ethyl-3-methylimidazolium Ethyl Sulfate + Water Solutions at 298.15 K

The effect of an ionic liquid, 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIm]ESO₄), on the thermophysical properties of aqueous D-glucose solutions including density, viscosity, and electrical conductivity have been investigated at 298.15 K. Using these properties, the apparent molar volumes, V _φ , the viscosity B-coefficients and the molar conductivities, Λ _m, have been computed for the ternary D-glucose + [EMIm]ESO₄+water solutions. The V _φ values were used to calculate the standard partial molar volumes, V_f⁰V_{\phi}^{0}, and transfer volumes, D_trV_f⁰\Delta_{\mathrm{tr}}V_{\phi}^{0}, of D-glucose from water to aqueous ionic liquid solutions. These volumetric parameters, for all the solutions studied, are positive and increase monotonically with increasing the concentration of [EMIm]ESO₄. These observations have been interpreted in terms of the interactions between D-glucose and ionic liquid in the aqueous solution. The viscosity data were analyzed in terms of the Jones-Dole equation to determine the values of the viscosity B-coefficients. The calculated conductometric parameters, the limiting molar conductivities, Λ ₀, the association constants, K _a, and the Walden products, Λ ₀ η, for [EMIm]ESO₄, decrease with increasing concentration of D-glucose. This trend suggests that the ions of an ionic liquid do not have the same hydrodynamic size in the presence of D-glucose molecules (ILs) and consequently provides evidence for the dehydration effect of the ionic liquid in aqueous D-glucose solutions.     

Oil-Water Partitioning of a Synthetic Tetracarboxylic Acid as a Function of pH

The oil-water partitioning of a synthetic tetraacid acting as a model compound for indigenous C₈₀-C₈₂ ARN acids has been studied as a function of pH, ionic strength and type of monovalent counterion. Experimental data obtained with ultraviolet-visible and HPLC/UV analyses have been fitted to thermodynamic models based on one, two or four dissociation steps to obtain o/w partition coefficients (K _wo ) of the fully protonated acid between chloroform and aqueous solutions, and its apparent acidity constant(s), pK _a. As the study is conducted above the CMC of the tetraacid, in general high apparent acidity constants were obtained in the range from 6 to 8 resulting from micellization equilibria. K _wo values were obtained in the range from 10^?3 to 10^?4, and decreasing with increasing salinity. At 50 mM K⁺, no conclusions could be made regarding the number of distinguishable dissociation steps, while at higher ionic strength (184 mM and 452 mM K⁺) and at 184 mM Na⁺ a model with two dissociation steps provided good fits to the experimental data. The first step was found to be given by a pK _a ≈ 6.6–6.8 and the second dissociation step at pK _a values ≈ 7.8–8.3. The two-step mechanism supports previous results obtained by potentiometric titrations. No significant difference in the o/w behavior was observed when changing the counterion from potassium to sodium. The main partitioning of the tetraacid in the aqueous phase occurred above pH 8, where the fully deprotonated acid was formed.     

Atom-atom potential functions for simulation of DNA—counterion interaction in aqueous solution

A system of atom-atom potential functions for computer simulation of aqueous solutions of DNA fragments and counterions was developed. Hydration of Na⁺, K⁺, and dimethyl phosphate (DMP⁻) ions was simulated by the Monte Carlo method. The obtained energy and structural characteristics of the solutions reproduce well the experimental data and are in good agreement with the results ofab initio calculations carried out by other authors. Translated fromIzvestiya Akademii Nauk Seriya Khimicheskaya, No. 11, pp. 2166–2173, November, 1998.     

Application of the Bjerrum Association Model to Electrolyte Solutions. III. Temperature and Pressure Dependencies of Association Constants

The Bjerrum association model, developed in 1926, is now incorporated in many conductance theories of electrolyte systems to extract ₀ and K _A from experimental data. The Bjerrum concept is simply a convenient way of taking into account short-range electrostatic interactions between ions. The equations of the Bjerrum model can be applied to the prediction of the temperature and pressure dependencies of K _A from the value of K _A at a reference T and P and from the dielectric properties of the solvent. This feature will be essential when the relaxation effect is taken into account when applying the model to heat capacities and compressibilities. These equations were tested against literature K _A values (obtained from treatment of conductance data by equations that incorporate the Bjerrum concept) in aqueous electrolyte solutions at high temperatures and pressures and in some electrolyte systems in acetonitrile, 2-butanone, propylene carbonate, -butyrolactone, and propanol. In the absence of specific interactions in solution, the agreement between experimental and predicted K _A are generally quite good. Notable exceptions are acids and bases in water, lithium perchlorate in most solvents, and the majority of electrolytes in propylene carbonate, suggesting that specific interactions in these systems may cause the model to fail.     

Ionic Association of the Ionic Liquids [C4mim][BF4], [C4mim][PF6], and [Cnmim]Br in Molecular Solvents

Considering the ionic nature of ionic liquids (ILs), ionic association is expected to be essential in solutions of ILs and to have an important influence on their applications. Although numerous studies have been reported for the ionic association behavior of ILs in solution, quantitative results are quite scarce. Herein, the conductivities of the ILs [C_nmim]Br (n=4, 6, 8, 10, 12), [C₄mim][BF₄], and [C₄mim][PF₆] in various molecular solvents (water, methanol, 1‐propanol, 1‐pentanol, acetonitrile, and acetone) are determined at 298.15 K as a function of IL concentration. The conductance data are analyzed by the Lee–Wheaton conductivity equation in terms of the ionic association constant (K_A) and the limiting molar conductance (Λ_m⁰). Combined with the values for the Br^? anion reported in the literature, the limiting molar conductivities and the transference numbers of the cations and [BF₄]^? and [PF₆]^? anions are calculated in the molecular solvents. It is shown that the alkyl chain length of the cations and type of anion affect the ionic association constants and limiting molar conductivities of the ILs. For a given anion (Br^?), the Λ_m⁰ values decrease with increasing alkyl chain length of the cations in all the molecular solvents, whereas the K_A values of the ILs decrease in organic solvents but increase in water as the alkyl chain length of the cations increases. For the [C₄mim]⁺ cation, the limiting molar conductivities of the ILs decrease in the order Br^?>[BF₄]^?>[PF₆]^?, and their ionic association constants follow the order [BF₄]^?>[PF₆]^?>Br^? in water, acetone, and acetonitrile. Furthermore, and similar to the classical electrolytes, a linear relationship is observed between ln K_A of the ILs and the reciprocal of the dielectric constants of the molecular solvents. The ILs are solvated to a different extent by the molecular solvents, and ionic association is affected significantly by ionic solvation. This information is expected to be useful for the modulation of the IL conductance by the alkyl chain length of the cations, type of anion, and physical properties of the molecular solvents.     

Thermodynamic quantities for the acid dissociation of alkylammonium ions in water over a wide temperature range

Enthalpies of protonation in aqueous solution have been determined from calorimetric measurements between 0 and 100°C for methylpentylamine, butyldimethylamine, and quinuclidine; between 0 and 125°C for di-iso-propylamine; and between 0 and 75°C for 1-azacycloheptane. Values ofpK_a at 25°C have been determined from pH measurements using a glass electrode for di-iso-propyl-, methylpentyl-, and butyldimethylamine. The calorimetric results are used together with values ofpK_a at 25°C to derive expressions which allow the calculation of accuratepK_a values over a large temperature range. The heat capacity and entropy changes for the protonation of aqueous alkylamines found in the present and in previous studies are discussed in terms of differences in hydrophobic solvation of the alkyl groups in the neutral amines and the ions.     

Complex Formation and Ionic Association in Methanol Solutions of Poly(Ethylene Oxide) and Alkaline Earth Salts

Complex formation of poly(ethylene oxide) (PEO) with divalent barium and strontium salts was investigated in methanol. In these systems the complexation was accompanied by a considerable degree of ionic association. An analytical model for the polymer-ion complexation based on a one-dimensional lattice model was proposed. According to this model, the electrostatic effects between the bound ions were separated from the total free energy change of the binding. Three binding constants, i.e., the ionic association constant K _A, the cation binding constant, K _c, and the anion binding constant, K _a, could be estimated. K _A for barium and strontium salts was comparable, and the effect of counteranions on K _A was not large. K _c for barium salts was almost independent of the kind of counteranion and larger than that for corresponding strontium salts, indicating stronger polymer-ion interaction for barium salts. The anion binding constant, K _a, was strongly dependent upon the kind of anion, and the order was CI^? ? ? 4 ^?. The pronounced ion binding for larger anions may be explained by the more favorable free energy change of desolvation. Finally, the concentration of free and bound ionic species was determined as a function of PEO concentration.     

Individual equivalent conductances of the major ions in seawater

A radioactive tracer method has been used to determine the trnasference numbers of eight major ions in seawater of salinity 38.4 g-kg ^–1 at 25°C. Data are presented for Na⁺, K⁺, Ca ²⁺ , and Mg ²⁺ cations and Cl^–, Br^–, SO ₄ ^2– , and HCO ₃ ^– anions, which contribute to the overall electrical conductance by more than 99%. The results have been checked by electrophoretic migrations on esters of cellulose strips providing independent values of the ratios of ionic mobilities to be compared to previous estimations. Through the individual ionic contributions to the electrical conductance, a method for calculating seawater density is proposed; this method is based on a nine-constitutents seawater model in which relative ionic concentrations can deviate from that of standard seawater.     

Theoretical Study of Firefly Luciferin pKa Values—Relative Absorption Intensity in Aqueous Solutions

Ground‐state vibrational analyses of firefly luciferin and its conjugate acids and bases are performed. The Gibbs free energies obtained from these analyses are used to estimate pK_a values for phenolic hydroxy and carboxy groups and the N–H⁺ bond in the N‐protonated thiazoline or benzothiazole ring of firefly luciferin. The theoretical pK_a values are corrected using the experimental values. The concentrations of these chemical species in solutions with different pH values are estimated from their corrected pK_a values, and the pH dependence of their relative absorption intensities is elucidated. With the results obtained we assign the experimental spectra unequivocally. Especially, the small peak near 400 nm at pH 1–2 in experimental absorption spectra is clarified to be due to the excitation of carboxylate anion with N‐protonated thiazoline ring of firefly luciferin. Our results show that the pK_a values of chemical species, which are contained in the aqueous solutions, are effective to assign experimental absorption spectra.     

A conductance study of the binding of benzo-15-crown-5 with alkali cations in acetonitrile

The decrease in the molar electrolytic conductance of Na⁺, K⁺, Rb⁺, and Cs⁺ tetraphenylborates, caused by the addition of benzo-15-crown-5 in acetonitrile at constant ionic strength, is analyzed according to a model involving 1:1 stoichiometry. The stability constant,K, and the limiting molar conductivity, _c , for each 1:1 complex are determined from the conductance measurements by using a nonlinear least squares curve fitting procedure. The stability sequence of the 1:1 complexes, as deduced from data at 288, 293, 298, 303, and 308 K, has the order Na⁺>K⁺>Rb⁺>Cs⁺. Values of H ⁰, S ⁰, and _c at 298 K are reported and their significance is discussed.     

Benchmarking and validating algorithms that estimate p<Emphasis Type="Italic">K</Emphasis><Subscript>a</Subscript> values of drugs based on their molecular structures

The REGDIA regression diagnostics algorithm in S-Plus is introduced in order to examine the accuracy of pK _a predictions made with four updated programs: PALLAS, MARVIN, ACD/pKa and SPARC. This report reviews the current status of computational tools for predicting the pK _a values of organic drug-like compounds. Outlier predicted pK _a values correspond to molecules that are poorly characterized by the pK _a prediction program concerned. The statistical detection of outliers can fail because of masking and swamping effects. The Williams graph was selected to give the most reliable detection of outliers. Six statistical characteristics (F _exp, R ², , MEP, AIC, and s(e) in pK _a units) of the results obtained when four selected pK _a prediction algorithms were applied to three datasets were examined. The highest values of F _exp, R ², , the lowest values of MEP and s(e), and the most negative AIC were found using the ACD/pK _a algorithm for pK _a prediction, so this algorithm achieves the best predictive power and the most accurate results. The proposed accuracy test performed by the REGDIA program can also be applied to test the accuracy of other predicted values, such as log P, log D, aqueous solubility or certain physicochemical properties of drug molecules.     

Determination of thermodynamic pKa values of pharmaceuticals from five different groups using capillary electrophoresis

A determination of the thermodynamic acid dissociation constants (pK_a) of 22 frequently used pharmaceuticals using capillary electrophoresis in aqueous media is presented in this work. The investigated pharmaceuticals belong to different pharmacological groups: macrolides, fluoroquinolones, sulfonamides, β‐lactams, tetracyclines, and other miscellaneous pharmaceuticals. The electrophoretic mobilities of the investigated analytes were monitored in a pH range from 2.00 to 10.82. The data were fitted with an appropriate mathematical model using a nonlinear regression analysis to obtain pK_a values. Experimentally obtained data were well described by the mathematical model chosen for each analyte that was confirmed by r² values higher than 0.99 for most of the investigated analytes. Extrapolations to zero ionic strength were used to determine the thermodynamic pK_a values. Experimentally obtained acid dissociation constants were interpreted using structural formulae of investigated analytes and the moieties corresponding to specific pK_a were identified.