Showing papers in "Journal of Solution Chemistry in 1995"

Excess partial molar enthalpies, entropies, Gibbs energies, and volumes in aqueous dimethylsulfoxide

Abstract: The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)−H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H2O The values of the excess partial molar Gibbs energies for both DMSO and H2O are negative over the entire composition range The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H2O Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H2O molecules from exposure to the nonpolar alkyl groups of DMSO

Magnetite solubility and phase stability in alkaline media at elevated temperatures

Abstract: A platinum-lined flowing autoclave facility was used to investigate the solubility behavior of magnetite (Fe3O4) in alkaline sodium phosphate and ammonium hydroxide solutions between 21 and 288°C. Measured iron solubilities were interpreted via a Fe(II)/Fe(III) ion hydroxo-, phosphato-, and ammino-complexing model and thermodynamic functions for these equilibria were obtained from a least-squares analysis of the data. A total of 14 iron ion species were fitted. Complexing equilibria are reported for 8 new species: Fe(OH)(HPO4)−, Fe(OH)2(HPO4)2−, Fe(OH)3(HPO4)2−, Fe(OH)(NH3)+, Fe(OH)2(PO4)3−, Fe(OH)4(HPO4)3−, Fe(OH)2(H2PO4)−, and Fe(OH)3(H2PO4)3−. At elevated temperatures, hydrolysis and phosphato complexing tended to stabilize Fe(III) relative to Fe(II), as evidenced by free energy changes fitted to the oxidation reactions. $$\begin{gathered} Fe(OH)_3^ - + H_2 O_ \leftarrow ^ \to Fe(OH)_4^ - + (1/2)H_2 (g) \hfill \\ Fe(OH)_2^{} (HPO_4 )^{2 - } + H_2 O_ \leftarrow ^ \to Fe(OH)_3 (HPO_4 )^{2 - } + (1/2)H_2 (g) \hfill \\ \end{gathered}$$ For temperatures below 83°C and for a dissolved hydrogen concentration of 234 μmol-kg−1, the activity of ferrous iron in aqueous solution is controlled by a hydrous Fe(II) oxide solid phase rather than magnetite.

Dielectric relaxation of electrolyte solutions in acetonitrile

Abstract: Complex permittivity spectra in the frequency range 0.95≤v (GHz)≤89 for acetonitrile and its solutions of LiBr, NaI, NaClO4, and Bu4NBr at 25°C show one Debye equation for the neat solvent whereas the superposition of a Debye process for the solute and a Cole-Cole distribution for the solvent is necessary to account for the dielectric relaxation behavior of the solutions. The reorientation of bulk acetonitrile is diffusive and only weakly coupled to viscosity. The number of solvent molecules irrotationally bound to the electrolyte is in good agreement with conventional solvation numbers for all electrolytes, when kinetic depolarization is assumed to be negligible. The solute relaxation process is dominated by the formation kinetics and reorientation of contact ion pairs. There is evidence for solvent-shared ion pairs in dilute NaClO4 solutions.

The enthalpies of interaction of glycine with some amides and ureas in water at 25°C

Abstract: Measurement of solution enthalpyiesof glycine in aqueous solutions of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), monomethylurea (MMU), 1,3-diethylurea (DEU) and tetramethylurea (TMU) at 25°C have been undertaken. On the basis of the results, enthalpic coefficients of heterotactic interactions between a glycine zwitterion and a molecule of organic substance in aqueous solutions have been calculated. Using the additivity of groups concept by Savage and Wood (SWAG), contributions of each functional group of studied amides and ureas have been estimated. In this model a zwitterion of glycine has been considered as an individual equal to a single functional group.

The second dissociation constant of sulfuric acid at various temperatures by the conductometric method

Abstract: The thermodynamic second dissociation constant K2 for sulfuric acid has been determined by conductivity measurements of aqueous sulfuric acid solutions at various temperatures. The data are treated by using two different methods developed with two independent assumptions due to Noyes et al. and Shedlovsky. Both methods require the knowledge of relevant ionic conductivities, which may be calculated from the Onsager limiting law. The values for K2 obtained with these two methods show excellent agreement. The value of 0.0103 mol-L−1 at 25°C agrees with the ‘best’ literature value of 0.0102 mol-L−1 within the experimental uncertainty, as also does the enthalpy of dissociation which is derived from the derivative of the temperature coefficient of K2.

Aqueous uranyl complexes. 3. Potentiometric measurements of the hydrolysis of uranyl(VI) ion at 25°C

Abstract: Potentiometric titrations of uranyl(VI) solutions were conducted using a standard glasslcalomel electrode combination over the pH range 3 to 12 at 0.1 molkg−1 ionic strength with tetramethylammonium trifluoromethanesulfonate as the supporting electrolyte. The electrodes were calibrated directly on the hydrogen ion concentration scale during the initial stage of each titration. The species, UO 2 2+ , (UO2)2(OH) 2 2+ , (UO2)3(OH) 5 + , (UO2)3(OH) 7 − , (UO2)3(OH) 8 2− , and (UO2)3(OH) 10 4− identified in an earlier Raman study were compatible with the analysis of the titration data. Based on this analysis and application of the extended Debye-Huckel treatment, the polynuclear species indicated above were assigned overall formation constants at 25°C and at infinite dilution of −5.51±0.04, −15.3±0.1, −27.77±0.09, −37.65±0.14, and −62.4±0.3, respectively. The results are discussed in reference to hydrolysis quotients reported in the literature for the first three species. Formation quotients for the last two species have not been reported previously.

Ion interaction approach to calculations of volumetric properties of aqueous multiple-solute electrolyte solutions

Abstract: The ion interaction approach developed by Pitzer was used for the prediction of volumetric properties of mixed electrolyte solutions at 25°C based on parameters calculated from experimental data for single-solute electrolyte solutions. Such an approach was shown to be especially effective for application to the calculation of volumetric properties of natural hypersaline brines and of industrial electrolyte solutions of large complexity. The use of the latest recommended sets of volumetric ion interaction parameters for single electrolyte solutions and symmetrical mixing parameters for Na−K−Cl ion combinations considerably improved the precision of the density calculations of highly concentrated mixed electrolyte solutions and of various natural waters.

Vapor pressure of dimethyl sulfoxide and water binary system

Abstract: The total vapor pressures of dimethyl sulfoxide (DMSO) and water mixtures have been measured at 15, 20, 25 and 30°C. The activity coefficients and molar excess Gibbs energies of the system have been calculated. Possible association interaction in the system are discussed.

Apparent molar volumes of aqueous calcium chloride to 250°C, 400 bars, and from molalities of 0.242 to 6.150

Abstract: Relative densities of CaCl2(aq) with 0.22≤ml(mol-kg−1)≤6.150 were measured with vibrating- tube densimeters between 25 and 250°C and near 70 and 400 bars. Apparent molar volumes VΦ calculated from the measured density differences were represented with the Pitzer ioninteraction treatment, with appropriate expressions chosen for the temperature and pressure dependence of the virial coefficients of the model. It was found that the partial molar volume at infinite dilution VΦo, and the second and third virial coefficients BV and CV, were necessary to represent VΦ near the estimated experimental uncertainty. The ionic-strength dependent β(1)v term in the BV coefficient was included in the fit. The representation for VΦ has been integrated with respect to pressure to establish the pressure dependence of excess free energies over the temperature range studied. The volumetric data indicate that the logarithm of the mean ionic activity coefficient, ln γ±(CaCl2), increases by a maximum of 0.3 at 400 bars, 250°C, and 6 mol-kg−1 as compared with its value at saturation pressure.

Activity coefficients for NaBr in ethanol-water mixtures at 25°C

Abstract: Activity coefficients for NaBr in ethanol-water mixtures with 0, 20, 40, 60, 70, 80, 90 and 99.9 weight% of ethanol have been determined at 25°C from the emf measurements of the galvanic cells $$Na - glass/NaBr(m),H_2 O(100 - Y),EtOH(Y),AgBr_{(S)} /Ag$$ The results obtained were analyzed by using the Pitzer equation. Those for the mixtures with ethanol content higher than 60 wt% were also analyzed by using a chemical model and their ion pair formation constants, K A 0 , determined and examined. The results were compared with those for NaCl reported in a previous paper.

Solvent effects on complexation of crown ethers with LiClO4, NaClO4 and KClO4 in methanol and acetonitrile

Abstract: The thermodynamic formation constants Kf for complexation of Li+, Na+ and K+ with the crown ethers 12C4 and 15C5 have been determined in methanol and acetonitrile at 25°C using precision conductivity data. The method permits evaluation of very small Kf values (e.g., Kf=6.98 mol−1-dm3 for LiClO4+12C4 in methanol) as well as fairly large values (e.g., Kf=2.73×104 mol−1-dm3 for NaClO4+15C5 in acetonitrile). The determination of Kf values from conductivity data takes into consideration the often neglected ion pair formation of both the uncomplexed and the complexed cations. Our results for Kf are generally consistent with previously reported values based on potentiometry, calorimetry and polarography, but there are significant differences in several cases which we attribute to neglect of ion association both for the uncomplexed or “free” cation Ka and the macrocyclic complexed cation Ka2. Our results are also consistent with the well known concepts relating the magnitude of Kf to both the cavity diameter and ion-solvent interactions. Limiting molar conductivities Λ 2 0 for the complex salt (M-crown ether) (ClO4) in both solvents were generally found to be smaller or very close to the corresponding quantity Λ 1 0 for the binary MClO4-solvent system. However, in methanol, single ion limiting molar conductivities for the cationic complexes λ 2 0 exhibit anomalous behavior which is attributed to solvation differences between “free” cations and complexed cations.

Excess molar volumes of diethyl carbonate with hydrocarbons or tetrachloromethane at 25°C

Abstract: The excess molar volumes V m E at atmospheric pressure and at 25°C for binary mixtures of diethyl carbonate with n-heptane, n-decane, n-tetradecane, 2,2,4-trimethylpentane, cyclohexane, benzene, toluene, or tetrachloromethane have been obtained over the whole mole-fraction range from densities measured with a vibrating-tube densimeter. The V m E are positive for all the systems investigated, except for the mixture with toluene which is negative. The results for V m E together with data previously published on excess molar enthalpies H m E and excess molar Gibbs energies G m E , suggest interactions between carbonate and hydrocarbons which are stronger with aromatic than with aliphatic hydrocarbons.

Variation ofET(30) polarity and the Kamlet-Taft solvatochromic parameters with composition in alcohol-alcohol mixtures

Abstract: Solvatochromic parameter values, namely ET(30) polarity, and Kamlet-Taft π*, α and β, in alcoholic binary mixtures have been determined and correlated with solvent composition. The studied mixtures involve ethanol, methanol, 2-methoxyethanol and 1,2-ethandiol. Theoretical and semiempirical equations which embody preferential solvation of solvatochromic probes and allow the accurate interpolation of solvatochromic parameters are proposed.

Thermodynamics of nucleic acid bases and nucleosides in water from 25 to 55°C

Abstract: Specific heat capacities, apparent molar heat capacities, densities, and apparent molar volumes have been determined for cytosine, uracil, thymine, adenine, cytidine, 2′-deoxycytidine, uridine, thymidine and adenosine at temperatures from 25°C to 55°C. The results of these measurements have been used to calculate for the first time, the thermodynamic quantities:C p,2 o , (∂C p,2 o /∂T)p, (∂2 C p,2 o ∂T 2)p,V 2 o , (∂V 2 o /∂T)p, and (∂2 V 2 o /∂T 2)p. The-CH2-group contribution has been calculated at different temperatures. It has also been observed from the data for the nucleic acid bases and nucleosides that the additivity ruleC p,2 o (nucleoside)-C p,2 o (base) +C p,2 o (water)=C p,2 o (ribose) does not hold in these cases.

Thermodynamic quantities for the protonation of amino acid amino groups from 323.15 to 398.15 K

Abstract: Flow claorimetry has been used to study the interaction of protons with glycine, DL-α-alanine, β-alanine, DL-2-aminobutyric acid, 4-aminobutyric acid, and 6-aminocaproic acid in aqueous solutions at temperatures from 323.15 to 398.15 K. By combining the measured heats for amino acid solutions titrated with NaOH solutions with the heat of ionization for water, the log K, ΔHo, ΔSo, and ΔCp o values for the protonation of the amino groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The ΔHo and ΔSo values increase while log K values decrease as temperacture increases. The trends for log K, ΔHo, ΔSo, and ΔCp o are discussed in terms of changes in long-range and short-range solvent effects. The trend in ΔHo, ΔSo, and ΔCp o values with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction between the two oppositely charged groups within the molecule.

Temperature and composition dependence of viscosity I. Propylene carbonate-dimethoxyethane mixtures and thermodynamics of fluid flow

Abstract: Concentration and temperature dependence of viscosity of propylene carbonate (PC)/dimethoxyethane (DME) mixtures were studied at temperatures between −45 and 25°C. Among various semiempirical ≠(x)-functions available for data analysis, only the McAllister equation fits the data precisely. Thermodynamic quantities obtained from such an analysis results in an interpretation of the structure of this binary system consistent with results from other methods.

Effect of temperature on the solubility of carnallite type double salts

Abstract: A method based on Pitzer's model has been used for calculation of the solubilities of carnallite type double salts crystallizing in the systems MeX−MgX2−H2O (Me=Li, NH4, K, Rb, Cs; X=Cl, Br). The solubility of congruently soluble double salts was also determined experimentally at 25–75°C. The results from studies of the solubility diagrams of ternary carnallite type water-salt systems over a wide temperature range are summarized. It is found that the width of the crystallization region for each of the salts can be explained by the relative differences in the solubilities of the ternary solution components (MeX, MgX2·6H2O and MeX·MgX2·6H2O) and by a change of temperature, and by the effect of temperature on the solubility. A method is proposed for calculating Pitzer's ternary parameters of interionic interaction (ΘMN and ΨMNX) on the basis of experimental data for the solubility in water of the double salts crystallizing in the corresponding ternary water-salt systems.

Differential interactions of cyclodextrins with hydrophobic derivatives of sepharose Cl-4B

Abstract: The partition coefficients Kav between the solution phase and octyl- and phenyl-sepharose CL-4B were determined for three cyclodextrins (CDs), α-CD, β-CD and γ-CD in various aqueous solutions, as a measure of their interactions with the two hydrophobic ligands. Kav of the CDs increased in the order of γ-CD<β-CD<α-CD for octyl-sepharose CL-4B and α-CD<γ-CD<β-CD for phenyl-sepharose CL-4B. In all cases, Kav increased by increasing NaCl concentration in the aqueous solution phase and also by lowering temperature, but in the presence of NaBr and NaSCN, both chaotropic salts, Kav decreased markedly. The spontaneity of the transfer of the CDs from the aqueous solution phases to the gel phases was due to the enthalpy decrease. It was shown that discrete separation of the three CDs can be achieved by the hydrophobic chromatography on a short column (1×25cm) of octyl-sepharose CL-4B by adjusting the NaCl concentration and temperature.

Inclusion complexes of alcohols with α-cyclodextrin

Abstract: Calorimetric studies of the inclusion complexes of straight and branched alcohols and of diols with alpha-cyclodextrin (α-CD) have been carried out in water solvent. The data suggest that straight and branched chain alcohols enter the cavity of α-CD alkyl end first. The hydroxyl group hydrogen bonds to the outer oxygen ring of the cyclodextrin. For branched chain alcohols the longer alkyl part of the molecule penetrates the α-CD cavity up to the hydroxyl group. Diols form two hydrogen bonds to the outer oxygen ring of the cyclodextrin with some penetration into its interior.

Acetonitrile + nitromethane: An example of ideal solvent mixtures

Abstract: Densities, viscosities, static dielectric constants and refractive indices of acetonitrile + nitromethane mixtures in the temperature range between 5 and 45°C are reported. Density and viscosity data, analyzed in terms of molar excess volumes and molar excess fluidities, show a weak interactions field between acetonitrile and nitromethane molecules. Lorentz-Lorenz molar refractivities and Kirkwood correlation factors, obtained from refractive indices and dielectric constants data, respectively, indicate that such weak interactions can be atributed to an antiparallel alignment between neighboring dipoles of unlike solvent molecules. Such specific features do not significantly perturb the local structures of the pure components so that acetonitrile + nitromethane mixtures can be considered, in a first approximation, one of the few ideal solvent mixtures for which the physical properties can be determined by applying the additivity rules to the pure components.

The solubility product of crystalline ferric selenite hexahydrate and the complexation constant of FeSeO 3

Abstract: The aqueous solubility of Fe2(SeO3)3·6H2O(c) was studied in deionized water adjusted to a range in pH values from 077 to 51 and in Na2SeO3 solutions ranging in concentrations from 00002 to 002 mol-dm−3 The studies were conducted from both the undersaturation and oversaturation directions, with equilibration periods ranging from 7 to 1725 days Stoichiometric dissolution of the solid was observed in solutions with pH values up to nearly 4 In general, concentrations of both Se and Fe decreased as pH increased from 1 to 4 Analyses of the equilibrated suspensions confirmed the equilibrium solid to be Fe2(SeO3)3·6H2O(c) and the aqueous Se to be selenite Pitzer's ion-interaction model was used with selected ion pairs to interpret the solubility data The logarithm of the solubility product of ferric selenite $$Fe_2 (SeO_3 )_3 6H_2 O(c) \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} 2Fe^{3 + } + 3SeO_3^{2 - } + 6H_2 O$$ was found to be −4158±011 This value is less than any reported in the literature for a ferric selenite by more than 10 orders of magnitude The solubility data and calculations show an extremely strong interaction between aqueous Fe3+ and SeO32−; interpretation of these data requires the inclusion of FeSeO3+ ie $$Fe^{3 + } + SeO_3^{2 - } \begin{array}{*{20}c} \to \\ \leftarrow \\ \end{array} FeSeO_3^ + , log K = 1115 \pm 011$$

Thermodynamic properties of water-β-cyclodextrin-dodecylsurfactant ternary systems

Abstract: Densities, heat capacities and conductivities of water-surfactant-β-cyclodextrin (β-CD) ternary systems were determined at 25°C. The surfactants studied were sodium dodecylsulfate (NaDS) and dodecyltrimethylammonium bromide (DTAB). From conductivity data, apparent critical micelle concentrations (cmc*) and degree of ionization of micelles were obtained at a fixed β-CD concentration (mCD). From the cmc* value and that in water (cmc) the stoichiometry of the surfactant-β-CD complex was calculated. At a given mCD, the apparent molar volume Vϕ,CD and heat capacity Cϕ,CD of β-CD in the two surfactants were calculated as functions of surfactant concentration mS. For both NaDS and DTAB, Vϕ,CD increases with mS up to about the cmc beyond which it decreases to a constant value at high mS, the opposite is observed for Cϕ,CD. With NaDS, a jump in the Cϕ,CD vs, mS trend was detected and ascribed to a structural NaDS micellar transition. The apparent molar volume VϕS and heat capacity CϕS of NaDS and DTAB in the water-β-CD mixture 0.017 m were also obtained. From these properties and those in pure water, the volume ΔVS and heat capacity ΔCS of transfer of the surfactant from water to water+β-CD mixture as functions of mS were calculated. For both surfactants, the ΔVS vs. mS trends increase to the cmc and then decrease in a monotonic manner, whereas ΔCS increases regularly with mS in the pre-micellar region and is essentially constant in the post-micellar region. The ΔVS vs. mS trends were qualitatively explained in terms of dispersed, complexed and micellized surfactant contributions.

Molar heat capacities and volumes of transfer of cytosine, thymine, caffeine and 1,3-diethylthymine to aqueous solutions of glycyl-glycine and L-α-alanyl-L-α-alanine at 25°C

Abstract: Densities and specific heat capacities of ternary aqueous systems containing dipeptides (glycyl-glycine or L-α-alanyl-L-α-alanine) and nucleic acid bases (cytosine or thymine) or their alkyl derivatives (1,3-diethylthymine or caffeine) were determined at 25°C by flow calorimetry and flow densimetry. The partial molar volumes and heat capacities of transfer at infinite dilution of the different nucleic acid bases from water to water+dipeptide solutions were obtained therefrom. Except for the case of the transfer of cytosine to aqueous glycyl-glycine solutions where a small positive dependence of the transfer quantities was observed with the dipeptide concentration, the values of the heat capacities of transfer were in general low, positive or negative, depending on the compensation of hydrophobic-hydrophilic interactions between the dipeptide and the base. The volumes of transfer of most of the bases are very small, within the limit of the experimental error.

Dissociation Quotient of Benzoic Acid in Aqueous Sodium Chloride Media to 250°C

Abstract: The dissociation quotient of benzoic acid was determined potentiometrically in a concentration cell fitted with hydrogen electrodes. The hydrogen ion molality of benzoic acid/benzoate solutions was measured relative to a standard aqueous HCl solution at seven temperatures from 5 to 250°C and at seven ionic strengths ranging from 0.1 to 5.0 molal (NaCl). The molal dissociation quotients and selected literature data were fitted in the isocoulombic (all anionic) form by a six-term equation. This treatment yielded the following thermodynamic quantities for the acid dissociation equilibrium at 25°C and 1 bar: logKa=−4.206±0.006, ΔH a o =0.3±0.3 kJ-mol−1, ΔS a o =−79.6±1.0 J-mol−1-K−1, and ΔC p;a o =−207±5 J-mol−1-K−1. A five-term equation derived to describe the dependence of the dissociation constant on solvent density is accurate to 250°C and 200 MPa.

Vapor pressure, speciation, and chemical activities in highly concentrated sodium borate solutions at 277 and 317°C

Abstract: The system H2O-B2O3-Na2O has been studied experimentally at 277 and 317°C. The activities of water and boric acid have been determined at mole ratios Na/B from 0 to 1.5, and total dissolved solids 3 to 80 weight percent. The activity of boric acid has been fitted to within experimental error using a speciation model with eight complex species. This model is consistent with the model previously published by Mesmer et al. The electrolyte properties of the liquid are modeled using the Pitzer-Simonson model of very concentrated electrolyte solutions. The calculated values of water activity agree with experiment, and the activity of NaOH and pOH have also been calculated. The potassium borate system also was briefly studied at 317°C, and is adequately described by a model with five complex species. The potassium borate liquid is more alkaline at K/B= 1 than a sodium borate liquid at the same mole ratio, but pOH in the two systems is the same at lower mole ratios.

Diffusion coefficients of the system α-cyclodextrin-n-butylurea-water at 25°C

Abstract: Diffusion coefficients were measured for the ternary system α-cyclodextrin(I)-n-butylurea(2)-water at three average concentrations. The cross coefficient D12 was found to be almost zero and D21 large and negative. These results are in agreement with the presence of an inclusion complex whose mobility is close to that of the host cyclodextrin molecules. The values of the four experimental diffusion coefficients are used to compute a value of the inclusion constant which is in reasonably good agreement with the calorimetric value.

Chiral recognition in aqueous solutions between enantiomeric α-amino acids bearing substituted alkyl chains at 25°C: The peferential configuration model

Abstract: Calorimetric, densimetric, and nuclear relaxation time measurements were carried out at 25°C on binary and ternary aqueous solutions containing L and D forms of the following amino acids bearing substituted alkyl chains: lysine, glutamine, arginine, asparagine, serine, and homoserine. For three of them (lysine, glutamine, and arginine), very high differences were found between the values of the homochiral and heterochiral pairwise enthalpic interaction coefficients. Volume and spectroscopic data showed that it is possible to detect chiral recognition by techniques other than calorimetric. The role of the zwitterionic interaction and of the substituted side chain is discussed to explain the enhanced chiral recognition of these α-aminoacids in respect to those bearing unsubstituted alkyl chains (alanine, α-aminobutyric acid, valines and leucines).

Electrical conductances of aqueous sodium trifluoromethanesulfonate from 0 to 450°C and pressures to 250 MPa

Abstract: The electrical conductances of dilute (0.001 to 0.1 mol-kg−1) aqueous sodium trifluoromethanesulfonate (NaCF3SO3) solutions have been measured from 0 to 450°C and pressures to 250 MPa. The limiting molar conductance\(\Lambda _0 \) increases with increasing temperature from 0 to 300°C and decreasing density from 0.8 to 0.3 g-cm−3. Above 300°C,\(\Lambda _0 \) is nearly temperature independent, but increases linearly with decreasing density. The logarithm of the molal association constant of NaCF3SO3 calculated at temperatures from 372 to 450°C is represented as a function of temperature (Kelvin) and density of water (g-cm−3) by $$\log K_m = 0.888 - 330.4/T - (12.83 - 5349/T)\log \rho _w $$

Ultrasonic study of the molecular encapsulation and the micellization processes of dodecylethyldimethylammonium bromide-water solutions in the presence of β-cyclodextrin or 2,6-di-o-methyl-β-cyclodextrin

Abstract: Aqueous solutions of β-cyclodextrin (β-CD) or 2,6-di-o-methyl-β-cyclodextrin (DM-β-CD) and dodecylethyldimethylammonium bromide (D12EDMAB) have been studied from speed of sound (u) data at 298.15 K, using a pulse-echo-overlap technique. The molecular encapsulation process of the surfactant monomer into the cyclodextrin cavity and its effect in the micellization process of the surfactant have been analyzed from theu measurements: I) as a function of [D12EDMAB] in the presence of several initial cyclodextrin concentrations (β-CD or.DM-β-CD); II) as a function of [cyclodextrin] (β-CD or DM-β-CD), for an initial micellar solution of D12EDMAB and; III) as a function of the [cyclodextrin]/[surfactant] stoichiometric concentrations. Both inclusion complexes formed (β-CD∶D12EDMAB) and (DM-β-CD∶D12EDMAB) have stoichiometries of 1∶1, and their association constantK have been determined using a model proposed in this work, based on the additivity of the different contributions of the involved species to the speed of sound. The apparent critical micellar concentration, cmc*, of D12EDMAB is found to increase linearly upon the addition of cyclodextrin (β-CD or DM-β-CD). The free surfactant concentration in the micellar region, [D12EDMAB]f, decreases in the presence of β-CD and slightly increases in the presence of DM-β-CD. The influence of the parcial methylation of the β-cyclodextrin (β-CD⇔DM-β-CD) and of the polar head of the surfactant (D12TAB ⇔D12EDMAB) on the complextion and micellar parameters are also discussed.

Thermodynamics of the solution equilibria of 3-hydroxypyridine and pyridoxine in water-dioxane mixtures

Abstract: The thermodynamics of tautomeric and microscopic ionization equilibrium constants of 3-hydroxypyridine and pyridoxine have been determined in waterdioxane mixtures (0–70% weight fraction in dioxane) ranging from 10°C to 50°C Generally, for both types of processes, the entropic contributions to the Gibbs energy predominate and this tendency increase concomitantly with the dioxane content in the media The thermodynamic parameters are consistent with the corresponding values obtained from macroscopic ionization processes An equation for the equilibrium constant K as a function of temperature and a solvent characteristic parameter is proposed to predict the K values throughout the whole range of the temperature and solvent compositions studied