Enthalpies of solution of urea in water-methanol mixtures at 298.15 K

The standard enthalpies of solution of urea in water-methanol mixtures were determined over the entire range of solvent compositions at 298.15 K. A comparison of the results obtained with the published data on mixtures of water with ethanol and n-propanol revealed a differentiating effect of the alcohol concentration on the enthalpy of solution of urea. Only for water-methanol solutions, an increase in the alkanol content in the mixture (0.6 mole fractions < x ₂ < 1.0 mole fractions) caused an increase in the degree of solvation of urea.     

Dissociation of salicylic acid, 2,4-, 2,5-and 2,6-dihydroxybenzoic acids in 1-propanol-water mixtures at 25°C

The dissociation constants and molar conductivities at infinite dilution of salicylic acid, 2,4-, 2,5-, and 2,6-dihydroxybenzoic acids have been determined in 1-propanol-water mixtures at 25°C. The experimental data have been analyzed by means of the Lee and Wheaton equation.     

氨基苯甲酸异构体在水-乙醇混合溶剂中的离解焓和熵

利用流动混合微量热法测定298.15K时单取代氨基苯甲酸邻,间,对三个异构体在水-乙醇混合溶剂中两个官能团分步离解的离解焓,结合相应的Gibbs自由能数据计算离解熵,从溶剂效应和取代基位置效应探讨其离解热力学行为。     

Solubility prediction of paracetamol in water-ethanol-propylene glycol mixtures at 25 and 30 degrees C using practical approaches

The solubility of paracetamol in water-ethanol-propylene glycol binary and ternary mixtures at 25 and 30 degrees C was determined using flask shake method. The generated data extended the solubility database for further computational investigations and also was used to assess the prediction capability of the Jouyban-Acree model. A new version of the model was proposed for modeling the solubility data in water-cosolvent mixtures with the cosolvent concentration of <50% which is required in pharmaceutical formulations. The accuracy of the predicted solubilities was evaluated by the mean percentage deviation (MPD) between the predicted and experimental solubilities. The overall MPD of the Jouyban-Acree model and the log-linear model of Yalkowsky for the entire composition range of the cosolvents were 11.0+/-8.7 and 55.4+/-17.8%, respectively; the corresponding values for the predicted solubilities in mixtures having a cosolvent concentration of <50% were 12.0+/-9.1 and 22.0+/-11.0%.     

Physicochemical characterization of ion-exchange membranes in water-methanol mixtures

A complete physicochemical characterization of two ion-exchange membranes—CM2 and Nafion^®117—used in electrodialysis and in direct methanol fuel cells (DMFC) has been carried out. For each membrane, in different methanol-water mixtures—0%, 20%, 40%, 60%, 80% and 100%—and at different temperatures (25.0; 40.0 et 55.0 °C), we have measured the variations of the geometrical dimensions, the proton electrical conductivity, the swelling rate and the amount of methanol in the membrane. The FTIR analysis of Nafion^®117 was performed at different methanol contents of the external solution.The results show that the CM2 membrane presents the best geometrical stability, and the lowest conductivity at any methanol content. At high methanol contents, Nafion^®117 is 10 times more conductive than the CM2 membrane. It was found that the methanol is absorbed more by Nafion^®117, and its effect is more noticeable on the microstructure of this membrane, under standard conditions. The high methanol permeability of these membranes, particularly of the Nafion^®117, induces bad cell efficiencies and lifetimes.     

The conductance of tetraethylammonium salts in water-methanol mixtures

Conductance measurements are reported for tetraethylammonium halide and perchlorate salts in water-methanol mixtures over the complete composition range at 25° C. Limiting conductances and association constants are evaluated together with the distance parameters. The maxima in the limiting ionic Walden products are considered in some detail. Ionic association is generally weak, and the lack of reliable correlation betweenK _A and the solvent dielectric properties supports the assumption of specific ion-solvent interaction.     

3,5-二硝基水杨酸比色法在烷基多苷体系中的应用研究

将3,5-二硝基水杨酸比色定糖法(DNS法)引入烷基多苷体系中,研究了丁苷化过程中葡萄糖含量随时间的变化,表征了丁苷化反应产率,测定了烷基多苷产品中残留的葡萄糖含量,并将测定结果与传统方法进行了比较,结果表明该法是一种简便快速、准确度高的方法。     

Preparation and characterization of polypyrrole-silica colloidal nanocomposites in water-methanol mixtures

The effect of methanol cosolvent on the synthesis of polypyrrole-silica colloidal nanocomposites using ultrafine silica sols in combination with both FeCl3 and APS oxidants has been investigated. Two protocols were evaluated: the addition of methanol to an aqueous silica sol and the addition of water to a methanolic silica sol. The latter protocol proved to be more robust, since it allowed colloidally stable dispersions to be prepared at higher methanol content (up to 50 vol% using the APS oxidant). This allowed greater control over the particle size of the nanocomposite particles. In general, the spectroscopic data, the particle size range, silica contents and electrical conductivities of these nanocomposites were similar to those reported earlier for purely aqueous formulations. Polypyrrole contents ranged from 49 to 71% by mass and particle diameters varied from around 160 to 360 nm. In terms of colloid stability, the APS oxidant was preferred for nanocomposite syntheses in the presence of methanol. However, the FeCl3 oxidant generally gave higher conductivities and narrower size distributions under comparable conditions. HF etching experiments combined with transmission electron microscopy studies indicated that, to a first approximation, these nanocomposite particles had core-shell morphologies, with a hydrophobic polypyrrole core and a hydrophilic silica shell that compose approximately one monolayer of silica sol particles. Finally, aqueous electrophoresis measurements suggested that the polypyrrole-silica nanocomposites were silica-rich and that the methanolic silica sol was more hydrophobic (lower surface charge density) than the aqueous silica sol.     

Inverse polarographic determination of creatinine with alkaline picrate and 3,5-dinitrosalicylic acid

A selective and highly sensitive inverse polarographic method for the determination of creatinine has been developed, involving the measurement of disappearance of diffusion current for the picric acid wave ($\text{E}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= ?0.501}\text{V}$) when creatinine is added. Creatinine levels of 0–5 μg/ml can be measured accurately.Investigation of 3,5-dinitrobenzoic acid and 3,5-dinitrosalicylic acid revealed the latter to be more reactive and useful for the inverse polarographic determination of creatinine at levels of 0–30 μg/ml.     

Time-resolved emission of flavin adenine dinucleotide in water and water-methanol mixtures

Time-resolved fluorescence decay of flavin adenine dinucleotide (FAD) was studied at room temperature in water and water-methanol mixtures by a fluorescence upconversion technique. The observations were focused on the most initial decay phase (200 ps), before the residual fluorescence assumes a single exponential decay, typical for an extended conformation of the fluorophore. Within the first few picoseconds, where most of the electron transfer coupled quenching takes place, the emission decay curves could be fitted by a stretched exponent, compatible with the inhomogeneous distance dependent electron transfer model. This implies that the population of the excited FAD molecules exhibits a large number of non-identical states, each with its own separation between the donor (adenine) and acceptor (isoalloxazine) moieties, having its own rate of electron transfer. To evaluate the distribution of the separation between the donor-acceptor pair, we carried out molecular dynamics simulations of closed conformation of the FAD in water and water-methanol mixtures, sampling the structure at 10 fs intervals. The analysis of the dynamics reveals that within the 4 ps time frame, where most of the nonexponential fluorescence relaxation takes place, the relative motion of the donor-acceptor pair is consistent with a one-dimensional Brownian motion, where the diffusion coefficient and the shape of the confining potential well are solvent dependent. The presence of methanol enhances the diffusion constant and widens the width of the potential well. On the basis of these parameters, the relaxation dynamics was accurately reconstructed as an electron transfer reaction in an inhomogeneous system where the reactants are diffusing within the time frame of the observation.     

Excess volumes of propanol-water mixtures at 5, 15, and 25°C

This paper reports on equipment for the automatic measurement of the density of binary mixtures as a function of concentration. The equipment is applied to mixtures of 1-propanol and water at 5, 15, and 25°C. An equilibrium mixture model, in which an alcohol-water solution is regarded as an ideal associated mixture, is fitted to the experimental data. Thermodynamic parameters characterizing the association of water to nonpolar alkyl groups (hydrophobic solvation) and to polar hydroxyl groups (hydrophilic solvation) are estimated.     

Dynamics of ionic and hydrophobic solutes in water-methanol mixtures of varying composition

We have carried out a series of molecular-dynamics simulations of water-methanol mixtures containing either an ionic or a neutral atomic solute to investigate the effects of composition of the mixture on the diffusion of these solutes. Altogether, we have considered 17 different systems of varying composition ranging from pure water to pure methanol. The diffusion coefficients of ionic solutes are found to show nonideal behavior with variation of composition of the solvent mixture. The extent of nonideality of the solute diffusion is found to be similar to the nonideality that is observed for the diffusion and orientational relaxation of water and methanol molecules in these mixtures and is attributed to the enhanced stability of the hydrogen bonds and formation of interspecies complexes in the mixtures. The neutral solute shows characteristics of hydrophobic solvation and its diffusion decreases monotonically with increase of methanol concentration. The present simulation results are compared with those of experiments wherever available.     

Dissociation constants of phosphoric acid in dimethylformamide-water mixtures at 298.15 K

The dissociation constants of phosphoric acid (pK ₁ and pK ₂) in water-dimethylformamide (DMFA) mixtures (0–0.65 mole fractions of DMFA) were determined at 298.15 K by potentiometric titration. The extrapolation of these data to pure DMFA and the comparative calculation method were used to estimate the dissociation constants of the acid in DMFA.     

Spectroscopic studies on intra- and inter-molecular hydrogen bonding in 2,4-dinitrophenol and 3,5-dinitrosalicylic acid

Effect of solvent on the electronic spectral behaviour of 2,4-dinitrophenol (I) and 3,5-dinitrosalicylic acid (II) is examined and discussed. It is deduced that in case of (I) the intramolecular hydrogen bond to nitro breaks to form an intermolecular hydrogen bond depending upon the nature of solvent used whereas (II) remains intramolecularly hydrogen bonded to carbonyl group in solvents as strongly basic as DMF.     

Thermodynamics of ionization processes for chloro-substituted benzoic acids in water—dimethysulfoxide mixtures at 25°C

The ionization enthalpies of o-, m-, and p-chloro-benzoic acid were measured calorimetrically at 25°C in water—DMSO mixtures ranging from pure water up to 0.8 DMSO mole fraction. In the same mole fraction range, molar free energies and entropies were calculated for the ionization process.The different results obtained for the three acids were explained by taking into account the different effects displayed on the solvation and ionization phenomena by the chlorine atom in the various positions on the aromativ ring.Using the transfer enthalpy of benzoic acid in the same mixtures as a reference point, a solvation sequence is proposed both for the ions and undissociated molecules.     

Thermodynamics of ionization processes for hydroxy-substitute benzoic acids in water—dimethylsulfoxide mixtures at 25°C

The ionization and solution enthalpies of the hydroxybenzoic acids were measured calorimetrically at 25°C in water—dimethylsulfoxide mixtures ranging from pure water to 0.8 DMSO mole fraction.In the same solution, the ΔG° values for the ionization processes have been determined by potentiometric measurements.The different enthalpy solvation values for the three hydroxy isomers were explained taking into account the prevalence of the resonance or the inductive effect as a function of the solvent composition.The intramolecular hydrogen bond of the undissociated molecules seems to be responsible for the greater degree of ionization of o-hydroxybenzoic acid with respect to the m and p-hydroxy isomers.