Journal of Solution Chemistry

Stepwise protonation constants of quercetin were determined in aqueous solution at different conditions of ionic strength (0.1–0.7 mol·dm−3 NaCl) and temperature (25.0–40.0 °C) using a combination potentiometric–spectrophotometric method. The results obtained show that the first and the third protonation constants decrease with increasing ionic strength whereas the second protonation constant has a minimum around 0.6 mol·dm−3 NaCl. The values of protonation constants were also calculated using the models of Debye–Hückel, Guggenheim, Scatchard, Davies, and the SIT equations at 25.0 °C and different ionic strengths. Also, the protonation constants of quercetin decrease with increasing temperature indicating an exothermic process. The thermodynamic parameters of the protonations were calculated at different ionic strengths using a modified van’t Hoff equation and are discussed.

l-Aspartic acid (ASP) and l-glutamic acid (GLU) in aqueous solution of a food additive, sodium acetate (SA) have been taken for acoustic and conductometric studies. The experimental results are analysed to predict and assess the solvation behaviour and types of interactions occurring in these ternary solutions [ASP/GLU + SA + water]. Compressibility parameters like isentropic compressibility and apparent molar isentropic compressibility along with their values at infinite dilution have been computed. Transfer values and ion pair and triplet interactions have been determined. Other thermodynamic parameters like internal pressure, molar free volume, relative association and acoustic impedance, have also been derived from the acoustic data. Specific conductance values measured experimentally led to the calculation of molar conduction of electric charge. Walden product sheds light on structure building or breaking properties of the amino acids. Gibbs thermodynamic functions predict the spontaneity of ion association process. Arrhenius equation is used to calculate activation energy which is indicative of the formation of high-energy transition state required for migration of ions. Analysis of these results establishes the amino acids as water-structure forming agents and predominance of ion–solvent, ion–ion and ion–hydrophilic interactions in the studied systems.

The mixing enthalpies of maltose with several typical α-amino acids (glycine, L-alanine, L-serine, L-valine, L-threonine and L-proline) and dilution enthalpies of each compound have been determined in aqueous solutions at T=298.15 K by a flow-mixing microcalorimeter. The heterotactic enthalpic pairwise interaction coefficients, h xy , of each amino acid with maltose have been calculated by the McMillan–Mayer formalism, and are discussed in terms of intermolecular interactions of the hydrated solute species.

Densities of dilute solutions of the electrolytes tetraethylphosphonium iodide and tetraethylammonium iodide in the nonaqueous solvents methanol, ethanol, 1-propanol, 1-butanol, acetonitrile, and 2-propanone were measured at 25°C. Using published values for the Debye-Hückel limiting slopes A V, apparent molar volume data were fitted to the Pitzer equation yielding infinite-dilution partial molar volumes $$\overline V _{\text{i}}^\infty$$ and deviation parameters B V. It is found that the variation in the van der Waals volume of the cationic central atoms is about one half the experimental volume change in 2-propanone solutions, but twice that value in the other five solvents. This finding is interpreted in terms of openness of solute structure and solvent penetration. Parameter B V for each salt is shown to be solvent dependent. An interesting approximate linear variation between A V and B V parameters is suggested by the data. This empirical relationship would entail correlation of short- and long-range interionic interactions in solution.

The study of physical properties of binary liquid mixtures is of great importance for understanding and characterizing intermolecular interactions. Similarly, some models attempt to correlate viscosity in liquid mixtures in order to illuminate interacting structures and peculiar behaviors. Grunberg–Nissan parameters for viscosity (η) in isobutyric acid + water mixtures over the entire range of mole fractions under atmospheric pressure and from 302.15 to 313.15 K were calculated from experimental dynamic viscosities presented in previous works. Many experimenters investigate physicochemical properties using various models to develop interpretations and conclusions. The present work comes within the framework of correlating different equations. Relationships between the Grunberg–Nissan and Arrhenius and Jouyban–Acree parameters for viscosity are shown in one critical binary mixture.

The stability of ibuprofen in the binary and ternary solvents of polyethylene glycol 200, ethanol and water was measured at four different temperatures (298.15, 308.15, 318.15 and 328.2 K). A modified high performance liquid chromatography method was used for determining the residual concentrations of ibuprofen in the solvent mixtures. The mobile phase was composed of acetonitrile and water (adjusted to pH 1.81 with orthophosphoric acid) with the ratio of 60:40; it was passed through the column at the flow rate of 1 mL·min−1 for conditioning the system (25 min) and carbamazepine was used as the internal standard. The Jouyban–Acree model was used to fit the stability factors of ibuprofen at several temperatures and in different solvent mixtures and the obtained mean relative deviation value for 204 stability data points is 19.0 %.

The kinetics of alkaline hydrolysis of 2-chloroquinoxaline (QCl) with hydroxide ion was investigated spectrophotometrically at different percentages of aqueous–organic solvent mixtures with acetonitrile (10–60% v/v) and with dimethylesulphoxide (10–80%) over the temperature range from 25 to 45 °C. The reaction was performed under pseudo first order conditions with respect to 2-chloroquinoxaline (QCl). An increase in the percentage of organic solvent (v/v) has different effects on the reaction rate constants, presumably due to hydrogen bond donor and acceptor differences of the media and other solvatochromic parameters. The data were discussed in terms of the Kamelt-Taft parameter and E T(30). A nonlinear relation between the logarithm of the rate constant and reciprocal of the dielectric constant suggests the presence of selective solvation by the polar water molecules. Activation parameters ΔH #, ΔS # and ΔG # were determined and discussed.

The enthalpies of solution of sodium iodide in methanol, ethanol and acetone and in mixtures of methanol and ethanol with water were measured over wide ranges of electrolyte concentration and temperature. Standard enthalpies of solution, transfer enthalpies of NaI from alcohols to alcohol-water mixtures, and temperature coefficients of enthalpies of solution have been calculated. Thermodyanmic characteristics of solution and solvation of the Na+ and I− ions in acetone and ethanol were determined at 243–298 K. It is noted that at lower temperatures the disruption of solvent structure by ions is a local effect. The presence of negative solvation of the Na+ and I− ions in alcohol-water mixtures at lower temperatures is demonstrated.