Thermochemical characteristics of nicotinamide protolytic equilibria in water-dimethylsulfoxide mixtures

The heat effects of nicotinamide protonation in water-dimethylsulfoxide (DMSO) solutions over the concentration range 0–0.75 DMSO mole fractions were determined calorimetrically at 25.00 ± 0.01°C and ionic strength 0.25 (NaClO₄). Changes in the enthalpy of protonation as the content of DMSO increased were found to be described by an S-shaped curve. This curve shape was caused by the dynamics of reagent solvation contributions as the concentration of DMSO grew with the predominance of the nicotinamide solvation contribution.     

The thermochemical characteristics of cellulose and its mixtures with water

The heat capacity of cotton microcrystalline cellulose was measured on an adiabatic vacuum calorimeter over the temperature range 80–330 K, and the differential thermal analysis data on the substance were obtained from 80 to 550 K. The enthalpy of cellulose interaction with water was measured at 303 K using a differential microcalorimeter. The standard enthalpies of combustion and formation of microcrystalline cellulose and wood celluloses with different crystallinity indices were determined. The concentration of saturated water solution in microcrystalline cellulose at 273 K was determined calorimetrically from the enthalpy of fusion of the excess water phase.     

Monte Carlo simulations of the solution structure of simple alcohols in water-acetonitrile mixtures

Monte Carlo simulations have been performed to explore the solution structure of ethyl, isopropyl, isobutyl, and tertiary butyl alcohols in pure water, pure acetonitrile, and different mixtures of the two solvents. The explicit solvent studies in NpT ensembles at T = 298 K illustrate that the solute "discriminates" the solvent's components and that the composition of the first solvation shell differs from that of the bulk solution. Since the polarizable continuum dielectric method (PCM) does not presently model the solvation of molecules with both polar and apolar sites in mixed protic solvents, we suggest a direction for further program development wherein a continuum dielectric method would accept more than one solvent and the solute sites would be solvated by user-defined solvent components. The prevailing solvation model will be determined upon the lowest free energy calculated for a particular solvation pattern of the solute having a specific conformational/tautomeric state. Characterization of equilibrium hydrogen-bond formation becomes a complicated problem that depends on the chemical properties of the solute and its conformation, as well as upon the varying nature of the first solvation shell. For example, while the number of hydrogen bonds to secondary and tertiary alcohol solutes are nearly constant in pure water and in water-acetonitrile mixtures with at least 50% water content, the number of hydrogen bonds to primary alcohols gradually decreases for most of their conformations when acetonitrile content is increased. Nonetheless, the calculations indicate that O-H...O(water) hydrogen bonds are still possible in a small fraction of the arrangements for the solution models with water content of 30% or less. The isopentene solute does not form any observable hydrogen bonds, despite having an electron-rich, double-bond site.     

苯甲酸在水-乙醇混合物溶剂中的电离焓

本文测定了苯甲酸在水-乙醇混合溶剂中的电离焓, 并就此进行了讨论。     

Liquid-vapor interfaces of water-acetonitrile mixtures of varying composition

Detailed molecular-dynamics simulations are carried out to investigate the equilibrium and dynamical properties of water-acetonitrile mixtures of varying composition. Altogether, we have simulated eight different systems of different concentrations of acetonitrile. The inhomogeneous density and anisotropic orientational profiles at interfaces, surface tension, and also the distribution of hydrogen bonds are calculated for both water and acetonitrile molecules. The dynamical aspects of the interfaces are investigated in terms of the anisotropic diffusion and dipole orientational relaxation of interfacial water and acetonitrile molecules. For both structural and dynamical properties, the behaviors of the interfaces are compared with those of the corresponding bulk phases. A comparison between the present theoretical results and experimental findings, wherever available, is also made to verify the usefulness of the molecular models employed in the present study for predicting interfacial properties.     

The thermochemical characteristics of the solvation of amines and silver ions in methanol-dimethylformamide mixtures

The heat effects of solution of ethylenediamine, sodium perchlorate, and silver perchlorate in the nonaqueous methanol-dimethylformamide binary solvent were determined calorimetrically at 298.15 K. The suggestion of equal enthalpies of solvation of Ph₄P⁺ and PhB^? was used to calculate the enthalpies of transfer of the ClO ₄ ^? and Ag⁺ ions from methanol to dimethylformamide. The influence of the composition of methanol-dimethylformamide solvents on the energy characteristics of solvation of ethylenediamine, Ag⁺, and ClO ₄ ^? was considered.     

The aqueous phase nitration of phenol and benzoic acid studied through flow-gated capillary electrophoresis

The aqueous phase nitration of benzoic acid and phenol was investigated via on-line capillary electrophoresis (CE). The presence of nitrated benzoic acid and phenol was supported through appearance of corresponding molecular ion peaks in ESI-MS measurements, and speciation of the nitrated isomers is achieved via the on-line CE method. The nitrated isomers produced in both reactions were successfully separated in <4?min by addition of 15?mM β-cyclodextrin to the electrophoresis buffer. Sequential separations (on-line analysis) allowed the reaction kinetics to be described. For benzoic acid, reaction yields were low (2–3%) however, results suggest both 3- and 2-nitrobenzoic acid form in a 1–1.4 concentration ratio. In addition, 3-hydroxybenzoic acid also forms in significant quantity under our reaction conditions. For the nitration of phenol, the reaction occurred more rapidly with observed yields between ≈10–30% for individual isomers. The yield of 2-nitrophenol was higher than 4-nitrophenol by a ratio of ≈?1.7–2, but 3-nitrophenol was not detected. For both reactions, nitrated and hydroxylated aromatics were the major products and formation of higher molecular weight oligomers was not observed.     

Ionization enthalpy of benzoic acid in water—dimethylsulfoxide mixtures

The ionization enthalpy of benzoic acid has been measured calorimetrically at 25°C in H₂ODMSO mixtures ranging from pure water to a maximum DMSO molar ratio X_DMSO = 0.80. With the increase of DMSO content, the ionization becomes more and more endothermic, and for X_DMSO = 0.8 the ionization enthalpy is about 6 kcal mol^?1 higher than in water. By also measuring the solution enthalpy of crystalline benzoic acid in the mixtures, it has been shown that the solvation of the undissociated molecule is the main cause for the increase of the dissociation enthalpy. A comparison has been made between the relative enthalpies of benzoic and hydroxide ions in H₂ODMSO mixtures.     

Thermodynamic behavior of water-acetonitrile mixtures excess volumes and viscosities

Densities of water+acetonitrile mixture have been measured from 308.15 to 278.15 K over the entire composition range. Partial molal volumes of both components of the mixed system have been calculated and deviations from ideal volumes of mixing have been evaluated. Literature data on viscosity have also been treated by a least square method and activation thermodynamic functions of viscous flow computed. Structural correlations have been made with previous results on acid-base properties; they allow to distinguish several interaction regions between components of binary systems.     

Interaction of chitosan with benzoic acid in solution and films

Chitosan films are prepared from solutions containing benzoic acid. Depending on preparation conditions, films of two types can be prepared: homogeneous optically transparent films and biphasic films containing fractal inclusions in a transparent matrix. The presence of ionic interaction between chitosan and benzoic acid in films is confirmed with the use of IR spectroscopy. The algorithm of the analysis of fractal dimensionality based on halftone images is suggested.     

The thermochemical characteristics of solution of L-cysteine and L-asparagine in aqueous 1,4-dioxane and acetone

The integral heats of solution Δ_sol H ^m of L-cysteine and L-asparagine in mixtures of water with 1,4-dioxane and acetone were measured by solution calorimetry at organic solvent concentrations of up to 0.30 mole fractions. The standard enthalpies of solution (Δ_sol H ^o) and transfer (Δ_tr H ^o) of amino acids from water to mixed solvents were calculated. The calculated enthalpy coefficients of pair interactions of L-cysteine and L-asparagine with cosolvent molecules were positive. The results were interpreted from the point of view of the predominance of interactions of various types in solutions and the influence of the nature of cosolvents and amino acid side substituents on the thermochemical characteristics of solution.     

Viscosity behavior of benzoic acid and benzoate ion in aqueous solution

Relative viscosities of aqueous solutions of benzoic acid and benzoates of lithium, sodium, potassium, and ammonium are measured. In the temperature range 25–35°C, the Jones-Dole viscosityB coefficients of the benzoate ion decrease with increasing temperature, indicating a net structure-making effect. The somewhat larger value of theB coefficient for the benzoate ion than that for the benzoic acid molecule confirms similar behavior for the acetate ion and acetic acid in aqueous solutions although the effect is much smaller.     

The thermochemical characteristics of nicotinamide coordination by iron(III) and ligand protonation in aqueous-ethanolic mixtures

The heat effects of protonation of nicotinamide and formation of its complex with iron(III) were determined calorimetrically at 25.00 ± 0.01°C and ionic strength 0.25(NaClO₄) in water-ethanol solvent containing 0–0.75 ethanol mol fractions. The experimental and literature data were used to construct the dependences of the enthalpy, entropy, and Gibbs energy of the reaction on the composition of the water-ethanol solvent. The dependence of the enthalpies of both reactions passed an exothermic minimum at 0.1 ethanol mol fractions. The reagent solvation contributions to the thermodynamic characteristics of transfer were analyzed.     

Solute-Solvent interactions in water-rich mixtures. II. Ionic conductances in water-dimethylsulfoxide mixtures at 25°C

Conductances of sodium bromide, iodide, and perchlorate, potassium chloride, and tetraphenylboride (BPh ₄ ^– ) as well as triisoamyl-n-butylammonium iodide (i-Am₃BuNI) have been measured in aqueous mixtures containing up to 20 mole percent dimethylsulfoxide (DMSO) at 25°C. Experimental data were analyzed by the 1965 Fuoss-Onsager-Skinner (FOS) equation. Single-ion limiting equivalent conductances were calculated by assuming that ₀ (i–Am ₃ BuN ⁺=₀ (BPh ₄ ^– ). The variations of the limiting ionic Walden products are discussed on the basis of acid-base type interactions for cations, and on the basis of structural effects for anions.     

Electrochemical reduction of CO2 in water-acetonitrile mixtures on nanostructured Cu electrode

This communication studies the CO₂ reduction reaction in H₂O/CH₃CN mixtures on nanostructured copper. It was found that the nanostructured copper electrode presents a well-defined voltammogram in acetonitrile, where it can be seen three signals related to adsorbed or surface attached (thin films) species. Also, it was found that the current density of CO₂ reduction in mixtures H₂O/CH₃CN on nanostructured copper electrodes with a mole fraction around 0.25 is higher than those observed with mole fractions lower than 0.15 or higher than 0.35. Finally, nanostructured Cu electrodes show higher catalytic activity towards the CO₂ reduction than copper electrode.     

Keto-enol tautomerization rates of acetylacetone in water-acetonitrile and water-dimethyl sulfoxide mixtures: Rate-partition relationship

The rate of the keto-enol tautomerization of acetylacetone was studied in water-acetonitrile (AN) and water-dimethyl sulfoxide (DMSO) mixtures. The apparent partition coefficients of acetylacetone between n-heptane and these mixed solvents were measured to estimate the free energies of transfer of each tautomer from water to the mixed solvents. The rate constants of enolization and ketonization depended remarkably on the composition of the mixed solvents, and a maximum in the enolization reaction was observed at about 20 mol% DMSO in water-DMSO mixtures. These kinetic solvent effects were analyzed with “rate-partition relationship” (RPR) derived on the basis of the transfer free energy concept, and the unusual kinetic solvent effect observed in water-DMSO mixtures was interpreted in terms of the destabilization of the keto form in the mixed solvents. An applicability of the RPR to other reactions was suggested.     

The volume characteristics of solution of naphthalene in heptane-ethanol mixtures at 298.15 K

The solubility of naphthalene was studied by the isothermal saturation method, and the density of naphthalene-heptane-ethanol solutions at 298.15 K was measured using an oscillating tube densimeter. The results were used to calculate the volume characteristics of naphthalene in the systems specified. The solubility of naphthalene in heptane was substantially higher than in ethanol. The composition dependences of apparent partial molar volumes contained extrema. The reason for such nonlinear dependences can be a sharp change in the contribution to the entropy of mixing related to the packing of molecules of different diameters (naphthalene and heptane). Another possible mechanism of these anomalies can be related to the stabilization of the structure of alkanols in the solvation sheath of alkanes noticed earlier.