Molecular structure of a water-polyethylene glycol-KOH system, according to densitometry and viscometry data

The dynamic viscosity and density of a water-polyethylene glycol-KOH system are measured at temperatures of 293.15 to 323.15 K in concentrations ranging from 0.00001 and 0.001 (mole fractions). The activation parameters of viscous flow (ΔG _η ^≠ , ΔH _η ^≠ , and ΔS _η ^≠ ), structural temperature (T ₀), the partial molar volume of polyethylene glycol (PEG) in solution $\left( {\tilde V} \right)$ , intrinsic viscosity([η]), and the Huggins constant (K _H), are calculated. It is found that PEG has a structuring effect on water in water-PEG and water-PEG-KOH systems, with the PEG structuring effect in the latter being somewhat attenuated by the destructuring influence of KOH.     

Thermodynamic transfer functions for urea and thiourea from water to water-tetrahydrofuran mixtures from precise vapor-pressure measurements

The rate of change of the standard chemical potential with solvent composition, \(\partial \bar G_0 /\partial Z\) , has been calculated from precise vapor-pressure measurements for urea at three temperatures and for thiourea at 298.15°K in water-tetrahydrofuran (THF) mixtures. From these results the standard free energy of transfer ΔG _t ^o of the solutes from water to various water-THF mixtures has been obtained together with the standard molar entropy ΔS _t ^o and the standard molar enthalpy ΔH _t ^o of transfer at 298.15°K in the case of urea. The quantity ΔG _t ^o for urea is negative in the water-rich region and positive for mole fractions THF>0.2. There is a nearly complete compensation between ΔH _t ^o andTΔS _t ^o at 298.15°K up to mole fraction THF=0.5. These phenomena can be partly related to the structure in H₂O-THF mixtures.     

Thermodynamics of the gas-phase reactions in chemical vapor deposition of silicon carbide with methyltrichlorosilane precursor

The gas-phase reaction thermodynamics in the chemical vapor deposition system of preparing silicon carbide via methyltrichlorosilane pyrolysis is investigated with a relatively complete set of 226 species, in which the thermodynamic data of 163 species are evaluated in this work with accurate model chemistry G3(MP2) and G3//B3LYP calculations combined with standard statistical thermodynamics. The data include heat capacity (C _p,m ^θ ), entropy (S _m ^θ ), enthalpy of formation (Δ_f H _m ^θ ) and Gibbs free energy of formation (Δ_f G _m ^θ ). All the results are consistent with the available reliable experiments. Based on these thermodynamic data, the equilibrium concentration distribution of the 226 possible species in 300–2,000 K is evaluated with the chemical equilibrium principle under a typical experimental condition. It is shown that the theoretical results are in very good agreement with the experiments. We conclude that the present work is instructive for experiments with different conditions.     

Features of the Diels-Alder reaction between 9,10-diphenylanthracene and 4-phenyl-1,2,4-triazoline-3,5-dione

The Diels-Alder reaction between substituted anthracenes 1a?1j and 4-phenyl-1,2,4-triazoline-3,5 (2) is studied. In all cases except one, the reaction proceeds on the most active 9,10-atoms of substituted anthracenes. The orthogonality of the two phenyl groups at the 9,10-position of diene 1a is found to shield 9,10-reactive centers. No dienophiles with C=C bonds are shown to participate in the Diels-Alder reaction with 1a; however, the reaction 1a + 2 proceeds with the very active dienophile 2,4-phenyl-1,2,4-triazoline-3,5-dione. It is shown that attachment occurs on the less active but sterically accessible 1,4-reactive center of diene 1a. The structure of adduct 3a is proved by ¹H and ¹³C NMR spectroscopy and X-ray diffraction analysis. The following parameters are obtained for reaction 1a + 2 ? 3a in toluene at 25°C: K _eq = 2120 M^?1, ΔH _f ^≠ = 58.6 kJ/mol, ΔS _f ^≠ = ?97 J/(mol K), ΔV _f ^≠ = ?17.2 cm³/mol, ΔH _b ^≠ = 108.8 kJ/mol, ΔS _b ^≠ = 7.3 J/(mol K), ΔV _b ^≠ = ?0.8 cm³/mol, ΔH _r-n = ?50.2 kJ/mol, ΔS _r-n = ?104.3 J/(mol K), ΔV _r-n = ?15.6 cm³/mol. It is concluded that the values of equilibrium constants of the reactions 1a?1j + 2 ? 3a?3j vary within 4 × 10¹?10¹¹ M^?1.     

Solvent and external pressure effects on the ratio of the cyanoisopropyl radical recombination and disproportionation rates

A GC-MS analysis of the azobisisobutyronitrile thermal decomposition products of in solutions at 80°C showed that the ratio of recombination and disproportionation rates of the cyanoisopropyl radical does not depend on the medium viscosity, but increases when the internal pressure of the solvent increases according to the log(k _dispr/k _rec) = ?1.25 + 0.096 P _int ^0.5 law. This means that the activation volume corresponding to recombination is larger than that corresponding to disproportionation. It follows from the relationship log(k _dispr/k _rec) = (ΔV _rec ^≠ ? Δv _dispr ^≠ )ΔP/RT that, for the decomposition of the substrate in benzene under a pressure of 0.5–4.0 kbar, the difference between the activation volumes is ΔV _rec ^≠ ? ΔV _dispr ^≠ = 8 cm³/mol.     

Critical micelle concentration and self-aggregation of hexadecyltrimethylammonium bromide in aqueous glycine and glycylglycine solutions at different temperatures

Conductivities, densities and ultrasonic speeds measurements of hexadecyltrimethylammonium bromide (HTAB) in aqueous solutions of glycine (Gly) and glycylglycine (Gly-Gly) have been made at various temperatures. The critical micelle concentration (CMC), the degree of ionization (??) of the micelles, standard free energy, enthalpy, and entropy of the micellization process (??G _m ^° , ??H _m ^° , and ??S _m ^° ) for the present systems were estimated at different temperatures. The CMC values of HTAB in aqueous Gly and Gly-Gly were also evaluated by density and ultrasonic speed measurements. Apparent molar volumes, (V _?), apparent molar volumes at infinite dilution, (V _? ^° ), apparent molar compressibilities, (K _?), of HTAB in the pre- and post-micellar regions, and volume change on micellization (??V _? ^m ) were also estimated. Large positive values of T??S _m ^° and small negative values of ??H _m ^° suggest that micellization process is driven primarily by entropy increase. The increase in ??V _? ^m and K _? with rise in temperature is indicative of less compact micellar structure of HTAB in presence of amino acid additives. These data suggest that amino acids are solubilised probably in the palisade layer of the micelle.     

Thermochemistry of aqueous solutions of nucleic acid bases and their alkylated derivatives

Enthalpy of solution, ΔH _sol ^o , enthalpy of sublimation, ΔH _subl ^o , apparent partial molar volume and heat capacities,V ₂ ^o andC _p,2 ^o were determined for aqueous solutions of thirty alkylated derivatives of uracyl and adenine, eight derivatives of cytosine and guanine. Calculated accessible surface areas and molar volumes are presented, too. The values of enthalpy of solution, enthalpy of sublimation can be useful in the studies on the nature of interaction between these compounds and water molecules. Apparent partial molar volume and heat capacity give a new aspect on hydrophob properties of the examined nucleic acid base derivatives.     

The densities and viscosities of aqueous solutions of glycine and glycylglycine containing carbohydrates and structural properties of the ternary systems

Apparent molar volumes, V _? for glycine (Gly) and glycylglycine (Gly-Gly) in aqueous D(+)-glucose and sucrose solutions have been determined from solution density, ρ measurements at 298.15, 303.15, 308.15, and 313.15 K as a function of the concentration of solutes (Gly and Gly-Gly). The standard partial molar volume, V _? ⁰ , transfer volume, ΔV _?(tr) ⁰ , for Gly and Gly-Gly from water to aqueous carbohydrate solutions, partial molar expansibility, E _? ⁰ , and hydration number, n _H of solute have been calculated. The viscosity data have been analyzed by means of Jones-Dole equation to obtain A- and B-coefficients, free energy of activation of viscous flow per mole of solvent, Δμ ₁ ^0# , and solute, Δμ ₂ ^0# , enthalpy, ΔH ^0#, and entropy of activation, ΔS ^0# of viscous flow were evaluated. The behavior of these parameters has been interpreted in terms of solute-solute and solute-solvent interactions.     

Hexadecyltrimethylammonium bromide micellization in glycine,diglycine, and triglycine aqueous solutions as a function of surfactant concentration and temperatures

Micellization behavior of hexadecyltrimethylammonium bromide (HTAB) was investigated conductometrically in aqueous solutions containing 0.02 mol kg^?1glycine (Gly), diglycine (Gly-Gly), and triglycine (Gly-Gly-Gly) as a function of surfactant concentration at different temperatures. The critical micelle concentration (CMC) of HTAB exhibits a decreasing trend as the number of carbon atoms increases from Gly to Gly-Gly-Gly, favoring the micelle formation. The values of CMC and the degree of counterion dissociation of the micelles were utilized to evaluate the standard free energy for transferring the surfactant hydrophobic chain out of the solvent to the interior of the micelle, ΔG _HP ^○ , free energy associated with the surface contributions, ΔG _S ^○ , standard free energy, ΔG _m ^○ , enthalpy, ΔH _m ^○ , and entropy, ΔS _m ^○ of micellization were also calculated. The results show that the micellization of HTAB in aqueous solutions as well as in aqueous Gly/Gly-Gly/Gly-Gly-Gly solutions is primarily governed by the entropy gain due to the transfer of the hydrophobic groups of the surfactant from the solvent to the interior part of the micelle. The CMC obtained by fluorometric method is in close agreement with those obtained conductometrically. Furthermore, decrease in the I ₁/I ₃ ratio of pyrene fluorescence intensity suggests the solubilization of the additives by the surfactant micelles and that this solubilization increases as the hydrophobicity increases from Gly to Gly-Gly-Gly.     

Measurement and Correlation of Excess Molar Enthalpy of Binary Mixtures Containing Butyl Acetate + 1-Alkanols (C1–C6) at 298.15 K

Excess molar enthalpies, ?H _m ^E , for the binary mixtures of butyl acetate + 1-alkanols, namely (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, and 1-hexanol), were measured over the whole range of composition at 298.15 K using a Parr 1455 solution calorimeter. The excess partial molar enthalpies, ?H _m,i ^E , were calculated from the experimental excess molar enthalpies using the Redlich–Kister polynomial equation. The sign of ?H _m ^E for all systems are positive because of the disruption of hydrogen bonding and dipole–dipole interactions in the alkanols and esters, respectively. The magnitude of the ?H _m ^E values increases with increasing alkyl chain length. The behavior of ?H _m ^E was analyzed in terms of the length of the alkanol chain, the nature and type of intermolecular interactions and the balance between positive and negative effects on deviations from ideality. The experimental excess molar enthalpy data have also been correlated using the Redlich–Kister and SSF equations and two local composition models (UNIQUAC and NRTL).     

Determination of the reaction heat and studies of thermodynamic functions

The hydrolytic polymerization of Cr³⁺ at relatively high concentrations was studied by microcalorimetry. The thermal curves were determined with a 2277 thermal activity monitor. From the curves, the identified reaction heats (δrH _m ^σ ), hydrolysis constants (K) and thermodynamic functions (δrS _m ^σ ,δrH _m ^σ ) were calculated at different temperatures.     

Energies of the gas-phase deprotonation of nitro-substituted benzenesulfonic and benzoic acids: The role of the conformation isomerism of sulfonic acids

The Gibbs energies of deprotonation Δ_r G ₂₉₈ ^○ of gaseous benzoic acid (BA), benzenesulfonic acid (BSA) and their six mono-, di-, and trinitro-substituted derivatives are calculated by means of B3LYP/cc-pVTZ and MP2/6-311++G**. The dependences of Δ_r G ₂₉₈ ^○ on the number and the position of nitro groups in an aromatic ring are revealed, as is the possibility of intramolecular hydrogen bond (IHB) formation in ortho-substituted acids. It is found that the deprotonation of conformers of ortho-nitro-substituted BSA without IHBs requires less energy (by 4–5 kcal/mol) than for conformers with IHBs. It is shown that the Δ_r G ₂₉₈ ^○ values for substituted BA are ～22 kcal/mol higher than the corresponding values for substituted BSA. A trend of diminishing Δ_r G ₂₉₈ ^○ for nitro-substituted acids is observed when the number of nitro groups is increased, and di- and trinitro-substituted BSA may therefore be considered superstrong acids.     

A feature on ensuring safety of superfine explosives

To probe the dependence of particle size on the safety of nitroamine explosives, coarse RDX and HMX were comminuted to nanometer particles by an improved superfine mill. Their thermolysis characteristics were studied by thermal analysis and described via calculating some thermodynamic and kinetic parameters such as the activation free-energy (ΔG ^≠), activation enthalpy (ΔH ^≠), activation entropy (ΔS ^≠), apparent activation energy (E), critical temperature of thermal explosion (T _b), and critical heating rate of thermal explosion ( $ ({\text{d}}T/{\text{d}}t)_{{T_{\text{b}} }} $ ). After comminuted, the values of T _b and $ ({\text{d}}T/{\text{d}}t)_{{T_{\text{b}} }} $ were increased. However, the values of ΔH ^≠, ΔS ^≠, ΔG ^≠, and E for nanoexplosives were close to those of microexplosives, which mean decreasing particle size into nanometer did not distinctly influence the thermolysis characteristic of nitroamines. The safety of the nanoexplosives was practically assessed by testing their impact, friction, and shock sensitivities. Results indicated that nano nitroamines presented obviously higher safety than the micro-counterparts. Especially in Small Scale Gap Test, the shock sensitivity of nano-RDX and nano-HMX decreased by about 45 and 56% compared with that of micro-RDX and micro-HMX, respectively.     

Study of compatibility in polymeric system cellulose nitrate-cellulose acetate by reversed-phase gas chromatography

Therm odynamic compatibility in the polymeric system cellulose nitrate-cellulose acetate is studied by reversed-phase gas chromatography. The excess enthalpy ΔH _ji ^E , entropy ΔS _ji ^E , and Gibbs energy ΔG _ji ^E of mixing of the components, and also the Flory-Huggins interaction parameter χ_ji are estimated.     

The nature and the mechanism of ion transfer in tungstates Me2+{WO4} (Ca, Sr, Ba) and Me 2 3+ {WO4}3 (Al, Sc, In) according to the data acquired by the tubandt method

The Tubandt method of electrolysis is used for studying the nature of ionic carriers in ceramics of tungstates Me²⁺{WO₄} (Ca, Sr, Ba) and Me ₂ ³⁺ {WO₄}₃ (Al, Sc, In) which are solid electrolytes. These compounds have the salt-like islet structure with isolated tetrahedrons {WO ₄ ^2? } and are crystallized in the allied structural types of scheelite (CaWO₄) for Me²⁺ and Sc tungstate (Sc₂{WO₄}₃) for Me³⁺. The electrolysis is carried out in 2- or 3-section cells (?)Pt|M ₂ ^n/n+ {WO₄}|Me ₂ ^n/n+ {WO₄}|Pt(+) in air atmosphere at the temperature of ～900°C and cell voltage of 4 and 300 V. All experiments without exception demonstrate a decrease in the mass of the cathodic section of cells. This points to the negative charge of ionic mass carriers and their transfer towards the Pt⁽⁺⁾ electrode. The cathodic briquette mass loss Δm ^(?) depends linearly on the charge passed through a cell. In all experiments with MeWO₄ tungstates, the anodic disk mass remains constant. The electrolysis of Me₂(WO₄)₃ cells is always accompanied by an increase in the anolyte mass Δm ⁽⁺⁾; however, in all experiments, Δm ^(?) > Δm ⁽⁺⁾. All data on mass variation and the results of studying the composition of nearelectrode electrolyte layers by XRD and SEM methods correspond to the condition $t_{WO_4^{2 - } } > t_C $ (C is the cation), i.e., {WO ₄ ^2? } anions pertain to the major ionic carriers. The transport number $t_{WO_4^{2 - } } $ is calculated based on the Faraday law from Δm ^(?). It is shown that the second ionic carrier with the mobility even higher than that of {WO ₄ ^2? } is the O^2? ion. For middle values of transport numbers, their ratio is shown to be $t_{O^{2 - } } $ (0.5–0.8) > (0.2–0.5) $t_{WO_4^{2 - } } $ . No results that would confirm the involvement of Me²⁺ and Me³⁺ ions in conduction are obtained.     

The Standard Chemical-Thermodynamic Properties of Phosphorus and Some of its Key Compounds and Aqueous Species: An Evaluation of Differences between the Previous Recommendations of NBS/NIST and CODATA

The aqueous chemistry of phosphorus is dominated by P(V), which under typical environmental conditions (and depending on pH and concentration) can be present as the orthophosphate species H₃PO ₄ ⁰ (aq),H₂PO ₄ ^? (aq),HPO ₄ ^2? (aq) or PO ₄ ^3? (aq). Many divalent, trivalent and tetravalent metal ions form sparingly soluble orthophosphate phases that, depending on the solution pH and concentrations of phosphate and metal ions, can be solubility limiting phases. Geochemical and chemical engineering modeling of solubilities and speciation require comprehensive thermodynamic databases that include the standard thermodynamic properties for the aqueous species and solid compounds. The most widely used sources for standard thermodynamic properties are the NBS (now NIST) Tables (from 1982 and earlier, with a 1989 erratum) and the final CODATA evaluation (1989). However, a comparison of the reported enthalpies of formation and Gibbs energies of formation for key phosphate compounds and aqueous species, especially H₂PO ₄ ^? (aq) and HPO ₄ ^2? (aq), shows a systematic and nearly constant difference of 6.3 to 6.9 kJ?mol^?1 per phosphorus atom between these two evaluations. The existing literature contains numerous studies (including major data summaries) that are based on one or the other of these evaluations. In this report we examine and identify the origin of this difference and conclude that the CODATA evaluation is more reliable. Values of the standard entropies of the H₂PO ₄ ^? (aq) and HPO ₄ ^2? (aq) ions at 298.15 K and p?° =1 bar were re-examined in the light of more recent information and data not considered in the CODATA review, and a slightly different value of S _m ^o (H₂PO ₄ ^? , aq, 298.15 K) = (90.6±1.5) J?K^?1?mol^?1 was obtained.     

Activation parameters of supercritical and gas-phase β-pinene thermal isomerization

New data on enthalpy and entropy contributions to the energy barrier of β-pinene thermal isomerization were obtained. The rate of β-pinene conversion is higher in supercritical EtOH (P = 120 atm) than in the gas phase (P ≤ 1 atm, without solvent, or for inert carrier gas N₂) at equal temperatures. The highest activation energy E _Σ of total β-pinene conversion is also observed in reactions in the supercritical (sc) condition. Activation parameters ΔH _Σ ^# , ΔS _Σ ^# , and ΔG _Σ ^# depend strongly on the reaction pressure. Thus, at P ≤ 1 atm (gas-phase reaction) the values of ΔS _Σ ^# are negative, while at sc conditions at P = 120 atm is positive. The linear dependences lnk _Σ0 ? E _Σ and ΔS _Σ ^# ? ΔS _Σ ^# indicate an isokinetic relation (IKR) and enthalpy-entropy compensation effect (EEC). The isokinetic temperature was calculated (T _iso = 605.5 ± 22.7 K). It was shown that elevation of temperature reduces the value of ΔG _Σ ^# (T) upon sc thermolysis only, whereas in all gas-phase reactions ΔG _Σ ^# (T) increases. At equal reaction temperatures, the greatest values of K _eq ^# (T) proved to be typical for thermolysis in sc-EtOH. We hypothesize that the rate of total β-pinene conversion increases dramatically due to a considerable shift in equilibrium toward higher concentrations of activated complex y _TS ^# . A detailed analysis of activation parameters shows that the IKR and EEC coincide, evidence of a common mechanism of β-pinene conversion observed under different reaction conditions, including thermolysis in sc-EtOH.     

Stabilization of water structure in ammonium hydrogenphosphate solutions

Radio-frequency permittivity and dielectric loss of (NH₄)₂ HPO₄ solutions at 288–308 K in the water permittivity dispersion region (7.5–25 GHz) are studied. Low-frequency electrical conductivities of these solutions are measured, and ionic losses at high frequencies are calculated. The analysis is carried out in terms of the Cole-Cole relaxation model. Static dielectric constants ?_s and the activation times τ and thermodynamic parameters of activation (ΔH _? ⁺⁺ , ΔG _? ⁺⁺ , and TΔS _? ⁺⁺ ) for the dielectric relaxation of the solutions are calculated. The values of ?_s decrease in response to increasing salt concentration at all temperatures. The increasing τ and ΔH _? ⁺⁺ values indicate the stabilization of water structure in solutions where the anion and cation are hydrophilically hydrated.