Thermal study of the influence of chemical bond ionicity on the glass transformation in (Na2O, CaO, MgO)-Al2O3-SiO2 glasses

The relation between change of the specific heat (ΔC _p) accompanying the glass transformation and the chemical composition of glasses (Na₂O, CaO, MgO)-Al₂O₃-SiO₂ system has been studied. The exchange of modifiers in the glass structure causes the ΔC _p increase in the sequence Na>Ca>Mg. Change the glass network composition by introducing Al into it makes smaller increase of the ΔC _p values. It has been shown that degree of ΔC _p value changes is dependent on the iconicity/covalence of chemical bonds of cations with oxygen of glass structure network. This revised version was published online in July 2006 with corrections to the Cover Date.     

Fluorescence study on the interaction of bovine serum albumin with two coumarin derivatives

The interactions between bovine serum albumin (BSA) and two substituted hydroxychromone derivatives of coumarin, 3-hydroxy-7,8,9,10-tetrahydro-6H-benzo[c]chromen-6-on (C₃) and 1,3-dihydroxy-7,8,9,10-tetrahy-dro-6H-benzo[c]chromen-6-on (C_1.3), were investigated by fluorescence quenching spectra and UV-vis absorption spectra. It was proved that the fluorescence quenching of BSA by C₃ and C_{1, 3} was mainly a result of the formation of C₃ and C_1.3-BSA complexes. The Stern-Volmer quenching constants, binding constants, binding sites and the corresponding thermodynamic parameters ΔH ^o, ΔS ^o and ΔG ^o at different temperatures were calculated. The results indicated that van der Waals interactions and hydrogen bonds were the predominant intermolecular forces in stabilizing each complex. The detection limits of C₃ and C_1.3 were 5.08 × 10⁻⁷ and 1.11 × 10⁻⁷ M in the presence of BSA, respectively.     

Amino acids in aqueous solution. Effect of molecular structure and temperature on thermodynamics of dissolution

The enthalpies of dissolution of glycine (Gly) and L-α-alanine (Ala) in water at 288.15–318.15 K were measured. The results were compared with the earlier obtained data for L-α-phenylalanine (Phe) and L-α-histidine (His). The standard enthalpies of dissolution (Δ_soln H ₀) and differences (ΔC _p ⁰ ) between the limiting partial molar heat capacity of the amino acids in solution and the heat capacity of the amino acids in the crystalline state were calculated in the temperature interval 273–373 K. Changes in the entropy of dissolution (ΔΔ_soln S ₀) and reduced Gibbs energy [Δ (Δ_soln G ₀/T)] in the temperature interval from 273 to 373 K were determined from the known thermodynamic relationships. The ΔC _p ⁰ value is negative for hydrophilic glycine and positive for other amino acids. The ΔΔ_soln S ⁰ values increase with an increase in the hydrophobicity of the amino acids. The Δ(Δ_soln G ₀/T) values become more negative in the order Ala, Phe, Gly, His. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 711–714, April, 2007.     

Effect of temperature on thermodynamic characteristics of the dissociation of glycylglycine in aqueous solutions of electrolytes

Heats of reaction of glycylglycine with nitric acid and potassium hydroxide solutions are determined by two calorimetric procedures at 288.15, 298.15, 308.15 K and an ionic strength of solution of 0.25, 0.50, and 0.75 in the presence of KNO₃. Standard thermodynamic characteristics (Δ_r H°, Δ_r G°, Δ_r S°, and Δ_p C°) are calculated for the acid-base reactions in aqueous peptide solutions. The effects of the concentration of background electrolyte and temperature on the heats of dissociation of glycylglycine are considered.     

Spectroscopic study of protonation of oligonucleotides containing adenine and cytosine

Acidobasic properties of purine and pyrimidine bases (adenine, cytosine) and relevant nucleosides (adenosine, cytidine) were studied by means of glass-electrode potentiometry and the respective dissociation constants were determined under given experimental conditions (I = 0.1 M (NaCl), t = (25.0 ± 0.1) °C): adenine (pK _HL = 9.65 ± 0.04, pK _H2L = 4.18 ± 0.04), adenosine (pK _H2L = 3.59 ± 0.05), cytosine (pK _H2L = 4.56 ± 0.01), cytidine (pK _H2L = 4.16 ± 0.02). In addition, thermodynamic parameters for bases: adenine (ΔH ⁰ = (−17 ± 4) kJ mol⁻¹, ΔS ⁰ = (23 ± 13) J K⁻¹ mol⁻¹), cytosine (ΔH ⁰ = (−22 ± 1) kJ mol⁻¹, ΔS ⁰ = (13 ± 5) J K⁻¹ mol⁻¹) were calculated. Acidobasic behavior of oligonucleotides (5′CAC-CAC-CAC3′ = (CAC)₃, 5′AAA-CCC-CCC3′ = A₃C₆, 5′CCC-AAA-CCC3′ = C₃A₃C₃) was studied under the same experimental conditions by molecular absorption spectroscopy. pH-dependent spectral datasets were analyzed by means of advanced chemometric techniques (EFA, MCR-ALS) and the presence of hemiprotonated species concerning (C⁺-C) a non-canonical pair (i-motif) in titled oligonucleotides was proposed in order to explain experimental data obtained according to literature.     

Thermodynamics of phenylated polyphenylene between 0 and 340 K

In an adiabatic vacuum calorimeter, the temperature dependence of the heat capacity C _p of phenylated polyphenylene and initial comonomer 1,4-bis(2,4,5-triphenylcyclopentadienone-3-yl)benzene was studied between 6 and 340 K with an uncertainty of about 0.2%. In a calorimeter with a static bomb and an isothermal shield their energies of combustion DUcomb were measured. From the experimental data, the thermodynamic functions C _p ⁰ (T), H ⁰(T)-H ⁰(0), S ⁰(T)-S⁰(0), G ⁰(T)-H ⁰(0) were calculated from 0 to 340 K, and standard enthalpies of combustion ΔH _comb ⁰ and thermodynamic parameters of formation-enthalpies ΔH _f ⁰, entropies ΔH _f ⁰, Gibbs functions ΔG _f ⁰ - of the substances studied were estimated at T=298.15 K at standard pressure. The results were used to calculate the thermodynamic characteristics (ΔH _f ⁰ ,ΔS _f ⁰, ΔG _f ⁰) of phenylated polyphenylene synthesis in the range from 0 to 340 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermochemical investigation of acid-base interactions in peptide solutions

Heat effects of interaction of D,L-α-alanyl-D,L-α-alanine, glycil-γ-aminobutyric acid, glycil-L-asparagine and D,L-α-alanyl-D,L-asparagine with KOH, LiOH and HNO₃ solutions were measured by the direct calorimetry method at 288.15, 298.15, 308.15, 318.15 K and at several values of the ionic strength created by adding KNO₃ and LiNO₃. The standard dissociation enthalpies of the investigated ligands were obtained by the extrapolation to zero ionic strength. The standard thermodynamic characteristics (ΔG ⁰ , ΔH ⁰ , ΔS ⁰ , ΔC _p ⁰) of the processes of acid-base interaction in dipeptide solutions were calculated. Several peculiarities of acid-base interaction reactions in the solutions of biologically important ligands were found. The correlations between the thermodynamic characteristics of the protolytic equilibria in the dipeptide and aminoacids solutions and the structure of these compounds were determined.     

Thermochemical, volumetric and spectroscopic properties of lysozyme–poly(ethylene) glycol system

Investigations of lysozyme–polyethylene glycol system were made by differential scanning calorimetry, fluorescence and density techniques. The values of unfolding enthalpies, ΔH_N^U, unfolding temperatures, T_m, excess molar heat capacities, ΔC_p, and apparent molar volumes, V_Φ , were determined as functions of PEG concentration. The three PEGs of average molecular mass (MW) 6000, 10000, 20000 were used as macromolecular crowding agents. The concentration of polymers was changed in the range 0–30% mass per volume (w/v). The values of ΔH_N^U remained constant with no dependence on PEG concentration, while PEG addition to buffered lysozyme solutions caused linear decrease of T_m. The values of ΔC_p and V_Φ of lysozyme dramatically changed in the range of 8–10% of PEG concentration. The fluorescence spectroscopy was used in order to investigate the polymer influence on possible solvent–lysozyme interactions. The electrical properties of polymer–water and polymer–buffer systems, the dielectric constants of solutions were determined with use of impedance spectroscopy.     

Thermodynamic Properties of Hydrofullerene C60H36 from 5 to 340 K

The temperature dependence of the molar heat capacity (C⁰ _p) of hydrofullerene C₆₀H₃₆ between 5 and 340 K was determined by adiabatic vacuum calorimetry with an error of about 0.2%. The experimental data were used for the calculation of the thermodynamic functions of the compound in the range 0 to340 K. It was found that at T=298.15 K and p=101.325 kPa C⁰ _p (298.15)=690.0 J K⁻¹ mol⁻¹,H^o(298.15)−H^o(0)= 84.94 kJ mol⁻¹,S^o(298.15)=506.8 J K⁻¹ mol⁻¹, G^o(298.15)−H^o(0)= −66.17 kJ mol⁻¹. The standard entropy of formation of hydrofullerene C₆₀H₃₆ and the entropy of reaction of its formation by hydrogenation of fullerene C₆₀ with hydrogen were estimated and at T=298.15 K they were Δ_fS^o= −2188.4 J K⁻¹ mol⁻¹ and Δ_rS^o= −2270.5 J K⁻¹mol⁻¹, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermodynamic characteristics of the acid dissociation of dopamine hydrochloride in water-ethanol solutions

Enthalpies of the interaction of protonated dopamine with a hydroxide ion in water-ethanol mixtures in the concentration range of 0–0.8 EtOH mole fractions are measured calorimetrically. The neutralization process of dopamine hydrochloride is shown to occur endothermally in solvents with an ethanol concentration of ≥0.5 mole fractions. Standard thermodynamic characteristics (Δ_r H ^○, Δ_r G ^○, and Δ_r S ^○) of the first-step acid dissociation of dopamine hydrochloride in solutions are calculated with regard to the autoprotolysis enthalpy of binary solvents. It is found that dissociation enthalpies vary within 9.1–64.8 kJ/mol, depending on the water-ethanol solvent composition.     

The thermodynamic properties of 2-ethylhexyl acrylate over the temperature range from <Emphasis Type="Italic">T</Emphasis> → 0 to 350 K

The temperature dependence of the heat capacity C _p ^o= f(T) 2 of 2-ethylhexyl acrylate was studied in an adiabatic vacuum calorimeter over the temperature range 6–350 K. Measurement errors were mainly of 0.2%. Glass formation and vitreous state parameters were determined. An isothermic shell calorimeter with a static bomb was used to measure the energy of combustion of 2-ethylhexyl acrylate. The experimental data were used to calculate the standard thermodynamic functions C _p ^o(T), H ^o(T)-H ^o(0), S ^o(T)-S ^o(0), and G ^o(T)-H ^o(0) of the compound in the vitreous and liquid states over the temperature range from T → 0 to 350 K, the standard enthalpies of combustion Δ_c H ^o, and the thermodynamic characteristics of formation Δ_f H ^o, Δ_f S ^o, and Δ_f G ^o at 298.15 K and p = 0.1 MPa.     

The Influence of Drawing in Hot Water on The Morphological Properties of Pet Films as Measured by DSC and Modulated DSC

PET films uniaxially drawn in hot water are studied by means of conventional DSC and modulated DSC (MDSC).Glass transition is studied by MDSC which allows to access the glass transition temperature T _g and the variations of ΔC _p=C _p1−C _pg (difference between thermal capacity in the liquid-like and glassy states at T=T _g). Variations of T _g with the water content (which act as plasticizer) and with the drawing (which rigidifies the amorphous phase) are discussed with regard to the structure engaged in these materials. The increments of ΔC _p at T _g are also interpreted using a three phases model and the 'strong-fragile’ glass former liquid concept. We show that the ‘fragility’ of the medium increases due to the conjugated effects of deformation and water sorption as soon as a strain induced crystalline phase is obtained. Then, ‘fragility’ decreases drastically with the occurring rigid amorphous phase. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solvatochromism, prototropism and complexation of para-aminobenzoic acid

Effect of solvents, buffer solutions of different pH and β-cyclodextrin (β-CD) on the absorption and fluorescence spectra of p-aminobenzoic acid (pABA) have been investigated. The inclusion complex of pABA with β-CD is investigated by UV-visible, fluorimetry, semiempirical quantum calculations (AM1), ¹H NMR and Scanning Electron Microscope (SEM). The thermodynamic parameters (ΔH, ΔG and ΔS) of the inclusion process are also determined. The experimental results indicated that the inclusion processes is an exothermic and spontaneous. The large Stokes shift emission in solvents with pABA are correlated with different solvent polarity scales. The increase in the excited dipole moment values suggest that pABA molecule is more polar in the S₁ state. Solvent and β-CD studies indicates intramolecular charge transfer in pABA is less than ortho and meta isomers. Acidity constants for different prototropic equilibria of pABA in the S₀ and S₁ states are calculated. β-Cyclodextrin studies shows that pABA forms a 1:1 inclusion complex with β-CD. A mechanism is proposed to explain the inclusion process.     

Thermodynamic characteristics of protolytic equilibria of L-serine in aqueous solutions

The heat effects of the reaction of aqueous solution of L-serine with aqueous solutions of HNO₃ and KOH were determined by calorimetry at temperatures of 288.15, 298.15, and 308.15 K, and ionic strength values of 0.2, 0.5, and 1.0 (background electrolyte, KNO₃). Standard thermodynamic characteristics (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o, ΔC _p ^o) of the acid-base reactions in aqueous solutions of L-serine were calculated. The effect of the concentration of background electrolyte and temperature on the heats of dissociation of amino acid was considered. The combustion energy of L-serine by bomb calorimetry in the medium of oxygen was determined. The standard combustion and formation enthalpies of crystalline L-serine were calculated. The heats of dissolution of crystalline L-serine in water and solutions of potassium hydroxide at 298.15 K were measured by direct calorimetry. The standard enthalpies of formation of L-serine and products of its dissociation in aqueous solution were calculated.     

The Enthalpies of Solution and Solvation of L-Serine in Aqueous Alcoholic Solutions at 298.15 K

The integral enthalpies of solution (Δ_sol H ^m ) of L-serine in water-alcohol (ethanol, n-propanol, isopropanol) mixtures were measured over the range of alcohol concentrations up to 0.32 mole fractions. The standard enthalpy of solution (Δ_sol H°), enthalpy of transfer of L-serine from water into a mixed solvent (Δ_tr H°), and enthalpy of solvation (Δ_solv H°) were calculated. The dependences of Δ_sol H°, Δ_solv H°, and Δ_tr H° on the composition of mixtures had extrema. The calculated enthalpy coefficients of the pair interactions of L-serine with alcohol molecules were positive and increased along the series ethanol, n-propanol, isopropanol. The data obtained were interpreted in terms of different types of interactions in solutions and the influence of the nature of amino acid residues on the thermochemical solution characteristics. Original Russian Text ? I.N. Mezhevoi, V.G. Badelin, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 4, pp. 789–791.     

Thermochemistry of europium and dithiocarbamate complex Eu(C5H8NS2)3(C12H8N2)

A solid complex Eu(C₅H₈NS₂)₃(C₁₂H₈N₂) has been obtained from reaction of hydrous europium chloride with ammonium pyrrolidinedithiocarbamate (APDC) and 1,10-phenanthroline (o-phen⋅H₂O) in absolute ethanol. IR spectrum of the complex indicated that Eu³⁺ in the complex coordinated with sulfur atoms from the APDC and nitrogen atoms from the o-phen. TG-DTG investigation provided the evidence that the title complex was decomposed into EuS. The enthalpy change of the reaction of formation of the complex in ethanol, Δ_r H _m ^θ(l), as –22.214±0.081 kJ mol^–1, and the molar heat capacity of the complex, c _m, as 61.676±0.651 J mol^–1 K^–1, at 298.15 K were determined by an RD-496 III type microcalorimeter. The enthalpy change of the reaction of formation of the complex in solid, Δ_r H _m ^θ(s), was calculated as 54.527±0.314 kJ mol^–1 through a thermochemistry cycle. Based on the thermodynamics and kinetics on the reaction of formation of the complex in ethanol at different temperatures, fundamental parameters, including the activation enthalpy (ΔH _≠ ^θ), the activation entropy (ΔS _≠ ^θ), the activation free energy (ΔG _≠ ^θ), the apparent reaction rate constant (k), the apparent activation energy (E), the pre-exponential constant (A) and the reaction order (n), were obtained. The constant-volume combustion energy of the complex, Δ_c U, was determined as –16937.88±9.79 kJ mol^–1 by an RBC-II type rotating-bomb calorimeter at 298.15 K. Its standard enthalpy of combustion, Δ_c H _m ^θ, and standard enthalpy of formation, Δ_f H _m ^θ, were calculated to be –16953.37±9.79 and –1708.23±10.69 kJ mol^–1, respectively.     

Interaction Between Levamisole Hydrochloride and Bovine Serum Albumin and the Influence of Alcohol: Spectra

The interaction between levamisole hydrochloride (LH) and bovine serum albumin, BSA, has been studied by a spectral method under physiological conditions. For 1:n complexes, the relationship between fluorescence quenching intensity and concentration of the quenchers can be deduced on the basis of the modified Stern–Volmer equation. The binding constants and corresponding thermodynamic parameters ΔH _m, ΔG _m and ΔS _m at different temperatures were calculated. The experimental results demonstrated that the combination reaction of LH and BSA was a static quenching process because a 1:1 complex was formed, and the main dominant binding forces were hydrogen bonding and van der Waals forces. Meanwhile, the polarity of the tyrosine residue (Tyr) or tryptophan residue (Trp) micro-region was not obviously affected by the interaction. Furthermore, the binding constant increase when alcohol was added.     

Thermochemical study of acid-base interactions in L-asparagine aqueous solutions

The heats of reactions between an L-asparagine solution and HNO₃ solutions were measured calorimetrically within different pH ranges at 298.15, 308.15, and 318.15 K and an ionic strength of 0.5, 1.0, and 1.5 (KNO₃ and LiNO₃). The heats of stepwise dissociation of L-asparagine were determined. The standard thermodynamic characteristics (Δ _r H ^o, Δ _r G ^o, δ _r S ^o, and ΔC _p ^⪧) of the acid-base interactions in aqueous solutions of the amino acid were calculated. A relationship between the thermodynamic characteristics of the dissociation of L-asparagine and its structure was considered.     

A temperature study on critical micellization concentration (CMC), solubility, and degree of counterion binding of α-sulfonatomyristic acid methyl ester in water by electroconductivity measurements

 For a sodium salt of α-sulfonatomyristic acid methyl ester (14SFNa), one of the α-SFMe series surfactants, critical micellization concentration (CMC), solubility and degree of counterion binding (β) were determined by means of electrocon-ductivity measurements at different temperatures (at every 5 °C) ranging from 15 to 50 °C. The phase diagram of 14SFNa in pure water was constructed from the CMC- and solubility-temperature data, in which the Krafft temperature (critical solution temperature) was found around 0 °C. The changes in the Gibbs energy, ΔG ⁰ _m, enthalpy, ΔH ⁰ _m, and entropy, ΔS ⁰ _m, upon micelle formation as a function of temperature were evaluated taking βvalues into calculation. Received: 28 August 1996 Accepted: 5 November 1996     

Thermal stability of lysozyme and myoglobin in the presence of anionic surfactants

The interactions of lysozyme and myoglobin with anionic surfactants (hydrogenated and fluorinated), at surfactant concentrations below the critical micelle concentration, in aqueous solution were studied using spectroscopic techniques. The temperature conformational transition of globular proteins by anionic surfactants was analysed as a function of denaturant concentration through absorbance measurements at 280 nm. Changes in absorbance of protein-surfactant system with temperature were used to determine the unfolding thermodynamics parameters, melting temperature, T _m, enthalpy, ΔH _m, entropy, ΔS _m and the heat capacity change, ΔC _p, between the native and denatured states.