Thermodynamic properties of solutions of benzoic and citric acids in formamide at different tempertures

The standard thermodynamic quantities, ΔG⁰, ΔH⁰, and ΔS⁰, associated with the ionization process of benzoic acid in formamide have been evaluated. The standard potentials of the Ag(s)/AgCNS(s)/CNS⁻ and the Ag(s)/Ag₃Ci(s)/Ci³⁻ electrode, and the standard thermodynamic quantities for the electrode processes have been calculated in formamide, at different temperatures.     

Formation of cobalt, nickel and copper complexes with murexide in ethanol-water mixtures

The complexation reactions between murexide and Co²⁺, Ni²⁺ and Cu²⁺ in C₂H₅OH-H₂O mixtures have been investigated spectrophotometrically. The formation constants of the 1:1 complexes formed increase in the order Co²⁺ < Ni²⁺ < Cu²⁺ for all solvent mixtures studied, and log K_f is a linear function of the mole fraction of ethanol. The heat of complexation was determined calorimetrically for the nickel and copper complexes. The values of ΔH° and ΔS° are solvent-dependent, and all three complexes have negative ΔH° and positive ΔS° values.     

Solubility and thermodynamic data of manganese hydrogen phosphate in the system MnO---P2O5---H2O at 35, 40, 45 and 50°C

Manganese hydrogen phosphate monohydrate, MnHPO₄·H₂O, a new phase, is synthesized. Its solubility is investigated in the temperature range 35–50°C and pH range 3.4–7.5. K_sp, ΔH⁰, ΔS⁰ and ΔG⁰ for the dissolution are reported. The decrease in solubility with increase in pH is explained as due to a surface coating of insoluble basic phosphate.     

Influence of non-covalent interactions on the thermodynamic stereoselectivity of the protonation of some dipeptides

ΔG⁰, ΔH⁰ and ΔS⁰ protonation values of some pairs of diastereoisomeric dipeptides have been determined by potentiometry and calorimetry in aqueous solution at 25°C and I = 0.1 mol dm⁻³ (KNO₃). On the basis of the results obtained it has been possible to assess the role played by two different non-covalent interactions, namely the electrostatic interaction and the solvophobic interaction, on the thermodynamic stereoselectivity in the proton complex formation, shown by the systems investigated.     

Thermodynamics of silver salicylate in water and water + 10, + 20, and + 40 mass percent of dioxane at different temperatures

The solubility studies on silver salicylate at different temperatures were made to derive (a) the standard electrode potential of the silver—silver salicylate electrode, (b) the mean activity coefficient of silver salicylate, (c) the dissociation constant of salicylic acid, and (d) the standard thermodynamic quantities, ΔG⁰_t, ΔH⁰_t, ΔS⁰_t, and δC⁰_pt, for the transfer of silver salicylate from the standard state in water to the standard state in water + 10, + 20, and + 40 mass percent of dioxane. The results are discussed in terms of the preferential solvation of the ions.     

Thermodynamics of hydrogen adsorption on the zeolite Li-ZSM-5

Thermodynamic characterisation of the adsorption process (at a low temperature) of dihydrogen on the zeolite Li-ZSM-5 was carried out by means of variable-temperature infrared spectroscopy, with the simultaneous measurement of temperature and equilibrium pressure. Adsorption renders the H–H stretching mode infrared active, at 4092 cm⁻¹. The standard adsorption enthalpy and entropy resulted to be ΔH⁰=−6.5(±0.5) kJmol⁻¹ and ΔS⁰=−90(±5) Jmol⁻¹ K⁻¹, respectively. The adsorption enthalpy is significantly larger than the liquefaction heat, and this fact renders Li-ZSM-5 a potential cryoadsorbent for hydrogen storage.     

Thermodynamic studies of the solvation of Ph4AsPh4B in mixed solvents (MeOH—DMF)

The thermodynamic data (ΔG⁰, ΔH⁰ and TΔS⁰) of the solvation of tetraphenylarsonium-tetraphenylborate (Ph₄AsPh₄B) and its neutral parts, tetraphenylgermanium (Ph₄Ge) and tetraphenylmethane (Ph₄C) in methanol—N,N-dimethylformamide mixed solvents are discussed.

The values of the free energy of transfer, Δ^s_MG⁰, are calculated from measurements of the solubilities of Ph₄AsPh₄B, Ph₄Ge and Ph₄C in the successive fractions of MeOH in DMF at three different temperatures (15, 25, 35°C). The values of Δ^s_MH⁰ and TΔ^s_MS⁰ for the derivatives are calculated from Δ^s_MG⁰ values.

The values of Δ^s_MG⁰, Δ^s_MH⁰ and TΔ^s_MS⁰ of tetraphenylarsonium and tetraphenylborate ions have also been carefully calculated. The ratios of Δ^s_MG⁰ values (Δ^s_MG⁰ = ΔG⁰(+)/ΔG⁰(−)) were found to be greater than unity. Similarly, the ratios of Δ^s_MH⁰ and TΔ^s_MS⁰ for the positive and negative ions were found to be greater than unity.     

Chiral separation of enantiomers of amino acid derivatives by high-performance liquid chromatography on a norvancomycin-bonded chiral stationary phase

A novel norvancomycin-bonded chiral stationary phase (NVC-CSP) was synthesized by using the chiral selector of norvancomycin. The chiral separation of enantiomers of several dansyl-amino acids by high-performance liquid chromatography (HPLC) in the reversed-phase mode is described. The effects of some parameters, such as organic modifier concentration, column temperature, pH and flow rate of the mobile phase, on the retention and enantioselectivity were investigated. The study showed that ionic, as well as hydrophobic interactions were engaged between the analyte and macrocycle in this chromatographic system. Increasing pH of buffers usually improved the chiral resolution for dansyl--amino-n-butyric acid (Dns-But), dansyl-methionine (Dns-Met) and dansyl-threonine (Dns-Thr), but not for dansyl-glutamic acid (Dns-Glu) which contains two carboxylic groups in its molecular structure. The natural logarithms of selectivity factors (ln ) of all the investigated compounds depended linearly on the reciprocal of temperature (1/T), most processes of enantioseparation were controlled enthalpically. Interestingly, the process of enantioseparaton for dansyl-threonine was enthalpy-controlled at pH of 3.5, while at pH of 7.0, it was entropy-controlled according to thermodynamic parameters Δ_R,SΔH° and Δ_R,SΔS° afforded by Van’t Hoff plots. In order to get baseline separation for all the solutes researched, norvancomycin was also used as a chiral mobile phase additive. In combination with the NVC-CSP, remarkable increases in enanselectivity were observed for all the compounds, as the result of a “synergistic” effect.     

Determination of thermodynamic parameters and equilibrium constants for complexation reactions involving lanthanon(III) ions with 1,2-naphthoquinone-2-semicarbazone-4-sulphonic acid (sodium salt) (NQS · 4S)

The thermodynamic parameters and equilibrium constants of complexation reactions of 1,2-naphthoquinone-2-semicarbazone-4-sulphonic acid (sodium salt) (NQS · 4S) involving lanthanon ions were evaluated by potentiometric estimation of [H⁺], in aqueous medium at various ionic strengths (0.02, 0.05, 0.10 and 0.20 M NaClO₄) and at different temperatures (20, 30, 40 and 50±0.5°C). The −n and pL values calculated by the method of Bjerrum and Calvin, as modified by Irving and Rossotti, were used for the calculation of the equilibrium constants on an HCL Busy Bee PC/AT computer using the weighted least-squares method. The values of S_min were also evaluated. The order of equilibrium constants was found to be Ho(III) > Dy(III) > Tb(III) > Y(III) > Eu(III) > Sm(III) > Gd(III) > Nd(III) > Pr(III) > Ce(III) > La(III). The thermodynamic parameters ΔG, ΔH and ΔS were also calculated at a fixed temperature (30±0.05°C) and ionic strength (0.10 M NaClO₄).     

Enantioseparation of various amino acid derivatives on a quinine based chiral anion-exchange selector at variable temperature conditions. Influence of structural parameters of the analytes on the apparent retention and enantioseparation characteristics

The influence of temperature on the performance of an enantioselective anion-exchange type chiral selector (SO) was systematically investigated. The resolution of the enantiomers of 23 N-acylated amino acids (selectands, SAs) on a covalently immobilized quinine tert.-butylcarbamate chiral stationary phase (CSP) was studied under linear chromatographic conditions over a temperature range of 0–85 °C with hydro–organic buffers (pH_a 6.0) as mobile phases. The apparent enantioseparation factors increased considerably at low column temperatures, indicating that enthalpic contributions are the dominating thermodynamic driving force for chiral recognition for all investigated SAs. Retention factors gave non-linear van’t Hoff plots, while the corresponding apparent enantioseparation factors showed linear van’t Hoff behavior. Correlations between magnitude and sign of the relative thermodynamic parameters of enantioselective adsorption (ΔΔG, ΔΔH and ΔΔS) and specific structural features of the analytes, i.e., steric and electronic nature of the various side chains and the N-acyl groups, are discussed with the aim to rationalize their possible contributions to the overall chiral recognition.     

Fluorometric investigation on the interaction of oleanolic acid with bovine serum albumin

The interactions between oleanolic acid and bovine serum albumin (BSA) have been studied by fluorescence, circular dichroism (CD), UV–vis absorption and Fourier transform infrared spectroscopy (FTIR) under physiological conditions. Spectroscopic analysis of the emission quenching at different temperatures has revealed that the quenching mechanism of bovine serum albumin by oleanolic acid is static quenching mechanism. The binding sites number n and binding constants K are obtained at various temperatures. The distance r between oleanolic acid and the protein is evaluated according to the theory of Forster energy transfer. The results by FTIR, CD and UV–vis absorption spectra experiment indicate that the secondary structures of protein have been perturbed in the presence of oleanolic acid. The thermodynamic parameters ΔH⁰, ΔG⁰, and ΔS⁰ are calculated according to van’t Hoff equation, which indicates that the hydrogen bonds and van der-waals are the intermolecular forces stabilizing the complex. Molecular modeling studies the interaction BSA with oleanolic acid.     

Solvent extraction of Fe(III) by dehydrated castor oil fatty acids

The effect of temperature on the extraction of FE(III) by dehydrated castor oil fatty acids (DCOFA) has been studied in the temperature range 283–313 K at 1.0M constant ionic strength (NaClO₄). The temperature dependence of the conditional constant of extraction is given in the form: ln K_ext=31.95 − 12800(1/T). Also, it was found that the average thermodynamic parameters, ΔH°_ext, ΔG°_ext, and ΔS°_ext are 106.5 kJ/mole, 27.3 kJ/mole, and 0.3 kJ. mole⁻¹.K⁻¹, respectively. The extracted species in toluene solution were identified as FeR₃.HR and Fe(OH)R₂, where HR represents the fatty acid used.     

Study of the effect of Cal-Red on the secondary structure of human serum albumin by spectroscopic techniques

The effect of Cal-Red on the structure of human serum albumin (HSA) was studied using Resonance light scattering (RLS), Fourier transformed Infrared (FT-IR) and Circular dichroism (CD) spectroscopic methods. The RLS spectroscopic results show that the RLS intensity of HSA was significantly increased in the presence of Cal-Red. The binding parameters of HSA with Cal-Red were studied at different temperatures of 289, 299, 309 and 319 K at pH 4.1. It is indicated by the Scatchard plots that the binding constant K decreased from 4.03 × 10⁸ to 7.59 × 10⁷ l/mol and the maximum binding number N decreased from 215 to 152 with increasing the temperature, respectively. The binding process was exothermic and spontaneous, as indicated by the thermodynamic analyses, and the major part of the binding energy is hydrophobic interaction. The enthalpy change ΔH⁰, the free energy change ΔG⁰ and the entropy change ΔS⁰ of 289 K were calculated to be −42.75 kJ/mol, −47.56 kJ/mol and 16.66 J/mol K, respectively. The alterations of protein secondary structure in the presence of Cal-Red in aqueous solution were quantitatively calculated from FT-IR and CD spectroscopy with reductions of -helices content about 5%, β-turn from 10% to 2% and with increases of β-sheet from 38% to 51%.     

A calorimetrical study of the polynuclear Zn(II) and Pb(II) and the polymeric Ni(II) and Cd(II) complexes of 2-mercaptoethanol and 3-mercapto-1,2-propanediol

The ΔG, ΔH and ΔS values for the dissociation of 2-mercaptoethanol (MEL) and 3-mercapto-1,2-propanediol and for the formation of complexes between these ligands and the metal ions Ni²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ have been determined calorimetrically in 0.5 M KNO₃ and at 25°C.     

Heat capacities of NaNO3 and KNO3 from 350 to 800 K

The heat capacities of NaNO₃ and KNO₃ were determined from 350 to 800 K by differential scanning calorimetry. Solid-solid transitions and melting were observed at 550 and 583 K for NaNO₃ and 406 and 612 K for KNO₃, respectively. The entropies associated with the solid-solid transitions were measured to be (8.43± 0.25) J K⁻¹ mole⁻¹ for NaNO₃ and (13.8±0.4) J K⁻¹ mole⁻¹ for KNO₃. At 298.15 K the values of C⁰_P S⁰_P, {H⁰(T)-H⁰(0)}/T and -{G⁰(T)-H⁰(0)}/T, respectively, are 91.94, 116.3, 57.73, and 58.55 J K⁻¹ mole⁻¹ for NaNO₃ and 95.39, 133.0, 62.93, and 70.02 J K⁻¹ mole⁻¹ for KNO₃. Values for S⁰_T, {H⁰(T)-H⁰(0)}/T, and -{G⁰(T)-H⁰(0)}/T were calculated and tabulated from 15 to 800 K for NaNO₃ and KNO₃.     

Standard molar enthalpies of formation of lithium alkoxides

The standard (p⁰ = 0.1 MPa) molar enthalpies of formation of several crystalline lithium alkoxides, ΔH_f⁰(LiOR, cr), have been determined by reaction-solution calorimetry at 298.15 K. A linear correlation has been found between ΔH_f⁰(LiOR, cr) and ΔH_f⁰(ROH, 1) for R = n-alkyl, enabling the prediction of data for other lithium alkoxides. The deviations from the linear correlation observed for R =ⁱPr and ^tBu were tentatively explained in terms of the electronegativities of the OR groups. The experimental data were also used to derive the lattice energies and the thermochemical radii of the anions OR⁻. The results were compared with those derived from the enthalpies of formation of the analogous sodium alkoxides, reported in a previous publication.     

Study on the retention behaviour of low-molar-mass polystyrenes and polyesters in reversed-phase liquid chromatography by evaluation of thermodynamic parameters

Polymers can be characterized under sorption conditions, to obtain information on molar mass and chemical composition. In order to get a better understanding of their retention behaviour under such conditions, the evaluation of thermodynamic parameters obtained from van't Hoff analyses on low-molar-mass polystyrenes (PS) and polyesters (PE) in various THF–water mixtures on a C₁₈ column is described in this study. Linear van't Hoff behaviour was observed in almost all cases. Negative values for both ΔH and ΔS were found for both PS and PE oligomers, which increase with increasing %THF. For ΔS this is explained from multi-site attachment effects. For PS, the non-linear relations between ΔH and ΔS, and degree of polymerization (p) could be properly described by the Stockmayer–Fixman equation. Although less clear, similar trends were found for PE. For PS, evidence for penetration effects of oligomer chains into the bonded chains was obtained. Martin plots for both PS and PE were shown to be non-linear in all investigated eluent compositions. The extent of non-linearity is suggested to depend on the conformation of a polymer in solution. No distinct enthalpy–entropy-compensation temperature (EECT) independent of p was found for PS, thus confirming the findings of an earlier study in which no exact molar mass independence was found under critical conditions. Further evaluation of EECT for PS oligomers revealed a retention mechanism independent of the binary eluent composition. This indicates that conclusions from this study can also be used for a qualitative understanding of sorption mechanisms in the gradient elution mode. Finally, for PS it was shown that ΔG equals zero under critical conditions, thus confirming theoretical predictions.     

Calorimetric studies of binary systems of 1,3,5-trinitrobenzene with naphthalene, anthracene and carbazole. II. Phase diagrams

The phase diagrams of the binary systems of 1,3,5-trinitrobenzene (TNB) with naphthalene, anthracene and carbazole have been determined by differential scanning calorimetry and optical microscopy over the temperature range 180 K to just above the melting point. All systems show the same features: (i) systems form nearly ideal double simple-eutectic type phase diagrams with 1 : 1 complex, (ii) each one of three known modifications of TNB may exist as a component of the complex—TNB eutectic mixtures. (iii) measured liquidus lines of complexes agree within experimental error with those calculated by the Vieland equation for a completely dissociated complex in the liquid phase, whereas the experimental liquidus lines for the parent components deviate slightly from those predicted by the Schröder—van Laar equation, indicating some degree of complexing in the liquid phase.

The solubility parameter theory has been used to clarify this discrepancy. Applying this theory to the liquidus lines of complexes, we have found that these TNB complexes are still stable upon fusion, and an approximate degree of dissociation amounted to 90% at the melting point in all three cases.

The enthalpy of complex formation, ΔH⁰, both in the liquid and solid state has been determined. The values of ΔH⁰ show that in the solid state the carbazole—TNB complex is the most stable, and the naphthalene—TNB complex is the least stable.     

'Genome order index' should not be used for defining compositional constraints in nucleotide sequences

A “genome order index,” defined as S = a² + c² + t² + g², where a, c, t, and g are the nucleotide frequencies of A, C, T, and G, respectively, was used to suggest that there exist genome-specific constraints on nucleotide composition. We show that the “evidence” for constraint, S < 1/3, is in fact a mathematical property that is always true regardless of data. Moreover, we show that S is strictly equivalent to and derivable from the Shannon H-function and has no advantage over it.     

Surface activity and thermodynamic of micellization and adsorption for isooctylphenol ethoxylates, phosphate esters and their mixtures with N-diethoxylated perfluorooctanamide

Three nonionic surfactants; p-isooctylphenol ethoxylates p-[i-OPE10], p-[i-OPE15], and p-[i-OPE20], were phosphorylated to produce three anionic phosphate ester surfactants. In addition, N-diethoxylated perfluorooctanamide (N-DEFOA) was also prepared. The surface and thermodynamic properties of the three types of surfactants and mixtures of the fluorocarbon surfactant (FC) with the hydrocarbon surfactants (HC) have been investigated. Surface tension as a function of concentration of the surfactant in aqueous solution was measured at 30, 40, 50 and 60°C, using the spinning drop technique. From these measurements the critical micelle concentration (CMC), the surface tension at the CMC (γ_CMC), the maximum surface excess concentration (Γ_max), the minimum area per molecule at the aqueous solution/air interface (A_min), and the effectiveness of surface tension reduction (π_CMC), were calculated. The thermodynamic parameters of micellization (ΔG_mic, ΔH_mic, ΔS_mic) and of adsorption (ΔG_ad, ΔH_ad, ΔS_ad) for these surfactants and their mixtures were also calculated. Structural effects on micellization, adsorption and effectiveness of surface tension reduction are discussed in terms of these parameters. The results show that the FC surfactant and its mixtures with HC surfactants enhance the efficiency in surface tension reduction and adsorption in the mixed monolayer at the aqueous solution/air interface, and also, reduce γ_CMC and the tendency towards micellization.