The solvation of L-serine in mixtures of water with some aprotic solvents at 298.15 K

The integral enthalpies of solution Δ_sol H ^m of L-serine in mixtures of water with acetonitrile, 1,4-dioxane, dimethylsulfoxide (DMSO), and acetone were measured by solution calorimetry at organic component concentrations up to 0.31 mole fractions. The standard enthalpies of solution (Δ_sol H°), transfer (Δ_tr H°), and solvation (Δ_solv H°) of L-serine from water into mixed solvents were calculated. The dependences of Δ_sol H°, Δ_solv H°, and Δ_tr H° on the composition of aqueous-organic solvents contained extrema. The calculated enthalpy coefficients of pair interactions of the amino acid with cosolvent molecules were positive and increased in the series acetonitrile, 1,4-dioxane, DMSO, acetone. The results obtained were interpreted from the point of view of various types of interactions in solutions and the influence of the nature of organic solvents on the thermochemical characteristics of solutions.     

The dependence of the enthalpy of solution of L-methionine on the composition of water-alcohol binary solvents at 298.15 K

The integral enthalpies of solution of L-methionine in water-methanol, water-ethanol, water-n-propanol, and water-iso-propanol mixtures were measured calorimetrically at alcohol concentrations x ₂ = 0–0.4 mole fractions. The standard enthalpies of solution (Δ_sol H ^o) and transfer of L-methionine (Δ_tr H ^o) from water to a binary solvent were calculated. The influence of the structure and properties of L-methionine and the composition of aqueous-organic mixtures on its enthalpy characteristics was considered. The enthalpic pair interaction coefficients (h _xy ) between L-methionine and alcohol molecules were calculated; they were positive and increased in the series methanol (MeOH), ethanol (EtOH), n-propanol (n-PrOH), iso-propanol (i-PrOH). The enthalpy characteristics of solution and transfer of L-methionine were compared with those of glycine, L-threonine, L-alanine, and L-valine in similar binary solvents.     

Thermochemical investigation of interaction of L-serin with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

By the method of dissolution calorimetry integral enthalpies of dissolution Δ_sol H ^m of L-serine are measured in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of the organic solvent up to 0.42 mole fraction. The standard values of enthalpies of dissolution (Δ_sol H ⁰) and transfer (Δ_tr H ⁰) of amino acids from water to mixed solvents are calculated. The calculated values of the enthalpy coefficients of pair interactions of L-serine with the molecules of co-solvents are positive. The data obtained are interpreted in terms of prevalence of different types of interactions in solutions and the influence of nature of co-solvents on the thermochemical characteristics of the dissolved amino acids.     

Enthalpy characteristics of solution of L-cysteine and L-asparagine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution Δ_sol H ^m of L-cysteine and L-asparagine in mixtures of water with ethanol, n-propanol, and isopropanol at a mole fraction of alcohol of up to 0.32 were determined by calorimetry of solution. The standard enthalpies of solution (Δ_sol H ⁰) of L-serine and of its transfer (Δ_tr H ⁰) from water to a mixed solvent were calculated. The dependences of Δ_sol H ⁰ and Δ_tr H ⁰ on the composition of water-alcohol mixtures pass through a maximum. The calculated enthalpy coefficients of pair interaction of amino acids with alcohol molecules are positive and increase in the order ethanol, n-propanol, isopropanol. The data obtained were interpreted from the viewpoint of various types of interaction in solution and effect of the amino acid residue on the thermochemical characteristics of solution.     

Dependence of the solution and transfer enthalpies of DL-α-alanyl-DL-α-asparagine on the composition of water-amide binary solvents at 298.15 K

Solution enthalpies of DL-α-alanyl-DL-α-asparagine (AlaAsn) in water-formamide, water-N-methylformamide, water-N,N-dimethylformamide, and water-N,N-dimethylacetamide mixtures were measured in the range of amide mole fractions x ₂ = 0–0.3. The standard enthalpies of solution (Δ_sol H°) and transfer (Δ_tr H°) of AlaAsn from water to the binary solvent and enthalpy coefficients of pair-wise interactions (h _xy ) of AlaAsn with amide molecules were calculated. The influence of the composition of the water-organic mixture on the enthalpy characteristics of AlaAsn is discussed. It is shown that the enthalpy characteristics of solution and transfer of AlaAsn are related to the structure of amides.     

The thermochemical characteristics of solution of DL-α-alanylglycine and DL-α-alanylalanine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution of DL-α-alanylglycine and DL-α-alanylalanine in water-ethanol, water-n-propanol, and water-isopropanol mixtures were measured calorimetrically at alcohol concentrations x ₂ = 0?0.4 mole fractions. The standard enthalpies of solution (Δ_sol H°) of the peptides and their transfer (Δ_tr H°) from water into the mixed solvents were calculated. The influence of the structure and properties of the solutes and mixture composition on the enthalpy characteristics were considered. The Δ_sol H° = f(x ₂) and Δ_tr H° = f(x ₂) dependences were found to have extrema. The enthalpy coefficients of pair interactions (h _xy ) between the peptide and alcohol molecules were calculated. The coefficients were positive and increased in the series ethanol, n-propanol, isopropanol.     

The enthalpy of solution of DL-α-alanyl-DL-α-valine depending on the composition of water-alcohol binary solvents at 298.15 K

The integral enthalpies of solution of DL-α-alanyl-DL-α-valine in water-ethanol, water-n-propanol, and water-isopropanol mixtures at alcohol concentrations x ₂ = 0–0.4 mole fractions were measured calorimetrically. The enthalpies of solution of the peptide Δ_sol H° and transfer from water to a mixed solvent Δ_tr H° were calculated. The effect of the structure and properties of the peptide and mixture composition on the enthalpy characteristics of the peptide are discussed. The enthalpy coefficients of pair interactions h _xy of DL-α-alanyl-DL-α-valine with alcohol molecules were calculated. It was found that they were positive and increased in the series ethanol, n-propanol, isopropanol. An analysis of the results allows the general features of changes in the thermodynamic parameters of solution of peptides of the DL-α-alanine series with different amino acid residues in water-alcohol mixtures to be established.     

Thermochemical characteristics of the interaction of L-cysteine with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

Integral enthalpies of dissolution Δ_sol H ^m of L-cysteine in mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at concentrations of organic solvents of up to 0.37 mole fraction were measured by calorimetry of dissolution. The standard values of the dissolution enthalpies (Δ_sol H°) and the transfer enthalpies (Δ_tr H°) of amino acid from water to the mixed solvent were calculated. It was shown that the calculated enthalpic coefficients of the pair interactions of L-cysteine with cosolvent molecules have positive values. The obtained data are interpreted from the viewpoint of the prevalence of different types of interactions in the solutions and influence of the cosolvents nature on the thermochemical characteristics of amino acid dissolution.     

Dissolution properties of oxymatrine in citric acid solution

In this article, the enthalpy of dissolution for oxymatrine in 0.15 M citric acid solution is measured using a RD496-2000 Calvet Microcalorimeter at 36.5 °C under atmospheric pressure. The differential enthalpy (Δ _dif H _m) and molar enthalpy (Δ _sol H _m) were determined for oxymatrine dissolution in 0.15 M citric acid solution. On the basis of these experimental data and calculated results, the kinetic equation, half-life, Δ _sol H _m, Δ _sol G _m, and Δ _sol S _m of the dissolution process were also obtained.     

The thermochemistry of solution of L-α-alanyl-L-α-alanine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution of L-α-alanyl-L-α-alanine in water-ethanol, water-n-propanol, and water-isopropanol mixtures were measured calorimetrically at alcohol concentrations x ₂ ranging from 0 to 0.4 mole fractions. The standard enthalpy of peptide solution Δ_sol H ^o and transfer Δ_tr H ^o from water into a mixed solvent were calculated. The effect of the structure and properties of peptides and mixture composition on the enthalpy characteristics is discussed. The enthalpy coefficients of pair interactions h _xy between L-α-alanyl-L-α-alanine and alcohol molecules were calculated; these coefficients were positive and increased in the series ethanol, n-propanol, isopropanol. The analysis performed allowed the differences in the thermodynamic characteristics of solution of L-α-alanyl-L-α-alanine and DL-α-alanyl-DL-α-alanine in water-alcohol mixtures to be determined.     

The Enthalpies of Solution of L-cysteine, L-serine and L-asparagine in Aqueous Solutions of Some Alcohols at 298.15 K

Heats of solution, Δ_sol H _m , of L-cysteine, L-serine and L-asparagine amino acids have been measured at different concentrations of aqueous ethanol, propanol and 2-propanol at 298.15 K using solvation calorimetry. These data are compared with the results reported earlier for L-alanine in ethanol. The enthalpic coefficients, h _xy , of the solute-organic cosolvent pair interaction in water have been obtained from the McMillan-Mayer approach and the data have been interpreted in terms of various interactions and changes in solvent structure.     

Enthalpy characteristics of dissolution of <Emphasis Type="SmallCaps">l</Emphasis>-cysteine and <Emphasis Type="SmallCaps">l</Emphasis>-asparagine in aqueous solutions of acetonitrile and dimethyl sulfoxide at 298.15 K

The integral enthalpies of dissolution Δ_sol H ^m of l-cysteine and l-asparagine in mixtures of water with acetonitrile and dimethyl sulfoxide at the concentration of organic solvent up to 0.32 molar fractions were measured by means of dissolution calorimetry. The standard enthalpies of dissolution (Δ_sol H°) and transfer (Δ_trans H°) of the amino acids from water to a mixed solvent were calculated. The enthalpy coefficients of pair interactions for L-cysteine and L-asparagine with cosolvent molecules are positive, except for the L-asparagine-water-acetonitrile system. The concepts on the prevailing effect of specific interactions in solutions and the influence of the nature of the cosolvents and lateral substituents of the amino acids on the thermochemical characteristics of dissolution were used to explain the data obtained.     

Enthalpies of Solution of L–L- and D–D- Isomers of Valine in Aqueous Alkanols at 298.15 K

The enthalpies of solution of L- and D-valines in water-ethanol, water-n-propanol, and water-i-propanol mixtures were measured calorimetrically at 298.15 K at alcohol mole fractions, x ₂, ranging up to 0.4. Enthalpies of transfer, Δ_tr H°, from water to aqueous alkanol were calculated for each of the system studied. The enthalpic coefficients, h _xy , of the solute-cosolvent pair-wise interaction in water proved to be positive and increasing in the series: ethanol, n-propanol, and i-propanol. It was shown that both the nature of the amino acid L–L- and D–D-isomerization and dimensions of linear or branched cosolvent molecules define the energetics of interaction between valine and alkanol molecules.     

Thermophysical Investigations of the Polymorphous Phases of Calcium Carbonate

1. Results of thermodynamic and kinetic investigations for the different crystalline calcium carbonate phases and their phase transition data are reported and summarized (vaterite: V; aragonite: A; calcite: C). A→C: T _tr=455±10°C, Δ_tr H=403±8 J mol^–1 at T _tr, V→C: T _tr=320–460°C, depending on the way of preparation,Δ_tr H=–3.2±0.1 kJ mol^–1 at T _tr,Δ_tr H=–3.4±0.9 kJ mol^–1 at 40°C, S _V ^Θ= 93.6±0.5 J (K mol)^–1, A→C: E _A=370±10 kJ mol^–1; XRD only, V→C: E _A=250±10 kJ mol^–1; thermally activated, iso- and non-isothermal, XRD 2. Preliminary results on the preparation and investigation of inhibitor-free non-crystalline calcium carbonate (NCC) are presented. NCC→C: T _tr=276±10°C,Δ_tr H=–15.0±3 kJ mol^–1 at T _tr, T _tr – transition temperature, Δ_tr H – transition enthalpy, S ^Θ – standard entropy, E _A – activation energy. 3. Biologically formed internal shell of Sepia officinalis seems to be composed of ca 96% aragonite and 4% non-crystalline calcium carbonate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of organic co-solvents on the solvation enthalpies of amino acids and dipeptides in mixed aqueous solutions

Transition enthalpies (Δ_tr H ^o) of substances from water to binary solutions were calculated at 298.15 K on the basis of standard dissolution enthalpies (Δ_sol H ^o) for six amino acids and five dipeptides in the mixtures of water with organic solvents of various chemical natures. The enthalpy pair coefficients of interaction h _xy for biomolecules with organic component of mixture were estimated within the formalism of the McMillan-Mayer theory. The change in the character of the interaction of the components of solution was demonstrated in dependence on the physicochemical properties of the solvent and nature of the side radical of the dissolved bioorganic substance. Quantitative estimation of the type of interaction of the substance with the solvent was performed on the basis of correlation ratios relating the enthalpy characteristics of bioorganic substances to properties of organic cosolvents. It was shown that in the solutions under study, the effects of both the specific (mainly electron donor) and non-specific solvation of amino acids and peptides are observed.     

Enthalpic characteristics of solution of amino acids and aliphatic dipeptides in aqueous solutions of KCl

The integral enthalpies of dissolution Δ_sol H ^m of L-α-serine and L-α-asparagine in mixtures of water with KCl were measured in electrolyte concentrations of up to 4 mol/kg at 298.15 K. The standard enthalpies of dissolution (Δ_sol H ^o) and transfer (Δ_tr H ^o) of amino acids from water to aqueous solutions of KCl were calculated. The enthalpic pair interaction coefficients h _xy of biomolecules with KCl were estimated within the McMillan-Mayer theory. The changing nature of the interaction between the components of the solution (depending on the structure of the dissolved biosubstance side substituents) is shown on the basis of data we obtained earlier for amino acids and dipeptides series. Estimates of the contributions from the electrostatic and other interactions of dipolar ions of amino acids and dipeptides with ions of electrolyte KCl in the enthalpic pair interaction coefficients h _xy are obtained using the Kirkwood approach.     

Effects of water-alcohol binary solvents on the thermochemical characteristics of L-tryptophane dissolution at 298.15 K

The enthalpies of L-tryptophane solution in water-methanol, water-ethanol, water-1-propanol, and water-2-propanol mixtures at alcohol concentrations of x ₂ = 0–0.4 mole fractions were measured by calorimetry. The standard enthalpies of L-tryptophane solution (Δ_sol H ^°) and transfer (Δ_tr H ^°) from water to the binary solvent were calculated. The influence of the composition of the water-alcohol mixture and the structure and properties of L-tryptophane on the enthalpy characteristics of the latter was considered. The enthalpy coefficients of pair interactions (h _xy ) of L-tryptophane with alcohol molecules were calculated. The coefficients were positive and increased in the series: methanol (MeOH), ethanol (EtOH), 1-propanol (1-PrOH), and 2-propanol (2-PrOH). The solution and transfer enthalpies of L-tryptophane were compared with those of aliphatic amino acids (glycine, L-threonine, DL-alanine, L-valine, and L-phenylalanine) in similar binary solvents.     

Thermal properties of tetraalkylammonium bromides in several solvents

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

The enthalpies of solution of DL-α-alanine in water-organic solvent mixtures at 298.15 K

The integral enthalpies of solution (298.15 K) of DL-α-alanine in water-organic solvent mixtures were measured at organic component concentrations x ₂ = 0–0.4 mole fractions. The organic solvents used were acetonitrile (ACN), formamide (FA), N,N-dimethylformamide (DMFA), and N,N-dimethylsulfoxide (DMSO). The standard enthalpies of solution Δ_sol H ^o, solvation Δ_solv H ^o, and transfer (Δ_tr H ^o) of DL-α-alanine from water to mixed solvents were calculated. The influence of the structure and properties of solutes and mixture composition on solute thermochemical characteristics was considered. The solution of DL-α-alanine in the mixtures studied was endothermic over the whole range of organic component concentrations. The Δ_sol H ^o, Δ_tr H ^o, and Δ_solv H ^o values as functions of x ₂ can pass extrema (DMSO and DMFA), be almost independent of mixed solvent composition (FA), or be exothermic and monotonic functions (ACN). The enthalpy coefficients of pair interactions (h _xy ) between DL-α-alanine and organic solvent molecules were calculated. The linear Kamlet-Taft equation was used to correlate the h _xy values with the properties of organic solvents.     

The enthalpies of solution and solvation of glycine in mixed water-formamide solvents at 298.15 K

The enthalpies of solution (ΔH _sol ^o ) of glycine in aqueous formamide, N-methylformamide, N,N-dimethylformamide, and N,N-diethylformamide were determined by calorimetry at 298.15 K over the concentration range x ₂=0–0.3 mole fractions. The enthalpies of glycine solvation (ΔH _solv ^o ) and transfer from water to mixed solvents (ΔH _tr ^o ) were calculated. The ΔH _sol ^o =f(x ₂) dependences for glycine in water-N-and water-N,N-substituted amide mixtures had extrema and, in water-formamide mixtures, this dependence was a smooth function, whose values changed in the opposite direction. The enthalpy coefficients of pair glycine-amide interactions were calculated. The interrelation between the enthalpy characteristics of solution, transfer, and solvation of glycine and the structure and physicochemical characteristics of solvents, on the one hand, and the composition of mixtures, on the other, was revealed.