Enthalpic Pair Interaction Coefficients Research Articles

Effects of l-α-amino acids side chains on their interparticle interactions with the dissociated potassium chloride in aqueous solutions

Dissolution enthalpies of several natural amino acids (l-α-asparagine, l-α-glutamine, l-α-aspartic acid, l-α-glutamic acid, l-α-arginine, l-α-lysine and l-α-histidine) were measured in aqueous solutions of potassium chloride at T = 298.15 K. Based on the resulting data the standard dissolution enthalpy of amino acids in aqueous solutions of potassium chloride have been determined. The standard dissolution enthalpy values were subsequently used to calculate the heterogeneous enthalpic pair interaction coefficients (hA-KCl) between zwitterions of l-α-amino acids and dissociated potassium chloride in water. Interparticle interactions in system (amino acid + dissociated KCl + water) were interpreted in terms of the hydrophobic - hydrophilic effects of the side chains of the tested encoded amino acids.

Thermodynamics of DL-alalnyl-DL-asparagine dissolution in water–organic mixtures at 298.15 K

Enthalpies of dissolution of DL-α-alanyl-DL-α-asparagine (AlaAsn) in aqueous solutions of acetonitrile, 1,4-dioxane, acetone, and dimethyl sulfoxide are measured via calorimetry at T = 298.15 K and organic solvent concentrations x2 = 0–0.3 mole fractions. Standard values of the enthalpy of dissolution (ΔsolH°), transfer (ΔtrH°) of AlaAsn from water into mixed solvent, and enthalpic pair interaction coefficients (hxy) of AlaAsn with molecules of organic solvents are calculated on the basis of experimental data. The effect the composition of a water–organic mixture has on the enthalpy characteristics of AlaAsn is considered. The correlation between the enthalpy of dissolution and the transfer of AlaAsn and the structure and properties of the solvents used is demonstrated.

Interaction of Some Aminophosphonic Acids with Urea in Aqueous Solutions at 298.15 K

The enthalpies of solution of aminomethylphosphonic acid (GlyP), 1-aminoethylphosphonic acid (AlaP), and 1-aminopropylphosphonic acid (HalaP) have been measured in water and aqueous urea solutions at 298.15 K. From the obtained experimental results, the standard enthalpies of solution of aminophosphonic acids (AP) in water–urea solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan–Mayer's formalism. These values were interpreted in the terms of the hydrophobic effect of the side chains of aminophosphonic acids on their interactions with a molecule of urea in water.

Study of the interaction between ethanol and natural amino acids containing ionic side groups in water at T = 298.15 K

The enthalpies of solution of l-α-aspartic acid, l-α-glutamic acid, l-α-arginine, l-α-lysine, and l-α-histidine have been measured in aqueous ethanol solutions at 298.15 K. From the obtained experimental results, the standard enthalpies of solution of amino acids in water–ethanol solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan–Mayer’s formalism. These values were interpreted in the terms of the ionic or no polar effect of the side chains of l-α-amino acids on their interactions with a molecule of ethanol in water.

Thermochemistry of electrolyte solutions

The results of calorimetric investigations of electrolyte solutions in the mixtures of water, methanol, N,N-dimethylformamide, and acetonitrile with numerous organic cosolvents are discussed with regard to the intermolecular interactions that occur in the solution. Particular attention is given to answer the questions how and to what extent the properties of the systems examined are modified by the cosolvent added and how much the properties of the cosolvent are revealed in the mixtures with the solvents mentioned above. To this goal, the analysis of the electrolyte dissolution enthalpies, single ionic transfer enthalpies, and enthalpic pair interaction coefficients as well as the preferential solvation (PS) model are applied. The analysis performed shows that in the case of the dissolution enthalpies of simple inorganic electrolytes in water–organic solvent mixtures, the shape of the dependence of the standard dissolution enthalpy on the mixed solvent composition reflects to a large extent the hydrophobic properties of the organic cosolvent. In the mixtures of methanol with organic cosolvents, the ions are preferentially solvated either by methanol molecules or by molecules of the cosolvent, depending on the properties of the mixed solvent components. The behavior of inorganic salts in the mixtures containing N,N-dimethylformamide is mostly influenced by the DMF which is a relatively strongly ion solvating solvent, whereas in acetonitrile mixtures, the thermochemical behavior of electrolyte solutions is influenced to a large extent by the properties of the cosolvent particularly due to the PS of cation by the cosolvent molecules.

Enthalpic interaction coefficients of NaI–alkanediol pairs in methanol and in water

Abstract The dissolution enthalpies of NaI in the mixtures of methanol with 1,2-alkanediols (1,2-propanediol, 1,2-butanediol, 1,2-pentanediol) and with α,ω-alkanediols (1,3-propanediol, 1,4-butanediol, 1,5-pentanediol), as well NaI in the mixtures of water with 1,3-propanediol and 1,2-pentanediol, were determined at 298.15 K. The energetic effect of interactions between the investigated alkanediols and NaI in methanol and in water was calculated using the enthalpic pair interaction coefficients (hxy) model. These results along with the other data concerning the NaI–non-electrolyte pairs taken from our earlier reports and from the literature were analyzed with respect to the effect of the non-electrolyte properties on the variations of the hxy values. The group contributions illustrating the interactions of NaI with selected functional groups in non-electrolyte (alkanediol and alkanol) molecules, namely: CH2 and OH groups were calculated and discussed.

Enthalpy of dilution and volumetric properties of N-glycylglycine in aqueous xylitol solutions at T = 298.15 K

The enthalpy of dilution of N-glycylglycine (Δ dil H m) in aqueous xylitol solutions has been determined by means of flow-mix isothermal microcalorimetry at the temperature of 298.15 K. The homogeneous enthalpic interaction coefficients ( h 2, h 3, and h 4) which characterize the interactions of examined N-glycylglycine in aqueous xylitol solutions have been calculated according to the excess enthalpy concept based on the values of dilution enthalpy. It has been found that the enthalpic pair interaction coefficients ( h 2) in the systems investigated are negative and become less negative as the molality of xylitol increases. Values of the density ( ρ) of the ternary homogeneous systems were also measured with a quartz vibrating-tube densimeter at the temperature of 298.15 K. The values of the apparent molar volume ( V ϕ ) of the ternary systems were calculated from the data of density, which have been used to deduce limiting partial molar volumes of N-glycylglycine ( V ϕ o ) and limiting partial molar volumes of transfer ( Δ trs V ϕ o ) from water to aqueous xylitol solutions at different concentrations. The results have been discussed based on solute–solute interactions and solvation effects.

Interactions between several l-α-amino acids and potassium chloride in aqueous solutions at 298.15 K

The enthalpies of solution of l-α-aminobutyric acid, l-α-valine, l-α-leucine, l-α-isoleucine, and l-α-cysteine have been measured in aqueous potassium chloride solutions at 298.15 K. From the obtained experimental results the standard dissolution enthalpies of amino acids in aqueous KCl solutions have been determined. These data were used to calculate the heterogeneous enthalpic pair interaction coefficients based on McMillan–Mayer’s theory. These values were interpreted in the terms of the hydrophobic or hydrophilic effects of the side chains of amino acids on their interactions with dissociated potassium chloride in water.

Enthalpic Interaction Coefficients of Several l-α-Amino Acids in Aqueous Sodium Chloride Solutions at 298.15 K

The enthalpies of solution of L-α-isoteucine, L-α-cysteine, L-α-aspartic acid, and L-α-gtutamic acid have been measured in aqueous sodium chloride solutions at 298.15 K. From the obtained experimental results the standard dissolution enthalpies of amino acids in aqueous NaCl solutions have been determined. These data were used to calculate of the heterogeneous enthalpic pair interaction coefficients based on McMillan-Mayer's theory. These values were interpreted in the terms of the hydrophobic or hydrophilic effects of the side chains of amino acids on their interactions with dissociated sodium chloride in water.

ChemInform Abstract: Enthalpies of Interaction of Some N-Acetyl Amino Acid Amides Dissolved in N-Methylformamide at 298.15 K.

Enthalpies of dilution of N-acetyl amides of glycine, L-alanine, L-valine, L-leucine and L-proline, as well as the N-acetyl-N′-methyl acids of the above amino acids dissolved in N-methylformamide (NMF) have been measured calorimetrically at 298.15 K. The results were used to calculate enthalpic pair interaction coefficients Bh2, which are a measure of the enthalpy of interaction between two solute molecules.The Bh2 coefficients of the above peptides are compared with those in N,N-dimethylformamide (DMF) and N-methylacetamide (NMA). The possible role of these solvents to be used as model environments for the inner parts of globular proteins is discussed. With regard to the latter, all three amidic solvents appear to be equally suitable, although DMF seems to be slightly more favourable than the other two. Finally, the Bh2 coefficients obtained here have been combined with previously reported ones and analysed in terms of the excess group additivity model. By taking into account the intramolecular hydrogen-bond formation of the peptides the predictive power of this model is enhanced.

Thermochemical Examination of NaI, NaBPh4, and Ph4PI in the Mixtures of Acetonitrile with N,N-Dimethylformamide and Dimethylsulfoxide at 298.15 K: Properties of Acetonitrile-Containing Mixtures as Solvents for Electrolytes

Dissolution enthalpies of NaI, NaBPh4, and Ph4PI in the mixtures of N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) with acetonitrile (AN) over the whole composition range were measured at 298.15 K. From these data the single ion (Na+, I−, BPh4−, Ph4P+) enthalpies of transfer from AN to AN + cosolvent mixtures and the enthalpic pair interaction coefficients, hxy (NaI−nonelectrolyte), in AN were determined and analyzed together with our earlier data concerning the mixtures of AN with methanol (MeOH), N, N-dimethylacetamide (DMA), propylene carbonate (PC), and 2-methoxyethanol (2-ME). The changes in electrolyte solvation were described with the use of Covington’s preferential solvation (PS) model. The linear relationship between interaction coefficients, hxy, of NaI−nonelectrolyte in AN and ln K1/n values from PS model for NaI molecule in the mixture of AN and DMA, 2-ME, DMF, and DMSO as well as for Na+ ion in the mixture containing DMA, DMF, 2-ME, and MeOH was demonstrated. The behavior of the mix...

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

The integral enthalpies of dissolution Δsol 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 (Δsol H o) and transfer (Δtr 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.

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 (Δsol H o) and transfer of L-methionine (Δtr 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.

Enthalpic interactions of N,N-dimethylformamide in aqueous glucose and sucrose solutions at 298.15 K

Enthalpies of dilution of N,N-dimethylformamide (DMF) in aqueous sucrose and glucose solutions have been determined using an isothermal calorimeter (4400 IMC) at 298.15 K. The values of dilution enthalpy were used to determine homogeneous enthalpic interaction coefficients which characterize the interactions of examined DMF in aqueous sugar solutions. The results show that enthalpic pair interaction coefficients h 2 of DMF are all positive in aqueous sugar solutions and pass through a maximum, respectively, at m ≈ 0.5 mol kg −1 of sucrose and at m ≈ 0.55 mol kg −1 of glucose in mixed solvents. In the meantime the h 2 coefficients of DMF in aqueous sucrose solutions are more positive than that in aqueous glucose solutions. The variations of the enthalpic pairwise interaction coefficients with the mass fraction of sugar in mixtures are interpreted in terms of solute–solute and solute–solvent interactions.

Enthalpic interactions between some amino acids and cyclohexanone in aqueous solutions at 298.15 K

The enthalpies of mixing of glycine, l-α-alanine, l-γ-aminobutyric acid, l-α-valine, l-α-serine and l-α-threonine with cyclohexanone in aqueous solutions and their respective enthalpies of dilution have been measured by calorimetry at 298.15 K. Experimental enthalpies of dilution and mixing have been correlated with the virial expansion equation that was obtained with the McMillan–Mayer theory. The enthalpic interaction parameters h xy , h xxy and h xyy of the amino acids studied with cyclohexanone in aqueous solutions have been evaluated, and the heterotactic enthalpic pair interaction coefficients ( h xy ) are discussed in terms of solute–solute interactions.

Enthalpies of Dilution of D-p-Hydroxyphenylglycine in Buffer Solutions at Different pH

The effect of pH on the dilution enthalpies of D-p-hydroxyphenylglycine in phosphate buffer solutions has been investigated by isothermal titration microcalorimetry at 298.15 K. The corresponding homogeneous enthalpic interaction coefficients have been calculated according to the excess enthalpy concept. The results show that the enthalpies of dilution of D-p-hydroxyphenylglycine in phosphate buffer solutions at different pH are all positive. The overall trend is that enthalpies of dilution become more positive with an increase of pH, but there is a minimum of the enthalpy of dilution at pH = 7.0. The enthalpic pair interaction coefficients, h2, all have negative values. The results are interpreted from the point of view of solute-solute and solute-solvent interactions involved in the solvent effects.

Enthalpic Interaction Coefficients of Formamide in Aqueous Methanol and Ethanol Solutions at 298.15 K

The dilution enthalpies of formamide in aqueous methanol and ethanol solutions have been determined using a CSC-4400 isothermal calorimeter at 298.15 K. The homogeneous solution enthalpic interaction coefficients have been calculated over a range of alcohol concentrations according to the excess enthalpy concept. The results show that the enthalpic pair interaction coefficients h 2 of formamide are negative in aqueous alcohol solutions and pass through a minimum in mixed solvents, whereas the h 2 coefficients of formamide in aqueous ethanol solutions are more negative than those in aqueous methanol solutions. The results are discussed in terms of solute-solute and solute-solvent interactions.

Thermochemical properties of ions in acetonitrile–organic cosolvent mixtures at 298.15 K

Dissolution enthalpies of NaI, NaBPh 4 and Ph 4PI in the mixtures of propylene carbonate (PC), 2-methoxyethanol (ME) and N, N-dimethylacetamide (DMA) with acetonitrile (AN) were measured over the whole composition range at 298.15 K. From these data, the single ion (Na +, I −, BPh 4 −, Ph 4P +) enthalpies of transfer from AN to AN–cosolvent mixtures and the enthalpic pair interaction coefficients, h xy (NaI—nonelectrolyte), in solvents were determined and analyzed. The changes in electrolyte solvation were described with the use of Covington's model.

Calorimetry—an important tool in solution chemistry

Calorimetry belongs to the most important experimental techniques. It is the only experimental method allowing for direct measurements of various physical and chemical processes and reactions. When appropriate model is used, analysis of results on molecular level is possible. This paper is devoted to use of solution calorimetry for investigation of two and three component liquid systems. Special attention is paid on results of dissolution enthalpy and heat capacity measurements. Several ways for analysis of the obtained results are presented. The course of standard enthalpies of solution versus mixed solvent composition can serve as a principal basis of the physico-chemical characteristics of investigated solutions. A quantitative measure of energetic effect of solute–solute interactions in solution can be obtained from analysis of the so-called enthalpic pair interaction coefficients derived from McMillan–Mayer theory. The Mastroianni, Pikal and Lindenbaum “cage model” and model of selective solvation proposed by Covington are useful in discussion of the dissolution enthalpies of hydrophobic solutes in water–organic cosolvent mixtures. Finally, the application of two-point scaling approach to the description of heat capacity function in microheterogenic systems is presented and discussed.

Thermochemical properties of electrolyte solutions in {2-(2-methoxyethoxy)ethanol + water} and (2-isopropoxyethanol + water) mixtures at 298.15 K

Dissolution enthalpies of NaCl and NaI in aqueous solution of 2-(2-methoxyethoxy)ethanol (C 1E 2) and 2-isopropoxyethanol ( iC 3E 1) have been measured and appropriate enthalpic pair interaction coefficients, h xy (electrolyte–non-electrolyte), in water have been determined at 298.15 K. Our investigations are a continuation of earlier studies concerning the influence of the alkyl group size in the 2-alkoxyethanol (2-methoxyethanol, 2-ethoxyethanol or 2-butoxyethanol) molecule on solution properties. The standard solution enthalpies, Δ sol H m ∞ of the salts in all the investigated systems pass through a maximum in the water-rich region. The presented maxima appear to be a result of the endothermic effect of the destruction of the 2-alkoxyethanol molecules hydration shell and the exothermic effect of replacement of water molecules by cellosolve ones in ionic solvation shells. The position and height of the maximum depend both on the nature of the salt and of the 2-alkoxyethanol molecule. The h xy values are positive for all the systems investigated here. It indicates that in the infinitely diluted aqueous solution, the endothermic effect resulting from the destruction of the hydrophobic cage around the 2-alkoxyethanol molecule predominates over the exothermic effect of the electrostatic ion–cellosolve interaction.