Hydration of α-alanine in aqueous sodium chloride and urea solutions

The hydration number of α-alanine in aqueous urea solutions is greater than in aqueous NaCl solutions; the ratio of the hydration numbers increases from 0.2 (m = 1) to ≈2 (m = 6). Given the same partial volumes of water, the hydration numbers of α-alanine in the two systems are close to each other.     

Effect of temperature on the thermodynamic characteristics of the protolytic equilibria of DL-α-alanyl-β-alanine in aqueous solutions in the range of 298 to 318 K

Protolytic equilibria in aqueous solutions of DL-α-alanyl-β-alanine are studied by calorimetry. Measurements are performed at temperatures of 298.15, 308.15, and 318.15 K; and at ionic strengths of 0.5, 1.0, and 1.5 (against a background of potassium nitrate). Thermodynamic characteristics (pK, ΔG, ΔH, and ΔS) of the stepwise dissociation of dipeptides both in water-salt and standard solutions are obtained for the first time.     

Hydration numbers of α-alanine in an aqueous urea solution

Literature data on the apparent molar volumes ϕ of alanine in water and aqueous urea solutions at 298 K are analyzed. It is shown that the slope of the ϕ dependence on the alanine concentration is not dependent on the urea concentration. The standard partial volume of alanine increases linearly with the increase in the urea concentration (wt.%). The structural characteristics of hydrated complexes of alanine (hydration number, molar volume of water inside and outside the hydration sphere, and proper volume of alanine in solution) are given. The hydration number of alanine decreases by a factor of two in passing from water to a saturated (20m) urea solution. The effects of urea additions on the hydration numbers of alanine and glycine are compared.     

Effect of sodium caproate on the volumetric and viscometric properties of glycine,dl-α-alanine,and dl-α-amino-n-butyric acid in aqueous solutions

The apparent molar volumes (V_m,2) and relative viscosities (η_r) at T=(298.15 and 308.15) K have been obtained for glycine, dl-α-alanine, and dl-α-amino-butyric acid in aqueous sodium caproate solutions from measurements of density and the flow time. The standard partial molar volumes (V^∘_m,2), standard volumes of transfer (Δ_tV^∘), the viscosity B-coefficients, and the activation thermodynamic quantities (Δμ₂^∘≠ and ΔS₂^∘≠) of viscous flow have been calculated for the amino acids. It is shown that the standard partial molar volumes, viscosity B-coefficients, and activation free energies for viscous flow increase with increasing number of carbon atoms in the alkyl chain of the amino acids. An increase in V^∘_m,2 and Δ_tV^∘ with increasing electrolyte concentrations have been explained due to the interactions of sodium caproate with the charged center of zwitterions for the amino acids. A comparison of the V^∘_m,2 values for glycine, dl-α-alanine, and dl-α-aminon-n-butyric acid in different aqueous salts solutions showed that carboxylate ions have stronger interactions with amino acid than chloride, thiocyanate, and nitrate ions. Results of viscosity are discussed in terms of changes in solvent structure.     

Chiral recognition in aqueous solutions: On the role of urea in hydrophilic and hydrophobic interactions of unsubstituted α-amino acids

Calorimetric and nuclear relaxation time measurements were carried out at 25°C on concentrated aqueous solutions of urea containing the L and D forms of the following -aminoacids: alanine, -aminobutyric acid, norvaline, and norleucine. Glycine was also studied under the same experimental conditions. The enthalpic interaction coefficients were rationalized according to the preferential configuration model. The trends of the homochiral coefficients at increasing concentration of urea underline an interaction mechanism not different from that occurring in water. Chiral recognition is the quantity more affected by the nature of the solvent. Among the amino acids studied, only norleucine presents this effect, which vanishes at the highest concentrations of urea because the attenuated electrostatic interactions cannot impose preferential configurations.     

Enthalpy parameters of molecular interactions of dl-α-alanyl-dl-α-alanine with polyhydric alcohols in the aqueous solution

Russian Journal of General Chemistry - Integral enthalpies of dissolution Δsol H m of DL-α-alanyl-DL-α-alanine in aqueous solutions of glycerol, ethylene glycol, 1,2-propylene...     

Thermochemical study of aqueous solutions of lithium diclofenac at 293.15–318.15 K

The enthalpies of solution and dilution of aqueous solutions of lithium diclofenac (LiDC) are measured in the concentration range of 0.002–0.047m at 293.15, 298.15, 308.15, and 318.5 K using an isoperibolic calorimeter. The heat capacity of solid LiDC in the temperature range of 273.15–373.15 K is determined using a DSC 204 F1 Phoenix differential scanning calorimeter (NETZSCH, Germany). The virial coefficients of the enthalpy characteristics of a water-LiDC solution are derived in terms of the Pitzer model to calculate a wide range of thermodynamic properties of both the solution and its components. Changes in these characteristics as a function of concentration and temperature are discussed.     

Liquid–liquid phase separation in aqueous solutions of poly(ethylene glycol) methacrylate homopolymers

Here, the liquid–liquid phase separation (LLPS) in aqueous solutions containing poly(ethylene glycol) (PEG) methacrylate homopolymers is reported for the first time. In this study, the thermoresponse of concentrated solutions of DEGMA₆₀ (two ethylene glycol, EG, groups) TEGMA₇₁ (three EG groups), OEGMA300_x (4.5 in average EG groups) of varying molar masses (MM), and OEGMA500₂₈ (nine in average EG groups) is discussed. Interestingly, the temperature of LLPS (T_LLPS) is controlled by the length of the PEG side chain, the MM of the OEGMA300_x and the polymer concentration. More specifically, the transition temperature decreases with: (i) Decrease in the length of the PEG side chain, (ii) increase in MM of the OEGMA300_x, and increase in concentration. In addition, LLPS is also observed in mixtures of OEGMA300_x with Pluronic® F127. In conclusion, these systems present a thermally induced LLPS, with the transition temperature being finely tuned to room temperature when DEGMA is used. These systems find potential use in numerous applications, varying from purification to “water-in-water” emulsions.     

Effect of temperature on the dilution enthalpies of α,ω-amino acids in aqueous solutions

Dilution enthalpies of aqueous solutions of 3-amino propanoic acid, 4-amino butanoic acid, 5-amino pentanoic acid, and 6-amino hexanoic acid were determined at T = (293.15, 298.15, 303.15, and 308.15) K using an LKB flow microcalorimeter. The homotactic interaction coefficients were obtained according to the McMillan–Mayer theory from the experimental data. For all the systems studied, the dilution of α,ω-amino acids in water is an exothermic process; the pair coefficients have positive values which increases with chain length. The obtained values of the interaction coefficients are interpreted in terms of solute–solvent and solute–solute interactions and are used as indicative of hydrophobic behavior of the amino acid studied.     

Solubility of naphthalene in aqueous solutions of poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) triblock copolymers and (2-hydroxypropyl)cyclodextrins

The solubility of naphthalene was investigated in aqueous solutions of triblock copolymers poly(ethylene glycol)–poly(propylene glycol)–poly(ethylene glycol) (PEG–PPG–PEG) and (2-hydroxypropyl)cyclodextrins. The results with solutions of the individual solubilizers were as expected: the solubility enhancement was much higher with a micelle-forming copolymer than with the non-micellizing one and with (2-hydroxypropyl)--cyclodextrin (HPBCD) than with (2-hydroxypropyl)--cyclodextrin (HPACD). Although the formation of inclusion complexes between HPACD and PEG and between HPBCD and PPG is well established, the naphthalene solubility in mixed solutions does not significantly deviate from that predicted for a mixture of independent solubilizers. Thus the interactions between HPCD and PEG–PPG–PEG copolymers are not strong enough to disrupt micelles and aggregates formed by those copolymers. In fact, slight synergetic deviations were observed with the micellizing copolymer, indicating the existence of ternary naphthalene/HPCD/copolymer interactions. For pharmaceutical applications, it is important that the solubilization efficacy of PEG–PPG–PEG copolymers and that of cyclodextrins modified by the 2-hydroxypropyl group would not be compromised if these two types of solubilizers were co-administered.     

A piezo-resonator as a detector of α- and β-alanine in aqueous solutions

The results of analysis of aqueous solutions of α- and β-alanine using a piezo-resonator are presented. Analytical signals from analytes were found to significantly differ from each other, which is connected with the peculiarities of the piezo-resonator operation in the solution as a viscosimetric rather than a mass-sensitive device.     

Thermodynamics of interaction of L-α-phenylalanine with urea and dimethylformamide in aqueous solution

The thermal effects of solution of L-phenylalanine in aqueous solutions of urea and dimethylformamide (DMF) at 25°C were determined. The solubility of L-phenylalanine in water and aqueous DMF solutions was measured. The standard enthalpies, free energies, and entropies of solution of the amino acid in aqueous solutions of amides were calculated. The parameters of pair and ternary amino acid-amide interactions were determined within the framework of the McMillan-Mayer theory. The amino acid-amide pair interaction is accompanied by a decrease in the Gibbs free energy, controlled by the entropy term with DMF and by the enthalpy term with urea. The interaction of L-phenylalanine with two amide molecules is repulsive, which in the case of DMF leads to an increase in the standard free energies of solution of the amino acid at the amide mole fraction X ₂ > 0.05.     

Experimental study on the effect of copper oxide nanoparticles on thermophysical properties of ethylene glycol–water for using in indirect heater at city gate stations

This study aimed to investigate the increase in heat transfer in the indirect heater at a city gate station (CGS) with the addition of copper oxide (CuO) nanoparticles to water–ethylene glycol base fluids. Indirect heaters are typically used at CGSs to raise the heat transfer coefficient of output gas flow from ? 5 to 15 °C. Moreover, manufacturing laboratory equipment in the presence of water–ethylene glycol base fluid and the nanoparticle in volume fractions of 0.05, 0.1, 0.2, and 0.3 at a temperature of 40–70 °C was discussed using dimensional simulation and analysis. The physical properties of the base fluid and nanofluid were measured using precise devices. Heat transfer tests for the base and nanofluid, as well as replacing of the air by gas, were conducted in a simulated and developed device. According to the obtained results with respect to the changes in convection and conduction heat transfer, enhancement of temperature difference at a rate of 36% was observed in the indirect heater with nanoparticle volume concentration of 0.2% at a temperature of 70 °C. Moreover, the Nusselt number showed a relatively good agreement with theoretical discussions.

     

Chiral effect in the sechenov parameters of argon solubility in aqueous α-valine and α-phenylalanine solutions

Data on the solubility of argon in water and aqueous solutions of L and D enantiomers of α-valine and α-phenylalanine at T = 283–328 K and partial pressure of argon p ₂ = 0.1 MPa are used to calculate the standard parameters of the Sechenov salt effect. Parameters of the solute-solute pair interaction are estimated within the formalism of the McMillan-Mayer theory. Evidence is provided for the presence of the chiral effect in the Sechenov parameters.     

Effects of acrylic acid incorporation on the pressure–temperature behavior and the calorimetric properties of poly(N-isopropylacrylamide) in aqueous solutions

 We studied the effects of pH on the pressure–temperature dependence of coil–collapse transition for aqueous solutions of copolymers of N-isopropylacrylamide and acrylic acid (Ac). At low pressures, the transition temperature (T _tr) increased with pressure, but T _tr decrease with increasing pressure at pressures higher than 50–100 MPa. By increasing the pH, the transition contour shifted to a higher temperature. When the Ac content was increased, the effects of pH became more evident. From a calorimetric study at atmospheric pressure, ΔH _tr was found to become smaller by increasing the portion of the ionized residues in the copolymer. The ratio to the van't Hoff enthalpy changes became larger with an increase in pH, which indicated that the production of charge decreased the cooperative domain size. Received: 19 July 1999 /Accepted in revised form: 7 September 1999     

Experimental study on rheological behavior of water–ethylene glycol mixture in the presence of functionalized multi-walled carbon nanotubes

Journal of Thermal Analysis and Calorimetry - This paper examines the rheological behavior of water (60%vol.)–ethylene glycol (40%vol.) mixture in the presence of functionalized multi-walled...     

Ion-solvent and ion-ion interactions of NaCl aqueous and aqueous maltose solutions at 298–323 K on viscosity data

Viscosities of sodium chloride in concentration range 1 × 10^?2 to 9 × 10^?2 ± 0.001 mol dm^?3, have been determined in aqueous and aqueous maltose systems (1.0 to 9.0 wt %) at different temperatures (298 to 323 K). The viscosity data have been analyzed by using Jones-Dole equation and the derived parameters A and B coefficients were also calculated. The data obtained from viscometric studies has been used to investigate the ion-solvent interaction and ion-ion interaction. Thermal effects on the ionic interactions were also examined under the light of transition state theory.     

Theoretical studies on the substitution effect of fluorine on ethylene

The substitution effect of fluorine on ethylene is investigated by means of studyingthe properties of the charge distribution at the bond critical points with the theory of atomsin molecules.It is found that fluorine atom acts not only as a σ electron acceptor,but also asa π electron donor,and these double effects are reflected in the quantity of ellipticity,Lap-lacian and the charge density of charge distribution at the bond critical points.For C—C,C—Fbonds,the major axis of elliptical contours is perpendicular to the molecular plane,but forC—H bond,it is parallel to the molecular plane.Other effects originating from the substi-tution have also been discussed.     

Volume properties and structure of aqueous solutions of urea at 263–348 K

The apparent molar volume of urea ? in aqueous solution in the range T = 273–323 K and m = 1–10 (molality) depends linearly on m ^1/2. An equation for ?(m, T) was derived. The partial molar characteristics of urea ? ₂ and water ? ₁ (volume, dilatability, and temperature coefficients of volumes) were calculated. The ?(T) dependences have characteristic points (extrema, inflection points), shifted to the region of lower temperatures for dilute solutions. The ? ₁(T) dependences for 2m and 4m of the urea solution retain the characteristics of the Y ₁(T) of pure water. In these solutions, the proper structure of water is preserved.