Apparent Molar Heat Capacities Research Articles

Some features of heat capacity changes in aqueous solutions of low molecular weight alkanols at T = 298.15 K and ambient pressure

The specific (cp) and molar (Cp) heat capacities of dilute aqueous solutions of methanol (MeOH), ethanol (EtOH), 1-propanol (n-PrOH) and 2-propanol (i-PrOH) were measured using an adiabatic calorimetry at T = 298.15 K and ambient pressure. Based on the obtained results, the excess molar heat capacities, CpE, as well as the apparent molar heat capacities of each of components, Cp,∅,12, of the system {water (1) + alkanol (2)} were calculated. The standard (at infinite dilution) Cp,2o values were also estimated. All the CpE values are positive and pass through maxima, which shift to the range of a lower mole fraction of alcohol, x2, when the molecular weight of the latter increases. The same goes for the Cp,∅,1max values, too. In turn, the order of changing Cp,∅,2max within the narrow high-diluted range of (∼0.01 <x2 < 0.035) m.f. is oppositely directed. It is established that the x2 value at dCpE/dx2 = 0 is approximately half of the sum of the x2 values at Cp,∅,1max and Cp,∅,2max, a fact indicating the parity contribution of both interacting components of the solution to the structure-energy transformations occurring in it.

On the interpretation of the unusual behavior of aqueous-organic mixtures in concentration regions with intersecting isotherms of heat capacity properties

Based on our own and some literature results, we have carried out a detailed analysis of concentration regions of the aqueous-organic mixture in which the temperature dependences of both excess molar heat capacity, CpE, and apparent molar heat capacity of each component, Cp,ϕ,i, are intersected. It is established that the ∂CpE/∂T and ∂Cp,ϕ,w/∂Tderivatives (W is water) for the {(W + hexamethylphosphoramide (HMPA)} and {(W + tert‑butanol (t-BuOH)} mixtures intersect in a rather narrow concentration interval between (0.30 and 0.35) and (0.35 and 0.40) mole fractions of the organic component, respectively. This fact we have interpreted as evidence that the decisive role in the mutually overlapping heat-capacity contributions to CpEresulting in such a situation plays the abnormal change in the isobaric molar heat capacity of W (Cp,w), which has a minimum at T ≈ 308 K, when the temperature increases.

Thermodynamic modeling of MDEA(aq)-NH3(aq)-K2CO3(aq)-CO2(aq) using the Extended UNIQUAC model

The development of advanced processes for carbon dioxide capture requires reliable thermodynamic models to ensure accurate process design. This work presents an optimized parameter set of the Extended UNIQUAC model for advanced mixed solvent solutions containing CO2-MDEA-NH3-K2CO3 in water. The model was parameterized using vapor–liquid equilibrium, pure component saturation pressure, excess enthalpy, heat of absorption, molar heat capacity, apparent molar heat capacity, and solid–liquid equilibrium. These thermal and equilibrium properties cover temperatures between 253.6K to 600K and pressures up to 75.6bar. The current model represents an extension of previous versions that were suitable for modeling aqueous NH3 and K2CO3 solutions. This parametrization accounts for molecular interactions between MDEA and these two components, which allows for the design of advanced capture processes. This paper demonstrates that the new model parameters are valid within the range of operational conditions typically found in CO2 capture processes.

Apparent molar heat capacities of aqueous solutions of LiBO2, NaBO2, KBO2, Na2B4O7 and K2B4O7 at T = (298.12, 313.13 and 328.14) K using a Picker flow calorimeter

The apparent molar heat capacities of LiBO2(aq), NaBO2(aq), KBO2(aq), Na2B4O7(aq) and K2B4O7(aq) solutions at molalities from 0.01 to 2.6 mol·kg−1, and at temperatures T = (298.12, 313.13 and 328.14) K have been determined using a Picker flow calorimeter built in this work. Further, the standard heat capacities of these solutes were derived from the experimental data by extrapolation to infinite dilution using the Pitzer ion-interaction model. The group contribution method is used to calculate the standard molar heat capacities of tetraborate ion in three equilibria for different assumptions, among which the prediction on the assumption that the standard molar heat capacity of reaction for the association reaction is zero is the most reasonable, and the results are in good agreement with the experimental data.

Heat capacity of six glymes in N,N-dimethylformamide + water mixtures. Solvation of glymes

The paper presents the heat capacities of glyme solutions: monoglyme, diglyme, triglyme, tetraglyme, pentaglyme and hexaglyme, in binary solvents N,N-dimethylformamide + water at four temperatures (293.15 K, 298.15 K, 303.15 K, 308.15 K) obtained using differential calorimeter Micro DSC III, Setaram – France. The concentration of glymes was approximately 0.25 mol/kg. On the basis of the obtained experimental results, the apparent isobaric heat capacities of the glymes were calculated. It was noted that the larger the glyme molecule was, the more pronounced the increase in the value of the apparent molar heat capacity function with the increase in water content in the two-component solvent. The observed changes in apparent isobaric heat capacity as a function of the water content in the mixed solvent are discussed in terms of the hydrophobic nature of the glymes. Moreover, it was shown that the apparent molar isobaric heat capacity of the glymes in pure solvents, i.e. in water and in N,N-dimethylformamide, increases linearly with the increase in the number of oxygen atoms in the glyme molecules i.e. with the size of glyme molecules. Furthermore, a linear correlation was observed between the apparent isobaric molar heat capacity and the enthalpic effect of the hydrophobic hydration of the studied glymes at 298.15 K. The results obtained in this paper are compare with the same obtained for selected cyclic ethers.

REGULARITIES OF CHANGES IN THERMODYNAMIC PARAMETERS INDUCED BY THE COMPLEXES FORMATION OF URACIL WITH SOME AROMATIC AMINO ACIDS IN A BUFFER SOLUTION AT PH 7.4

The increasing interest in the physicochemical studies of nucleic acid bases and their derivatives is due mainly to their biological and pharmaceutical importance. Despite increasing data on the interaction of DNA with proteins, the molecular mechanism of recognition of particles in the resulting DNA-protein complexes is not well understood yet. The investigation of the nature and the forces that stabilize nucleic acid structures and their complexes with other molecules is not only of fundamental significance but may also find practical applications. In this paper, we discussed the interaction of model compounds of nucleic acids and proteins using the example of complexes of uracil with aromatic amino acids: L-histidine, L-tryptophan, and L-phenylalanine in aqueous buffer solutions with physiological value of pH to be 7.4. The available literature data on the thermochemical, volume, and heat capacity properties of these systems were used. It should be noted that a change in the acidity of the medium induces a shift in the equilibrium of ionic forms, tautomers, and this leads to a variation in the reactivity of the reagents. The obtained characteristics of the complex formation process differ markedly from the results obtained for an aqueous solution without the use of buffer. A generalization of data on the change in thermodynamic parameters (equilibrium constants, Gibbs energy, enthalpy, entropy, apparent molar volume, and apparent molar heat capacity) under the formation of uracil complexes with above aromatic amino acids depending on their physicochemical properties is given. The dependence of the thermodynamic parameters of the formation of complexes on the acid-base properties of amino acids as a characteristic of the electron density on the carboxylate group has been established. Changes in the standard apparent molar heat capacity and in the standard apparent molar volume of uracil under the addition of amino acids correlate with the hydrophobicity of the added amino acids.

Thermodynamics of (water + hexamethylphosphoramide) mixtures: Heat capacity properties in the temperature range between 283.15 K and 298.15 K at ambient pressure

The specific (cp) and molar (Cp) heat capacities of the mixtures of hexamethylphosphoramide (HMPA, or h) with water (w) have been measured using an adiabatic calorimetry at T = (283.15, 288.15, 293.15, and 298.15) K and ambient pressure. Based on the results obtained, the excess molar heat capacities of the mixtures, CpE, as well as apparent molar heat capacities of each component, Cp,ϕ,i (down to its infinite dilution, Cp,i°), have been calculated. The CpE values are positive up to HMPA mole fractions being xh ≈ 0.9 and the dCpE/dT derivative undergoes a positive-to-negative sign inversion at xh = 0.33. Both concentration and temperature dependences of Cp,ϕ,i have also a complex character. The correlations of molar heat capacity properties with volume-related those for (H2O + HMPA) at the same temperatures have been discussed, too.

HEAT CAPACITY PROPERTIES OF AQUEOUS BUFFER SOLUTIONS OF L-HISTIDINE IN A WIDE TEMPERATURE RANGE

The influence of temperature and concentration of L-histidine on the heat capacity properties of its aqueous buffer solutions was studied by differential scanning calorimetry. The investigations were carried out in aqueous buffer solutions (pH 7.4) containing monobasic sodium phosphate and dibasic sodium phosphate, which brings the environment closer to the conditions of real biological systems. The pH values of the solutions were fixed with a digital pH meter Mettler Toledo, model Five-Easy (Switzerland). The differential scanning microcalorimeter SCAL-1 (Biopribor, Pushchino, Russia) was used for measure the specific heat capacity of the system under study. It was equipped with Peltier thermoelectric elements, two measuring glass cells with an internal volume of 0.377 cm3, as well as a computer terminal and software for calculating heat capacity. The standard error of measurement of the specific heat for the studied solutions was within ±7·10-3 J·K-1·g-1. The experimental values of the specific heat of solutions of the amino acid in a phosphate buffer solvent in the temperature range (283.15 – 343.15) K were obtained. The concentration of histidine was varied from (0.00215 to 0.03648) mol·kg-1. All the studied solutions were prepared by the gravimetric method using Sartorius-ME215S scales (with a weighing accuracy of 1·10-5 g). The apparent molar heat capacities of L-histidine in the buffer solution, as well as its partial molar heat capacities at infinite dilution, were determined. The calculated molar parameters increase with an increase in both temperature and amino acid concentration. It was shown that the partial molar heat capacities transfers of L-histidine from water to the buffer solution have positive values in the temperature range studied. The results are discussed on base of the Gurney model.

Generalized Quasi-Random Lattice model for electrolyte solutions: Apparent and partial molal heat capacities

Ionic solutions play an important role in many areas of technology and science, therefore understanding and predicting their thermodynamic behavior has been an essential research topic for an extended period of time. However, complexity and variety of real systems and the need for accurate representation of thermodynamic data hamper the development of models that are truly predictive over wide ranges of concentration, temperature and pressure conditions. In this regard, predictive potentialities of the Quasi-Random Lattice (QRL) model seem worthy of consideration.The QRL model, originally proposed for mean-activity/osmotic coefficients, was recently presented in a generalized form inclusive of formulae for partial/apparent molal enthalpies and volumes. This article follows and presents some further developments concerning partial and apparent molal heat capacities with application to various aqueous 1:1, 2:1, and 3:1 electrolytes. The semi-predictive character of QRL will be confirmed by present results and an improved procedure that yields the main QRL parameterization within a few steps will be illustrated. As a general remark, the experimental information required by QRL is very modest compared to the number of data points typically used with regression analysis.

Thermodynamically Traceable Calorimetric Results for Dilute Aqueous Potassium Chloride Solutions at Temperatures from 273.15 to 373.15 K. Part 2. The Quantities Associated with the Partial Molar Heat Capacity

In previous articles (Partanen J. Chem. Eng. Data 2019, 64, 16−33 and Partanen J. Chem. Eng. Data 2019, 64, 2519–2535), we presented traceable and transparent two-parameter Hückel equations (with parameters B and b1) for the activity coefficients of the salt and for the osmotic coefficients of water in aqueous KCl solutions in the temperature range of 273.15–383.15 K. The latter article is the first part (Part 1) of the calorimetric study. We showed in these articles that our equations for these solutions explain within experimental error the literature data on almost all thermodynamic quantities including the partial molar enthalpies at least up to a molality of 0.2 mol·kg–1 and up to 373 K. In this model, parameter B is regarded as a constant but parameter b1 has a quadratic temperature dependence. No calorimetric data were needed in the parameter estimation. In the second part (Part 2) of the calorimetric study, now, the results obtained for the heat capacity quantities of KCl (aq.) are considered. We show here that all heat capacity literature available for KCl solutions at least up to 0.5 mol·kg–1 is possible to explain within experimental error using exactly the same Hückel equations as those considered previously in our studies for dilute KCl solutions from 273 to 373 K. Because of the success of the used model, we supplement the existing thermodynamic tables with new values for the relative apparent and partial molar heat capacities for KCl solutions. It is likely that the new tables contain the most reliable values available for these heat capacity quantities.

Heat Capacity and Volume Properties of Ammonium Thiocyanate in N-Methylpyrrolidone Solution at 298.15 K

The heat capacity and volumetric properties of ammonium thiocyanate solutions in N-methylpyrrolidone are measured at 298.15 K by calorimetric and densimetric means. Concentration dependences of the partial and apparent molar heat capacities and volumes of the investigated systems are discussed. The standard partial molar heat capacity and volume of NH4CNS, and the standard heat capacity and volume of ammonium and thiocyanate ions in N-methylpyrrolidone at 298.15 K are calculated.

Two-point scaling analysis of the isobaric heat capacity of dodecyltrimethylammonium bromide aqueous solutions in the presence of the n-hexan-1-ol as a co-surfactant

The specific heat capacities at constant pressure cp of aqueous solutions of cationic surfactant: dodecyltrimethylammonium bromide, with addition of co-surfactant: n-hexan-1-ol with the molar ratio of surfactant to co-surfactant 1:1 have been measured within temperature range (278.15–328.15) K over wide concentration range. The apparent and partial molar heat capacities of the surfactant/co-surfactant were calculated to analyse the aggregation process taking place in the solution. In order to study some additional information on the solution structure and verify the shape of the aggregates the two-point scaling theory was used for the first in the case of the surfactant/co-surfactant system in which prolate micelles are formed.

Heat capacity and phase behaviour of {pentaethylene glycol monoheptyl ether + water} system. Two-point scaling approach

The miscibility in the {pentaethylene glycol monoheptyl ether (C7E5) + water} system was investigated by differential scanning calorimetry (DSC). The specific heat capacities cp of chosen nonionic amphiphile (C7E5) aqueous solutions have been measured within temperature range 273.15–338.15 K over wide concentration range. The apparent and partial molar heat capacities were calculated to analyze the aggregation process taking place in the solution. In order to study some additional information on the solution structure two-point scaling theory was used. On the base of temperature dependences of differential heat flow, phase coexistence curve was determined.

Heat Capacities of Aqueous Solutions of K4Fe(CN)6, K3Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, and KAg(CN)2 at 298.15 K

Isobaric volumetric heat capacities of aqueous solutions of K4Fe(CN)6, K3Fe(CN)6, K3Co(CN)6, K2Ni(CN)4, and KAg(CN)2 have been measured at 298.15 K over the approximate concentration range 0.02 to 0.4 mol·kg–1 using a Picker flow calorimeter. These data were combined with measured densities to calculate the corresponding apparent molar isobaric heat capacities, Cpϕ. The values so obtained were fitted as a function of concentration using an extended Redlich–Rosenfeld–Meyer-type equation to provide the standard state (infinite dilution) quantities, Cpϕo, for each salt. The Cpϕ values for all the salts studied showed similar dependences on concentration, with a slight upward curvature at higher molalities, possibly due to anion aggregation. Values of Cpϕo for the aquated cyanometallate anions were estimated using the tetraphenylphosphonium tetraphenylborate extrathermodynamic assumption and were little affected by ion size but were strongly dependent on ionic charge, ranging from −191 J·K–1·mol–1 for [Fe(CN)...

Standard partial molar heat capacities and volumes of aqueous N-methylpiperidine and N-methylpiperidinium chloride from 283 K to 393 K

Densities and volumetric heat capacities for dilute solutions of N-methylpiperidine (N-MP, 0.2mol·kg−1) and N-methylpiperidinium chloride (N-MPH+Cl−, 0.1mol·kg−1) were measured relative to water at 0.1MPa using a vibrating-tube densimeter, and at 0.4MPa using a twin fixed-cell, power-compensation, differential temperature-scanning nano calorimeter, over the temperature ranges 283.15K≤T≤363.15K and 283.15K≤T≤393.15K, respectively. The resulting apparent molar volumes, Vϕ, and heat capacities, Cp,ϕ, were corrected for speciation effects using Young’s rule and extrapolated to infinite dilution using the Debye-Hückel limiting law, Yo≈Yϕ−AYIs1/2, to yield the standard partial molar properties, Yo, for N-MP(aq) and N-MPH+Cl−(aq). The standard partial molar volumes, Vo, and heat capacities, Cpo, were represented by a semi-empirical function of solvent density and temperature, Yo(ρw,T), the “density” model, which can be used to extrapolate these results, as well as other thermodynamic properties for the ionization of N-methylpiperidine, to higher temperatures.

Molar Volumes and Heat Capacities of Aqueous Solutions of Potassium Hydroxide and for Water Ionization up to 573 K at 10 MPa

Densities (ρ) and isobaric volumetric heat capacities (σp) of carefully purified aqueous solutions of potassium hydroxide have been measured at concentrations up to 6 mol·kg–1 at a pressure of 10 MPa over the temperature range 323 ≤ T/K ≤ 573 using a vibrating-tube densimeter and a Tian–Calvet differential calorimeter, respectively. Apparent molar volumes (Vϕ) and isobaric heat capacities (Cpϕ) calculated from these data were fitted with modified Redlich–Meyer type equations to derive the corresponding standard molar volumes and heat capacities at infinite dilution. Exact comparisons with literature data were not possible because of pressure differences but the present values, which greatly extend the database for KOH(aq) in terms of precision, temperature and concentration, are broadly compatible with most of the earlier results. Comparison of the present values of Vϕ and Cpϕ for KOH(aq) with corresponding data for solutions of the other alkali metal hydroxides indicated that all of these systems show si...

INTERACTION OF NICOTINIC ACID WITH L-PHENYLALANINE IN BUFFER SOLUTIONS: HEAT CAPACITY AND VOLUME PROPERTIES STUDY

Thermodynamic and physicochemical properties of multicomponent aqueous solutions containing biologically active solutes are important in various areas of applied chemistry and are essential for understanding the chemistry of biological systems. Interactions between nicotinic acid (NA) and L-phenylalanine (Phe) were studied in aqueous phosphate buffer solutions (pH=7.35) by differential scanning calorimetry and volume methods. Heat capacities and densities of nicotinic acid-buffer, L-phenylalanine-buffer, and nicotinic acid-L-phenylalanine-buffer mixtures were determined at T=(288.15, 298.15, 308.15 and 318.15) K using the microdifferential scanning calorimeter SCAL-1 (Pushchino, Russia) and the density meter DSA 5000 M (Anton Paar). The apparent molar heat capacities (fCp) and apparent molar volumes (Vφ,NA) of nicotinic acid in buffer solution and in buffer 0.0120 mol×kg-1 amino acid solutions were evaluated. The concentration of NA was varied from (0.0079 to 0.036) mol×kg-1. The first and second differentials values were determined for NA in an aqueous buffer solution and for NA in an aqueous amino acid buffer solution. The interaction of NA with Phe is accompanied by complex formation. NA molecules in an aqueous buffer solution are water structure breakers, then the structure breaking effects of NA decrease as a result of interactions with Phe molecules during the complex formation in an aqueous amino acid buffer solution. The results were discussed in terms of various interactions taking place in this system.Forcitation:Tyunina E.Yu., Badelin V.G., Egorkina V.S. Interaction of nicotinic acid with L-phenylalanine in buffer solutions: heat capacity and volume properties study. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 4. P. 33-39.

Thermodynamics of aqueous adenine: Standard partial molar volumes and heat capacities of adenine, adeninium chloride, and sodium adeninate from T = 283.15 K to 363.15 K

Apparent molar volumes, Vϕ, and apparent molar heat capacities, Cp,ϕ, were measured for aqueous solutions of neutral adenine (AdH) and adeninium chloride (AdH2Cl) at temperatures from 288.15K to 363.15K, and for sodium adeninate (NaAd) at temperatures from 283.15K to 313.15K. Density measurements used in the determination of the standard partial molar volume, Vo, were made with a vibrating-tube densimeter at ambient pressure. Heat capacity measurements used in the determination of the standard partial molar heat capacity, Cpo, were made with a twin fixed-cell, differential output, power compensating, temperature-scanning calorimeter at a pressure of 0.4MPa. The contributions from minor equilibrium species were removed using Young’s Rule to yield values for the standard partial molar volume and heat capacities of the ionic species AdH2+(aq) and Ad−(aq), and the equilibrium mixture of neutral species AdH∗(aq). Semi-empirical equations describing the temperature dependence of Vo and Cpo for each species were fitted by regression to the experimental results. From these equations, the standard partial molar volume change ΔrVom and heat capacity change ΔrCpom for the ionization reactions were calculated. The standard partial molar volumes presented in this study for neutral adenine are the first to be experimentally determined at temperatures above 330K, while the standard partial molar heat capacities are the first to be reported at temperatures above 298.15K. The experimental values for the adeninium cation and adeninate anion are the first to be reported in the literature.

Study on the Interaction of Nicotinic Acid with l-Phenylalanine in Buffer Solution by Heat Capacity Measurements at Various Temperatures

The interaction between nicotinic acid (NA) and l-phenylalanine (Phe) was studied in aqueous phosphate buffer solutions (pH = 7.35) by differential scanning calorimetry. Heat capacities of nicotinic acid–buffer, l-phenylalanine–buffer, and nicotinic acid–l-phenylalanine–buffer mixtures were determined at (283.15, 288.15, 293.15, 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15) K using the microdifferential scanning calorimeter SCAL-1 (Pushchino, Russia). The apparent molar heat capacities, ϕ C p , of nicotinic acid in buffer solution and in buffer 0.0216 mol·kg−1 amino acid solutions were evaluated. The concentration of NA was varied from (0.0106–0.0701) mol·kg−1. The interaction of NA with Phe is accompanied by complex formation. The partial molar heat capacities of transfer of nicotinic acid from buffer to buffer amino acid solutions are positive. The results are discussed in terms of various interactions operating in this system.

Heat capacity and phase behaviour of {1-propoxypropan-2-ol–water} system: Two-point scaling analysis

The miscibility in the {1-propoxypropan-2-ol–water} system was investigated by differential scanning calorimetry. The specific heat capacities of 1-propoxypropan-2-ol aqueous solutions have been determined within the temperature range 274.15–338.15K, over the wide concentration range by DSC. The apparent and partial molar heat capacities were calculated. Observed maxima on the concentration dependences of the partial molar heat capacities were interpreted as the result of aggregation taking place in the solution. Two-point scaling theory was used to analyze obtained results. Influence of the temperature on the values of critical indices were discussed in terms of possible structural changes in the solution. The mixing scheme was determined on the basis of the scanning calorimetry experiment.