Gibbs Energy Of Hydration Research Articles

Correlation strategy for determining the parameters of the revised Helgeson-Kirkham-Flowers model for aqueous nonelectrolytes

The main goal of this study is to revise the correlation algorithm for estimating parameters of the revised Helgeson-Kirkham-Flowers (HKF) model for aqueous nonelectrolyte solutes. The basis for the revision is the analysis of a large body of experimental values of the standard partial molar heat capacities and volumes of aqueous nonelectrolytes published mainly during the last decade. Unlike earlier estimation methods, we show that one of the most useful properties for developing correlations for uncharged species is the standard Gibbs energy of hydration of a solute at 298.15 K and 0.1 MPa, ΔhGo, which reflects the strength of water-solute interactions. Explicit correlations with ΔhGo seem to provide reliable means to estimate the “solvation” parameter ω and the “caloric” parameter c2 in the revised HKF model. “Volumetric” parameters a1 to a4 depend on both the size of a solute and its standard Gibbs energy of hydration. It is expected that the revised estimation strategy will improve the reliability of predictions of thermodynamic properties of aqueous nonelectrolytes in the framework of the revised HKF model. A related problem is the temperature and density ranges of the applicability of the model for uncharged species. A comparison of experimental properties and those fitted in the framework of the revised HKF model made in this study or in articles published elsewhere shows that the model can be used along the saturation vapor-liquid curve of pure water in the density region sufficiently remote from the critical point of water, say up to 630 K. At densities above 500 to 600 kg · m−3 the range of applicability of the revised HKF model for uncharged species may extend up to higher temperatures. At temperatures up to 500 K at pressures up to 50 MPa, the revised HKF model is capable of excellent description of the standard thermodynamic properties of aqueous nonelectrolytes, except in the narrow temperature range below 280–290 K.

Estimation of the Krichevskii parameter for aqueous nonelectrolytes

The Krichevskii parameter determines the sign and magnitude of the near-critical divergence of the partial molar thermodynamic properties of infinitely dilute solutions. Estimation of the values of the Krichevskii parameter, A Kr, would be helpful in improving the quality of predictions of the infinite dilution partial molar properties of aqueous nonelectrolytes at near-critical and supercritical conditions. In this paper, we discuss the methods to obtain values of A Kr from various types of experimental information on the thermodynamic properties of binary systems, and use these methods to evaluate the Krichevskii parameter for more than 30 aqueous nonelectrolytes. The possibility of calculating the values of the Krichevskii parameter for solutes at infinite dilution in water from the van der Waals and Peng–Robinson equations of state was also checked. It is shown that the classical cubic equations of state may be used to predict the values of A Kr for small nonpolar solutes in water, but grossly overestimate A Kr for larger compounds. As an alternative, we propose an empirical correlation between the Krichevskii parameter and the Gibbs energy of hydration at 298.15 K and 0.1 MPa for the corresponding compound. This correlation seems to hold for solutes with values of the Krichevskii parameter ranging from −200 to +200 MPa, and values of the Gibbs energy of hydration extending over 100 kJ mol −1, and appears to be useful for estimating values of the Krichevskii parameter for many solutes of different sizes and polarities, from inert gases to aqueous silica. Predictions, related to the sign of the Krichevskii parameter for a number of aqueous inorganic and organic nonelectrolytes, are made, which can be verified experimentally.

Group Contribution Values of the Infinite Dilution Thermodynamic Functions of Hydration for Aliphatic Noncyclic Hydrocarbons, Alcohols, and Ketones at 298.15 K and 0.1 MPa†

A compilation of experimental values of the infinite dilution partial molar Gibbs energy, enthalpy, and heat capacity of hydration, together with molar volumes in water at 298.15 K and 0.1 MPa, is presented for aliphatic noncyclic ketones. These data, combined with the related results for aliphatic noncyclic hydrocarbons and monohydric alcohols, were treated in the framework of a simple first-order group additivity scheme. Numerical values of the contributions to each of the thermodynamic properties are obtained by the least-squares procedure for the following groups: CH3, CH2, CH, C, OH, and CO.

Some Structural and Thermodynamic Aspects of Solvation of Single Ions. 1. Thermodynamic Characteristics of Solvation of Single-Charged Ions in Water and Methanol and Their Structural Terms

The Gibbs energies of hydration of monoatomic single-charged ions (298.15 K) were calculated based on the previously developed concept about the real thermodynamic properties of single ions. The structural terms of the thermodynamic functions of ion solvation in water and methanol, defining structural changes in the solvent in the near solvation region, were evaluated.

Hydration Thermodynamics of N-Methylacetamide

Reliable values for the thermodynamic functions associated with the hydration of N-methylacetamide, NMA, in the temperature range 5–100°C, are obtained using experimental data from different investigations. They are analyzed by means of a suitable theoretical approach. The large and negative hydration entropy values are determined by the excluded volume effect owing to cavity creation in water in order to host the NMA molecule. The large and negative hydration enthalpy values are determined by the NMA-water van der Waals interaction and H -bond energies. The latter are so large in magnitude to overwhelm the work of cavity creation, causing the good solubility of NMA in water. The analysis assumes that the reorganization of H -bonds among water molecules in the hydration shell of NMA is characterized by a nearly perfect enthalpy-entropy compensation, so that such process does not affect the hydration Gibbs energy change.

Standard state Gibbs energies of hydration of hydrocarbons at elevated temperatures as evaluated from experimental phase equilibria studies

Experimental results of phase equilibria studies at elevated temperatures for more than twenty hydrocarbon-water systems were uniformly correlated within the framework of the Peng-Robinson-Stryjek-Vera equation of state in combination with simple mixing rules. This treatment allows evaluation of the Gibbs energy of hydration for many alkanes, 1-alkenes, cycloalkanes (derivatives of cyclohexane) and alkylbenzenes up to 623 K at saturated water vapor pressure and up to 573 K at 50 MPa. Results for homologous series show regular changes with increasing carbon number, and confirm the applicability of the group contribution approach to the Gibbs energy of hydration of hydrocarbons at elevated temperatures. The temperature dependence of group contributions to the Gibbs energy of hydration were determined for CH 3, CH 2, and CH in aliphatic hydrocarbons; C=C and H for alkenes; c-CH 2 and c-CH in cycloalkanes; and CH ar and C ar in alkylbenzenes (or aromatic hydrocarbons). Close agreement between calculated and experimental results suggests that this approach provides reasonable estimates of Gibbs energy of hydration for many alkanes, 1-alkenes, alkyl cyclohexanes and alkylbenzenes at temperatures up to 623 K and pressures up to 50 MPa.

Infinite dilution partial molar properties of aqueous solutions of nonelectrolytes. II. equations for the standard thermodynamic functions of hydration of volatile nonelectrolytes over wide ranges of conditions including subcritical temperatures

The volumetric equation proposed previously (Plyasunov et al., 2000), for estimating the infinite dilution Gibbs energy of hydration of volatile nonelectrolytes at temperatures exceeding the critical temperature of pure water, T c , is extended to subcritical temperatures. The basis for the extension without inclusion of new fitting parameters besides the experimental values of the thermodynamic functions of hydration at 298.15 K, 0.1 MPa, is an auxiliary function, Δ h Cp 0( T, P r ), for the variation of the infinite dilution partial molar heat capacity of hydration of a solute in liquid-like water between temperatures T = 273.15 K and T = T s = 658 K along the isobar P r = 28 MPa. The analytical form of Δ h Cp 0( T, P r ) was found by globally fitting all available data for the seven best-studied solutes (CH 4, CO 2, H 2S, NH 3, Ar, Xe, and C 2H 4). Four constraints were used to determine the values of four terms of the Δ h Cp 0( T, P r ) function: the numerical values of the temperature increments between T = 298.15 K and T = T s = 658 K for the Gibbs energy and the enthalpy of hydration, and numerical value of the heat capacity at T s and at 298.15 K, all at the selected isobar P r . This approach, in combination with the volumetric equation, may be used to describe and predict all the infinite dilution thermodynamic functions of hydration for nonelectrolytes over extremely wide ranges of temperature and pressure. The model allows calculation of the standard state partial molar properties, including the Gibbs energy of aqueous solutes in a single framework for conditions from high-temperature magmatic processes through hydrothermal phenomena to low-temperature conditions of hypergenesis.

Predictions of thermodynamic properties at infinite dilution of aqueous organic species at high temperatures via functional group additivity

In this study, a functional group additivity scheme was used for correlation of the partial molar volumes, V 2 0, compressibilities, κ 2 0, heat capacities, C P,2 0, hydration enthalpies, Δ hyd H 2 0, and hydration Gibbs energies, Δ hyd G 2 0, at infinite dilution of aqueous organic electrolytes and non-electrolytes. The results were used for predicting the chemical potentials of these solutes. At present, the groups included are C (hydrocarbon), H (hydrocarbon), COOH, CONH 2, NH 2, OH, COO −, NH 3 +, and an amino acid functional group. An equation of state based on the Fluctuation Solution Theory was employed which allows predictions for numerous acyclic organic compounds composed of these functional groups to 623 K and 100 MPa. Although the uncertainty normally associated with a group additivity scheme is larger than the experimental uncertainty at ambient conditions, at high temperatures and pressures the two uncertainties become comparable. The functional group contributions were based on almost 1400 experimental points and are reported as a function of solvent density and temperature.

Thermodynamic functions of hydration of hydrocarbons at 298.15 K and 0.1 MPa

An extensive compilation of experimental data yielding the infinite dilution partial molar Gibbs energy of hydration Δ h G O , enthalpy of hydration Δ h H O , heat capacity of hydration Δ h Cp O , and volume V 2 O , at the reference temperature and pressure, 298.15 K and 0.1 MPa, is presented for hydrocarbons (excluding polyaromatic compounds) and monohydric alcohols. These results are used in a least-squares procedure to determine the numerical values of the corresponding properties of the selected functional groups. The simple first order group contribution method, which in general ignores nearest-neighbors and steric hindrance effects, was chosen to represent the compiled data. Following the precedent established by Cabani et al. (1981), the following groups are considered: CH 3, CH 2, CH, C for saturated hydrocarbons; c-CH 2, c-CH, c-C for cyclic saturated hydrocarbons; CH ar, C ar for aromatic hydrocarbons (containing the benzene ring); C=C, C≡C for double and triple bonds in linear hydrocarbons, respectively; c-C=C for the double bond in cyclic hydrocarbons; H for a hydrogen atom attached to the double bond (both in linear and cyclic hydrocarbons) or triple bond; and OH for the hydroxyl functional group. In addition it was found necessary to include the “pseudo”-group I(C-C) to account for the specific interactions of the neighboring hydrocarbon groups attached to the benzene or cyclic ring (in the latter case only for cis-isomers). Results of this study, the numerical values of the group contributions, will allow in most cases reasonably accurate estimations of Δ h G O , Δ h H O , Δ h Cp O , and V 2 O at 298.15 K, 0.1 MPa for many hydrocarbons involved in geochemical and environmental processes.

A new equation of state for correlation and prediction of standard molal thermodynamic properties of aqueous species at high temperatures and pressures

A new expression for partial molar volumes of aqueous solutes at infinite dilution has been developed in an empirical form based on Fluctuation Solution Theory for use over wide ranges of temperature and pressure. The solvent density and compressibility characterize the solvent properties, providing a much better description, especially in the critical region, than do models based on the solvent dielectric constant. The formulation has been integrated and differentiated to obtain analytic expressions for the Gibbs energy of hydration and Henry's constant, as well as the infinite dilution partial molar enthalpy and heat capacity of hydration at supercritical temperatures. For partial molar heat capacities at subcritical temperatures, an additional temperature-dependent function is used. Using newly established comprehensive databases of experimental V 2 0 and C 0 p,2 for nonelectrolytes and 1-1 electrolytes, supplemented with smaller databases for Δ hyd H 2 0 and Δ hyd G 2 0 for nonelectrolytes and κ 2 0 for electrolytes, it is shown that accurate correlations of all available data are obtained with the new model. For nonelectrolytes, the accuracy of predictions for all hydration and derivative properties at densities greater than 250 kg m −3 and at temperatures from ambient to over 700 K using only data at 298 K is almost as good as fitting the model's five parameters to all the data. For 1-1 electrolytes, predictions of C 0 p,2 near the critical temperature from V 2 0 near the critical temperature are satisfactory for NaCl(aq).

Calculation of Gibbs Energies of Hydration of Monovalent Ions: Examination of Born Equation and its Reevaluation

The limitations of Born equation and its different modifications for the calculation of Gibbs energies of hydration of ions have been briefly reviewed. A modified method for the calculation of hydration energies of ions has been suggested. The hydration energies thus obtained have been compared with the data in the literature.

Hydration thermodynamics of aliphatic alcohols

The hydration thermodynamics of five linear aliphatic alcohols in the temperature range 5–100°C is carefully analysed using a suitably modified version of the theoretical approach developed by Lee. The hydration Gibbs energy change is determined by the balance of three contributions: the direct alcohol–water van der Waals interaction energy, the direct alcohol–water H-bond energy, and the excluded volume effect due to solute insertion. The analysis shows that the direct alcohol–water H-bond energy is fundamental in determining the negative values of the hydration Gibbs energy over the whole temperature range investigated, whereas the excluded volume effect determines the large and negative hydration entropies. The reorganization of H-bonds in the hydration shell of aliphatic alcohols proves to be a compensating process, not affecting the Gibbs energy change, as in the case of the hydration of nonpolar molecules. However, H-bond reorganization is the main molecular origin of the large and positive hydration heat capacity change, a signature of hydrophobic hydration, determining the temperature dependence of the hydration enthalpy and entropy changes. We show that H-bond reorganization can be reliably described by means of the modified Muller's model, indicating that the hydration shell is not akin to an iceberg: hydration shell H-bonds are energetically slightly stronger but more broken than those in bulk water. This finding allows the rationalization of the puzzling experimental data on the temperature dependence of the water proton NMR chemical shift in solutions of aliphatic alcohols.

On the temperature dependence of hydration thermodynamics for noble gases

The hydration thermodynamics of five noble gases in the temperature range 0–60°C have been carefully determined. We analyse these data using the theoretical approach developed by Lee that emphasizes that there are different physical causes for the large and positive hydration Gibbs energy change and the large and positive hydration heat capacity change. The analysis confirms that the hydrophobicity of noble gases is caused by the excluded volume effect due to solute insertion and exaggerated by the small size of water molecules at any temperature. The reorganization of H-bonds in the hydration shell of noble gases is a compensating process that does not contribute to the Gibbs energy change, but it is the cause of the large and positive hydration heat capacity change and determines the temperature dependence of the hydration enthalpy and entropy changes. The modified Muller's model, despite its simplicity, proves able to satisfactorily describe the reorganization of H-bonds, indicating that the hydration shell does not resemble an iceberg. The H-bonds in the hydration shell are energetically slightly stronger but more broken than those in the bulk water.

On the size dependence of hydrophobic hydration

The solubility in water of noble gases, at room temperature, increases with their size, whereas that of gaseous aliphatic hydrocarbons decreases on increasing their size. This puzzling experimental observation is a unique feature of water as solvent. No real explanation of the phenomenon exists, even though it has been suggested that it is evidence of clathrate-type structure formation around nonpolar molecules. In this paper, we show that the experimental data can be reproduced well by means of Lee's theory of hydrophobic hydration. A fundamental ingredient of this theory is the demonstration that the purely structural reorganization of H-bonds in the hydration shell of a nonpolar solute is a compensating process. The solubility is determined by the balance of two contrasting factors: the excluded volume entropy change due to cavity creation in the solvent, and the direct solute–solvent van der Waals interactions. The work of cavity creation is dominant, determining the poor solubility of nonpolar compounds in water. However, for noble gases, on increasing the hard-sphere diameter, the van der Waals interactions increase, in absolute value, more rapidly than the work of cavity creation, enhancing the solubility. On the contrary, for aliphatic hydrocarbons, on increasing the hard-sphere diameter the van der Waals interactions increase, in absolute value, less rapidly than the work of cavity creation, lowering the solubility. The experimental data of hydration Gibbs energies, therefore, can be accounted for without invoking an enhancement of water structure in the hydration shell of a nonpolar solute.

Electrochemistry of C 70 fullerene films in aqueous solutions

The electrochemical behavior of C 70 fullerene films on gold and glassy carbon substrates was studied in aqueous solutions. The neutral C 70 film behaved as an intrinsic semiconductor electrode, and it could be transformed to a conductor by electrochemical reduction. We observed that C 70 could be reduced in the presence of various cations, and the reduction process shifted parallel with the hydration Gibbs energy of the cations. In each case the reduction consumed about three electrons per C 70 molecule. The potential of the reduction changed with the activity of the cations, indicating the formation of some ion-exchanging transient species. The C 70 fullerene electrodes had good stability at ambient conditions.

Evaluation of the contribution to hydration of nonelectrolytes from the hydrophobic effect

A new method was suggested for estimating the hydrophobic effect of contributions to the Gibbs energies and enthalpies of hydration of hydrocarbons, inorganic gases and rare gases. In accordance with this method the hydrophobic effect contribution to the Gibbs energy was evaluated from the difference between the hydration Gibbs energy of a solute and the non hydrophobic contribution. To estimate the latter value, the known dependence connecting the Gibbs energies of solvation of a solute in a number of aprotic solvents to the Hildebrand solubility parameter for these solvents was used. The non hydrophobic contribution to the Gibbs energy of hydration was calculated for various solutes from such dependences extended to water as solvent. The Hildebrand solubility parameter for water used in the calculation was corrected for the effect of association through hydrogen bonding. This correction was made by subtraction of the water self-association enthalpy from the enthalpy of vaporization of water. The evaluated Gibbs energies of the hydrophobic effect are positive for saturated hydrocarbons, inorganic gases and rare gases and linearly depend on the solute molecular refraction. The hydrophobic contribution to the hydration enthalpies of the solutes was calculated in the same manner as was made to calculate the hydrophobic contribution to Gibbs energies of hydration. Enthalpies of the hydrophobic effect for the solutes under study are negative.

Specific effect of alkali-metal cations on the kinetics of peroxodisulfate–iodide reactions. Influence of approach distance, Gibbs energy of hydration and equivalent ionic conductivity

The reaction rate between peroxodisulfate and iodide ions for different alkali-metal iodides have been obtained. The specific effect of alkali-metal cations can be understood by means of several correlations between the rate constant and the approach distance, Gibbs energy of hydration of the cation and the cationic conductivity. The deviation of lithium from linearity in the correlations has been discussed. The logarithm of activation energy is a linear function of the Gibbs energy of hydration of the alkali-metal cation. The model proposed has been tested for the reaction between bromoacetate and thiosulfate.

Energetics of interactions of aromatic hydrocarbons with water

The thermodynamics of transfer of aromatic (benzene, toluene) and aliphatic (ethane, propane, butane) hydrocarbons from the gas phase into water in the temperature range 5–125°C have been analyzed in order to determine the net hydration effect of these compounds. In the case of the aromatic hydrocarbons the enthalpic contribution predominates over the entropic contribution to the Gibbs energy of hydration. This results in a negative value of the hydration Gibbs energy of aromatic hydrocarbons, in contrast to the positive Gibbs energy of hydration of aliphatic hydrocarbons. The different sign of the hydration Gibbs energies indicates that the mechanism causing hydrophobicity of aromatic hydrocarbons has different nature than that causing the hydrophobicity of aliphatic hydrocarbons. The comparison of hydration of aliphatic and aromatic hydrocarbons leads to the following thermodynamic parameters for these additional interactions between the benzene ring and water at 25°C: enthalpy −5.4 kJ/mol, entropy 26.8 J/K mol and Gibbs energy −13.4 kJ/mol. The large enthalpic contribution to the Gibbs energy of hydration of aromatic hydrocarbons probably comes from the ability of the aromatic ring to accept hydrogens from water, forming hydrogen bonds.

Electronic structure, energy of hydration, and stability of metal aquo ions

The stability of metal aquo ions with respect to redox reactions is determined by the ionization energies of the atoms and the Gibbs energies of hydration for the ions(−ΔhG0). We present critically selected values of −ΔhG0 for 55 metal ions, determined from electrochemical, thermochemical, and spectra data. We consider the factors determining the values of −ΔhG0 (charges, ionic radii, electronic structure, and relativistic effects). For isoelectronic ions, we observe correlations between the ratios of the Gibbs energies of hydration for these ions with different charges and the ratios of their ionic radii. Based on the use of these correlations, we find −ΔhG0 for a number of aquo ions not observed experimentally and we estimate the unknown oxidation-reduction potentials for the pairs of ions M3+/M2+. We formulate the principles for stabilization of unstable oxidation states of the metals by including the corresponding ions in complexes with certain classes of ligands.

Contribution of Hydration to Protein Folding Thermodynamics: II. The Entropy and Gibbs Energy of Hydration

The entropy of hydration of non-polar and polar groups upon protein unfolding has been estimated for four globular proteins in the temperature range 5 to 125°C, using structural information on the groups of these proteins exposed to water in the native and unfolded states and the entropies of transfer of various model compounds from the gaseous phase to water. The latter was calculated using the molar scale. It is shown that the entropies of hydration of non-polar and polar groups are both negative, but change in different directions with increasing temperature: the entropy of hydration of polar groups increases in absolute magnitude, while the entropy of hydration of non-polar groups decreases and becomes zero at 122°C for aliphatic groups and at 104°C for aromatic groups, with a change in sign at higher temperature. The configurational entropy was estimated by comparing the entropy of overall hydration with the total entropy of protein unfolding. Using previously determined enthalpies of hydration of polar and non-polar groups and disruption of the internal bonds, the Gibbs energies of hydration of protein groups and breaking of the hydrogen bonds and van der Waals interactions have been estimated. It was shown that hydration of the polar groups and aromatic non-polar groups destabilizes the native protein structure, while hydration of aliphatic non-polar groups (hydrophobic hydration) stabilizes it; however, the main stabilization effect comes from the internal van der Waals interactions and hydrogen bonding. Analysis of the factors contributing to the stability of the folded protein conformation shows that it is the hydration of polar groups that is mainly responsible for the cold denaturation of proteins.