Juan A. R. Renuncio

Combination rules for intermolecular potential parameters. I. Rules based on approximations for the long‐range dispersion energy

Combination rules for intermolecular potential parameters based on different approximations for the long‐range dispersion energy are derived and applied to the Lennard‐Jones (12–6) and Kihara potential functions. The resulting group of rules is given by the expressions σ12 = <σ≳j, and e12 = <eσ6γ≳i/‖<σ≳6j<γ≳k ‖, where the i,j, and k subscripts may adopt the values a, g, or h indicating the type of mean (arithmetic, geometric, or harmonic, respectively) to be taken for the magnitude within the brackets. Expressions for γ depend on the approximation chosen for the van der Waals coefficient c6. This group of rules includes most of those previously proposed and others which are new. Experimental values of the interaction virial coefficient and unlike‐pair potential parameters obtained from viscosity data are used to test the validity of the rules. Six related rules are shown to be satisfactory for both potential functions and for accurate correlation of virial and viscosity data as well.

Combination rules for intermolecular potential parameters. II. Rules based on approximations for the long‐range dispersion energy and an atomic distortion model for the repulsive interactions.

Combination rules for intermolecular potential parameters based on the identification of the attractive term of the potential function with different expressions for the long‐range dispersion energy and the introduction of an improved model for the combination of repulsive potentials are derived and applied to the Lennard‐Jones (12–6) and Kihara potentials. The ability of the rules to predict interaction virial coefficients and unlike‐pair potential parameters obtained from viscosity data is examined. Results are compared to those obtained in similar tests in part I of this study. Combination rules based on the London dispersion formula and an assumption of a geometric mean rule for the distances at which the repulsive forces are in equilibrium are shown to have advantages over previously proposed rules for the two potential functions assumed.

Regression of vapor-liquid equilibrium data based on application of the maximum-likelihood principle

Abstract Rubio, R.G., Renuncio, J.A.R. and Diaz Pena, M., 1983. Regressin of vapor-liquid equilibrium data based on application of the maximum-likelihood principle. Fluid Phase Equilibria, 12: 217-234. A method first proposed by Anderson et al. (1978) has been applied to evaluate the excess Gibbs energy from vapor-liquid equilibrium (VLE) data. This method, which is based on the maximum-likelihood principle, is shown to be more accurate and to provide more information than classical methods based on the least-squares principle. The influences of experimental errors, the number of data points, and the vapor pressures of the pure components on the fitting are studied using reliable experimental data obtained for several systems. The selection of the set of variables to be considered ((p, T, x, y), (p, T, x) or (p, T, y)) and the criteria for choosing the equation which best fits the excess Gibbs energy are also discussed.

Vapour-liquid equilibrium of the ethanol–propanal system

A Gibbs–Van Ness type apparatus for total vapour-pressure measurements of binary mixtures has been developed. Vapour–liquid equilibrium data for the ethanol–cyclohexane system at 298.15 K have been obtained and compared with previous literature data. Data reduction has been carried out by a modified Barkers method using a Pade approximant to describe to excess Gibbs energy. The vapour–liquid equilibrium data for the ethanol–propanal systems have been measured over the whole concentration range at 298.15, 308.15 and 318.15 K. These mixtures show negative deviations from ideality. The calculated values for the excess functions are large and negative, and the calculated values for the concentration–concentration correlation function are lower than ideal values. This experimental behaviour is analysed in terms of the UNIQUAC model and several versions of the UNIFAC model.

Thermodynamic quantities for the protonation of amino acid amino groups from 323.15 to 398.15 K

Flow claorimetry has been used to study the interaction of protons with glycine, DL-α-alanine, β-alanine, DL-2-aminobutyric acid, 4-aminobutyric acid, and 6-aminocaproic acid in aqueous solutions at temperatures from 323.15 to 398.15 K. By combining the measured heats for amino acid solutions titrated with NaOH solutions with the heat of ionization for water, the log K, ΔHo, ΔSo, and ΔCpo values for the protonation of the amino groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The ΔHo and ΔSo values increase while log K values decrease as temperacture increases. The trends for log K, ΔHo, ΔSo, and ΔCpo are discussed in terms of changes in long-range and short-range solvent effects. The trend in ΔHo, ΔSo, and ΔCpo values with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction between the two oppositely charged groups within the molecule.

Excess Gibbs energies of (benzene + n-pentadecane) at 298.15 and 323.15 K

Vapour pressures for (benzene + n-pentadecane) at 298.15 and 323.15 K are reported. A new degassing method is described which makes the reproduction of vapour-pressure measurements possible. Liquid-phase values of GmE have been determined from vapour pressures. Analysis using the maximum-likelihood principle enables the evaluation of the variances of the calculated parameters, and the selection, between two or more fits, of the one providing a better representation of the results.

Effect of temperature and pressure on the protonation of glycine.

Flow calorimetry has been used to study the interaction of glycine with protons in water at temperatures of 298.15, 323.15, and 348.15 K and pressures up to 12.50 MPa. By combining the measured heat for glycine solutions titrated with NaOH with the heat of ionization for water, the enthalpy of protonation of glycine is obtained. The reaction is exothermic at all temperatures and pressures studied. The effect of pressure on the enthalpy of reaction is very small. The experimental heat data are analyzed to yield equilibrium constant (K), enthalpy change (DeltaH), and entropy change (DeltaS) values for the protonation reaction as a function of temperature. These values are compared with those reported previously at 298.15 K. The DeltaH and DeltaS values increase (become more positive), whereas log K values decrease, as temperature increases. The trends for DeltaH and DeltaS with temperature are opposite to those reported previously for the protonation of several alkanolamines. However, log K values for proton interaction with both glycine and the alkanolamines decrease with increasing temperature. The effect of the nitrogen atom substituent on log K for protonation of glycine and alkanolamines is discussed in terms of changes in long-range and short-range solvent effects. These effects are used to explain the difference in DeltaH and DeltaS trends between glycine protonation and those found earlier for alkanolamine protonation.

Vapor-liquid equilibrium of the methanol[1,1-dimethylethyl methyl ether (MTBE) or 1,1-dimethylpropyl methy ether (TAME)] systems

Abstract Isothermal vapor-liquid equilibria (VLE) for methanol-1,1-dimethylethyl methyl ether ( tert -butyl methyl ether or MTBE) and for methanol-1,1-dimethylpropyl methyl ether ( tert -amyl methyl ether or TAME) measured at temperatures ranging from 288.15 to 338.15 K have been correlated by means of the UNIQUAC model and by means of the Peng-Robinson equation of state and the Wong-Sandler mixing rule. The systems show positive deviations from Raoults law with an azeotrope, whose coordinates have been interpolated and compared with experimental values. Predictions of VLE data and azeotrope coordinates have been made by means of several versions of the group contribution UNIFAC model and by means of the modified Huron-Vidal second order (MHV2) model used in conjunction with the same UNIFAC model versions. In order to study the effect of the hydrogen-bond interaction in these mixtures, the lattice-fluid associated solution (LFAS) model and the extended real associated solution (ERAS) model have been used to simultaneously describe excess enthalpy and VLE data. Results from these calculations have been compared with those obtained by means of the purely physical lattice-fluid model (LF) of Sanchez-Lacombe.

The effect of temperature and pressure on the protonation ofo-phosphate ions at 348.15 and 398.15 K, and at 1.52 and 12.50 MPa

Flow calorimetry has been used to study the interaction of HPO42− and H2PO4− with H+ in water at temperatures of 348.15 and 398.15 K and at pressures of 1.52 and 12.50 MPa. The protonations of HPO42− and H2PO4− are exothermic and endothermic, respectively, under these experimental conditions. Under the conditions of this study, the effect of pressure on the enthalpy changes for both reactions is small. Equilibrium constant K, enthalpy change ΔH, and entropy change ΔS values are given for the protonation reactions at each temperature. These values are compared with those reported in the literature. Incorporation into the calculation procedure of reactions involving association between protonated phosphate species to form hydrogen-bonded dimers does not result in better fits of the experimental data.

Order effects in the excess thermodynamic properties of benzene + alkane mixtures

Mixtures of benzene and an alkane have been analysed using the method of Patterson. Order contributions in pure benzene, pure n-alkane and benzene + n-alkane mixtures and sterichindrance effects in some benzene + branched-alkane mixtures are able to explain qualitatively the excess thermodynamic properties of these systems. Order contributions are studied in terms of the model of Heintz and Lichtenthaler. It is shown how the value of X12 obtained from HE data may be predicted from the value of X12 obtained from GE data for a system without order contributions.