Grupo Especializado en Termodinámica de los Equilibrios entre Fasesenglish versionGETEF emblema


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Año 1997

  • Gonzalez, JA; de la Fuente, IG; Cobos, JC; DISQUAC behaviour close to critical points application to methanol plus alkane mixtures; Ber Bunsen-Ges Phys Chem Chem Phys, 101 (2) 1997 219-227
Interactions present in binary mixtures of methanol+alkanes have been previously characterized in terms of the DISQUAC group contribution model. It was shown that DISQUAC is a reliable tool to represent thermodynamic properties such as molar excess functions: Gibbs energies, GE, enthalpies, HE, or heat capacities at constant pressure, CPE. In this work, we examine the ability of the model to represent liquid-liquid equilibria, LLE, for the whole series of methanol+n-alkanes mixtures, as well as for methanol+cyclohexane, +cycloheptane, or +cyclooctane. Up to now, we had only considered LLE of those solutions involving n-hexane, n-heptane, or cyclohexane.
Using the interaction parameters previously reported for methanol+n-alkanes mixtures, which are independent of the non-polar compound, DISQUAC predicts UCSTs (upper critical solutions temperatures) of the methanol+n-alkanes mixtures which are lower than the experimental values for those systems containing the longer n-alkanes. This is in contradiction with the critical exponents theory. The possibility of improving results by modifying only the third interchange coefficients is discussed. So, two groups of these parameters are given depending on n, hereafter the number of carbon atoms in the n-alkane (n less than or equal to 7; n greater than or equal to 8). The coordinates of the critical points are then fairly well represented. Nevertheless, the shape of the LLE curves (the experimental ones being much flatter than those calculated) is rather poorly described, mainly for mixtures containing the longer n-alkanes. The new interaction parameters are tested predicting vapor-liquid equilibria, VLE, at very high temperatures (up to 473.15 K) and at moderate pressures (up to 40 atm) of methanol+n-alkanes mixtures. It is noteworthy that DISQUAC correctly predicts the formation of azeotropes for systems with the lower n-alkanes, and the absence of azeotropes for mixtures with the longer n-alkanes (from n-nonane).
Properties of mixtures containing cycloalkanes are more difficult to represent because these compounds do not form an homologous series in terms of the c-CH2 groups. Nevertheless, it has been shown that the interaction parameters of 1-alkanols (methanol)+cyclohexane systems can be applied to calculate HE Of mixtures with other cycloalkanes. In the case of LLE of methanol+cycloalkanes mixtures, different first dispersive parameters are needed for each cycloalkane. The quasichemical parameters are, as usually, independent of the n-alkane. The LLE coexistence curves are better represented than in the case of solutions including n-alkanes.
The possibility of modyfing the scaling temperature (equal to 298.15 K in DISQUAC) in order to improve LLE results is also briefly examined.
As usually, the experimental HE curves are much flatter than the calculated ones at temperatures close to the critical. Results for methanol+n-propane, or +n-heptane are shown in order to complete information previously given.
  • Domanska, U; Gonzalez, JA; Solid-liquid equilibria for systems containing long-chain 1-alkanols. III. Experimental data for 1-tetradecanol, 1-hexadecanol, 1-octadecanol or 1-icosanol plus 1-butanol, 1-hexanol, 1-octanol or 1-decanol mixtures, characterization in terms of DISQUAC; Fluid Phase Equilibr, 129 (1-2) 1997 139-163
Solid-liquid equilibrium temperatures for 1-tetradecanol, 1-hexadecanol, 1-octadecanol or 1-icosanol + 1-butanol, 1-hexanol, 1-octanol or 1-decanol systems have been measured by a dynamic method from 275 K to the melting point of the long-chain 1-alkanol. An eutectic point was encountered for those mixtures containing 1-decanol. However, first-order transitions between different crystal forms (α, β, γ) of the long-chain 1-alkanols were observed. It was noted that these solid-solid transitions are very stable in alcohols unlike in hydrocarbons or other solvents. The solubility of a given long-chain 1-alkanol is nearly the same in all alcohols tested from 1-butanol to 1-decanol; Only below the second phase transition point does the solubility change, increasing with the molecular weight of the shorter alcohol.
The hydroxyl/hydroxyl interactions present in the investigated systems were characterized in terms of DISQUAC using dispersive interchange coefficients for such contacts. Calculations were developed taking into account the solid-solid transitions of the 1-alkanols. The mean relative standard deviations for the equilibrium temperatures are 0.006 for mixtures with an alcohol in the absence of 1-icosanol; and the experimental data are fairly well represented by ideal solubility curves. For mixtures including 1-icosanol, the mean relative standard deviations are 0.021, as the model cannot reproduce, using the proposed interaction parameters, their negative deviations from Raoult's law.
  • Serna, A; de la Fuente, IG; Gonzalez, JA; Cobos, JC; Excess molar volumes of 1-propanol plus n-polyethers at 298.15 K; Fluid Phase Equilibr, 133 (1-2) 1997 187-192
Excess molar volumes VmE at 298.15 K and atmospheric pressure for 1-propanol + 2,5-dioxahexane, 3,6-dioxaoctane, 2,5,8-trioxanonane, 3,6,9-trioxaundecane or 5,8,11-trioxapentadecane have been calculated from densities measured with an Anton-Paar DMA 602 vibrating-tube densimeter. All the excess molar volumes are negative over the whole mole-fraction range, nearly symmetrical for mixtures with diethers and slightly skewed towards the region of high mole fraction of 1-propanol for mixtures with triethers. The value of VmE decreases as the n-alkyl chain end length of the diethers or the triethers increases. When the n-alkyl chain end of the polyethers is the methyl group (-CH3), VmE is very small in absolute value and similar for the diethers and triethers, whereas when the end group is larger than the methyl group, the value of VmE is more negative for the diethers than for the triethers. These results, together with previously published excess molar enthalpies, suggest the formation of hydrogen bonds between the functional group -OH of the 1-alkanol and the -O-atoms of the polyethers.
  • Cobos, JC; An exact quasi-chemical equation for excess heat capacity with W-shaped concentration dependence; Fluid Phase Equilibr, 133 (1-2) 1997 105-127
A reliable equation for the excess heat capacity CVE at constant volume is derived from Guggenheim's quasi-chemical lattice theory of liquid mixtures for the case of equal-sized molecules, with the assumption that the interchange energy depends on the temperature. Using suitable parameters, a W-shaped concentration dependence for CVE is predicted. The availability of this exact quasi-chemical equation permits the study of the theoretical dependence of CVE On temperature and concentration, a comparison with previous approximate results and better analysis of the molecular meaning of the W-shapes found in the experimental excess heat capacities CPE at constant pressure. Moreover, within this theory, the long-wavelength limit of the Bhatia- Thornton concentration-concentration partial structure factor Scc(0) is also derived and its correlation with CVE or CPE is discussed.
  • Gonzalez, JA; de la Fuente, IG; Cobos, JC; Thermodynamics of mixtures containing linear monocarboxylic acids. II. Binary systems showing cross-association between components: DISQUAC characterization of linear monocarboxylic acid plus 1-alkanol, or plus linear monocarboxylic acid mixtures; Fluid Phase Equilibr, 135 (1) 1997 1-21
Binary mixtures containing compounds which show cross-association between them are investigated in terms of DISQUAC: namely, systems with two linear monocarboxylic acids, or with one acid and one 1-alkanol. In the former, the interactions between the COOH groups of the acids are represented by dispersive parameters only. Binary systems involving two l-alkanols behave similarly. In the linear monocarboxylic acids + 1-alkanol mixtures, the COOH/OH interactions are represented by structure-dependent dispersive and quasichemical parameters. It is shown that those solutions with methanol and ethanol do not fit into the general scheme followed by the higher members of each homologous series considered here. A similar behaviour is found when mixtures containing methanol and benzene or CCl4 are compared with those involving higher alkanols in the frameworks of DISQUAC or of the Barker's theory.
Vapor-liquid equilibria, VLE, and excess enthalpy, HE, data are consistently described by DISQUAC. Discrepancies are analysed.
The UNIQUAC association model or an equation of state (Carnahan-Starling) with the association built in have been applied in the literature as pure correlation equations of the experimental data for acids + l-alkanols systems. Their results are compared with those reported in this work by DISQUAC.
  • Gonzalez, JA; de la Fuente, IG; Cobos, JC; Thermodynamics of mixtures with strongly negative deviation from Raoult's law. I. Application of the DISQUAC model to mixtures of alkan-1-ols and propanal or linear alkanones and trichloromethane; J Chem Soc-Faraday Trans, 93 (21) 1997 3773-3780
Thermodynamic properties: vapour-liquid equilibria (VLE), molar excess Gibbs energies, HE, molar excess enthalpies, HE, or the mole fraction structure factors, Sce(O), of systems containing alkan-1-ols and propanal, or linear alkanones and CHCl3, i.e. strongly associated mixtures which show compound formation, are studied in the framework of the DISQUAC (dispersive quasichemical) model. Interaction parameters for the contacts OH-CO (in propanal) and Cl (in CHCl3)-CO (in alkanones) are reported. For alkan-1-ol-propanal systems, DISQUAC gives better results for VLE than other models previously applied, such as lattice fluid (LF), lattice fluid associated solution (LFAS), extended real associated solution (ERAS), or the universal quasichemical (UNIQUAC) association theory. For HE, only the latter improves results meaningfully but DISQUAC gives good agreement for Scc(0). It is shown that heterocoordination is an esssential characteristic of this class of systems. For linear alkanones with CHCl3, DISQUAC consistently describes GE and HE. Only the symmetry of the GE curve Of the propan-2-one-CHCl3 mixtures is not entirely satisfactory. The calculated Scc(0) also shows that heterocoordination plays an important role in these mixtures. As a trend heterocoordination seems to be represented by large and negative enthalpic parameters for the selected contacts.
The ability of DISQUAC to predict VLE and HE for the complex ternary systems propan-2-one-butan-2-one-CHCl3 and methanol-propan-2-one-CHCl3 is examined. Previously, the highly non-ideal mixture methanol-CHCl3 was studied with the necessity for interaction parameters for the OH-CI contact. DISQUAC accurately describes the VLE of this system, and reproduces reasonably the S-shaped HE. Scc(0) shows that this system is characterized by homocoordination. For the ternary mixtures considered, DISQUAC yields good results for VLE at different temperatures but HE values are only qualitative.
  • Gonzalez, JA; de la Fuente, IG; Cobos, JC; Thermodynamics of mixtures containing the CO and OH groups. III. DISQUAC predictions on VLE and HE for ternary mixtures containing 1-alkanols, n-alkanones, and one organic solvent; Can J Chem-Rev Can Chim, 75 (10) 1997 1424-1433
Thermodynamic properties: vapor-liquid equilibria, VLE, or excess enthalpies, HE, for a set of 21 ternary mixtures of the type 1-alkanol + n-alkanone + organic solvent are studied in the framework of the DISQUAC group contribution model. This treatment is extended to the binaries involved. The DISQUAC analysis is developed on the basis of binary interactions only, that is, ternary interactions are neglected. Most of the interchange coefficients needed are available in the literature. The average relative standard deviations are 0.026 for pressure in the VLE (12 systems) and 0.098 in the HE (9 systems). The discrepancies observed are briefly discussed.
  • Gonzalez, JA; Thermodynamics of mixtures containing the CO and OH groups. I. Estimation of the DISQUAC interchange coefficients for 1-alkanol plus n-alkanone systems; Can J Chem-Rev Can Chim, 75 (10) 1997 1412-1423
1-Alkanol + n-alkanone mixtures are treated in terms of the DISQUAC group contribution model, reporting the interaction parameters for hydroxyl-carbonyl contacts. The quasichemical interchange coefficients are independent of the compounds in the mixture; the dispersive interchange coefficients depend on the intramolecular environment of the hydroxyl and (or) carbonyl groups. Mixtures of a given 1-alkanol with isomeric ketones are characterized by the same first dispersive interaction parameter, which is constant from 2-pentanone. This type of system, when including an alcohol up to 1-pentanol, needs different dispersive enthalpic parameters depending on the symmetry of the ketone. In this case, such parameters are constant from 2-pentanone or 3-pentanone. A detailed comparison is presented between DISQUAC results and data available in the literature on vapor-liquid equilibria, VLE (including azeotropic data), molar Gibbs energies, GE, molar excess enthalpies, HE, solid-liquid equilibria, SLE, natural logarithms of activity coefficients, In γi, and partial molar excess enthalpies at infinite dilution, HiE,∞. For 54 systems, the mean relative standard deviation in pressure is 0.018; for 61 systems, this magnitude in the case of the HE is 0.059. It is noteworthy that the model yields good predictions over a very wide range of temperature for VLE and SLE. HE is also reasonably well represented at different temperatures. Larger discrepancies are encountered, as usual, for partial molar quantities at infinite dilution.



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