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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.

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.

Thermodynamic quantities for the interaction of Cl− with Mg2+, Ca2+ and H+ in aqueous solution from 250 to 325°C

The aqueous reactions, Mg2++Cl−=MgCl+, Ca2++Cl−=CaCl+, and H+ +Cl−=HCl(aq), were studied as a function of ionic strength at 250, 275, 300, and 325°C using a flow calorimetric technique. The logK, ΔH, ΔS and ΔCp values were determined from the fits of the calculated and experimental heast. The data were reduced assuming a known functionality of the activity coefficient. Hence, the logK, ΔH, ΔS and ΔCp values determined in this study are dependent on the activity coefficient model used. These thermodynamic values were compared with literature results. The logK values for the formation of MgCl+ agree reasonably well with those reported in the literature. The logK values for CaCl+ formation agree reasonably well with those reported in the literature at 300 and 325°C. At lower temperatures, the agreement is poorer. The logK values for the formation of HCl(aq) are generally lower than those reported in the literature. The logK, ΔH, ΔS and ΔCp values for all three ion association reactions are positive and increase with temperature over the temperature range studied. These values are the first determined calorimetrically for the formation of MgCl+ and CaCl+ in the temperature range 275–325°C.

Thermodynamic quantities for the ionization of nitric acid in aqueous solution from 250 to 319°C

The aqueous reaction, HNO3(aq)=H++NO3− was studied as a function of ionic strength I at 250, 275, 300 and 319°C using a flow calorimeter and the equilibrium constant K and enthalpy change (ΔH) at I=0 were determined. Using these experimental values, equations describing logK, ΔH, the entropy change ΔS and the heat capacity change ΔCp of reaction at I=0 and temperatures from 250 to 319°C were derived. The increasing importance of ion association as temperature rises was discussed. The use of an equation containing identical numbers of positive and identical numbers of negative charges on both sides of the equal sign (isocoulombic reaction principle) was applied to the logK values reported here and to those determined by others. The resulting plot of logK for the isocoulombic reaction vs. 1/T was fairly linear which supports the postulate that the principle is a useful technique for the extrapolation of logK values from low to high temperatures.

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.

Preparation of pharmaceutical co-crystals through sustainable processes using supercritical carbon dioxide: a review

The preparation of pharmaceutical co-crystals using supercritical CO2 (scCO2) is reviewed. Co-crystallization is an emerging and powerful technique to improve the physicochemical properties of an active pharmaceutical ingredient (API). The solid-state and solution co-crystallization methods usually employed present several disadvantages that may be overcome using the supercritical methods. All the methods employing scCO2 have low environmental impact and operate at relatively low temperature avoiding API degradation and producing solvent-free pharmaceuticals. The role of the fluid varies from one method to another. In the rapid expansion of supercritical solutions (RESS) the API and the coformer are dissolved in scCO2. In the co-crystallization with supercritical solvents (CSS), the fluid also acts as a solvent that facilitates molecular interactions in a suspension of the API and coformer powders. However, in the supercritical antisolvent (SAS) and the gas antisolvent (GAS) crystallizations scCO2 acts as an antisolvent that induces precipitation of the API and the coformer previously dissolved in an organic solvent. In the atomization and antisolvent (AAS) technique and supercritical fluid enhanced atomization (SEA) the fluid may act either as a spray enhancer or an antisolvent depending on the process conditions. Each method is described and its application, advantages and disadvantages are discussed. Future perspectives and areas to be investigated are outlined.