Activity Coefficients Of Mixtures Research Articles

Prediction of parameters of group contribution models of mixtures by matrix completion.

Group contribution (GC) methods are widely used for predicting the thermodynamic properties of mixtures by dividing components into structural groups. These structural groups can be combined freely so that the applicability of a GC method is only limited by the availability of its parameters for the groups of interest. For describing mixtures, pairwise interaction parameters between the groups are of prime importance. Finding suitable numbers for these parameters is often impeded by a lack of suitable experimental data. Here, we address this problem by using matrix completion methods (MCMs) from machine learning to predict missing group-interaction parameters. This new approach is applied to UNIFAC, an established group contribution method for predicting activity coefficients in mixtures. The developed MCM yields a complete set of parameters for the first 50 main groups of UNIFAC, which substantially extends the scope and applicability of UNIFAC. The quality of the predicted parameter set is evaluated using vapor-liquid equilibrium data of binary mixtures from the Dortmund Data Bank. This evaluation reveals that our approach gives prediction accuracies comparable with UNIFAC for data sets to which UNIFAC was fitted, and only slightly lower accuracies for data sets to which UNIFAC is not applicable.

The potential of head-space gas chromatography for VLE measurements

Head-space gas chromatography (HS-GC) is thought to allow the performance of (vapour+liquid) equilibrium (VLE) measurements in a fast and automated way. However, two decades after the first applications of HS-GC for this purpose, the potential of this technique is not fully developed yet. Measurements of isothermal VLE and activity coefficients of mixtures can be obtained in a high throughput scenario. However, several considerations have to be taken into account before starting the analysis, such as the equilibration time or the minimum sample volume and the GC response factors. These aspects can strongly influence on the validity of the results and should therefore be determined for each mixture.In this paper, four azeotropic mixtures of interest in the pharmaceutical and chemical industry, i.e., (ethylacetate+water), which forms a heterogeneous azeotrope, (ethylacetate+isooctane), (acetonitrile+toluene) and the ternary mixture (acetonitrile+toluene+tetrahydrofuran), are considered to show the potential of HS-GC for VLE measurements. The thermodynamic analysis of VLE data leads to activity coefficients for the mixtures at (35, 50, and 70)°C. In addition, the experimental data are compared with thermodynamic models and data from the literature, when available.

Refinement of COSMO−SAC and the Applications

In dielectric continuum solvation models (CSMs), the solvation free energy is essential for accurate calculation of the thermodynamic properties of liquids and liquid mixtures. Here, we present a refinement to the COSMO−SAC (where SAC denotes segment activity coefficient) model to calculate the solvation free energy, using statistical thermodynamics and information from quantum mechanics. After the surface charge density for each pure compound has been obtained from a one-time quantum mechanics calculation, the COSMO−SAC model with 23 parameters can be used to predict both pure and mixture thermodynamic properties of almost any nonmetallic molecule in a computational time comparable to that of group contribution methods, but without their hundreds of functional-group-specific parameters. Combining perturbation theory and a mean field model for the van der Waals (vdW) free-energy calculation, this COSMO−SAC model can be used to predict the properties of pure compounds. The overall deviation in vapor pressure for 1432 compounds is ∼63%, and that in the normal boiling temperature prediction is ∼20 K; these values are comparable to those of group contribution methods, which require a much larger set of parameters. This model can also be used to predict activity coefficients in mixtures with the same parameters, and here we have calculated vapor−liquid equilibria for more than 500 binary systems with over 8000 experimental data points, and for seven ternary systems. For these systems, the overall average deviation in vapor-phase compositions is 2.14%, and the deviation in total pressure at a fixed liquid composition is 6.03%.

Improved thermodynamic calculations for concentrated mixed electrolyte systems including ion pairing (or the absence of it)

Knowledge of the activity coefficients of solutes under temperature and pressure ranges of interest is generally required for calculations of the physicochemical properties of aqueous multicomponent electrolyte solutions such as seawater. Whilst these activity coefficients are well characterized for the predominant salts individually, fewer data are available for minor components under a sufficiently wide range of conditions and there are significant interactions between some of the chemical species involved which can substantially alter the required activity coefficients in mixtures. For these reasons, accurate thermodynamic predictions for seawater and other multi-electrolyte solutions in moderate or high concentration are more difficult than is often supposed. In particular, progress in this area has been slowed by uncertainty regarding the nature and extent of ion pairing in concentrated strong electrolyte solutions, a problem that has been debated for decades. Zdanovskii's rule, described originally almost a century ago but widely neglected, provides a fundamentally sound method for calculating the properties of mixed electrolyte solutions. Modelling developments are described, which accord with various types of experiment, from our laboratory and from the chemical literature. It has become clear that, when treated in an appropriate way based on solvent activity, linear mixing behaviour of strong electrolyte solutions can be considered the norm. This is an inevitable consequence, first recognized by Guggenheim, of the cancellation of ion pairing and other interactions as, conceptually, one ion replaces another. Stronger solute–solute interactions, described by equilibrium constants, can then easily be coupled in a thermodynamically consistent manner.

Water activities, osmotic and activity coefficients in aqueous mixtures of sodium and magnesium chlorides at 298.15 K by the hygrometric method

Sodium and magnesium are important cations found in atmospheric aerosols. Their salts of chlorides are hygroscopic. In this research, the mixed aqueous electrolyte system of magnesium and sodium chlorides has been studied with the hygrometric method at the temperature 298.15 K. The relative humidities of this system are measured at total ionic strength from 0.45 to 6 mol kg −1 for different ionic strength fractions y of NaCl with y=0.33, 0.50, and 0.67. The obtained data allow the deduction of water activities and osmotic coefficients. The experimental results are compared with the predictions of our extended composed additivity (ECA) law proposed in our previous work, the Zdanovskii–Stokes–Robinson (ZSR), the Robinson–Stokes (RS), the Reilly–Wood–Robinson (RWR), the Pitzer, and the Leitzke–Stoughton (LS II) models. Predictions made using these models are, in general, consistent with our results. From these measurements, new Pitzer mixed ionic parameters are determined and used to predict the solute activity coefficients in the mixture at total ionic strength for different ionic strength fractions.

Thermodynamic analysis of effluent tailing observed during dissolution of binary oil mixtures

Tailing phenomena observed during the dissolution of binary oil mixtures are manifestations of non-equilibrium effects and are not induced by non-ideal partitioning. This conclusion follows a thermodynamically-based analysis of the dissolution process in a chromatographic framework. A criterion, both components of binary oil mixtures having enhanced or unaffected solubilities relative to Raoult's law, serves as an indicator for systems in which non-linear-partitioning induced tailing is precluded; tailing must be the result of non-equilibrium processes. This condition is the rule, since chemical dissimilarities tend to increase activity coefficients in mixtures.

Thermodynamics of concentrated electrolyte mixtures. Activity coefficients in aqueous sodium bromide-calcium bromide mixtures at 25.degree.C

The activity coefficients of NaBr and CaBr/sub 2/ in NaBr-CaBr/sub 2/ aqueous mixtures have been measured at 25/sup 0/C over the ranges of ionic strengths 0.1 to 7.5 m. The measurements were made with a dual electrometer amplifier using Na and Ca ion selective electrodes against a Br ion selective electrode. The data were fitted to Harned's equation of the form log ..gamma../sub i/ = log ..gamma../sub i//sup 0/ - ..cap alpha../sub AB/y/sub B/ - ..beta../sub AB/y/sub B//sup 2/ to compute the trace activity coefficients and were also analyzed by using the Pitzer formation. Deviations from the Pitzer formalism were observed for the experimental activity coefficients in this mixture.

An improved group method for hydrocarbon-hydrocarbon systems arising from a comparative study of asog and unifac

A comparison of the formalism of ASOG, in the version of Kojima and Tochigi, and of UNIFAC shows the total formal equivalence of both empirical group methods for calculating activity coefficients in mixtures. The most advantageous features of each method are taken into account in the formulation of an improved method for hydrocarbon-hydrocarbon systems. Parameters for CH 2/ArCH, CH 2/CyCH, CH 2CH, ArCH/CyCH, ArCH/CH and CyCH/CH are evaluated from total pressure- liquid composition and heats of mixing data using the SIGMA procedure. The results obtained with the three empirical group methods are compared with experimental data. The ability to predict activity coefficients at infinite dilution is discussed.

A modification of regular solution theory to incorporate an improved approximation for pair distribution functions in mixtures

AbstractThe radial distribution functions for pairs in a fluid mixture can be closely approximated from known data on pair distribution functions pure components evaluated at a mean density determined by a properly defined average of molecular interaction parameters in the mixtures. This approximation has been used to replace the assumption about structure in mixtures inherent in the traditional regular solution theory. The result is a modified regular solution theory which gives an improvement in predicting excess thermodynamic functions and activity coefficients in mixtures involving appreciable differences in molecular size and shape.