This study has used a new polynomial expression with temperature-dependent coefficients as a function based on the so-called active fraction of the mixture components, z(x i ), which enabled the thermodynamic vapor-liquid equilibrium quantities, the Gibbs excess energy function and corresponding activity coefficients, and the excess enthalpies all to be correlated simultaneously. Correlation is carried out by using a least-squares procedure implemented in a genetic algorithm that optimized an objective function selected for that purpose. The model is applied to a set of thermodynamic data for binary systems composed of ethanol + one of five n-alkanes (hexane to decane) collected as part of the study. Good results were obtained using the model, which yielded good estimates of the mixing properties, g E and h E , which in turn were used for comparison with the results obtained using certain more conventional models. The expression that yielded the best correlation of the excess Gibbs energy function for the binary systems considered as a function of the concentration of one of the components and temperature was (g E /RT)(T,x 1 ) ) z 1 z 2 [(A 01 T + A 02 /T + A 03 ) + (A 21 T + A 22 /T + A 23 )z 1 2 ].

The behavior was investigated for several binary mixtures that were composed of five methyl esters (from methanoate to pentanoate) with n-heptane, by using different thermodynamic quantities as obtained from experimental results such as isobaric vapor-liquid equilibria at the pressure 101.32 kPa and the corresponding excess properties. The equilibrium data show that all the mixtures reveal a positive deviation from ideality and that they are consistent with a point-to-point test. All the mixtures studied, with the exception of methyl pentanoate + n-heptane, reveal an azeotrope. Various equations were used to treat the equilibrium data. The use of a new polynomial correlation with temperature-dependent coefficients was deemed to be convenient. Various theoretical models were used in this study, including UNIFAC and ASOG, but the Gmehling et al. version was found to be more acceptable when estimating excess enthalpies with differences of between (3 and 17)%.

A new empirical mathematical model for the Gibbs excess function, g E ) ψ(p,T,x), is presented for a multicomponent system. Dependence on the composition is achieved through the so-called active fraction, z i , which, in turn, is related to the molar fraction x i of the components of a solution and a parameter k ij , the determination of which is also indicated. The efficacy of the model in relation to its extension of application is discussed, considering various cases and three possible ways to calculate the parameter k ij . This produces different versions of the model for data correlation the advantages of which are discussed. The model proposed for the Gibbs excess function adopts the following generic expression, g E (P,T,x) ) z(x)[1z(x)]∑ i)0 g i (P,T)z i where g i (P,T) ) g i1 + g i2 P 2 + g i3 PT + g i4 /T + g i5 T 2 , which can be applied to a general case of vapor-liquid equilibrium with variation of the three main variables x i , p, and T, or by considering the experimental values for two important situations, isobaric and isothermal, which are also studied here. Other mixing properties are obtained via mathematical derivation, and a simultaneous correlation is carried out on several of them. The model has been applied to various binary systems for which experimental data are available in the literature and over a wide range of p and T. The results obtained can be considered acceptable.

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This paper presents the experimental excess properties H m E and V m E obtained at different temperatures and the vapor-liquid equilibrium values at 101.32 kPa for four binary mixtures composed of four ethyl esters (methanoate to butanoate) and tert-butyl alcohol. A point-to-point test applied to the equilibrium values indicated that the systems studied were consistent. The binary mixture ethyl ethanoate + tertbutyl alcohol exhibits an azeotropic point at x az ) 0.832 and T ) 349.59 K. All values were correlated using a new equation with temperature-dependent coefficients fitting simultaneously the vapor-liquid equilibria and H m E values. Good fits were obtained in all cases. Application of a modified version of the UNIFAC model to the mixtures containing a tertiary alkanol yielded rather poor estimates.