Modeling of the mixing energies of binaries (methyl ester - alcohol) according to different versions of the UNIFAC method

Abstract

On a hE vs x database of 81 binary systems formed by nine methyl esters and nine normal alcohols, three versions of the UNIFAC group contribution method are used to represent the corresponding mixing energies. Representations of the hEs versus the variables u and v, corresponding to the acid chains of methyl alkanoates (CuH2u+1CO2CH3) and alcohols (CvH2v+1(OH)), are used to verify the results. The 2D-plots show that the equimolar hE values exhibit a non-regular distribution with u and v, a consequence of effects of different nature. The simplest version of the method used, named UNIFAC-DM, estimates hEs with an average error of 8% or 16%, depending on whether methyl esters are considered as ethanoates or non-ethanoates. The UNIFAC-ST version qualitatively and quantitatively improves the results since it considers the auto- association of alcohols. The UNIFAC-NK version is more complex, because, in addition to the above, it takes into account the interactions that form the ester-alcohol complexation, giving more satisfactory results, with an average global error of less than 4%. It is indicated that ester-alcohol solutions present specific interactions of three ways, one physical due to the molecular size and shapes, in addition to van der Waals forces, and two quasi-chemical due to intramolecular ester-ester actions and to the intermolecular ester-alcohol actions.