Thermodynamic Analysis of Systems Formed by Alkyl Esters with alpha,omega-Alkyl Dibromides: New Experimental Information and the Use of a Dense Database to Describe Their Behavior Using the UNIFAC Group Contribution Method and the COSMO-RS Methodology

Abstract

This is a theoretical study carried out on the experimental data obtained for 90 binary systems comprised of alkyl esters with dibromoalkanes, using data from previous publications and those corresponding to 20 mixtures presented here. The experimental part of this work corresponds to experimental measurements of HmE and VmE at T = 298.15 K and atmospheric pressure for systems containing propyl alkanoates (methanoate to butanoate) with 1,ω-dibromoalkanes (ω = 2-6). The systems containing propyl methanoate present HmE > 0, while the remaining ones give values of HmE that follow a sigmoid distribution. The VmE of these mixtures of propyl esters are either positive or negative, depending on the dihalide involved. Positive ones correspond to mixtures with short-chain dibromoalkanes, and contraction effects are observed with the increasing chain length of the dihalide compound. On the whole, experimental information suggests a specific behavior with either positive or negative structural mixing effects depending on the chain length of the compounds. HmE values were estimated theoretically by applying two different types of models. The UNIFAC group contribution model gives good results in the prediction, but only when the Br, Br/carboxylate interaction is considered to vary with the chains of the ester and dibromoalkane; mathematical expressions are presented to describe these variations for the entire set of mixtures. The COSMO-RS quantum-chemical method is also applied to predict HmE values for the systems studied and the results extrapolated to a set of 90 systems (alkyl alkanoates + dibromoalkane), optimizing the model parameters, especially to obtain an adequate description of the Van der Waals interactions. As a result, the optimized COSMO-RS method can be used to interpret the behavior of the HmE for the whole group of alkyl esters + dibromoalkane in terms of the different intermolecular interactions taking place inside the pure compounds and mixtures, especially in relation to the variable effects of the alkyl chains of the components on the mixing enthalpies. © 2010 American Chemical Society.