Thermodynamic study of (alkyl esters + α,ω-alkyl dihalides) I: $H_{\mathrm{m}}^{\mathrm{E}}$ and $V_{\mathrm{m}}^{\mathrm{E}}$ for 25 binary mixtures {xC_u − 1H_2u − 1CO₂C₂H₅ + (1 − x)α,ω-ClCH₂(CH₂)_v − 2CH₂Cl}, where u = 1 to 5, α = 1 and v = ω = 2 to 6

Abstract

This article presents the experimental data of $H_{\mathrm{m}}^{\mathrm{E}}$ and $V_{\mathrm{m}}^{\mathrm{E}}$, obtained at atmospheric pressure and at a temperature of 298.15 K, for a set of 25 binary mixtures composed of the first 5 ethyl alkanoates (methanoate to pentanoate) and five α,ω-dichloroalkanes (1,2-dichloroethane to 1,6-dichlorohexane). Quantitatively, and with only a few exceptions, small values are obtained for the excess properties and the results imply that specific interactions exist between both types of compounds, with exothermic process for most mixtures, but with the exception of some that contain ethyl methanoate and ethanoate. The change in enthalpies with increasing length of the dichloroalkane chain for the same ester is regular, and also the change in $H_{\mathrm{m}}^{\mathrm{E}}$ with the acid portion of the ethyl ester. However, the change in excess volumes does not present such a regular variation. A behavioural structural model is established to explain the results of the excess properties. Experimental values of $H_{\mathrm{m}}^{\mathrm{E}}$ and $V_{\mathrm{m}}^{\mathrm{E}}$ were correlated, as a function of ester concentration, x with a new expression which uses the so-called active fraction as a variable and which, in turn, is a function of this concentration.

The application of two versions of the UNIFAC group contribution models produces no good estimations of $H_{\mathrm{m}}^{\mathrm{E}}$.

Introduction

This article is the first of a series of papers we intend to carry out on binary systems of aliphatic esters with alkyl dihalides. More specifically, we initially aim to compile sufficient information which permit us to study the behaviour of a set of binary mixtures that contain alkyl alkanoates and α,ω-dihaloalkanes (Cl, Br, I). The studies of these binary mixtures are of great theoretical interest owing to the probable existence of specific interactions between the molecules of the compounds involved, making it difficult to interpret the behaviour and application of theoretical models. Another objective is to study the so-called proximity effect between the halogen atoms in the α,ω-dihaloalkanes and its repercussion on excess thermodynamic quantities. As antecedents, our research team has published data in the literature on excess properties for binary mixtures of α,ω-dihaloalkanes + alkanes [1], [2], [3], and other authors have measured values for α,ω-dichloroalkanes + alkanes [4], or + alkanols [5], or + ketones [6], [7]. Studies carried out in our laboratory with mixtures of ethyl esters with monochloroalkanes are also recorded in the literature [8], [9]. However, for the systematization of these studies, experimental values are required and a preliminary analysis of primary systems, such as those corresponding to mixtures of α,ω-dihaloalkanes + alkanes [1], [2], [3], and esters + alkanes [10], [11], [12], requiring exhaustive work by our research team.

Therefore, the present work focuses on studies of binary mixtures of ethyl esters + α,ω-dichloroalkanes, providing data of $H_{\mathrm{m}}^{\mathrm{E}}$ and $V_{\mathrm{m}}^{\mathrm{E}}$ for a set of 25 binary mixtures, which will be used to complete contributions made in previous works and to verify the structural model proposed for other similar binary systems. Hence, the literature offers experimental data of $H_{\mathrm{m}}^{\mathrm{E}}$ for some mixtures of alkyl esters + α,ω-dichloroalkanes [13], [14], although there is no systematic study of a sufficient number of systems for their behaviour to be fully understood. In the cited papers, enthalpy data are explained on the basis of the CH₂Cl/COO interaction and the so-called intramolecular proximity effect. The behavioural change of the molecule of α,ω-dichloroalkane in solution was attributed to this effect because of the different proximity of the two Cl atoms [15], [16], [17], [18]. For the present study, experimental data of $H_{\mathrm{m}}^{\mathrm{E}}$ from a set of 12 mixtures, previously measured in our laboratory [19], will be used, showing here the measurements made for the other 13 systems. For this work, the $V_{\mathrm{m}}^{\mathrm{E}}$s were measured for the mixtures of the same set of 25 mixtures for which the actual literature does not give any values.

Finally, in this work on ester + dichloroalkane mixtures, we intend to verify the theoretical models considering two perspectives. On the one hand, to study the utility of an expression [11], [12] for the treatment/correlation of experimental data which, in the longer time could be extended using other thermodynamic quantities for different conditions of temperature and pressure. On the other hand, we consider important to know in greater depth the nature of the inter/intramolecular interactions that affect the mixing process of these solutions, to be able to correctly apply these predictive models. Moreover, with the experimental data we can verify the validity of the UNIFAC group contribution model in two of its versions, that of Dang and Tassios [20], using the parameters obtained for the ester/chloride interaction by Ortega et al. [21] for mixtures of esters with monochloroalkanes, and that of Gmehling et al. [22].