Thermodynamic properties of (an ester + an alkane). XVII. Experimental HmE and VmE values for (an alkyl propanoate + an alkane) at 318.15 K

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Abstract

In this article, we record the experimental values of HmE and VmE, obtained at a temperature of T = 318.15 K and atmospheric pressure for a set of 30 binary mixtures comprised of five alkyl propanoates (methyl to pentyl) with six odd alkanes (heptane to heptadecane). The net values obtained for these properties are the result of different effects of the selected compounds on the mixing process. These effects and the variation with temperature are studied. The HmE are positive in all cases and increase with the saturated hydrocarbon chain and diminish with the alkanolic portion of the ester. The variation in VmE is similar to that occurring in the HmE. For the data correlation, a new form of polynomial equation is used in which the variable is the so-called active fraction which, in turn, is a function of the concentration of the mixture, giving acceptable estimations for simultaneous correlations between the values of Gibbs function obtained in the isobaric (liquid + vapour) equilibria and the enthalpies of the mixture, for some of the mixtures studied. The results are explained with the molecular model proposed for (ester + alkane) mixtures. Finally, the application of two versions of the UNIFAC groups contribution method to estimate enthalpies of the mixtures does not give satisfactory results, although the modified UNIFAC gives somewhat better results.

Introduction

This article is one more of the series belonging to a research project on the general behaviour of (ester + alkane) binary systems. Here, among other factors, variations in some properties of the mixture with temperature are analysed in order to interpret correctly the mixing process and to proceed to formulating the model. In previous works [1], [2], [3], [4], [5], [6], [7], mixing quantities enthalpies HmE and volumes VmE have been determined at T = 298.15 K for binary systems of propanoates (methyl to pentyl) with several alkanes. Also, in the literature (vapour + liquid) equilibria (VLE) have been found for mixtures of (methyl propanoate + heptane) [8] and (propyl propanoate + heptane or nonane) [9]. In this line, in a previous work a similar study was carried out with systems containing different alkyl ethanoates [10]. For all (ester + alkane) mixtures studied to date the behaviour can be standardised, in that the mixing processes presents endothermic effects, where HmE is negatively correlated with the increasing number of ester carbons, for the acid part R1 or the alkanolic part R2 in R1COOR2. Regarding the second component, the CnH2n + 2 hydrocarbon, the HmE increase with n. A similar change also occurs in VmE. In the previously mentioned work [10], it was established for binary mixtures with alkyl ethanoates that (HmE/T)p,x>0 and (VmE/T)p,x>0.

For this work, values of HmE and VmE have been experimentally determined for a set of 30 binary mixtures of alkyl propanoates (methyl to pentyl) with odd alkanes (heptane to heptadecane), measured at 318.15 K. It is important to establish the influence of temperature since this permits different structural aspects to be defined and the results obtained in other conditions to be corroborated as well as the hypothesis of the structural behavioural model already proposed [10]. In the empirical sphere, the data can be used to study the value of a new simple polynomial expression to correlate thermodynamic properties, which is expressed in relation to the so-called active fraction with temperature-dependent coefficients. With this expression [11], isobaric VLE and HmE data can be simultaneously correlated and in this work VLE data from the literature are used and other values determined experimentally for this work.

Finally, another aspect to take into consideration in this work is that measurements made at a temperature of 318.15 K can be used to verify the value of some group contribution models [12], [13] at a different temperature than that used in most cases to obtain acceptable parameters for the (ester + alkane) interaction.

Section snippets

Experimental

Alkyl esters and alkanes used for this work were acquired from either Aldrich or Fluka, depending on the commercial purity of these products offered by each manufacturer. Before use, all compounds were degasified with ultrasound for several hours and then passed through a molecular sieve of 0.3 nm Fluka to eliminate any traces of moisture. Compounds were characterised by verifying the purity of each of them with a GC model HP9860 equipped with FID, giving similar values to those specified by the

Results and treatment of experimental data

TABLE 2, TABLE 3 show, respectively, the experimental values of HmE/(J·mol-1) and VmE/(m3·mol-1) obtained at a temperature of 318.15 K for a set of 30 binary mixtures of {xalkyl propanoate (methyl to pentyl) + (1  x) odd alkanes (C7 and C17)}. Both quantities, represented generically by YmE, were correlated with the equation:YmE=z(1-z)(A0+A1z+A2z2),wherez=xx+k(1-x)is the active fraction of one of the compounds of the binary mixture which is a function of the molar fraction x of the compound studied

Discussion

The results obtained for the (alkyl propanoate + alkane) mixtures reveal endothermic and expansion effects for all of them, with the exception of some mixtures with pentane at 298.15 K. The change in temperature has little influence on the change in enthalpy and the value of the (HmE/T)p,x coefficient is almost zero in mixtures of compounds with a greater number of carbon atoms and this tendency even inverts in mixtures of the higher propanoates with heptadecane. The changes in the expansive

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