Thermodynamic properties of (an ester + an alkane). XVII. Experimental and values for (an alkyl propanoate + an alkane) at 318.15 K
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 and volumes 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 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 increase with n. A similar change also occurs in . In the previously mentioned work [10], it was established for binary mixtures with alkyl ethanoates that and .
For this work, values of and 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 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 and 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 , were correlated with the equation: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 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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