Enthalpies of vaporization of a series of aliphatic alcohols: Experimental results and values predicted by the ERAS-model
Introduction
Thermodynamic properties of short-chained linear alcohols C1–C6 have been studied extensively [1], [2], [3], [4], [5], [6], [7], [8]. Vapor pressures and enthalpies of vaporization, ΔlgH°m, of the longer straight-chained aliphatic alcohols, however, are scarce, especially for the C11–C16 members of the series [4], [5], [6], [7], [8]. Besides, the available results are not always consistent, as they should be when using them for comparison with values predicted by any theoretical or empirical method.
In this work, vaporization enthalpies ΔlgH°m of a series of linear alcohols have been obtained from vapor pressure measurements. We have used our new experimental results together with data already available from the literature for testing the ability of the extended real associated solution model (ERAS-model) to predict values for ΔlgH°m (298.15 K) of alcohols. We also included recently measured data of the vaporization enthalpies of pure branched alcohols [9] into the evaluation procedure.
The ERAS-model [10], [11] was developed in its original version for predicting thermodynamic excess properties as well as vapor–liquid and liquid–liquid equilibria data for mixtures containing an associating and a non-associating compound. The ERAS-model is an equation of state based on the combination of Flory’s theory of liquids and a model of chain-like association. Flory’s equation of state is not applicable to strong polar liquids such as alcohols, but its combination with the associated solution model leads to a generalized equation of state applicable to pure alcohols as well as mixtures of alcohols with hydrocarbons. It turns out that the equations obtained for the thermodynamic functions are split into two additive terms which arise from hydrogen bonding effects (“chemical” contributions) and non-polar van der Waal’s interactions including free volume effects (“physical” contributions). It was assumed that only consecutive linear association of the alkanol occurs, which can be described by a chemical equilibrium constant KA being independent of the chain-length i of the associated species according to the following:The dependence of KA on temperature is given by van’t Hoff’s relation.where Δh∗ is the molar association enthalpy related to the intermolecular hydrogen bond formation. KA0 is the association constant at the temperature T0. Values of the parameter Δh∗ and KA are required for calculating the chemical contribution to the excess properties. The value of Δh∗ is regarded to be independent of the type of alcohol [12]. A further molecular parameter used in the ERAS-model is the hydrogen bonding volume which is assumed to be a negative value (−5.6 cm3 mol−1) for alcohols indicating that hydrogen bonding formation of alcohol molecules is associated with a contraction of the hard core volume of the molecules. KA is a property of the individual alkanol. One of the advantages of the ERAS-model in comparison to empirical or semi-empirical methods is that only a restricted number of experimental data of the properties of the pure components are required, as presented in Table 1. Values of the density ρ, the isobaric thermal expansion coefficient α, and the isothermal compressibility κ are available in the literature. The association constant KA and the formation enthalpy of hydrogen bonding Δh∗ as well as reaction volume of the hydrogen bonding have been obtained from adjusting these values simultaneously to experimental data of excess enthalpies, HE; Gibbs excess energies, GE; and excess volumes, VE, of alcohols+hydrocarbon mixtures [10], [11].
Using the parameters KA, Δh∗, and , the vaporization enthalpy of the pure alcohol can be predicted by the ERAS-model without adjusting any additional parameters. Exceptions are only those cases where no values of KA are available in the literature.
Section snippets
Materials
Pure hexanol-1, octanol-1, decanol-1, undecanol-1, dodecanol-1, tridecanol-1, tetradecanol-1, pentadecanol-1, and hexadecanol-1 were purchased from Aldrich, Acros, and Merck and have been used without further purification. The degree of purity was controlled using a Hewlett-Packard gas chromatograph 5890 Series II equipped with a flame ionization detector and a Hewlett-Packard 3390A integrator. The carrier gas (nitrogen) flow was 12.1 cm3 s−1. A capillary column HP-5 (stationary phase
Hexanol-1
Reliable values of the standard enthalpy of vaporization ΔlgH°m (298.15 K) of hexanol-1 have been determined by Mansson et. al. [6] (61.85±0.20 kJ mol−1) and by Wadso [14] (61.63±0.17 kJ mol−1) using a direct calorimetric method. These values are in fair agreement with those reported by Green [15] (62.8±1.3 kJ mol−1) obtained from vapor pressure measurements. The value obtained in this work, 61.1±0.3 kJ mol−1 (see Table 3), is in agreement with those available from the literature. The agreement of our
Acknowledgements
One of us (D. K.) acknowledges gratefully a research scholarship from the Deutscher Akademischer Austauschdienst (DAAD).
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