Excess enthalpies of binary mixtures of 2-ethoxyethanol with four hydrocarbons at 298.15, 308.15, and 318.15 K: An experimental and theoretical study
Introduction
The thermodynamic properties of highly non-ideal mixtures of associated or hydrogen-bonded fluids and their correlation/analysis are not only of key interest in the development and testing of statistical thermodynamic models capable of correlating the macroscopic properties of the fluid systems, but also of importance for the rational design of industrial chemical processes involving such systems. Hydrogen bonding may be one of the most important interactions in these systems and intra-molecular hydrogen bonding may also be an important contribution to the overall hydrogen bonding.
2-Ethoxyethanol (widely known by its trade name, cellosolve) is a common solvent in laboratory practice and in the chemical industry and is able to form two types of hydrogen bonds, inter- and intra-molecular ones. This capacity keeps attracting the interest of many researchers for the theoretical and experimental study of miscellaneous properties of systems of cellosolve. The excess molar enthalpies of aqueous solution of 2-ethoxyethanol were measured by Tamura et al. [1] and Davis et al. [2]. Other systems of 2-ethoxyethanol include binaries with various carboxylates [3], [4], with di-n-butylether [5], and with 1,4-dioxane or 1,2-dimethoxyethane or various hydrocarbons [6], [7]. More recently, Lee and Lee [8] have introduced intra-molecular association explicitly in their correlations of vapor–liquid equilibria of pure alkoxyalkanols and their mixtures with inert substances.
In our previous works we provided new experimental spectroscopic data on 2-methoxyethanol and 2-ethoxyethanol + n-hexane systems in very dilute mixtures [9], and high pressure vapor–liquid equilibrium (VLE) data of the binary mixture of CO2 + 2-ethoxyethanol at three temperatures [10], and discussed the extent of the two competing types of hydrogen bonds as well as the validity of equations-of-state models, such as the LFHB (lattice-fluid hydrogen bonding) [11] and the NRHB (non-random hydrogen bonding) [15] models.
In the present work, we report measurements of the excess enthalpies of 2-ethoxyethanol with four hydrocarbons, two normal (n-hexane and n-octane), one cyclic (cyclohexane), and one aromatic (benzene), at three temperatures: 298.15, 308.15, and 318.15 K. The NRHB model with the known hydrogen bonding parameters [9] and the scaling constants [10] was used to correlate/predict the heats of mixing of the studied systems. The calculated heats of mixing are compared with the experimental ones and the contributions from the different molecular interactions to the heats of mixing are shown and discussed. The same parameters were used for all external conditions and for calculating the effect of temperature on the heats of mixing and the experimental data corroborated the calculations.
We, also, measured the relative permittivities, ɛ, the refractive indices, nD, and the densities, ρ, of the binary of 2-ethoxyethanol with n-hexane at 298.15 K. From these values, the Kirkwood correlation factors, g, over the full composition range were derived. This factor is correlated with our present calorimetric findings and the previous spectroscopic ones [9] and the combined results shed light on the nature of the molecular interactions of 2-ethoxyethanol.
Section snippets
Description of the apparatus
Excess enthalpies were measured with a flow mixing unit consisting of the calorimeter, the mixing cell, and auxiliary devices. A schematic view of the calorimetric system is shown in Fig. 1. The calorimeter is a commercial Setaram (France), model C80, differential heat-flux apparatus based on the Calvet principle. Two identical wells used for inserting the two mixing cells, one empty (the reference) and one for the flow mixing process (the sample) are located in the calorimetric block. In the
Experimental results
The results for the test systems are given in Table 2 and compared with the literature in Fig. 2, Fig. 3. From these diagrams it can be seen that our results agree within ±1% with those obtained by different research groups.
The new experimental data for the four binaries at three temperatures are reported in Table 3, Table 4, Table 5, Table 6 and in Fig. 4, Fig. 5, Fig. 6, Fig. 7. In Fig. 8 one can see the effect on the excess enthalpy of the type of the hydrocarbon (normal, cyclic, or
The non-random hydrogen bonding (NRHB) model
In this section we will briefly present the essentials of an equation-of-state model, which will be used for correlating our experimental data. The details of the model can be found in our previous publications [11], [14], [15]. It is based on the lattice-fluid rationale [11], [17], on the quasi-chemical approach [14], [18], it incorporates a number of molecular features that augment its capacity, and is referred to as NRHB (non-random hydrogen bonding) model [15].
Discussion
The experimental measurements as well as the theoretical calculations indicate that the heats of mixing for the four binary mixtures studied in this work are positive (endothermic) over the full composition range and at the studied temperature range. In addition, the HE values of all studied mixtures increase with increasing temperature. This endothermic process was rather expected and could be explained on the basis of the breaking of intermolecular hydrogen bonds upon mixing of
Conclusions
The main conclusions of this work could be summarized as follows:
- 1.
New experimental data are reported for the heat of mixing of 2-ethoxyethanol with four hydrocarbons at 298.15, 308.15 and 318.15 K, measured with a flow calorimeter, and for the Kirkwood correlation factor, g, of 2-ethoxyethanol + n-hexane at 298.15 K.
- 2.
The hydrogen bonding parameters, obtained previously from the spectroscopic measurements [9] and applied successfully on the VLE data of the 2-ethoxyethanol + CO2 mixture [10], correlate
Acknowledgments
We are thankful to Dr. Vladimir Hynek for his help in the design and installation of the preheater, and to Michael Bridakis and Triantafyllos Tsilipiras for the construction and installation of all home-made parts of our setup. We also thank Dr. Georgios Ritzoulis for his help in the measurements of relative permittivity.
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