Elsevier

Fluid Phase Equilibria

Volume 262, Issues 1–2, 15 December 2007, Pages 121-136
Fluid Phase Equilibria

Temperature dependence of limiting activity coefficients and Henry’s law constants of cyclic and open-chain ethers in water

https://doi.org/10.1016/j.fluid.2007.08.013Get rights and content

Abstract

Limiting activity coefficients (γ1) of seven selected ethers (diethyl ether, tetrahydrofuran, tetrahydropyran, dimethoxymethane, 1,2-dimethoxyethane, 1,3-dioxolane, and 1,4-dioxane) in water were measured at several temperatures in the range from 273 to 373 K. Six experimental techniques were employed for the purpose. A comprehensive review is presented of experimental data on the limiting activity coefficients, infinite dilution partial molar excess enthalpies and heat capacities of these aqueous solutes. For each ether, the compiled data were critically evaluated and together with the data measured in this work correlated with a suitable model equation providing adequate simultaneous description of the equilibrium measurements and the calorimetric information. As a result, a recommended thermodynamically consistent temperature dependence of γ1 of superior accuracy was established in the range from the melting point to the normal boiling point of water. Analogous recommendations were derived also for the temperature dependence of their Henry’s law constants (KH). Variations of the infinite dilution thermodynamic properties of aqueous ethers with temperature and ether molecular structure are subsequently discussed. The performance of three group contribution approaches, namely the modified UNIFAC, and the methods of Cabani et al. and Plyasunov et al., to predict γ1 for the aqueous ethers was tested and found quite unsatisfactory.

Introduction

Ethers are nonassociated amphiphilic oxygenated compounds of moderate polarity. These substances are typically volatile liquids with good chemical stability, low viscosity and strong solvency exhibiting a substantial compatibility with water and a number of organic solvents. Such properties make ethers valuable process media and solvents in many industries. The proton accepting ability of ethers makes them also efficient solvent stabilizers and their favorable combustion characteristics promote their massive and still increasing use as automotive fuel additives. Many ethers are large volume production chemicals which, due to their high vapor pressures and aqueous solubilities, are released in significant amounts to the environment. The environmental contamination by ethers, in particular of surface and ground water, is therefore growing concern. The adverse impact of ether contamination ranges from bad taste characteristics of drinking water to acute toxic and possibly carcinogen effects [1]. To model and predict phase and chemical equilibria, transport effects and other phenomena involved in environmental processes, remediation of contaminated sites, and industrial separations, the thermodynamic properties of highly dilute aqueous solutions of ethers, such as ether limiting activity coefficients (γ1) or Henry’s law constants in water (KH), are of essential importance. Accurate knowledge of the thermodynamic quantities of the dissolution and hydration of ethers and their variation with temperature is of extreme interest also for theoretical reasons, in particular for understanding the hydrophobic effect.

Recently, we have presented several detailed studies on the temperature dependence of limiting activity coefficients and Henry’s law constants of various oxygen- or nitrogen-containing aqueous solutes of semihydrophobic character [2], [3], our systematic effort being especially devoted to alkanols [4], [5], [6]. In this work, we extend the scope of this systematic investigation to ethers. Literature data on γ1(T) or KH(T) for these aqueous solutes are often fragmentory or insufficiently accurate, which calls for experimental reexamination [7]. Seven environmentally important and large volume production ethers were selected for the present study, namely: diethyl ether (DEE), tetrahydrofuran (THF), tetrahydropyran (THP), dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), 1,3-dioxolane (DXL), and 1,4-dioxane (DOX). As indicated in Fig. 1, the involvement of these various cyclic and open-chain monoethers and diethers in this selection makes possible various molecular structure effects on thermodynamic behavior of aqueous ethers to be captured. For the selected aqueous ethers we report here results of our accurate measurements of limiting activity coefficients which were performed by several suitable experimental techniques as a function of temperature. The present experimental work is further amended by a comprehensive compilation and critical evaluation of literature experimental data on limiting activity coefficient and related thermal dissolution properties-limiting partial molar excess enthalpy, H¯1E,, and heat capacity, C¯P,1E,. All the data, measured in this work and taken from literature, are subsequently processed by a simultaneous thermodynamically consistent correlation. The treatment results in a recommended temperature dependence of these infinite dilution properties which has superior accuracy and is valid in the range from the melting to the normal boiling temperature of water. Analogous recommendations are further generated for the temperature dependence of the Henry’s law constants, hydration enthalpies, and heat capacities. Finally, variation of γ1 and KH with temperature and ether molecular structure is discussed and their prediction tested using three group contribution schemes.

Section snippets

Materials

The solutes studied in this work were analar grade commercial chemicals used without further purification: diethyl ether (Penta, p.a. 99.7%); dimethoxymethane (Aldrich, 99.9%); tetrahydrofuran (Aldrich, 99.9%); tetrahydropyran (Fluka, 99%); 1,2-dimethoxyethane (Fluka, >99.5%); 1,3-dioxolane (Aldrich, 99%); 1,4-dioxane (Aldrich, 99.8%). Water used as the solvent was distilled and subsequently treated by Milli-Q Water Purification System (Millipore, USA).

Apparatus and procedure

To measure the limiting activity

Results of measurements

Primary measurements by the various techniques were processed to obtain values of limiting activity coefficients. The calculations have been described in detail previously and respective relations can be found in our papers cited above. Due attention was paid to the selection of sufficiently accurate solute vapor pressure data in order to minimize the resulting uncertainty of γ1values. The selected vapor pressure data are given in Table 1 as parameters of the Wagner equation or the Antoine

Recommended γ1(T) data

To establish a truly reliable temperature dependence of the limiting activity coefficients, the newly measured values for each aqueous ether studied were supplemented by additional critically evaluated data on γ1 and related thermal properties H¯1E, and C¯P,1E, compiled from the literature and all these data were then correlated simultaneously by a suitable fitting equation. Comprehensive databases gathered on respective properties and used in this treatment are given in Appendix A, Appendix

Data and correlation assessment

The treatment described in Section 4.1 revealed that most γ1 data collected in Appendix A agree within a reasonable scatter. Some data, however, deviate grossly (>0.2inlnγ1) from the fits. According to the evaluation policy we adopted, such data were not strictly rejected, but rather labelled with a larger uncertainty, which reduced appropriately their statistical weight in the treatment. The grossly deviating points, which are indisputably subject to large errors, are encountered especially

Conclusion

Thanks to our new systematic air-water partitioning measurements combined with corresponding calorimetric information from the literature, recommended thermodynamically consistent temperature dependences could be established for γ1 and KH of seven environmentally important ethers in water. These recommendations of superior accuracy substantially improve our knowledge of thermodynamic behavior of dilute aqueous ethers and meet respective data needs for environmental and industrial applications.

Acknowledgments

We thank to Professor E.M. Woolley from Brigham Young University, Provo, USA for making available to us C¯P,1 values measured in his laboratory before publication. We also thank to our colleagues from ICT Prague, Dr. M. Bureš for performing the quantum mechanics computations and Dr. P. Vrbka for helpful advices with the measurements. Financial support to this project from the Ministry of Education of the Czech Republic (grant MSM 604 613 7307) is gratefully acknowledged.

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