Elsevier

Fluid Phase Equilibria

Volume 448, 25 September 2017, Pages 99-104
Fluid Phase Equilibria

Thermodynamics of phase transfer for polar molecules from alkanes to deep eutectic solvents

https://doi.org/10.1016/j.fluid.2017.05.008Get rights and content

Abstract

Deep eutectic solvents (DESs) have been used for the purification of oils and the extraction of active ingredients from natural products but little is known about the mechanism of the extraction process. In this study a variety of molecular solutes are dissolved in alkanes and the thermodynamics of transfer into six DESs have been quantified. It is shown that the transfer of most solutes into the DES is endothermic and driven by entropy. The largest partition coefficients were demonstrated by the liquids with the lowest surface tensions and this is thought to arise because the enthalpy of hole formation controls the rate of solute transfer. Accordingly, it was shown that the size of the solute has an effect on the partition coefficient with smaller solutes partitioning preferably into the DES. As expected, solutes capable of strongly hydrogen bonding partitioned much better into the DES as the enthalpy of transfer was negative.

Introduction

Deep Eutectic Solvents, DESs, are mixtures of Lewis and Brønsted acids and bases which produce low melting point systems due to complex formation [1], [2]. The eutectic formation is endothermic and the process is driven by the large entropy change of going from two ordered solids to a more disordered liquid.

DESs have been primarily used for a wide variety of metal processing applications such as electrolytic, immersion, electroless deposition and electropolishing. This is due to their high solubility of metal salts, conductive nature and relatively wide electrochemical windows. They have however also been shown to be suitable solvents for a number of other processes such as liquid-liquid extraction, synthesis and gas capture. A review by Smith et al. [3]. has covered this extensively.

One area where DESs are receiving the most interest is in their use as specific extractants. The first reported application was in the purification of biodiesel to extract excess glycerol and water from the reaction mixture. A Lewis basic mixture (1:1) of choline chloride and glycerol were used to complex polar species from crude biodiesel [4].

A variety of studies have investigated the extraction of sulfur containing compounds from alkanes using ionic liquids and deep eutectic solvents. This has been proposed as a methodology to enable desulfurization of oil [5], [6], [7], [8], [9], [10], [11], [12]. Despite numerous groups studying the partition coefficients of thiophenic compounds no calculations have been made for the thermodynamics of phase transfer. It has also been used for the extraction of phenolic compounds from olive oil [13]. The use of ionic liquids and DESs for natural product extraction has also recently been reviewed [14], [15], [16]. Ionic liquids and DESs have both been shown to be candidates for extraction of a wide variety of natural product types, such as phenolic compounds, anthraquinones, flavonoids, alkaloids, essential oils, carbohydrates and lignin. They can be tailored towards specific extraction tasks by making slight variations to the anions, cations and hydrogen bond donors (HBDs). Ionic liquid and DES extraction processes have also been used in conjunction with microwave and ultrasound techniques to enhance extraction rates and some exhibit enhanced extraction efficiency compared to some organic solvents [14], [16]. Efficient recovery of the natural products from ionic liquids and DESs has been carried out using supercritical fluid, anti-solvent precipitation, recrystallization and back extraction techniques.

In the current study the thermodynamics for the extraction of a range of compounds from alkanes is studied. Initially 3 thiophenic compounds are extracted from decane with six DESs. This study is then extended to investigate the extraction of iodine followed by a variety of alkanes with different functional groups to determine the importance of hydrogen bonding in enabling selective extraction.

Section snippets

Experimental

All materials and reagents employed in this work were used as received and their sources and purities are listed in Table 1:

The deep eutectic solvents were prepared using similar approaches described by literature methods [1], [2], [3], [4], [5]. The hydrogen bond donor (HBD) and salt are mixed with respect to the specified HBD: salt molar ratio at a particular mixing speed and temperature until a clear homogeneous liquid phase was produced. Choline chloride (ChCl): ethylene glycol (EG) was

Extraction of organosulfur species by type III DESs

DESs have been studied for the extraction of sulfur containing compounds from mineral oil. Most sulfur in oil is in the form of thiophene (Th), benzothiophene (BT) and dibenzothiophene (DBT). All of these compounds are polarisable and should be good hydrogen bond acceptors through the sulfur moiety on the heterocyclic ring. The thermodynamics of Th partition can be determined by measuring the partition coefficient of the compound between the DES and oil phase as a function of temperature.

These

Conclusion

The extraction of a range of solutes from alkanes to a variety of DESs was studied. It was found that in general the largest partition coefficients were obtained for the liquids for the lowest surface tensions as these will be the easiest to form a cavity in. From this it was concluded that the partition of solutes is governed by hole formation. This tends to make the transfer of solutes to DESs endothermic which means that it is the entropy change which enables transfer presumably because the

Acknowledgements

The authors would like to thank the Higher Committee for Education Development in Iraq for studentships for (JHK) and (IBQ), the Saudi Arabian Cultural Bureau (OAOA) and Iraqi Ministry of Higher Education and Scientific Research (AYMA). They also thank the University of Kufa, University of Ha'il, Erbil Polytechnic University and Hawler Medical University for allowing study leave to carry out this research.

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