Molar heat capacities and electrical conductivities of two ammonium-based deep eutectic solvents and their aqueous solutions
Introduction
The interest in deep eutectic solvents (DESs) as promising green media for a number of chemical and industrial processes has grown in recent years. This is because they have characteristics, which are usually unique to ionic liquids (ILs) including non-volatility, wide liquid range, high conductivity, thermal stability, and high solvation capacity [1], [2]. In addition, they have several advantages over conventional ILs because they are biodegradable, non-toxic, and relatively easy and economical to prepare [3]. DESs can be made by simply mixing a quaternary ammonium salt with a hydrogen bond donor (HBD) to form a eutectic mixture, which has a substantially low melting point [4]. The HBD forms a complex with the simple anion of the salt, leading to a reduction of the lattice energy in the system and depression in the freezing point [5]. Moreover, many of the available ammonium salts (e.g. N,N-diethylethanolammonium chloride) and HBDs (e.g. glycerol, ethylene glycol) are relatively inexpensive; thus, DESs could be manufactured for bulk applications – a key advantage of DESs over conventional ILs [6].
Recent studies demonstrated potential uses of DESs in a wide range of applications. Some DESs have been considered as choice solvents in electrodeposition, electropolishing, and electroplating of metals [7], [8]. They have also been characterized for utilization as drug solubilization vehicles [9], and as solvents for materials processing and synthesis [10], [11], [12]. They are also candidate solvents for enzyme-based biotransformations and biocatalytic processes [13], [14]. Furthermore, DESs have been suggested as excellent and sustainable media for efficient biodiesel purification [15], [16], [17], and have also been investigated as potential absorbents for carbon dioxide capture [18], [19], [20]. For such applications, process design and engineering would require data on thermophysical properties of these solvents. However, fundamental physical, thermal, and chemical properties of DESs including heat capacity and electrical conductivity are rather scarce in the current literature. In fact, for the studied DESs, reported data are limited only to freezing point and density at 298.15–368.15 K [21], [22]. Thus, we aimed to contribute to the database of properties of these solvents, which would be useful for future applications.
In this work, the molar heat capacities, CP, and electrical conductivities of the DESs N,N-diethylethanolammonium chloride-glycerol and N,N-diethylethanolammonium chloride-ethylene glycol and their aqueous solutions were determined. Molar heat capacity measurement was done at atmospheric pressure and temperatures from 303.15 to 353.15 K, while electrical conductivity was measured from 298.15 to 343.15 K. The aqueous solutions were in the composition range 0.1–0.9 mole fraction. The temperature dependence of the CP of the DESs was represented by a first-order empirical equation. For the aqueous solutions, the excess molar heat capacities, CPE, were determined, and the data were correlated with temperature and composition using a Redlich–Kister-type equation. The dependence of κ on temperature and composition was represented by an Arrhenius-type equation.
Section snippets
Preparation of DES
The DES was prepared according to the method described by Abbott et al. [1]. The quaternary ammonium salt N,N-diethylethanolammonium chloride was mixed with the HBD (glycerol or ethylene glycol) at 1:2 mole ratio, and heated at 343 K with continuous stirring until a homogeneous colorless liquid was observed. The sources and purities of the chemicals used in the preparation of the DESs are given in Table 1. The prepared DES was then dried under vacuum at 343 K for 48 h, and kept in a dry box prior
Molar heat capacities
The molar heat capacities of the DESs N,N-diethylethanolammonium chloride-glycerol and N,N-diethylethanolammonium chloride-ethylene glycol at temperatures from 303.15 to 353.15 K are presented in Table 3, and also shown as points in Fig. 1. It can be observed that the values of the CP of the DESs increase with increasing temperature, which is expected due to the availability of more degrees of freedom at higher temperature. It can also be noted that at the same temperature, the CP of N,N
Conclusions
Molar heat capacities of the deep eutectic solvents N,N-diethylethanolammonium chloride-glycerol and N,N-diethylethanolammonium chloride-ethylene glycol and their aqueous solutions at temperatures 303.15–353.15 K and mole fractions 0.1–0.9 were reported. The molar heat capacities of the DESs varied linearly with temperature. The molar heat capacities of the aqueous DES solutions increased with increasing temperature and DES mole fraction in the solution. Positive excess molar heat capacities
Acknowledgements
This research was supported by a grant, NSC 102-3113-P-007-007, from the National Science Council of the Republic of China.
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