Densities of aqueous mixtures of (choline chloride + ethylene glycol) and (choline chloride + malonic acid) deep eutectic solvents in temperature range 283.15–363.15 K
Introduction
With the introduction of green chemistry in early 1990s, room temperature ionic liquids (RTILs) have emerged as ‘green solvents’ owing to their advantageous physical and chemical properties, e.g., extremely low vapor pressure, high thermal stability, high solvation capacity, high thermal conductivity, and wide liquid range [1], [2], [3], [4]. However, the main disadvantages to their use are their high cost and issues such as non-biodegradability and toxicity [5], [6], [7]. As greener alternative to RTILs, a new family of solvents, known as deep eutectic solvents (DESs) has emerged as novel media in chemistry. DES is a fluid generally composed of two or three cheap components that are capable of self-association, often through H-bonding interactions, to form a eutectic mixture with much lower melting point than either of the individual components [8], [9]. DESs possess similar physicochemical properties to the traditionally used RTILs, while possessing several advantages over the later. Many of the reported DESs are made from biodegradable components and are non-toxic. So, they are considered as better substitute to conventional volatile organic solvents and RTILs. DESs find major applications in the field of organic synthesis [10], electrochemistry [11], [12], [13], nanomaterial [14], biochemistry [15], [16] and other chemical and industrial processes. A DES can be prepared by forming a complex between a H-bond acceptor (HBA) such as an ammonium salt and a H-bond donor (HBD) such as an acid, alcohol or an amide. DESs are easier to prepare with high purity at a relatively cheaper cost. The first DES was investigated by Abbott et al. in 2003 containing choline chloride and urea in 1:2 mole ratio [17].
Unlike certain PF6– and (CF3SO2)2N– RTILs, among others, the most popular class of DESs constituted of choline chloride, exhibit complete water miscibility. Subsequently, water, an obvious choice as environmentally-benign substance, can be used as a cosolvent to effectively and favorably modify properties of a choline chloride DES. In this paper, we have chosen two common and popular DESs, named ethaline and maline, that are composed of (choline chloride + ethylene glycol) and (choline chloride + malonic acid), respectively, combined in the mole ratio 1:2. Their melting points are significantly lower than the room temperature (ethaline: Tf = 213.15 K and maline: Tf = 283.15 K) [18]. Both the DESs are non-flammable and non-toxic and exhibit complete water miscibility. To assess and design the possible industrial-scale applications of DESs, it is essential to know the thermophysical properties of their aqueous mixtures. Among the key physicochemical properties, density has enormous importance as far as various industrial applications are concerned.
Densities of aqueous mixtures of ethaline have been measured by other researchers. In a recent study, Leron et al. reported the densities of (choline chloride + ethylene glycol) and their aqueous mixtures; however, the temperature range (298.15–333.15 K) of this report was significantly narrower [19]. In another study, the same group reported high pressure volumetric properties of choline chloride–ethylene glycol based deep eutectic solvent and its mixtures with water covering the temperature range of 298.15–323.15 K and pressure range of 0.1–50 mPa [20]. The temperature range in this study again is rather restricted. In a recent contribution towards temperature-dependent density measurement of ammonium- and phosphonium-based DESs was from Shahbaz et al. in which the densities were predicted using artificial intelligence and group contribution techniques [21]. In another study, Shahbaz et al. reported predicted densities of ethaline using atomic contributions at 298.15 K [22]. The same group also reported the predicted densities of ionic liquids analogues by using Eotvos and Guggenheim empirical rules in the temperature range 298.15–328.15 K [23]. Siongco et al. reported the densities, viscosities, and refractive indices of (N,N-diethylethanol ammonium chloride + glycerol) and (N,N-diethylethanol ammonium chloride + ethylene glycol) and their aqueous mixtures from 298.15 to 343.15 K [24]. In a recent study, Hsieh et al. reported the diffusivity, density and viscosity of aqueous solutions of choline chloride/ethylene glycol and choline chloride/malonic acid; however, the temperature range (303.15–323.15 K) of this report was significantly narrower [25]. Lin et al. also reported Henry’s constant of carbon dioxide-aqueous DES (choline chloride/ethylene glycol, choline chloride/malonic acid) systems at the pressure 101.00 kPa in the temperatures range 303.15–323.15 K [26]. The temperature ranges of this report is also significantly narrower. We present densities of ethaline, maline and their aqueous mixtures (over the entire composition range) and covering the temperature range from 283.15 to 363.15 K.
Section snippets
Materials
Ethaline 197 (mol wt. 88.03 g/mol) and maline 200 (mol wt. 115.80 g/mol), a mixture of (choline chloride + ethylene glycol) and (choline chloride + malonic acid) in 1:2 mole ratio, respectively, were purchased from Scionix Ltd., and used as received. Alternatively, ethaline and maline were also prepared by mixing choline chloride (≥99%, from Sigma–Aldrich) with ethylene glycol (≥99.8%, from Sigma–Aldrich) and choline chloride (≥99%, from Sigma–Aldrich) with malonic acid (≥99%, from Sigma–Aldrich) in
Temperature dependence of densities of ethaline and maline and their aqueous mixtures
Experimentally measured densities of ethaline and maline, respectively, and their water mixtures as a function of temperature in the range 283.15–363.15 K over the entire composition range are reported in Table 3, Table 4, respectively. As expected, densities of ethaline, maline, water, and aqueous mixtures of DESs, respectively, are found to decrease with increase in temperature. Thermal expansion usually results in decreased density of a substance as the temperature is increased. The
Conclusions
Densities of aqueous mixtures of DESs ethaline and maline and their temperature and composition dependence in the temperature range 283.15–363.15 K reveal interesting information on interactions present within these systems. Variation of density with temperature for these systems is found to be quadratic in nature. Negative and significant values of excess molar volume at all temperature and composition hint at the presence of stronger interactions between water and ethaline/maline as compared
Acknowledgements
This work is generously supported by the Department of Science and Technology (DST), Government of India through a grant to SP [grant number SB/S1/PC-80/2012]. AY would like to thank CSIR, Government of India for her fellowship. MV and SN would like to thank Summer Faculty Research Fellow Programme 2014 at IIT Delhi.
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