Henry’s constant of carbon dioxide-aqueous deep eutectic solvent (choline chloride/ethylene glycol, choline chloride/glycerol, choline chloride/malonic acid) systems
Introduction
A deep eutectic solvent (DES) is an ionic solvent that can be prepared by forming a complex between an ammonium salt and a hydrogen-bond donor (HBD), the product (DES) being one with melting point that is much lower than the melting point of the ammonium salt and HDB precursors [1]. In addition, DESs have low vapour pressure and low flammability, non-toxic, non-reactive with water, and biodegradable. It is for these reasons that DESs have great potential to be environment-friendly. Consequently, DESs have gained attention because, as recent studies have claimed, such solvents can be viable and low-cost alternatives to room-temperature ionic liquids in a variety of possible applications such as in biocatalysis [2], [3], [4], pharmaceuticals [5], electrochemistry [6], [7], biodiesel purification [8], [9], [10], [11], carbon dioxide absorption [12], [13], and other chemical and industrial processes. For any future application of DESs, the solvent’s physical properties will be indispensable in the design of the corresponding processes. In the literature, so far, only the following properties are available, and they are scant: density [14], [15], [16], [17], [18], surface tension [19], molar heat capacity [5], [20], conductivity [1], [21], [22], and vapour pressure [23]. To date, there are no reported data on Henry’s constant for the system CO2-aqueous DES. Such data may be important particularly when the application for which a process design is to be made is absorption of carbon dioxide using DES as solvent.
In this study, we present Henry’s constant, a thermophysical property that is related to gas solubility in liquid, for the system CO2-aqueous DES. The DESs considered are those that have been prepared from choline chloride (as ammonium salt) and HBDs: ethylene glycol (ethaline), glycerol (glyceline), or malonic acid (maline). Since density data are required in the estimation of the Henry’s constant, densities of aqueous maline systems in the high concentration range, which has not yet been reported in the literature, were also measured and presented. Values of the Henry’s constant were then correlated with temperature and composition using a differential Henry’s coefficient model.
Section snippets
Chemicals
All DESs used were commercially available from Scionix Ltd. (United Kingdom): ethaline (1 mole choline chloride: 2 moles ethylene glycol), glyceline (1 mole choline chloride: 2 moles glycerol), and maline (1 mole choline chloride: 1 mole malonic acid). Each of these DESs has purity >98 wt%, and were used without further purification.
High purity distilled water, which was deionised (resistivity = 18.3 MΩ · cm) using a Barnstead Thermolyne water purification module (model EASYpure LF) was used in the
Density
The density of maline and its aqueous solutions at (50 to 80) wt% DES were measured at temperatures from (303.15 to 323.15) K, and the values are presented in table 1. These data would serve as additions to those we recently reported for aqueous maline solutions at (10 to 40) wt% DES [28]. We likewise present figure 1 to describe the dependence of the density on DES concentration and temperature. The density decreased with temperature, and increased with DES concentration, as would be expected.
Conclusions
The Henry’s constant of carbon dioxide in aqueous solutions of the deep eutectic solvents ethaline, glyceline, and maline were measured at temperatures (303.15, 308.15, and 313.15) K and concentrations from (20 to 80) wt% DES. Additional measurements for the densities of aqueous maline solutions, which were used in the estimation of Henry’s constants, were also presented. Results show that the Henry’s constants increase with temperature, but decrease as the concentration of the DES in the
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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