Chemical and electrochemical properties of a hydrophobic deep eutectic solvent
Introduction
A Deep Eutectic Solvent (DES) is a eutectic mixture formed by a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA) [1,2]. The interactions between these components lower the melting point of the mixture with respect to those of the individual components, leading to a liquid phase at room temperature. The odd nature of the hydrogen bond formed in similar compounds is discussed in ref. 1.
DESs are easy to prepare and offer several advantages [1,2], such as capability to solubilise different organic and inorganic species, high biodegradability, low toxicity and low cost. These features make them suitable for large-scale production, being effective alternatives to other solvents possessing some similar properties, such as ionic liquids.
Among the different types of DESs, the hydrophobic ones have emerged in recent years as promising alternatives to toxic and flammable organic solvents. However, only a few examples of such DESs have been reported so far (see ref. 3–10 and Tables S1 and S2 in Supplementary Materials). Hydrophobic DESs are based on poorly water-soluble components, such as tetraalkylammonium salts [3], long chain carboxylic acids [4], menthol [5] and lidocaine [6]. The hydrophobic nature has been exploited mostly to extract species from biological materials or aqueous media, e.g., artemisin and polyprenyl acetates from Artemisia annua [7] and Ginkgo biloba leaves [4], synthetic pigments from beverages [8], as well as volatile fatty acids [3], pesticides [9], and few alkali and transition metal ions, e.g. In3+ [10] and Co2+ [6], from aqueous solutions.
In the present paper, a prototypical hydrophobic DES, namely a 1:2 M mixture of tetrabutylammonium chloride (TBACl) and decanoic acid (DA), has been prepared and its relevant physico-chemical properties characterised. Although the role of water content is largely underrated for both hydrophobic and hydrophilic DESs, we could demonstrate that it induces dramatic changes in DESs' properties.
For the first time for hydrophobic DESs, the electrochemical behaviour has been studied. We demonstrate here that the addition of small water quantities to the hydrophobic DES dramatically improves its electrical conductivity, without the addition of any salt. Thanks to this property, DESs constitute particularly appealing green systems in electrochemistry, e.g. in sensing, electroremediation, and electrodeposition. In view of the possibilities in similar applications, we explored the electrochemical behaviour of ferrocene, a benchmark redox probe, and of Cr(VI) species, extracted from aqueous solution, in this solvent. Cr(VI) species have been selected in view of their importance in various industrial processes and their impact on the environment [11,12].
Section snippets
DES preparation
DESs with different water contents (DES1-4) were prepared using TBACl (≥98%, TCI) and DA (≥98%, Sigma Aldrich) in the 1:2 M ratio [3]. A mixture of the components was heated in a water bath at 65 °C for ca. 3 h leading to the liquefaction of the two solids, and the formation of a liquid, homogeneous phase. Then, the mixture was left cooling to room temperature.
The driest DES (DES1 in Table 1) was prepared in an Ar atmosphere (Glovebox mBraun UNIlab Pro), where water and oxygen levels were
Electrochemical behaviour of hydrophobic DESs
In order to prepare DESs containing different amounts of water, four procedures have been adopted, as detailed in Section 2.1. Electrical conductivity and dynamic viscosity of the hydrophobic DES under investigation, at different water contents, are reported in Table 1. The electrical conductivity of the solvents with water content below 2% w/w was found to be too low and their viscosity too high for a medium to be used in electrochemistry. Only DES4, with a water content of 2.4% w/w, displayed
Conclusions
New insights on the physico-chemical properties of a prototypical hydrophobic DES, based on TBACl and DA, are reported. In particular, the viscosity and electrical conductivity are strongly dependent on the water content of the DES. The electrochemical behaviour of a hydrophobic DES is investigated for the first time and the DES is employed to extract Cr(VI) species from an aqueous phase. The extracted metal species are electroactive in DES, opening the possibility to develop in-line
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