Solvatochromic parameters of deep eutectic solvents formed by ammonium-based salts and carboxylic acids

Abstract

Deep eutectic solvents (DES) have been studied in a wide range of applications, and despite their potential as sustainable solvents, detailed knowledge on their solvatochromic parameters is still lacking. To overcome this problem, in this work, the Kamlet Taft (KT) solvatochromic parameters, namely the hydrogen-bond acidity, hydrogen-bond basicity and dipolarity/polarizability, of a wide range of DES composed of ammonium-based salts as hydrogen bond acceptors (HBAs), and carboxylic acids as hydrogen bond donors (HBDs), were determined aiming at better understanding the influence of the chemical structure of the DES components on their polarity. It is shown that the high acidity of the DES investigated is mainly provided by the organic acid present in the mixture, and that an increase of the alkyl side chain of both the HBA and the HBD species leads to a lower ability of the solvent to donate protons. On the other hand, the ammonium salt plays the major role on the hydrogen-bond basicity of DES. Contrarily to the hydrogen-bond acidity, an increase in the length of the aliphatic moieties of both the carboxylic acid and salt cation results in solvents with higher ability to accept protons. The dipolarity/polarizability of DES is mainly defined by the ionic species present, and tend to decrease with the increase of the aliphatic moiety of the organic acid. In general, DES composed of ammonium-based salts and carboxylic acids present a higher capacity to donate and accept protons when compared to most of the ionic liquids or organic molecular solvents.

Design and Characterization of Sugar-Based Deep Eutectic Solvents Using Conductor-like Screening Model for Real Solvents

“…The Kamlet-Taft parameter α describes the hydrogen bond acceptor capacity or the solvent acidity. The values obtained are in general higher than those obtained for tetraalkylammonium-based DES 36 and, as in the other parameters, similar to those obtained for [Ch]Cl-based DES. 35 All mixtures studied present α's higher than 1 and thus, higher than most of the organic solvents used nowadays ( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 wt%.…”

“…When comparing the β values obtained in this work with other [Ch]Cl-based DES, 35 similar 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 22 values are obtained, while tetraalkylammonium-based DES 35,36 present higher values of β.…”

A closer look into deep eutectic solvents: exploring intermolecular interactions using solvatochromic probes

“…18 Nevertheless, polarity scales have been shown to be very useful to understand a wide range of phenomena such as rate constants, equilibrium constants, solubilities and spectral frequencies, based upon data acquired for a few solvents. 19,20,21 Only four studies [21][22][23][24] on the characterization of DESs polarities have been reported. The first study of determined the solvatochromic parameters for the most common and popular DESs, namely cholinium chloride: urea (1:2), cholinium chloride: glycerol (1:2) and cholinium chloride: ethylene glycol (1:2).…”

“…It was also shown that addition of water to DESs resulted in increased dipolarity/polarizability and a decrease in H-bond accepting basicity. Very recently, Teles et al 24 published a more detailed study of solvatochromic parameters of DES formed by ammonium-based salts and carboxylic acids. It was showed that the high acidity of the studied DESs was mainly due to the organic acid present in the mixture, and that an increase of the alkyl side chain of both the HBA and the HBD species leads to a lower ability of the solvent to donate protons.…”

Systematic evaluation of hydrophobic deep-melting eutectics as alternative solvents for the extraction of organic solutes from aqueous solution

“…28 These favorable characteristics have led to a range of applications in a variety of elds, including synthesis, 29,30 catalysis, 31 and chemical analysis, 32 among others. While the vast majority of deep eutectic solvents described to date are hydrophilic, 33,34 recent work has shown that a hydrophobic DES (designated hereaer as an HDES) can be readily obtained by combining menthol [35][36][37][38] or an appropriate quaternary ammonium halide [39][40][41] with a longchain carboxylic acid. The resultant liquids can then be employed, for example, as the basis of biphasic systems for the extraction of metal ions.…”