Ionic liquids (ILs), salts liquid below a generally agreed 100 °C limit, are important in many research fields, such as chemical synthesis, cost efficiency of production, reduction of waste, and toxic reagents. ILs are known for their attractive properties such as high chemical and thermal stability, negligible vapor pressure and interesting electrochemical as well as solvating properties. These properties can be tailored by variation of the composition of cations and anions [1
]. Even more variability is offered by using mixtures of ILs, an option that is considered the fourth evolution of ILs [2
]. In this contribution, we deal with mixtures of choline carboxylate ILs. We anticipate that these mixtures will widen the application range of this class of ILs. We see potential applications in the medical [3
] and environmental fields [1
] where these systems offer a combination of low toxicity, adjustable solvation properties, and tunable ionic conductivity [2
One of the most practically relevant properties of IL mixtures is their low melting point. Given the limited thermal stability of organic ions, a low melting point leads to a wide liquidus range in the resulting ionic system. A viable alternative for low melting ILs and IL mixtures are deep eutectic solvents (DESs) [4
], which are defined as eutectic mixtures formed from Lewis or Brønsted acids and bases containing ionic species [5
] whose real eutectic point is lower than their ideal eutectic point [6
]. Advantages of DESs compared to pure ILs include easy preparation and therefore low production costs, increased biocompatibility in case of certain components, and bigger variety for customization concerning acidity, hydrophobicity, or polarity. DES systems have demonstrated their practical use in several applications, such as interfacial polymerizations [7
], electrochemistry [8
], and organic reactions [9
So far, the published research describing IL mixtures has mainly focused on imidazolium [10
], pyrrolidinium [12
], and pyridinium [13
] ILs. In these works, significant melting point suppression effects were found for the respective eutectic mixtures. However, imidazolium, pyrrolidinium, and pyridinium ILs and their mixtures face some limitations when it comes to large-scale applications in contact with microorganisms or living matter [14
]. Challenges include toxicity aspects and the cost for the organic cation synthesis. This is why a more detailed investigation of choline salts is highly interesting. Based on the available literature, choline salts are characterized by their low toxicity for different cell-lines [15
], bacteria [17
], and other organisms [18
]. Moreover, choline salts are industrially available on a large scale.
The most widely studied choline salt used in eutectic mixtures is choline chloride [19
]. One of the first reported DESs was a mixture of choline chloride (liquefaction temperature 302 °C), and urea (melting point 134 °C) that resulted in a 2:1 molar mixture with a remarkable melting point suppression to 12 °C [4
]. The typical strategy was to combine choline ILs with molecular compounds such as carboxylic acids, alcohols, and urea derivates [27
], and such choline-based DESs have been used in various applications, e.g., as drug solubilization vehicles [28
]. Note, however, that by mixing a choline IL with molecular substances, some of the very attractive IL properties, e.g., non-volatility, non-flammability, and high ionic conductivity can get lost to a large extent. This is why the mixing of two choline ILs is a very attractive, yet widely unexplored, alternative.
In the present study, we focus on IL mixtures based on choline carboxylates with a different alkyl chain length at the cation, and various anions. The combination of the choline cation with anions derived from carboxylic acids leads to biologically benign ILs, which have been used e.g., in separation processes [31
] and microemulsions with low toxicity [32
]. As temperature influences the properties of IL mixtures [21
], tailoring the melting point of mixtures opens interesting options for performance optimization. Thus, knowledge of the phase behavior of these IL mixtures is needed to leverage the full application potential of the approach. The purpose of our study is to identify eutectic mixtures of choline carboxylates salts with melting points close to room temperature and high thermal stability. We use differential scanning calorimetry (DSC) to determine the eutectic composition of the mixtures under investigation together with the thermophysical properties of the individual components.
The phase behavior of binary mixtures of choline carboxylate ILs has been investigated. Eight phase diagrams have been reported from mixtures of six different choline carboxylates. Five of the obtained phase diagrams presented eutectic points with melting point suppression between 13 and 45 °C. Anions with different levels of bulkiness like, for example, in the mixture of [Ch][Ac] and [Ch][2mb], led to a higher decrease in the melting point at the eutectic composition compared to anions of similar size, like in [Ch][Iv] and [Ch][2mb]. More drastic changes in the anion structure, such as the addition of carboxylate or hydroxyl group in the case of [Ch][Mal] and [Ch][Lac] resulted in mixtures without eutectic behavior.
From the thermogravimetric analysis, we conclude that the tested choline ILs can be applied at temperatures below 100 °C under inert as well as under oxygen-containing atmospheres. The thermal stability measurements with a steep heating rate of 10 °C·min−1 gave decomposition temperatures between 129 and 206 °C with a clear dependence on the IL anion. Choline lactate was found to be the most stable salt among the tested choline salts.
The present study adds useful data for the wider applications of choline salts and their mixtures. As many choline salts are solid at ambient temperature, their usage as low-toxicity alternatives for conventional ILs has been limited so far. Our study proves that mixing different choline salts can lead to significantly lower melting points while fully maintaining the typical IL properties of extremely low vapor pressure and high ionic conductivity. Excellent availability and low toxicity of both the choline cation and the short-chain carboxylate anions make the here-described choline salt mixtures interesting candidates for solvent applications in the food and pharmaceutical industry.