Design of Improved Deep Eutectic Solvents Using Hole Theory

Abstract

The development of ambient temperature ionic fluids has recently received intense interest due to their potential as environmentally benign solvents and their unusual solvent properties. [1][2][3] Studies have focused primarily on quaternary ammonium salts (QAS) with special emphasis on imidazolium cations. Decreased viscosities and increased conductivities have been achieved using fluorinated anions such as PF 6 À and (CF 3 SO 2 ) 2 N À , which are able to effectively shield the charge from the cation.[1] An alternative approach to the formation of ionic fluids is to start with a quaternary ammonium halide and shield the charge on the anion by complexing it with hydrogen-bond donors (HBD) such as carboxylic acids or amides. [4,5] These liquids have physical and solvent properties that are similar to ionic liquids formed with discrete ions. They are, however, easy to produce by simply mixing common commodity chemicals such as choline chloride and urea. To differentiate these liquids from other ionic liquids, the term deep eutectic solvents (DESs) has been coined as the mixtures are effectively eutectics formed between the two components, but unlike most molecular eutectic mixtures, the interaction between the ammonium salt and the hydrogen-bond donor results in a very large depression of freezing point which can be in the region of 200 8C. DESs are already used in the large-scale application for the electrowinning of metals from complex oxide matrices and the electropolishing of stainless steel due to their unexpected solvent properties which enable them to dissolve metal oxides. [6,7] Like other ionic liquids, one issue that is associated with the use of DESs is their viscosity compared to molecular solvents. This has a concomitant effect upon the conductivity, which is important for electrochemical applications such as those described above.The cause of the relatively high viscosity in ambient temperature ionic liquids is that the solvent species (ions) have relatively large radii ( % 3-4 ) compared to the average radius of the voids ( % 2 ). It has recently been shown that the viscosity of a fluid is related to the free volume and the probability of finding holes of suitable dimensions for the solvent molecules/ ions to move into.[8] The free volume of a liquid can be increased by decreasing the surface tension and the conductivity of an ionic fluid can be increased by using smaller ions. [8,9] In the current investigation we show that less viscous liquids with higher conductivities can be obtained using small quaternary ammonium cations such as ethylammonium and fluorinated hydrogen-bond donors such as trifluoroacetamide. Figure 1 shows the temperature at which the first solid starts to form on cooling the EtNH 3 Cl/amide mixtures as a function of composition. Most of the systems form glasses on freezing and the exact freezing point is difficult to identify but the values given in Figure 1 provide comparative information about the relative component interactions and the eutectic composition. Previously it was...

Theoretical study on the structures and properties of mixtures of urea and choline chloride

“…To differentiate it from ILs, the term deep eutectic solvent (DES) has been adopted. Up to now, many kinds of DESs have been reported, composed mostly of substituted quaternary ammonium salts, e.g., choline chloride, and uncharged hydrogen bond donors such as amine, amide, carboxylic acid, and alcohol [14][15][16]. Meanwhile, these DESs have been applied widely as solvents and reaction mediums in many areas, including electrochemistry [17,18], biochemistry [19][20][21] and material science [22], etc.…”

Viscosity of Imidazolium-Based Ionic Liquids at Elevated Pressures: Cation and Anion Effects

“…Most of these methods are based on the principle of corresponding states, on the absolute rate theory of Eyring [27], and, more recently, on molecular dynamic calculations [28]. Abbott and coworkers [29][30][31] used a modified "hole theory" to predict the viscosities of ionic liquids at ambient conditions. The Vogel-Fulcher-Tammann (VFT) equation [32][33][34] and Litovitz [35,36] equation are empirical equations that have been used, in general, at ambient pressure for liquids with a temperature-dependent flow activation energy.…”

Investigations on a series of novel ionic liquids containing the [closo-B12Cl12]2− dianion

“…8 Among all the boron cluster anions studied, the 5,6,8 the melting points could be regulated by optimiz-appropriate H-bonds donors, which are generally named as 'deep eutectic ILs'. [25][26][27] In our present work, we synthesized a series of novel [closo-B 12 Cl 12 ] 22 dianion containing salts in order to investigate their abilities as ILs and the studied cations included imidazolium, ammonium, phosphonium and pyridinium, shown in Scheme 1. Furthermore, we characterized the salts and analysed the reasons attributed to their higher melting points both from the alkylation pattern on the cation and from the anion.…”