The Role of Charge Transfer in the Formation of Type I Deep Eutectic Solvent-Analogous Ionic Liquid Mixtures

Abstract

It was recently shown that tetramethylammonium chloride presented negative deviations to ideality when mixed with tetraethylammonium chloride or tetrapropylammonium chloride, leading to a strong decrease of the melting points of these salt mixtures, in a behavior akin to that observed in the formation of deep eutectic solvents. To better rationalize this unexpected melting point depression between two structurally similar compounds devoid of dominant hydrogen bonding capability, new solid–liquid equilibria data for tetramethylammonium-based systems were measured and analyzed in this work. Molecular dynamics was used to show that the strong negative deviations from ideality presented by these systems arise from a synergetic share of the chloride ions. A transfer of chloride ions seems to occur from the bigger cation in the mixture (which possesses a more disperse charge) to the smaller cation (tetramethylammonium), resembling the formation of metal–chloride complexes in type I deep eutectic solvents. This rearrangement of the charged species leads to an energetic stabilization of both components in the mixture, inducing the negative deviations to the ideality observed. The conclusions presented herein emphasize the often-neglected contribution of charge delocalization in deep eutectic solvents formation and its applicability toward the design of new ionic liquid mixtures.

Liquefying Compounds by Forming Deep Eutectic Solvents: A Case Study for Organic Acids and Alcohols

“…The [N 4,4,4,4 ]Cl, however, presents either a near-ideal behavior or even weak positive deviations to ideality. This behavior is reminiscent of that previously observed in the [N 1,1,1,1 ]Cl/ [N 4,4,4,4 ]Cl system, 27 supporting the idea that chloride anions are being transferred from the [N 4,4,4,4 ] cation to the carboxylic acid head, engaging in stronger hydrogen bonds than those present in the pure acid liquid phases.…”

“…[N 4,4,4,4 ]Cl has been shown to behave as a chloride donating agent, with its cation being able to form apolar domains depleted in chloride anions. 27 In this work, the solid− liquid phase diagrams of the systems composed by [N 4,4,4,4 ]Cl and the model fatty acids were measured and are depicted in Figure 9.…”

“…Since the cation of [N 1,1,1,1 ]Cl is much more densely charged, it is unable to similarly donate chloride anions, functioning as the receiving end of chloride anions in the [N 1,1,1,1 ]Cl/[N 4,4,4,4 ]Cl system, as previously shown. 27 Considering that [N 1,1,1,1 ]Cl shows severe negative deviations to ideality and considering that the The Journal of Physical Chemistry B pubs.acs.org/JPCB Article acids and alcohols presented severe positive deviations to ideality (which indicates that the new interactions established in the mixture are weaker than those present in their pure liquid phase), it is concluded that the cation of [N 1,1,1,1 ]Cl is interacting with the negative zones of the organic molecules (lone pairs of the oxygen atoms), further stabilizing itself by withdrawing additional negative charge but preventing the hydrogen bonding between the organic molecules, leading to the asymmetrical deviations to ideality seen in these systems.…”

“…Hence, thymol and menthol establish a stronger hydrogen bond network than those present in the liquid phase of the pure components. We have also identified mixtures in which tetrabutylammonium chloride ([N 4,4,4,4 ]Cl) behaves as a chloride donning agent, 27 analogous to the mechanism of formation of type I DES. An example is the system [N 1,1,1,1 ]Cl/[N 4,4,4,4 ] Cl with [N 1,1,1,1 ]Cl standing for tetramethylammonium chloride, which presents large negative deviations to ideality 27 due to a synergetic share of the available chloride ions, in which the coordination number of chloride anions around the [N 1,1,1,1 ] + cation increases (when compared to its pure liquid phase) and decreases around the [N 4,4,4,4 ] + cation (in comparison to its pure liquid phase).…”

Synthesis of Purine-Based Ionic Liquids and Their Applications

“…The introduction of the bulky [N 4444 ] + cation disrupts this efficient packing, lowering the lattice energy and melting point of the salts, in comparison to the purines. Nevertheless, the melting point of the purine-based ILs is still higher than the melting point of other conventional ILs, such as [N 4444 ]Cl (344 K) [51]. As for the decomposition temperature of the synthesized salts, this property increased in the following order: 2).…”