How Water Dissolves in Protic Ionic Liquids

Abstract

In recent years, ionic liquids (ILs) have emerged as useful chemical solvents for an enormous number of processes and technologies. [1,2] Their ions have more complex chemical structures than inorganic salts; by incorporating large, sterically mismatched anions and cations, ILs melt at low temperatures because, compared to typical inorganic salts, Coulombic attractions are weakened and lattice-packing arrangements frustrated. [3] ILs are regarded as "designer solvents", as molecular control over liquid properties is possible depending on how the ions are functionalized. Hydrogen bonding can play a key role in IL chemistry. [4][5][6] Whereas most inorganic salts cannot form hydrogen bonds and are dominated by electrostatic interactions between ions, many ILs have extensive H-bonding capacity. For example, H-bond donor and acceptor sites are created during synthesis of protic ionic liquids (PILs). [2] This enables some PILs to develop dense Hbond networks and thus mirrors a number of remarkable structural [5] and solvent [2] properties of water. Finally, ILs have the capacity to self-assemble, forming well-defined nanostructures in the bulk phase [7][8][9][10][11][12][13] as well as at interfaces. [3,[14][15][16] IL nanostructure arises because at least one of the ions (frequently the cation) is amphiphilic, with distinct charged and uncharged moieties. [9] This drives segregation of ionic and nonionic groups in ILs, reminiscent of self-assembly in aqueous surfactant mesophases. [3,12] Here we elucidate the bulk solvent structure of mixtures of a PIL, ethylammonium nitrate (EAN), and water ( Figure 1). EAN is one of the oldest known [17] and most extensively studied PILs. As EAN is completely miscible with water, this raises questions such as: how do EAN and water mix? Are the forces that lead to self-assembly in pure EAN [9] sufficient to maintain a solvophobic nanostructure? What is the nature of ion solvation in such mixtures? If nanostructure persists in aqueous mixtures and key solvent properties are retained, this will increase PIL utility by offering an additional mechanism for tuning liquid behavior and lowering the overall cost of the solvent medium.While primitive (continuum solvent) models of dilute aqueous electrolyte solutions are generally successful, understanding ion-water interactions and concentrated solutions has proved challenging, and is complicated in part by the absence of a satisfactory model for liquid water. [18,19] The structure in aqueous electrolyte solutions is understood in terms of Hofmeister [20] and hydrophobic [21] effects, which can only be probed using sophisticated experimental [22][23][24] and computational [25][26][27] techniques. Solvated ions induce a different local structure of water molecules in the first, and even the second or third solvation shells, to accommodate the dissolved species. This leads to ions being classified as either "structure making" or "structure breaking" through the creation of "solute cavities". [28] Recent, growing interest in IL/water mixtures h...

Combining wide-angle and small-angle scattering to study colloids and self-assembly

“…In protic ionic liquids this periodic charge ordering can lead to a nanostructure that is more reminiscent of a well dispersed microemulsion or disordered sponge phase [27]. This extended nanostructure, that is periodically concomitant with the size of the molecules in the liquid, can then play an interesting role when additional solutes are added to the ionic liquid, for example the addition of water to protic ionic liquids has been found to adjust the charge-ordered molecular arrangement to modify its distribution from a reasonably uniform distribution to one where there is some evidence of a branching network of higher curvature [30].…”

Equilibrium of Contact and Solvent‐Separated Ion Pairs in Mixtures of Protic Ionic Liquids and Molecular Solvents Controlled by Polarity

“…Possible proton transfer in PILs (Brønsted acids and bases) provides proton-acceptor and -donor sites and results in strong hydrogen bonds. [24][25][26][27][28][29][30][31] This holds in particular for PILs including trialkylammonium cations. A strong and directional hydrogen bond between the NH bond of the cation and the anion provides strongly bound CIPs for the pure Coulomb fluid.…”

Ion Speciation of Protic Ionic Liquids in Water: Transition from Contact to Solvent‐Separated Ion Pairs

“…A key feature in PILs is the possible proton transfer from the Brønsted acid to the Brønsted base, leading to the formation of proton acceptor and donor sites and possible hydrogen bonding. [17][18][19][20][21][22][23][24] The typical definition of distinguishing ion pairs from complexes does not hold here. CIPs in protic ionic liquids are held together by long-range, non-directional electrostatic forces as well as short-range, spatially directed donor-acceptor-type interactions.…”