Nanostructured Protic Ionic Liquids Retain Nanoscale Features in Aqueous Solution While Precursor Brønsted Acids and Bases Exhibit Different Behavior

Abstract

Small- and wide-angle X-ray scattering (SWAXS) has been used to investigate the effect that water has on the nanoscale structure of protic ionic liquids (PILs) along with their precursor Brønsted acids and bases. The series of PILs consisted of primary, secondary, and tertiary alkylammonium cations in conjunction with formate, nitrate, or glycolate anions. Significant differences were observed for these systems. The nanoscale aggregates present in neat protic ionic liquids were shown to be stable in size on dilution to high concentrations of water, indicating that the water is localized in the ionic region and has little effect on the nonpolar domains. The Brønsted acid-water solutions did not display nanostructure at any water concentration. Primary amine Brønsted bases formed aggregates in water, which generally displayed characteristics of poorly structured microemulsions or a form of bicontinuous phase. Exceptions were butyl- and pentylamine with high water concentrations, for which the SWAXS patterns fitted well to the Teubner-Strey model for microemulsions. Brønsted base amines containing multiple alkyl chains or hydroxyl groups did not display nanostructure at any water concentration. IR spectroscopy was used to investigate the nature of water in the various solutions. For low PIL concentrations, the water was predominately present as bulk water for PIL molar fractions less than 0.4-0.5. At high PIL concentrations, in addition to the bulk water, there was a significant proportion of perturbed water, which is water influenced in some way by the cations and anions. The molecular state of the water in the studied amines was predominately present as bulk water, with smaller contributions from perturbed water than was seen in the PILs.

Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics

“…We now have task specific ionic liquids, [1][2][3][4][5][6][7][8][9][10][11] ionic liquid drugs, 12-16 ionic liquids for energy applications such as batteries and supercapacitors, 17-31 those that capture carbon dioxide 3,32-38 and those that dissolve cellulose [39][40][41][42][43][44] and proteins. [45][46][47][48][49] In terms of their structure, interesting work has surfaced on protic ionic liquids, [50][51][52][53][54][55][56][57][58][59] ILs that are highly fluorinated, 59-63 ILs with polar tails, [64][65][66][67] ILs with paramagnetic ions, 68,69 those containing silicon, [70][71][72][73][74] and some that are triphillic 59,61-63 just to mention a few examples. Not all ionic liquids but a vast number of them can be categorized as having both charged components and also apolar components.…”

Tertiary and Quaternary Ammonium-Phosphate Ionic Liquids as Lubricant Additives

“…More recently, ammonium-based ILs have been examined as neat lubricants or additives [15,[30][31][32][33][34][35]. Other recent work has revealed the merits of protic ILs [36][37][38][39][40][41].…”

Small-Angle X-Ray Scattering of Ionic Liquids

“…Specific to ILs, such ordering may be inherent to the IL itself, whether it be a consequence of the cation [20,21], anion [22], or the aggregation of amphiphilic molecules in ILs [23,24]. In these cases, the structural order is analyzed by resolving the repeat distance, d, of the phases.…”