Diversity Observed in the Nanostructure of Protic Ionic Liquids

Abstract

The nanostructure of a series of 20 protic ionic liquids (PILs) has been investigated using small- and wide-angle X-ray scattering (SAXS and WAXS). The PILs contained alkylammonium, dialkylammonium, trialkylammonium, and cyclic ammonium cations combined with organic or inorganic anions. The presence of hydroxyl and methoxy substituents on the alkyl chains of the cations was also explored. Many of the PILs showed a nanostructure resulting from segregation of the polar and nonpolar components of the ionic liquid. It was found that this segregation was enhanced for longer alkyl chains, with a corresponding increase in the length scale, whereas the presence of hydroxyl groups on the alkyl chains led to much less ordered liquids. The broad range of protic ionic liquids studied allowed several structure-property relationships to be established. The solvophobic effect was shown to be dependent on the nanostructure of the PILs. These PILs support amphiphile self-assembly, and it was shown that the less structured PILs had more "water-like" behavior in the diversity of lyotropic liquid-crystal phases supported, and the thermal stability ranges for these phases.

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.…”

Structure of protic (HCnImNTf2, n=0–12) and aprotic (C1CnImNTf2, n=1–12) imidazolium ionic liquids: A vibrational spectroscopic study

“…Similar behavior (the appearance of the low-Q peak in X-ray scattering measurements) is also followed in protic ionic liquids with short alkyl groups such as ethylammonium and propylammonium nitrates [20,21]. Detailed scattering studies indicate that the charged heads are associated with electrostatic and hydrogen bonding attractions while the short alkyl chains aggregate together through solvophobic interactions, suggesting a disordered smectic or "sponge" like structure [22,21,[23][24][25][26].…”

“…Scattering experiments from piperidinium [16] and pyrrolidinium [17,18] ILs show that the characteristic length of the nanoscale heterogeneity area does not follow the trend of the imidazolium melts, but their values propose an interdigitate system, in the first case, and an intermediate one for the pyrrolidinium [19]. Similar behavior (the appearance of the low-Q peak in X-ray scattering measurements) is also followed in protic ionic liquids with short alkyl groups such as ethylammonium and propylammonium nitrates [20,21]. Detailed scattering studies indicate that the charged heads are associated with electrostatic and hydrogen bonding attractions while the short alkyl chains aggregate together through solvophobic interactions, suggesting a disordered smectic or "sponge" like structure [22,21,[23][24][25][26].…”

Room-Temperature Molten Salts: Protic Ionic Liquids and Deep Eutectic Solvents as Media for Electrochemical Application

“…Ionic liquid cations usually consist of an ionic and an alkyl component which, similar to amphiphile self-assembly, drives the segregation. 2 ], and correlated this lack of ordering of the ions as an influential factor in small molecule amphiphile self-assembly [107,108]. In contrast, highly structured PILs have an increased propensity toward amphiphile solubilization as show in Fig.…”