Nanostructure changes in protic ionic liquids (PILs) through adding solutes and mixing PILs

Abstract

The effect of adding primary n-alcohols with aliphatic chains and hexane on the nanostructure of a series of 14 protic ionic liquids (PILs) was explored using small and wide angle X-ray scattering (SAXS and WAXS). PILs were investigated with primary, secondary and tertiary ammonium cations containing different alkyl chain lengths, with and without hydroxyl substitution of the alkyl chain. Formate or nitrate anions were paired with these cations. The PILs which had no identified intermediate range order between 5-16 Å had very low solubilities of the solutes. The other PILs, which had non-polar domains present, were mostly miscible with the primary alcohols of ethanol, propanol and butanol. When the alkyl chain length of the alcohols was similar to the PILs then the alcohols co-partitioned with the alkylammonium cation components of the PILs and caused either an increase or decrease in the size of the non-polar domains, depending on whether the alcohol chain length was longer or shorter than that of the cation in the PIL respectively. For ethylammonium nitrate (EAN) with propanol or butanol and propylammonium nitrate (PAN) with butanol, the difference between the alcohol chain length and the alkyl chain length was too great to lead to a modified nanostructure, and instead large aggregates were present. The solubility of hexane in the alkylammonium nitrate PILs had a very strong correlation to the alkyl chain length. The addition of hexane had very little effect on the non-polar domain sizes, which was attributed to it not being orientated in alignment with the alkylammonium cations in the non-polar domains. Lastly, seven binary PIL-PIL solution series were investigated using SAXS and WAXS to show how the nanostructure of these systems can be fine tuned to control the size and structure of the non-polar domains.

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.…”

Interaction of a long alkyl chain protic ionic liquid and water

“…This spatial scale can be easily modified acting on the cation alkyl chains, simply increasing their length while maintaining little interdigitation, such as recently observed for the protic ionic liquid series: ethyl-, propyl-, butyl-and pentyl-ammonium nitrate. 6 Governing the intermediate range order is crucial to control the interaction of ILs with other solvents, 7 since the presence of large non polar domains greatly enhances the ability of ILs to dissolve non polar species. For this reason, and considered that many ILs are completely miscible in water, 8,9,10 it is interesting to study the effect of water as solvent.…”

Heterogeneity of propyl-ammonium nitrate solid phases obtained under high pressure

“…This has been verified also for protic ionic liquids (PILs) [4][5][6], an important subclass of ILs characterized by the presence of proton donors and acceptor sites, which gives rise to a network of hydrogen-bonds. This network strongly influences the physicalchemical properties of the system [7][8][9][10][11] and can also induce several phase transitions and polymorphism upon a temperature or pressure change [12][13][14].…”