12-Hydroxyoctadecanoic acid forms two kinds of supramolecular gels in nanostructured protic ionic liquids
George P T Roddy1, Livia Salvati Manni2, Rob Atkin3
1School of Chemistry and University of Sydney Nano Institute, The University of Sydney, Sydney, NSW 2006, Australia.
Hypothesis:
We postulate that the amphiphilic nanostructure of ionic liquids, consisting of interpenetrating networks of polar and apolar domains, may enable them to support distinct self-assembled organogel-like and hydrogel-like structures.
Experiments:
The structures of gels formed by the low molecular weight gelator 12-hydroxystearic acid (12HSA) and its ammonium salts have been investigated from the molecular to the microscale by a combination of powder X-ray diffraction, SAXS/WAXS, FTIR, CD, and optical microscopy, together with rheological characterisation of the gels formed.
Findings:
12HSA is shown to form long-lived ionogels in ethylammonium and propylammonium nitrate ionic liquids at low concentrations via two distinct mechanisms; supramolecular, hydrogen-bond driven aggregation of the acid and amphiphilic assembly of the conjugate base. 12HSA gel structures were shown to consist of high aspect-ratio twisted crystalline fibrils assembled from H-bonded dimers, similar to organogels, while 12HS salts form an elongated rectangular ribbon of solvophobically-associated lamellar stacks with an opposite twist to the acid form. Partial neutralisation of 12HSA gels with base can generate coexisting mixtures of both types of gel in these ILs.
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