12-Hydroxyoctadecanoic acid forms two kinds of supramolecular gels in nanostructured protic ionic liquids

Abstract

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.