Effects of Cooling Rates on Self-Assembling Structures of 12-Hydroxystearic Acid in an Ionic Liquid

Abstract

We investigated effects of cooling rates on self-assembling structures and mechanical and electrochemical properties of 12-hydroxystearic acid (12-HSA) in an ionic liquid (IL), 1-allyl-3-butylimidazolium bis(trifluoromethanesulfonyl) imide ([ABIm][TFSI]). The mixture of 12-HSA with [ABIm][TFSI] had an upper critical solution temperature (UCST) above the solgel transition temperature, and the microstructure of the ionogel was significantly affected by cooling rates, where it was prepared. The twisted self-assembling structure was formed during a slow cooling process at a rate of 0.4 ∘ C/min, whereas spherical domains caused by the liquid-liquid phase separation and radiate fibrous structure were observed for the quenched gel. The real-time smallangle X-ray scattering (SAXS) measurements for the ionogel during a slow cooling process at a rate of 0.4 ∘ C/min presented three different (001) peaks arising from long spacings of 46.5, 42.4, and 39.7Å, which were also observed for SAXS curves of a neat 12-HSA. These results suggest that three polymorphic forms of 12-HSA are formed in the IL. The polymorphic form significantly affected the mechanical properties of the ionogel, whereas it did not affect the ionic conductivity. The ionic conductivity of the ionogel was close to that of a neat [ABIm][TFSI] irrespective of the polymorphic forms of 12-HSA.

(R)-10-Hydroxystearic Acid: Crystals vs. Organogel

“…Considering the presence of polymorphs observed for positional isomers of (R)-10-HSA, such as 12-HSA and 9-HSA [7,8,14,18], the compound was crystallized from a supersaturated cyclohexane solution and analyzed using powder diffraction techniques at room temperature (Figure 8, black line). The comparison of the resulting pattern with the simulated pattern obtained from the single-crystal structure at 100 K (Figure 8, blue line) shows a clear similarity between the two forms.…”

“…Organogels of (R)-12-HSA are obtained by cooling a hot solution prepared with a solvent where the hydroxy fatty acid is insoluble at a lower temperature. Solvent, temperature, and cooling rate are relevant variables that affect the properties of the final material and, in turn, the supramolecular architecture of the gel network [7][8][9]. The most common features observed in (R)-12-HSA gels are the presence of fibers or spherulitic aggregates [7,9].…”