Insights into the Gelating Abilities of Ricinelaidic Acid and its Ammonium Salts: How do Stereochemistry, Charge, and Chain Lengths Control Gelation of a Long-Chain Alkenoic Acid?

Abstract

The gelating abilities of ricinelaidic acid (d-REA), the trans-isomer of ricinoleic acid (d-RA), and a series of its alkylammonium and alkane-α,ω-diammonium salts have been examined in a wide range of organic liquids. The gelation efficiency of the trans acid is much better than that of the cis, although neither is as efficient as is the completely saturated molecular gelator analogue, (R)-12-hydroxystearic acid (d-12HSA). The formation of ammonium salts also improves the gelation ability of d-REA in high polarity liquids. The gelating properties are highly dependent upon the chain length of the alkyl group of the alkylammonium salts, but not very dependent on the chain length of the alkane-α,ω-diammonium salts. Structural insights from Fourier transform infrared spectroscopy and powder X-ray diffraction indicate that the absence or presence of unsaturation, the incorporation of (charged) ammonium centers, and the different chain lengths of the alkylammonium salts lead to different packing arrangements and different strengths of H-bonding interactions within the gel assemblies of the d-REA derivatives. Insights into the relationships among the various systematic structural changes to d-REA and the properties of their aggregated structures, including the gel states, are provided.