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.

Gelating abilities of metal salts of ricinelaidic acid. Structural and rheological considerations

“…The metal salts tend to have lower solubilities and stronger driving forces for aggregation in most of the liquids examined than the parent acid, D-REA. [9] They are stable thermally to >120°C, and the initial weight loss from the Ni • H 2 O salt is consistent with its being a monohydrate ( Figure S1). Whether SAFiNs form from these salts is related to the different cations that are attached to the carboxylate group.…”

“…The gelation properties of 5 wt% of metal salts of D‐REA— sodium ricinelaidate ( D‐REA‐Na ), calcium ricinelaidate ( D‐REA‐Ca ), copper(II) ricinelaidate ( D‐REA‐Cu ), nickel(II) ricinelaidate hydrate ( D‐REA‐Ni • H 2 O ), zinc(II) ricinelaidate ( D‐REA‐Zn )—in a wide variety of organic liquids are summarized in Table . The metal salts tend to have lower solubilities and stronger driving forces for aggregation in most of the liquids examined than the parent acid, D‐REA . They are stable thermally to >120°C, and the initial weight loss from the Ni • H 2 O salt is consistent with its being a monohydrate (Figure S1).…”

“…The indexings of the diffraction peaks indicate that only D‐REA‐Ca and D‐REA‐Na pack in the same arrangement (orthorhombic, Cmma); it is different from the packing arrangment of the other metal salts that are inefficient as gelators. The longest d ‐spacing of D‐REA‐Ca , 49.5 Å, is slightly shorter than the sum of the lengths calculated for 2 extended D‐REA molecules, 47.2 Å, and twice the van der Waals radius of a calcium(II) ion, ca. 1.12 Å .…”

“…Molecular lengths are the sum of the lengths calculated for 2 extended D‐REA molecules and twice the van der Waals radius of a cation …”

“…We have found 2 reports of the gelating ability of the acid with vegetable oils . Recently, we reported the gelating ability of the acid and its ammonium salts in a wide range of liquids . The parent compound, stearic acid, is not an efficient gelator, although there is a recent report on its gels at high concentrations in sunflower oil, and sodium stearate is the major component of many soaps .…”