Impact of the molar ratio and the nature of the counter-ion on the self-assembly of myristic acid.

HYPOTHESIS: In fatty acid systems, the role of the nature of the counter-ion on their solubility is well described. However, the effect of the molar ratio (R) between the fatty acid and its counter-ion is less explored. We investigated the effect of R as a function of the temperature in aqueous solution between myristic acid and two organic amines under hydroxide form: tetramethylammonium (TMAOH) and benzyltrimethylammonium hydroxide (BTAOH). We compare with the results previously obtained for choline hydroxide as counter-ion (Arnould et al., 2015).

EXPERIMENTS: We characterized the phase behavior by coupling phase-contrast microscopy, SANS, DSC and WAXS experiments. The myristic acid ionization state was determined by pH, conductivity and infra-red spectroscopy measurements.

FINDINGS: Our results highlight that R tunes the phase behavior. The amount of hydroxide groups in solution fixes the ionization state of the fatty acids, which governs the headgroup interactions. At low R, the counter-ion hydrophobicity plays a role on the phase behavior: TMAOH and choline hydroxide shows a broad polymorphism (facetted and unilamellar vesicles, lamellar phases) due to their hydrophilicity while the more hydrophobic BTAOH gives fatty acid crystals at low temperatures and vesicles at high temperatures. At high R, spherical micelles are observed for all counter-ions.