Morphological Transition in Fatty Acid Self-Assemblies: A Process Driven by the Interplay between the Chain-Melting...

In surfactant systems, the major role of the nature of the counterion in the surfactant behavior is well-known. However, the effect of the molar ratio between the surfactant and its counterion is less explored in the literature. We investigated the effect of the molar ratio (R) between 12-hydroxystearic acid (12-HSA) and various alkanolamines as a function of the temperature in aqueous solution from the molecular scale to the mesoscale. By coupling microscopy techniques and small-angle neutron scattering, we showed that 12-HSA self-assembled into multilamellar tubes and transitioned into micelles at a precise temperature. This temperature transition depended on both the molar ratio and the alkyl chain length of the counterion and could be precisely tuned from 20 to 75 °C. This thermal behavior was investigated by differential scanning calorimetry and wide-angle X-ray scattering. We highlighted that the transition at the supramolecular scale between tubes and micelles came from two different mechanisms at the molecular scale as a function of the molar ratio. At low R, with an excess of counterion, the transition came from the chain-melting phenomenon. At high R, with an excess of 12-HSA, the transition came from both the chain-melting process and the surface-melting process of the hydrogen bonds. At the mesoscale, this transition of supramolecular assemblies from tubes to micelles delimited a regime of high bulk viscosity, with a regime of low viscosity.