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.