Self-assembly in surfactant-based liquid mixtures: Octanoic acid/Bis(2-ethylhexyl)amine systems

The physico-chemical properties of Bis(2-ethylhexyl)amine (BEEA) plus octanoic acid (OA) mixtures have been investigated by IR, SAXS, WAXS, viscosimetry, and AC complex impedance spectroscopy in the whole composition range. Mainly driven by proton transfer from the acidic OA to the basic BEEA, the formation of stoichiometrically well-defined adducts takes place in the mixtures. This causes the slowing down of molecular dynamics and the increase in charge carrier number density. Interestingly, while the pure components possess no significant conductivity (about 10−12 S cm−1 at 25 °C), their mixtures show a composition-dependent enhanced conductivity (up to about 10−5 S cm−1), i.e., more than seven orders of magnitude higher than that of the pure components. The comparison of the composition dependence of viscosity, direct-current conductivity, and static permittivity indicates the concurrence of contributions of different adducts and that the dynamics controlling molecular reorientation and momentum and charge transfer, even if ultimately related to the proton transfer from OA to BEEA, are different. The results can be used not only to design novel materials for application purposes, but also to shed more light on the principles regulating molecular self-assembly in surfactant-based liquid systems.