Emerging dynamics in surfactant-based liquid mixtures: Octanoicacid/bis(2-ethylhexyl) amine systems

This work focuses on the dynamic phenomena emerging in self-assembled transient intermolecularnetworks formed when two different surfactants are mixed. In particular, the relaxation processesin liquid mixtures composed by bis(2-ethylhexyl)amine (BEEA) and octanoic acid (OA) inthe whole composition range has been investigated by dielectric spectroscopy and Brillouin spectroscopy.A thorough analysis of all the experimental data consistently suggests that, mainly drivenby acid-base interactions arising when the two surfactants are mixed, supra-molecular aggregatesformation causes the slowing down of molecular dynamics. This, in turn, reflects to longer-rangerelaxations. These changes have been found to be composition-dependent, involving strong departuresof the mixture physico-chemical properties from an ideal behaviour, and reflecting the structuraland dynamical features of local structures. In particular, the peculiar dynamic processes occurringin these local inter-molecular structures, have been found to be the factors responsible forthe observed and quite surprising increase of direct-current conductivity which occurs when twodifferent (and pretty non-conductive) surfactants are mixed. The discovery can be used not onlyto design novel materials for application purposes but also to shed more light on the basic principlesregulating charge migration in structured liquid systems.