Anti-Arrhenian behaviour of conductivity in octanoic acid-bis(2-ethylhexyl)amine systems: a physico-chemical study

Pure surfactant liquids and their binary mixtures, owing to the amphiphilic nature of the molecules involved, can exhibit nano-segregation and peculiar transport properties. The structural and dynamic properties of octanoic acid (OA)-bis(2-ethylhexyl) amine (BEEA) liquid mixtures at various compositions have been studied by Wide Angle X-ray Scattering (WAXS), H-1-NMR and broadband dielectric spectroscopy as a function of temperature. It was found that the self-assembly occurs via proton exchange between the OA COOH group and the BEEA NH one; such self-assembled local structures are affected anisotropically by a temperature increase so that the thermal dilatation is more marked in a direction perpendicular to the molecular axis whereas it is practically unaffected along its axis. Interestingly, an anti-Arrhenian behaviour of the conductivity has been found at the OA molar ratio (X-OA) of around 0.66. This behaviour has been interpreted as the overall result of the competition between the tendency of OA surfactant molecules to aggregate through OA-OA H-bond formation and the establishment of hetero-adducts OA-BEEA joined to the temperature disordering effect. The potentialities of such systems as proton conducting water-free organic-based liquid systems enable them to be tailored for their direct use as liquid membranes in fuel cells but also, from a more general perspective, for the piloted design of smart materials for specific applications.