Decoupling of Dynamic Processes in Surfactant-Based LiquidMixtures: The Case of Lithium-Containing Bis(2-ethylhexyl)phosphoric Acid/Bis(2-ethylhexyl)amine Systems

Pure surfactant liquids and their binary mixtures, because of theamphiphilic nature of the molecules involved, can exhibit nanosegregation andpeculiar transport properties. The idea that inspired this work is that the possibilityof including in such media salts currently used for technological applications shouldlead to a synergy between the properties of the salt and those of the medium.Therefore, the dynamic features of bis(2-ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) liquid mixtures were investigated as afunction of composition and temperature by 1H nuclear magnetic resonance (NMR)spectroscopy and rheometry. Inclusion of litium trifluoromethanesulfonate (LiT)has been investigated by infrared spectroscopy, pulsed field gradient NMR, andconductimetry methods to highlight the solubilizing and confining properties ofthese mixtures as well as the lithium conductivity. It was found that BEEA/HDEHPbinary liquid mixtures show zero-threshold percolating self-assembly with amaximum in viscosity and a minimum in molecular diffusion at a 1:1 composition. Dissolution of LiT in such system canoccur via confinement in the locally self-assembled polar domains. Despite this confinement, Li+ conduction is scarcelydependent on the medium composition because of the possibility of a field-induced hopping decoupled by the structural anddynamical features of the medium.