From proton transfer to ionic liquids: How isolated domains of ion pairs grow to form extended network in diphenyl phosphate-bis(2-ethylhexyl) amine mixtures

Hypothesis: Diphenyl phosphate (DPP)/bis(2-ethylhexyl) amine (BEEA) liquid mixtures are expected to show peculiar self-assembly behavior due to their, respectively, acidic and basic nature which can trigger acid-base reaction. Experiment: The properties of DPP/BEEA mixtures of different compositions are explored by a combined theoretical (Ab initio calculations) and experimental (rheology, X-ray and light scattering) approach. Findings: (i) a proton transfer from DPP to BEEA takes place with formation of cationic and anionic species in liquid phase (ion pair formation); (ii) due to the strong and long-range electrostatic interactions, each of the two charged species is preferentially surrounded by the other one, in a picture resembling the structure of ionic materials; (iii) this gives rise to a striking viscosity increase takes place as DPP concentration increases; (iv) the composition dependencies of all measured parameters show clear deviations around DPP molar fraction of 0.2, indicating unequivocally the formation of isolated domains, formed by DPP-BEEA pairs which coalescence/percolate into a unique, extended, network at higher DPP concentrations.