Arising of magnetically induced optical birefringence in ionic liquid/dielectric systems: The case of diphenyl phosphate / bis(2-ethyl hexyl) amine

Addition of diphenyl phosphate (DPP) to bis(2-ethylhexyl) amine (BEEA) causes the proton transfer from the acidic DPP to the basic BEEA with formation of ionic liquid DPP/BEEA domains. Such mixtures show significant optical birefringence if subjected to a magnetic field. When DPP amount is increased, as highlighted by molar refraction and viscosity measurements, a structural change occurs, most probably due to the percolation of the DPP/BEEA domains. Despite of this, the intensity of the magnetically-induced birefringence monotonically depends on the amount of DPP, with no peak at any specific composition. In addition, the birefringence appears immediately after the magnetic field is switched on and immediately disappears after the magnetic field is switched off, with a reversible and reproducible on/off switching. All this suggests that this property needs to be intended as deriving from a local anisotropy, not implying big supra-molecular rearrangements. Although there is effective modelling of the induced birefringence in well-defined molecular aggregates, there is not yet a theoretical description of the phenomenon shown by the nanoheterogeneities of surfactant-based liquid mixtures. So, our findings open new ways for the comprehension of the dynamics in these liquids.