## 1 Introduction  
Simple liquids usually respond to external magnetic fields quite quickly thanks to the fast molecular orientational dynamics taking place in timescales of the order of nanoseconds. Despite the fast molecular dynamics occurring in simple liquids, the orientational order generated by the magnetic field is usually low at room temperature since the thermal energy surpasses the magnetic energy of individual molecule. For these reasons fast response and relatively low value of magnetically induced optical birefringence are usually observed in simple molecular liquids.  
From the technological point of view, only highly optically anisotropic molecules/particles (e.g., liquid crystals) can be exploited in order to induce a significant birefringence. This puts very restricting limitations on system behavior and the possible applications.  
This work follows recent investigations in surfactant-based complex systems [1] highlighting that a lot of unexpected and intriguing emerging properties arise when two neat liquid amphiphiles are mixed. The resulting surfactant-based liquid mixtures can exhibit local intermolecular self-assembly [2; 3], enhanced proton conductivity [4; 5], one-dimensional (1D) anomalous proton diffusion [6], peculiar solubilizing properties towards inorganic salts [7; 8; 9] or functional molecules [10] and anti-Arrhenian behavior of conductivity [11]. In some cases, even ionic liquids are produced [12]. In this ambit, not only these diversified properties promote their use in several research fields, for example as amphiphile self-assembly reaction media [13; 14] or reaction catalyst in organic synthesis [15; 16], as a solvent phase in microemulsions and emulsions or in the emerging field of nanostructured inorganic materials synthesis [17], but a peculiarly intriguing property deserving attention is the optical birefringence arising as a response to an external magnetic field [18].  
It has been shown that mixing specific amounts of dibutyl phosphate and propylamine, which themselves are diamagnetic molecules and have very low intrinsic anisotropy, results in a system with an unusual response to external magnetic fields [19]. Unlike the generally fast response and low induced birefringence expected for molecular liquids and their mixtures, an extremely slow response has been observed, with an induced birefringence significantly higher than that observed for mixtures of simple molecular liquids of anisotropic molecules [20]. Significant birefringence is, in fact, more often observed in dispersions of highly magnetic minerals [21].  
The origin of this feature yields on the opportune choice of amphiphiles: if one is basic (for example a short alkyl chain alkylamine) and the other is acidic (for example protic alkyl phosphate), then the direct acid-base interaction can give a strong H bond or a neat proton transfer, resulting anyway in a molecular pair which is inherently anisotropic. These pairs, although dynamical in nature, can further self-assemble into larger super-molecular aggregates whose net magnetic energy exceeds the thermal energy making them magnetically susceptible. For amphiphile binary systems, this process involves all the molecules present in the systems, which are all interacting with each other making the system dense, viscous and structurally and dynamically complex. Here, the system cannot be imagined as a dispersion of individual and spatially separated anisotropic entities in a uniform and magnetically neutral liquid matrix, but rather a somehow percolated structure of molecules subjected to steric hindrance, polar-polar, polar-apolar, van der Waals and magnetic interactions.  
The present work aims at shedding light in this phenomenon by studying the response of dibutyl phosphate (DBP)/propylamine (PA) binary mixture against an applied magnetic field as a function of time and of magnetic field intensity. The intriguing data will be modeled and interpreted from the molecular basis.