On the physico-chemical basis of self-nanosegregation giving magnetically-induced birefringence in dibutyl phosphate/bis(2-ethylhexyl) amine systems

Abstract

Dibutyl phosphate (DBP)/bis(2-ethylhexyl) amine (BEEA) liquid mixtures can show optical birefringence when exposed to a magnetic field. It has been hypothesized that this is a consequence of self-segregation characterizing these systems giving formation of anisotropic local assemblies which are able to re-orient under magnetic field. In this work, an ab-initio Density Functional Theory computer modeling coupled with synchrotron X-Ray Scattering (XRS) experiments carried out at various BEEA molar ratio (X) and temperatures (10 ≤ T ≤ 60 °C) permitted to highlight the physical origin of this effect: a DBP-to-BEEA proton transfer with formation of an inherently anisotropic DBP-BEEA ion pair constituting the building block of highly structured ionic liquid nanodomains. At non-equimolar compositions (X ≠ 0.5) such ionic liquids domains are dispersed in the component in excess. At X > 0.5 the ionic liquids nanodomains are dispersed in excess of poorly structured BEEA, creating the ideal assumptions for the formation of a two-phase dispersed system, as confirmed by Guinier analysis in the small angle regime of the XRS spectra, explaining the system capability to become birefringent in reaction to external magnetic field.These information can be transferred to other amphiphiles-based mixtures for the piloted design of new magnetic field responsive materials with advanced applications.