Mesoscopic Correlations in Aqueous Alkylamine Mixtures Between Molecular and Micro Emulsions

Abstract: Understanding how molecular correlations give rise to mesoscale organization is central to the physics of complex fluids such as hydrogen-bonded mixtures. In this work, we develop a mesoscale bridge formalism that connects the site-site Ornstein-Zernike (SSOZ) framework to the field theoretical Teubner-Strey (TS) approach. This bridge highlights how local orientational correlations, typically lost in the SSOZ closure, reemerge as effective long-range components at the mesoscale. The resulting theory provides a unified description of density fluctuations spanning molecular to mesoscopic length scales. The approach is illustrated using X-ray scattering spectra from simulated and experimental hydrogen-bonded fluids, showing that the TS representation captures the essential features of the mesoscale structure. Beyond this specific application, the proposed formalism offers a general route to interpret the structural crossover between microscopic interactions and collective mesoscale organization in complex fluids, including aqueous and amphiphilic systems.