Microscopic Structure of Aqueous Alkylamine mixtures: a Computer Simulation Study

We investigate the microscopic origin of the pronounced x-ray scattering pre-peaks observed in water-rich aqueous alkylamine mixtures. While neat amines display only weak pre-peaks compared to neat alcohols, and aqueous alcohols exhibit none, aqueous amines reveal a striking and unexpected signal. Molecular dynamics simulations of primary amines from propylamine to octylamine show that nitrogen head groups preferentially saturate the surfaces of water-rich domains, stabilizing both water and amine regions through dual hydrogen bonding. This interfacial organization prevents macroscopic demixing except at high water contents, where insufficient amines remain to cover the interfaces. The resulting disordered bilayer arrangement produces long-ranged domain oscillations that manifest as positive like-like and negative cross correlations, whose partial cancellation yields the experimental pre-peak. Model comparison highlights the sensitivity of this effect to both solute and water force fields, with the CHARMM-AA/SPC/E combination providing the best compromise. These findings establish aqueous amines as prototypical systems where stable micro-heterogeneity and supramolecular topology directly account for scattering pre-peaks, bridging molecular liquids and micro-emulsions.