Microscopic structure of aqueous alkylamine mixtures: A computer simulation study

We examine the microscopic origin of the pronounced x-ray scattering pre-peaks observed in water-rich aqueous alkylamine mixtures. While neat amines display weak pre-peaks compared to alcohols, and aqueous alcohols generally exhibit none, aqueous amines display 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 water-nitrogen donor hydrogen bonding. This interfacial anchoring prevents macroscopic demixing except at high water contents, where too few amines remain to cover the interfaces. The resulting disordered bilayer-like arrangement generates long-ranged domain oscillations that appear as positive like-like species and negative cross species correlations, whose partial cancellation produces the experimental pre-peak. Model comparisons further show that the effect is highly sensitive to both solute and water force fields, with the CHARMM-AA/SPC/E combination offering the most consistent description. These results establish aqueous amines as prototypical systems in which stable micro-heterogeneity and supramolecular topology directly give rise to scattering pre-peaks, providing a concrete bridge between molecular liquids and micro-emulsions.