Microscopic structure of aqueous alkylamine mixtures: A computer simulation study

Abstract

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.