First-order liquid-liquid phase transition in nitrogen-oxygen mixtures

The first-order liquid-liquid phase transition (LLPT) describes the counterintuitive nature between two distinct liquids in a single system. However, the physical understanding of LLPT in simple liquid mixtures has remained elusive. Here, a first-order LLPT for nitrogen-oxygen (N-O) mixtures is investigated via extensive ab initio molecular dynamics simulations. The first-order LLPT is characterized by discontinuities of a short-range order and transport properties, occurring at the transition pressures identified by looking at isotherms. The LLPT in N-O mixtures is dominated primarily by the molecular-to-polymeric transformation. The resulting phase boundaries and further structural analysis show the crucial influence of oxygen on weakening the LLPT of N-O mixtures by promoting chemical reactions with nitrogen and crippling polymerizations. Furthermore, noncovalent interactions of short- to long-range van der Waals have evident impacts on the LLPT boundary. The introduction of the strongly constrained and appropriately normed functional with revised Vydrov-van Voorhis nonlocal correlation (SCAN-rVV10) functional to describe the phase-transition behaviors improves the agreement with experimental data. The results highlight the characteristic features of the complex interplay in binary mixtures and revisit the first-order phase transition of dense nitrogen.

Automated summary

By conducting extensive ab initio molecular dynamics simulations on nitrogen-oxygen mixtures, this study investigates the nature of first-order liquid-liquid phase transitions. The results reveal that oxygen plays a crucial role in weakening the LLPT by promoting chemical reactions with nitrogen and hindering polymerizations. Additionally, noncovalent interactions, such as van der Waals forces, have significant impacts on the LLPT boundary.