Non-equilibrium pathway to mesoscale ordering in ethanol-water binary liquid

Abstract

Ethanol-water mixtures are a classic example of thermodynamic non-ideality, yet the structural origin of their pronounced anomalies, such as volume contraction and a large negative excess entropy, has remained a long-standing puzzle. Here, we demonstrate these anomalies are not equilibrium properties but calorimetric fingerprint of an arrested phase transition. By imposing periodic thermal oscillations, we drive a 50% (v/v) ethanol-water system along a complete hierarchical self-assembly pathway that progressed from ethanol clusters to water-containing droplets, then to acicular flakes, and finally to micron-scale ordered ethanol aggregates. Fluorescence spectroscopy, two-dimensional correlation analysis and nuclear magnetic resonance revealed the underlying non-equilibrium molecular mechanism: a periodic perturbation of the water-dominated hydrogen-bond network initiates a ethanol-water coexistence intermediate, ultimately leading to the stable ordered assembly of an ethanol-rich phase. Our finding demonstrated that periodic physical perturbations capable drive spontaneous ordering across multiple length scales in a simple binary mixture, providing a kinetic perspective on the structural origin of solution non-ideality, and carry general implications for self-assembly strategies in soft matter.