Fragile to strong crossover coupled to liquid-liquid transition in hydrophobic solutions

TL;DR

This study uses molecular dynamics simulations to explore how the fragile-to-strong transition in hydrophobic solutions is linked to liquid-liquid phase transitions, revealing a persistent thermodynamic-dynamic connection similar to bulk water.

Contribution

It demonstrates the coupling between dynamic and thermodynamic behaviors in hydrophobic solutions near the liquid-liquid critical point, extending understanding from bulk water to solutions.

Findings

Fragile-to-strong transition occurs near the liquid-liquid transition.

Diffusivity crossover aligns with the Widom line above critical pressure.

Hysteresis indicates crossing of stability limits below critical pressure.

Abstract

Using discrete molecular dynamics simulations we study the relation between the thermodynamic and diffusive behaviors of a primitive model of aqueous solutions of hydrophobic solutes consisting of hard spheres in the Jagla particles solvent, close to the liquid-liquid critical point of the solvent. We find that the fragile-to-strong dynamic transition in the diffusive behavior is always coupled to the low-density/high-density liquid transition. Above the liquid-liquid critical pressure, the diffusivity crossover occurs at the Widom line, the line along which the thermodynamic response functions show maxima. Below the liquid-liquid critical pressure, the diffusivity crossover occurs when the limit of mechanical stability lines are crossed, as indicated by the hysteresis observed when going from high to low temperature and vice versa. These findings show that the strong connection between dynamics and thermodynamics found in bulk water persists in hydrophobic solutions for concentrations from low to moderate, indicating that experiments measuring the relaxation time in aqueous solutions represent a viable route for solving the open questions in the field of supercooled water.