Core-softened water-alcohol mixtures: the solute-size effects

Area of Science:

• Physical Chemistry
• Materials Science
• Computational Chemistry

Background:

• Water exhibits anomalous thermodynamic, dynamic, and structural behaviors attributed to a potential liquid-liquid phase transition (LLPT) ending at a liquid-liquid critical point (LLCP).
• Previous work showed methanol addition suppresses water's density anomaly by inducing crystallization near the LLPT.

Purpose of the Study:

• To investigate the effect of longer-chain alcohols (ethanol, 1-propanol) on the anomalous behavior of water-alcohol mixtures in the supercooled regime.
• To understand how alcohol chain length and concentration influence the LLPT and density anomalies.

Main Methods:

• Utilized molecular dynamics simulations with a continuous shouldered well (CSW) potential to model water-alcohol mixtures.
• Analyzed phase diagrams, density anomalies, and solid phase formation (HCP, amorphous, BCC) as a function of alcohol concentration and chain length.

Main Results:

• Increasing non-polar chain length and alcohol concentration gradually reduced and eventually eliminated density anomalies.
• The liquid-liquid phase transition (LLPT) and liquid-liquid critical point (LLCP) persisted in all studied mixtures.
• Longer chains and higher concentrations favored hexagonal close-packed (HCP) and amorphous solid phases over body-centered cubic (BCC).

Conclusions:

• Density anomalies in water-alcohol systems are suppressed by increasing alcohol concentration and non-polar chain length due to favored short-range ordering.
• The liquid-liquid phase transition (LLPT) and liquid-liquid critical point (LLCP) can exist independently of density anomalies.
• Findings elucidate the behavior of supercooled water solutions and the conditions for observing LLCP.