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

TL;DR

This study investigates how longer-chain alcohols affect the density anomalies and phase behavior of water-alcohol mixtures using molecular dynamics simulations with a core-softened potential, revealing the gradual disappearance of anomalies with increasing chain length and concentration.

Contribution

It extends previous work by analyzing the impact of longer-chain alcohols on water-alcohol mixtures' anomalous behavior and phase transitions using a unified core-softened potential model.

Findings

Density anomaly diminishes with longer chains and higher concentrations.

Liquid-liquid phase transition persists despite anomaly disappearance.

Non-polar chain size influences solid phase preferences, favoring HCP and amorphous phases.

Abstract

In a recent work [\textit{J. Mol. Liq}, 2020, \textbf{320}, 114420], using molecular dynamics simulations and a core-softened potential approach, we have shown that adding a simple solute as methanol can "kill" the density anomalous behavior as the LLCP is suppressed by the spontaneous crystallization in a hexagonal closed packing (HCP) crystal near the LLPT. Now, we extend this work in order to realize how longer-chain alcohols will affect the complex behavior of water-alcohol mixtures in the supercooled regime. Besides core-softened (CS) methanol, ethanol and 1-propanol were added to a system of identical particles that interact through the continuous shouldered well (CSW) potential. We observed that the density anomaly gradually decreases its extension in phase diagrams until disappearing with the growth of the non-polar chain and the alcohol concentration, differently from the liquid-liquid phase transition (and the LLCP), which remained present in all analyzed mixtures, in according to \textit{Nature}, 2001, \textbf{409}, 692. For our model, the longer non-polar chains and higher concentrations gradually impact the competition between the scales in the CS potential, leading to a gradual disappearing of the anomalies until the TMD total disappearance is observed when the first coordination shell structure is also affected: the short-range ordering is favored, leading to less competition between short- and long-range ordering and, consequently, to the extinction of anomalies. Also, the non-polar chain size and concentration have an effect on the solid phases, favoring the hexagonal closed packed (HCP) solid and the amorphous solid phase over the body-centered cubic (BCC) crystal.