Colloidal mixtures help explain liquid-liquid phase transitions

Liquid-liquid phase transitions are uncommon and difficult to access experimentally. Because Brownian motion in colloids is similar to the thermal motion of atoms and molecules, Gußmann et al. sought to determine whether colloidal systems can be useful for studying liquid-liquid phase transitions.

“To date, there are many details of liquid-liquid phase transitions not fully understood, and so a new model system that, on the one hand, allows us to study liquid-liquid phase transitions theoretically and, on the other hand, can be realized experimentally can open up new possibilities to gain insight in liquid-liquid phase transitions in general,” said author Roland Roth.

The group found a colloidal model with three binodal branches, one of which indicates a liquid-liquid phase transition from low to high density. If the model can be used to investigate the possible presence of a metastable critical point that allows the phase transition to occur, it may shed light on the hypothesis that water undergoes a liquid-liquid transition on deep cooling.

“Since in water the second critical, if it exists, is metastable, it is extremely difficult to get close to it experimentally. Long before one reaches the state point where the second critical point is expected, water freezes,” Roth said. “If it is possible to stabilize the fluid phase, such as by a geometrical confinement, one can learn about the properties of the transition between the low- and high-density liquid and maybe, on a longer time scale, understand some of the unusual properties of water.”

The estimations provided by the colloidal model were corroborated by density functional theory calculations. The authors hope their findings can serve as a good guide for future experiments.

Source: “Liquid-liquid phase separation in an inhomogeneous ternary colloid-polymer mixture,” by Florian Gußmann, Hendrik Hansen-Goos, S. Dietrich, and Roland Roth, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0052586 .

This paper is part of the Depletion Forces and Asakura-Oosawa Theory Collection, learn more here .