Microscopic Mechanism and Kinetics of Ice Formation at Complex Interfaces: Zooming in on Kaolinite

TL;DR

This study uncovers the molecular mechanism and kinetics of ice formation on kaolinite, revealing a significantly faster nucleation process with a predominantly hexagonal ice structure, aligning with classical nucleation theory.

Contribution

It provides the first detailed molecular-level understanding of ice nucleation on kaolinite, highlighting the 2D character of nuclei and confirming classical nucleation theory applicability.

Findings

Ice nucleation on kaolinite is twenty orders of magnitude faster than homogeneous freezing.

Critical nuclei are smaller and have a strong 2D character.

Kaolinite promotes hexagonal ice formation exclusively.

Abstract

Most ice in nature forms thanks to impurities which boost the exceedingly low nucleation rate of pure supercooled water. However, the microscopic details of ice nucleation on these substances remain largely unknown. Here, we have unraveled the molecular mechanism and the kinetics of ice formation on kaolinite, a clay mineral playing a key role in climate science. We find that the formation of ice at strong supercooling in the presence of this clay is twenty orders of magnitude faster than homogeneous freezing. The critical nucleus is substantially smaller than that found for homogeneous nucleation and, in contrast to the predictions of classical nucleation theory (CNT), it has a strong 2D character. Nonetheless, we show that CNT describes correctly the formation of ice at this complex interface. Kaolinite also promotes the exclusive nucleation of hexagonal ice, as opposed to homogeneous freezing where a mixture of cubic and hexagonal polytypes is observed.