Bulk versus interface nucleation of CO$_2$ hydrates from computer simulations

TL;DR

This study uses molecular dynamics simulations to investigate CO₂ hydrate nucleation, finding that nucleation predominantly occurs in the bulk rather than at interfaces, challenging previous experimental assumptions.

Contribution

The paper provides computational evidence that hydrate nucleation is more likely in the bulk and not at interfaces, contrasting with prior experimental hypotheses.

Findings

Hydrate seeds in the bulk are more likely to grow than those at interfaces.

Nucleation rates are similar with and without interfaces, indicating the interface's limited role.

Hydrates nucleate in regions of high CO₂ concentration, spontaneously forming in the bulk.

Abstract

Gas hydrates are of great relevance to both the oil industry and the environment. Understanding how these solid structures nucleate from aqueous solutions is essential to controlling their formation. Experimental studies have often suggested that hydrate nucleation originates at the interface between the aqueous phase and the guest-molecule reservoir. To assess this hypothesis, we perform molecular dynamics simulations of CO$_2$ hydrate nucleation. First, we place hydrate seeds at different positions relative to the interface and monitor their evolution, finding that seeds embedded in the bulk are more likely to grow than those located near or at the interface. Second, we analyse spontaneous nucleation simulations with and without an interface. Our previous work showed that nucleation rates are indistinguishable in both systems, strongly indicating that the interface does not play a role. Here, trajectory analysis reveals that hydrates nucleate in regions of locally high CO$_2$ concentration, which arise spontaneously in the bulk and are not associated with the interface. Our results indicate that hydrate nucleation does not preferentially occur at the interface, at least at the at deep supercooling conditions explored in this work. Further work at higher temperatures, and considering alternative nucleation locations, is needed to reconcile experiments and simulations, and thereby reach a deep understanding of the mechanism of hydrate formation.