Molecular Insights into the Nucleation and Growth of CH<sub>4</sub> and CO<sub>2</sub> Mixed Hydrates from Microsecond Simulations

Hydrates of CH<sub>4</sub>/CO<sub>2</sub> gas mixture are widely involved in hydrate-based applications, such as CH<sub>4</sub>/CO<sub>2</sub> separation and exchange in exploitation of natural gas hydrate resource; nevertheless, their formation mechanisms remain elusive. In this study, microsecond simulations are performed to investigate the nucleation and growth of CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates from two-phase systems of water and CH<sub>4</sub>/CO<sub>2</sub> gas mixture. The simulation results show that CH<sub>4</sub>-occupied small 5<sup>12</sup> cages initiates hydrate nucleation in a local liquid phase with a high gas concentration, where CO<sub>2</sub> and CH<sub>4</sub> cooperatively adsorb to hydrogen-bonded water rings toward hydrate-like ordering and cage formation. The difference in hydrophobicity between CH<sub>4</sub> and CO<sub>2</sub> affects the stability of nanobubbles of CH<sub>4</sub>/CO<sub>2</sub> gas mixture in water and nucleation rate, and a high content (>75%) of CO<sub>2</sub> accelerates nucleation due to its high solubility. The formation kinetics reveals the preferential uptake of CH<sub>4</sub> into CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates during nucleation and the transition from fast to slow growth due to the rapid conversion of free water into hydrates. After hydrate growth, most water molecules in the systems are converted to CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates, composed of standard cages (5<sup>12</sup>, 5<sup>12</sup>6<sup>2</sup>, 5<sup>12</sup>6<sup>3</sup>, and 5<sup>12</sup>6<sup>4</sup>) and metastable cages (4<sup>1</sup>5<sup>10</sup>6<sup>2</sup>, 4<sup>1</sup>5<sup>10</sup>6<sup>3</sup>, and 4<sup>1</sup>5<sup>10</sup>6<sup>4</sup>). In these incipient hydrates, gas molecules always prefer to occupy the size-fitting cages, i.e., CH<sub>4</sub> in 5<sup>12</sup>, 4<sup>1</sup>5<sup>10</sup>6<sup>2</sup>, 5<sup>12</sup>6<sup>2</sup> cages and CO<sub>2</sub> in 4<sup>1</sup>5<sup>10</sup>6<sup>2</sup>, 5<sup>12</sup>6<sup>2</sup>, respectively. Interestingly, the abundance of 4<sup>1</sup>5<sup>10</sup>6<sup>2</sup> metastable cages (especially those CO<sub>2</sub> occupied), with a size between those of small 5<sup>12</sup> and large 5<sup>12</sup>6<sup>2</sup> cages, suggests their important role in the formation of incipient CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates. Multiple pathways are observed for the nucleation of CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates. In most of the systems, amorphous hydrates are formed with small sI and sII motifs exhibiting short-range orders, while only one system grows into partially ordered solids containing a large sI domain with long-range order spanning the whole simulation box. From bottom-up, this simulation study reveals the complex interplay between gas and water molecules at the condition of hydrate formation, and provides microscopic insights into the nucleation and growth of CH<sub>4</sub>/CO<sub>2</sub> mixed hydrates.