Resolving Temperature-Dependent Hydrate Nucleation Pathway: The Role of "Transition Layer"

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Natural gas hydrate (NGH) nucleation is critical for NGH recovery, gas storage, and separation.
• Understanding nucleation mechanisms under varying conditions is essential for industrial applications.
• Classical nucleation theory often describes rounded nuclei, but nonclassical pathways may exist.

Purpose of the Study:

• To investigate NGH nucleation events at different supercooling degrees using molecular dynamics (MD) simulations.
• To develop and apply an order parameter (OP) for characterizing hydrate nucleus size and shape.
• To explore the free energy landscapes of hydrate nucleation and elucidate the underlying mechanisms.

Main Methods:

• Extensive molecular dynamics (MD) simulations were performed to trace numerous hydrate nucleation events.
• A novel evolutionary order parameter (OP) was developed to precisely identify hydrate nucleus size and shape.
• Free energy landscapes were analyzed using the developed OP to understand nucleation pathways.

Main Results:

• At 270 K (0.92 Tm supercooling), near-rounded nuclei consistent with classical nucleation theory were observed.
• At stronger driving forces (0.85 and 0.88 Tm), nonclassical nucleation pathways leading to elongated nuclei became significant.
• A 'transition layer' between the nucleus and aqueous solution was proposed, retaining methane and facilitating collisions, promoting elongated nuclei formation at higher supercooling.

Conclusions:

• The 'transition layer' concept provides deeper insight into NGH nucleation, explaining the shift towards nonclassical pathways at higher supercooling.
• This layer's functionalities (methane retention and facilitated collision) promote elongated nucleus formation and offset surface free energy costs.
• The findings suggest the transition layer model could be applicable to other types of hydrate nucleation.