The Surprising Role of Urea in Promoting CO<sub>2</sub> Hydrate Formation: Enhanced Molecular Diffusivity via Weakening of the Hydrogen-Bond Network.

Abstract

Urea is known to act as a kinetic promoter for CO₂ hydrate formation by reducing the nucleation induction time. Recent molecular simulation studies suggest that urea facilitates CO₂ hydrate growth by lowering the mass transfer resistance of CO₂ in water, as evidenced by increased diffusivity of both CO₂ and water. However, the enhancement of water diffusivity is counterintuitive, given urea's relatively large size, strong affinity for water, and its tendency to increase the fraction of hydrate-like water structures─conditions typically associated with reduced mobility. To resolve this apparent contradiction, we employ molecular dynamics simulations to examine how urea modifies the hydrogen-bonding network of water, considering both structural and energetic aspects. Our results reveal that urea subtly disrupts the water hydrogen-bond network by competing for bonding sites. The resulting urea-water and adjacent water-water hydrogen bonds are weaker than those in bulk water-water interactions, leading to a locally weakened hydrogen-bond network. This reduction in hydrogen-bond strength lowers the energetic barrier for water diffusion, thereby enhancing molecular mobility. These findings reconcile the seemingly paradoxical increase in both water diffusivity and hydrate-like structuring in the presence of urea, offering new insight into how small organic solutes modulate water structure and dynamics in hydrate-forming environments.