Abstract
Urea is known to act as a kinetic promoter for CO(2) hydrate formation by reducing the nucleation induction time. Recent molecular simulation studies suggest that urea facilitates CO(2) hydrate growth by lowering the mass transfer resistance of CO(2) in water, as evidenced by increased diffusivity of both CO(2) 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.