# The Surprising Role of Urea in Promoting CO2 Hydrate Formation: Enhanced Molecular Diffusivity via Weakening of the Hydrogen-Bond Network

## TL;DR

Urea helps CO2 form hydrates faster by weakening water's hydrogen bonds, increasing molecular mobility despite its large size.

## Contribution

The study reveals how urea disrupts hydrogen-bond networks in water, resolving a contradiction in water diffusivity enhancement.

## Key findings

- Urea competes for hydrogen-bonding sites in water, weakening the hydrogen-bond network.
- Weakened hydrogen bonds lower the energetic barrier for water diffusion, increasing molecular mobility.
- Urea increases hydrate-like water structures while enhancing diffusivity, resolving a paradox in hydrate formation.

## 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.