Effect of surfactant crowding on clathrate hydrate growth

Abstract

The present study investigates the influence of surfactant crowding on hydrate growth and detachment of hydrate crystal from the interface in a droplet-based configuration. Experiments were conducted under a constant subcooling of 5 °C using a cyclopentane droplet of volume 5 μL immersed in the water pool. Sorbitan Monooleate (Span 80) was used as a surfactant at various concentrations below and above critical micelle concentration (CMC). Hydrate forming droplet images were captured in backlight illumination. The centroid of the droplet was also tracked to explain the morphological changes during hydrate formation. Surfactant crowding occurs due to rearrangement of surfactants at the guest-water interface during clathrate hydrate growth. The presence of surfactant encouraged radial hydrate growth and formation of hydrate columns. The effect of radial growth manifested in form of dramatic rise in the vertical position of the centroid, that is, 125.15% higher than no surfactant case for 0.00143% v/v surfactant concentration. The hydrate crystals detach after the initial growth period for surfactant concentrations closer to 10% CMC. The forces affecting the crystal detachment process are illustrated, and an approximate mathematical model was proposed to find the height of the crystal for initiation of detachment of the crystal. The analysis revealed that crystals with a narrow base tend to gain more height vertically before detachment, complementing experimental observations. Overall, the present study illustrates the significant role of surfactant crowding in modifying the hydrate growth process and morphology. The detachment of the hydrate crystals from the interface can serve as an alternative mechanism for rapid hydrate growth during the presence of surfactant.

Graphical Abstract

Keywords:

• Surfactant
• hydrate
• droplet
• crystal detachment

Acknowledgments

We gratefully acknowledge the funding and support by IIT Kanpur for this work.