Clathrate Hydrates
Clathrate hydrates are ice-like crystalline structures consisting of water and small non-polar guest molecules that form at high pressures typically above the freezing point of water (T > 273.15 K). The crystalline structures of hydrates consist of regular cages formed by water molecules encaging the guest molecules. The water forming the cages are stabilized via non-covalent hydrogen bonds. Clathrate hydrates of methane are abundant in the regions where the formation conditions occur, such as the ocean floor, the permafrost near the poles, and in some industrial processes. In the oil and gas industry, clathrate hydrates represent a security concern that could potentially cause environmental damages and economical loses. The goal of this project is to understand the fundamental science of hydrate nucleation to design the mechanisms to prevent their formation in industrial applications.
(a) Methane clathrate block embedded in the sediment of hydrate ridge, off Oregon, USA (source: Wikipedia). (b) Clathrate hydrate cage from molecular dynamics simulations.
Nucleation of Methane Hydrates at Moderate Subcooling by Molecular Dynamics Simulations
J. Phys. Chem. C 2014, 118, 21, 11310–11318
I employed molecular dynamics simulations to uncover the molecular mechanisms of clathrate hydrates nucleation and growth, which are difficult to resolve experimentally. The nucleation of crystals is a rare process difficult to capture in simulations. During nucleation, small nuclei of crystals randomly appear and disappear. To grow into a bulk crystal, the systems must overcome a free energy barrier which occurs when the nuclei are larger than certain critical nucleation radius. I performed the simulations at high methane supersaturation which dramatically reduces the nucleation time and allowed me to simulate nucleation at moderate subcooling. In the simulations, the crystallization initiates from the spontaneous formation of amorphous clusters wherein structures I, II, and other ordered defects emerge (see Figure and movie). I observed that the order of the crystalline structure increases by decreasing the subcooling while the crystalline structures I and II form and coexist at moderately low temperatures.
Snapshots from molecular dynamics simulations showing domains of structures I and II.
Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates
ACS Central Science 2018, 4, 7, 820–831
In the energy industry, clathrate hydrates may form swiftly during hydrocarbon transportation and obstruct the flowlines. The blockage may lead to severe infrastructure damages, economic loses, and casualties. Therefore, hydrate mitigation is necessary and often is performed by means of chemical additives classified as thermodynamic inhibitors (TIs) and low dosage inhibitors (LDIs). The adsorption of functional molecules on the surface of hydrates is necessary to develop hydrate inhibitors. The goal of this project is to understand the fundamental science to design the mechanisms to prevent the formation of clathrate hydrates in industrial applications. I used molecular dynamics simulations to investigate the adsorption of hydrophobic, polar, and ionic molecules on the surface of clathrate hydrates and I found a strong connection between the water ordering around solutes in bulk and the affinity for the hydrates surface. I distinguished two types of water ordering around solutes: (i) hydrophobic hydration where water molecules form a hydrogen bond network similar to clathrate hydrates, and (ii) ionic hydration where water molecules align according to the polarity of an ionic group. Surprisingly, the hydrophobic groups are the ones that better adsorb on the surface of hydrates.