Promotion mechanism of carbon dioxide hydrate formation by L-Methionine and its competitive effects with NaCl

The growth kinetics and macroscopic morphology evolution of CO2 hydrate promoted by L-Methionine (L-Met) in NaCl aqueous solutions in a wide range of mass fractions were systematically investigated. 0.05 wt% was the threshold mass fraction of L-Met to have prominent promotion effects in a batch mode. The addition of NaCl could suppress its promotion performance, while it could be regained by further addition of L-Met to 1.0 wt%. Porous feature and wall-climbing phenomena of CO2 hydrate were obvious in the presence of L-Met while it could be suppressed by NaCl. The tangential (lateral) growth rate of the hydrate film on planar pure water surface was the highest while both L-Met and NaCl could decrease it. For L-Met, it was the highest at 0.05 wt%. Special ball-like, mosaic-like and sword-like appearances of CO2 hydrates were found. Both the adsorption-capillary effects of L-Met and the hydration effects of L-Met and NaCl were thought to account for all the phenomena and a detailed scenario was proposed and discussed. A macroscopic growth model based on the tangential growth rates of hydrate films was proposed and evaluated.