以分子動力學模擬探討甲硫胺酸對二氧化碳水合物生長及成核的促進效果

Abstract

由於甲硫胺酸已被許多研究證實為二氧化碳水合物的促進劑，但目前仍尚未有研究結果表明其促進機制，因此本研究的目的是透過分子動力學（MD）模擬來探討甲硫胺酸對二氧化碳水合物成核和成長過程的影響，並進一步找出其促進機制。我們的模擬包含二氧化碳水合物的成核以及成長兩個部分，關於二氧化碳水合物的成長，從模擬結果可以看出當初始系統的液相中存在低濃度(0.56 wt%)的甲硫胺酸時對二氧化碳水合物的生長過程會有促進效果，而目前有發現甲硫胺酸具有促進效果的研究中所使用的甲硫胺酸濃度也都在1.0 wt%以下，因此我們的模擬結果和實驗結果相當一致。另外，我們觀察到當在模擬過程中甲硫胺酸存在於水合物和液體的界面附近的數量較少時，才能觀察到對二氧化碳水合物的生長的促進效果，同時我們也發現添加甲硫胺酸會增加二氧化碳分子的擴散係數，加快其質傳速率，這應為甲硫胺酸主要的促進原因。 而對於二氧化碳水合物的成核，在我們模擬的溫度和壓力條件下，並未觀察到甲硫胺酸對水合物成核速率有具有統計學意義的促進效果，然而我們發現在壓力為30 bar且過冷溫度約為26 K時，初始液相中含有0.82 wt%甲硫胺酸的系統在成核後水合物的生長速率相較於無添加甲硫胺酸的系統約有36.61%的提升，此濃度和先前二氧化碳水合物生長測試中觀察到的促進現象的甲硫胺酸濃度(0.56 wt%)相當接近，再次表現出了結果的一致性。經過分析後得知，在水合物生長過程中有甲硫胺酸靠近的那一側水合物會生長的較快，顯示出甲硫胺酸對於二氧化碳水合物生長的潛在貢獻。

Due to the fact that methionine has been confirmed by many studies as a promoter of CO2 hydrate formation, but the mechanism of this promotion has not yet been clarified, the purpose of this study is to investigate the effect of methionine on the nucleation and growth processes of CO2 hydrate through molecular dynamics (MD) simulations and to further identify its promoting mechanism. Our simulations include both the nucleation and growth of CO2 hydrate. Regarding CO2 hydrate growth, the simulation results show that the presence of low concentrations (0.56 wt%) of methionine in the initial liquid phase of the system promotes the growth of CO2 hydrate. The concentrations of methionine found to have a promoting effect in previous studies are also below 1.0 wt%, indicating a high degree of consistency between our simulation results and experimental findings. Additionally, we observed that the promotion of CO2 hydrate growth is evident when a smaller amount of methionine molecules is present near the hydrate-liquid interface during the simulation process. We also found that the addition of methionine increases the diffusion coefficient of CO2 molecules, accelerating their mass transfer rate, which is likely the main reason for methionine's promoting effect. For the nucleation of CO2 hydrate, our simulations did not show a statistically significant promoting effect of methionine on the nucleation rate under the temperature and pressure conditions simulated. However, we found that at a pressure of 30 bar and a subcooling temperature of approximately 26 K, the system with 0.82 wt% methionine in the initial liquid phase exhibited a 36.61% increase in the growth rate of hydrate after nucleation compared to the system without methionine. This concentration is very close to the methionine concentration (0.56 wt%) observed to have a promoting effect in previous CO2 hydrate growth tests, again demonstrating consistency in the results. Our analysis results show that during the growth process, the side of the hydrate with methionine nearby grows faster, indicating the potential contribution of methionine to the growth of CO2 hydrate.