利用分子動力學模擬探討水合物晶體界面特性

Interfacial Properties of Methane Hydrate and Water via Molecular Dynamics Simulations

何垚煌(Yao-Huang Ho)

指導教授 ： 林祥泰

國立臺灣大學/工學院/化學工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201902960

摘要

分子動態模擬被用來進行研究冰-h以及甲烷水合物晶體的表面性質。我們特別關注於其熱力學性質以及表面波動的動態性質。根據毛細管波動理論，我們測量出對於冰-h在270K以及甲烷水合物在285K下，其界面自由能各為29.24與34.60 mJ/m2。這個結果與實驗值非常相似。在實驗中，冰-h在250K至283K下其界面自由能為25至35mJ/m2；而甲烷水合物則是在260K至285K下，其介面能為31至36mJ/m2。我們的模擬表明，晶體面相對於界面自由能的影響很小，冰-h 和甲烷水合物分別只有1％和3％。隨著冰-h和甲烷水合物晶體的波長增加，表面波的弛豫會漸漸地被毛細波支配。此外，在液相擴散的時間尺度特徵中，甲烷水合物晶體的弛豫時間幾乎是冰-h-晶體的40倍。我們將這種差異歸因於復雜的氫鍵生成與斷裂和籠狀結構的水合物的存在。最後，我們通過模擬毛細波動力學來估計動力係數（晶體生長速率取決於過冷度），並將冰-h和甲烷水合物晶體的動力係數與之前的模擬值與實驗值做比較。

關鍵字

表面波動 ； 界面自由能 ； 晶體面相 ； 熱力學 ； 動力學 ； 分子動力學模擬

並列摘要

Molecular dynamics (MD) simulations were conducted to study the crystal-melt interface of ice and sI methane hydrate crystals. In particular, we focus on both the thermodynamics and dynamics of surface waves. Based on the capillary fluctuation theory, we determined the interfacial free energy of Ice-H (Ih)/water and sI methane hydrate/water to be 29.24mN/m2 at 270K and slightly higher value, 34.60 mN/m2 at 285K, respectively. The results are consistent with experiment, 25~35 mJ/m2 at 250K to 283K for Ih/water interface, and 31~36 mJ/m2 at 260K to 285K, for the sI methane hydrate/water interface. Our simulations show that the effect of orientation of crystal to interfacial free energy is small, only 1% and 3% for Ih/water and sI methane hydrate/water, respectively. The relaxation of surface waves are dominated by the slow process as the wavelength increases for both Ih and sI methane hydrate crystal. Moreover, in a time scale characteristic for the diffusion of the liquid phase, the relaxation time of the crystal-melt interface of sI methane hydrate crystal is almost 40 times slower than that of Ih¬ crystal. We ascribe this difference to the presence of complicate hydrogen bond network and cage-like configuration of hydrate. Finally, we estimate the kinetic coefficient (rate of crystal growth depends the degree of supercooling) from our simulation of the capillary wave dynamics and compare it with previous simulation studies and with experiments for the case of Ih and sI methane hydrate crystal.

並列關鍵字

Surface Wave ； Interfacial Free Energy ； Crystal Orientation ； Thermodynamics ； Dynamics ； Simulation

參考文獻

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