受限甲烷水合物相稳定性降低和快速生成/分解动力学的分子起源
Molecular origin of the reduced phase stability and faster formation/dissociation kinetics in confined methane hydrate
摘要：
利用先进的分子模拟技术结合中尺度热力学方法，揭示了纳米尺度甲烷水合物形成/分解的微观机理。通过原子尺度的模拟探测共存的限制和自由能计算，相稳定性的限制甲烷水合物被限制在一个较窄的温度和压力域比其散装对应。在给定的压力下，这是与现有的实验数据相一致的熔点抑郁症，示出使用吉布斯-汤姆逊形式主义，如果使用的孔隙/液体和孔隙/水合物表面张力的准确估计进行定量描述。水合物形成和分解时的亚稳势垒被发现减少后，约束，因此提供了一个分子尺度的图片，在实验中观察到的约束气体水合物的更快的动力学。通过考虑不同的形成机制-散装均匀成核，外表面成核，和孔隙内的限制成核-我们确定了一个交叉的成核过程中，在孔中形成的临界核对应于半球形帽或桥核取决于温度，接触角，和孔径。使用经典的成核理论，对于这两种机制，典型的诱导时间示出与孔表面体积比，因此，倒数孔径缩放。这些发现的临界核和成核速率与这种复杂的过渡提供了一种手段，合理化和预测甲烷水合物的形成在任何多孔介质从简单的热力学数据。
英文摘要：
The microscopic mechanisms involved in the formation/dissociation of methane hydrate confined at the nanometer scale are unraveled using advanced molecular modeling techniques combined with a mesoscale thermodynamic approach. By means of atom-scale simulations probing coexistence upon confinement and free energy calculations, phase stability of confined methane hydrate is shown to be restricted to a narrower temperature and pressure domain than its bulk counterpart. The melting point depression at a given pressure, which is consistent with available experimental data, is shown to be quantitatively described using the Gibbs--Thomson formalism if used with accurate estimates for the pore/liquid and pore/hydrate surface tensions. The metastability barrier upon hydrate formation and dissociation is found to decrease upon confinement, therefore providing a molecular scale picture for the faster kinetics observed in experiments on confined gas hydrates. By considering different formation mechanisms -- bulk homogeneous nucleation, external surface nucleation, and confined nucleation within the porosity -- we identify a crossover in the nucleation process; the critical nucleus formed in the pore corresponds either to a hemispherical cap or a bridge nucleus depending on temperature, contact angle, and pore size. Using the classical nucleation theory, for both mechanisms, the typical induction time is shown to scale with the pore surface to volume ratio and, hence, the reciprocal pore size. These findings for the critical nucleus and nucleation rate associated to such complex transitions provide a mean to rationalize and predict methane hydrate formation in any porous media from simple thermodynamic data.
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