Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima, Okayama 700-8530, Japan.
J Phys Chem B. 2011 Dec 8;115(48):14256-62. doi: 10.1021/jp205067v. Epub 2011 Sep 8.
We present a method that brings prediction of phase behaviors of various clathrate hydrates with firm statistical mechanical ground adopting a different reference state from the usual one. Accommodation of a large guest molecule makes the frequencies of the lattice vibrational motions higher, which is one of the breakdowns of the assumptions in the original van der Waals and Platteeuw theory. The frequency modulations are incorporated in the free energy of cage occupation in the present method. Moreover, the reference state, which is originally the corresponding empty clathrate structure, is alternated to a state where cages of at least one sort are fully occupied. This meets the stability condition of clathrate hydrates that most of the cages should be accommodated. Owing to this new reference state, the thermodynamic stability is evaluated with reasonable accuracy from the free energy of cage occupation especially by a large guest molecule without considering its dependence on the cage occupancy. This conversion is also beneficial to establish a relation between the chemical potential of water and the cage occupancy from grandcanonical Monte Carlo simulation. We show a new method indeed works well in predicting the dissociation pressures of clathrate hydrates containing isobutane, propane, ethane, Xe, and CF(4).
我们提出了一种方法，该方法采用与通常情况不同的参考状态，从坚实的统计力学基础出发，对各种笼形水合物的相行为进行预测。较大客体分子的容纳会使晶格振动的频率升高，这是范德华和普拉特尤理论原始假设的一个突破。在本方法中，频率调制被纳入笼占据的自由能中。此外，参考状态最初是相应的空笼形结构，被改变为至少一种类型的笼完全占据的状态。这满足了笼形水合物的稳定性条件，即大多数笼应该被容纳。由于这种新的参考状态，从笼占据的自由能，特别是从没有考虑其对笼占据依赖性的大客体分子，以合理的精度评估热力学稳定性。这种转换还有利于从巨正则蒙特卡罗模拟建立水的化学势与笼占据之间的关系。我们确实展示了一种新方法可以很好地预测包含异丁烷、丙烷、乙烷、Xe 和 CF4 的笼形水合物的离解压力。
