Miyoshi Tatsuya, Imai Masatoshi, Ohmura Ryo, Yasuoka Kenji
Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan.
J Chem Phys. 2007 Jun 21;126(23):234506. doi: 10.1063/1.2746324.
The free energy differences are calculated for various type-I and type-II clathrate hydrates based on molecular-dynamics simulations, thereby evaluating the thermodynamic stability of the hydrates depending on the chemical species of the guest substances. The simulation systems consist of 27 unit cells, that is, 1242 water molecules and 216 guest molecules for type-I hydrates, and 3672 water molecules and 648 guest molecules for type-II hydrates. The water molecules are described by TIP4P potential, while the guest molecules are described by one-site Lennard-Jones potential, U=4epsilon{(sigma/r)12-(sigma/r)6}, where U is the potential energy, r is the particle distance, sigma is the particle diameter, and epsilon is the energy well depth. The optimal values of sigma that yield the minimum free energy (the best thermodynamic stability) were determined to be 0.39 nm for the type-I hydrates and 0.37 nm for the type-II hydrates.
基于分子动力学模拟计算了各种I型和II型笼形水合物的自由能差，从而根据客体物质的化学种类评估水合物的热力学稳定性。模拟系统由27个晶胞组成，即I型水合物有1242个水分子和216个客体分子，II型水合物有3672个水分子和648个客体分子。水分子用TIP4P势描述，而客体分子用单位点Lennard-Jones势描述，U = 4ε{(σ/r)¹² - (σ/r)⁶}，其中U是势能，r是粒子距离，σ是粒子直径，ε是能量阱深度。对于I型水合物，产生最小自由能（最佳热力学稳定性）的σ的最佳值确定为0.39 nm，对于II型水合物为0.37 nm。
