Smith E J, Bryk T, Haymet A D J
Department of Chemistry, University of Houston, Texas 77204-5003, USA.
J Chem Phys. 2005 Jul 15;123(3):34706. doi: 10.1063/1.1953578.
Molecular-dynamics simulations of Cl(-) and Na(+) ions are performed to calculate ionic solvation free energies in both bulk simple point-charge/extended water and ice 1 h at several different temperatures, and at the basal ice 1 h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface. For the ions in bulk water in the NPT ensemble at 298 K and 1 atm, results are found to be in good agreement with experiments, and with other simulation results. Simulations performed in the NVT ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl(-) and Na(+) ions in ice at 150 K are found to be approximately 30 and approximately 20 kcal mol(-1), respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water is the same (within statistical error) as that measured at room temperature, and in the ice is equivalent and approximately 10 kcal mol(-1) less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl(-) ion. However, for the model of Na(+) the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast with the results observed in the bulk calculations.
进行了氯离子（Cl⁻）和钠离子（Na⁺）的分子动力学模拟，以计算在几种不同温度下，在体相简单点电荷/扩展水体系以及冰 Ih 中，以及在冰 Ih/水的基底界面处的离子溶剂化自由能。对于界面，我们计算了离子穿过冰/水界面的“转移”自由能。对于在 298 K 和 1 个大气压下处于 NPT 系综的体相水中的离子，结果与实验以及其他模拟结果吻合良好。在 NVT 系综中进行的模拟显示给出了等效的溶剂化自由能，并且该系综用于界面模拟。发现 Cl⁻和 Na⁺离子在 150 K 的冰中的溶剂化自由能分别比室温下水的溶剂化自由能大约低 30 和大约 20 kcal/mol⁻¹。在模型的熔点附近，水中离子的溶剂化与室温下测量的结果相同（在统计误差范围内），而在冰中的溶剂化与之等效且比液体中的大约低 10 kcal/mol⁻¹。计算了每个离子穿过冰/水界面的转移自由能，并且与 Cl⁻离子的体相观测结果吻合良好。然而，对于 Na⁺的模型，与体相计算中观察到的结果相反，冰中对自由能的长程静电贡献比液体中的更负。
