Holzmann Jörg, Ludwig Ralf, Geiger Alfons, Paschek Dietmar
Institut für Chemie, Abteilung Physikalische Chemie, Universität Rostock, Dr.-Lorenz-Weg 1,D-18051 Rostock, Germany.
Chemphyschem. 2008 Dec 22;9(18):2722-30. doi: 10.1002/cphc.200800544.
We perform molecular dynamics (MD) simulations of aqueous salt (NaCl) solutions using the TIP4P-Ew water model (Horn et al., J. Chem. Phys. 2004, 120, 9665) covering broad temperature and concentration ranges extending deeply into the supercooled region. In particular we study the effect of temperature and salt concentration on the solvation of methane at infinite dilution. The salt effect on methane's solvation free energy, solvation enthalpy and entropy, as well as their temperature dependence is found to be semi-quantitatively in accordance with the data of Ben-Naim and Yaacobi (J. Phys. Chem. 1974, 78, 170). To distinguish the influence of local (in close proximity to ions) and global effects, we partition the salt solutions into ion influenced hydration shell regions and bulk water. The chemical potential of methane is systematically affected by the presence of salt in both sub volumes, emphasizing the importance of the global volume contraction due to electrostriction effects. This observation is correlated with systematic structural alterations similar to water under pressure. The observed electrostriction effects are found to become increasingly pronounced under cold (supercooled) conditions. We find that the influence of temperature and salt induced global density changes on the solvation properties of methane is well recovered by simple scaling relation based on predictions of the information theory model of Garde et al. (Phys. Rev. Let. 1999, 77, 4966).
我们使用TIP4P-Ew水模型（Horn等人，《化学物理杂志》，2004年，第120卷，第9665页）对盐水（NaCl）溶液进行分子动力学（MD）模拟，涵盖了广泛的温度和浓度范围，深入到过冷区域。特别是，我们研究了温度和盐浓度对无限稀释下甲烷溶剂化的影响。发现盐对甲烷溶剂化自由能、溶剂化焓和熵的影响及其温度依赖性与Ben-Naim和Yaacobi的数据（《物理化学杂志》，1974年，第78卷，第170页）在半定量上一致。为了区分局部（靠近离子）和全局效应的影响，我们将盐溶液划分为受离子影响的水合壳层区域和本体水。在这两个子体积中，盐的存在都会系统地影响甲烷的化学势，强调了由于电致伸缩效应导致的全局体积收缩的重要性。这一观察结果与类似于受压水的系统结构变化相关。发现在寒冷（过冷）条件下，观察到的电致伸缩效应变得越来越明显。我们发现，基于Garde等人的信息理论模型（《物理评论快报》，1999年，第77卷，第4966页）的预测，通过简单的比例关系可以很好地恢复温度和盐引起的全局密度变化对甲烷溶剂化性质的影响。
