Holmberg Nico, Chen Jian-Cheng, Foster Adam S, Laasonen Kari
Department of Chemistry, Aalto University, P.O. Box 16100, FI-00076 Aalto, Finland.
Phys Chem Chem Phys. 2014 Sep 7;16(33):17437-46. doi: 10.1039/c4cp00635f.
The dissolution of NaCl has been systematically investigated by employing ab initio molecular dynamics (AIMD) on different NaCl nanocrystals as well as on a surface system immersed in water. We discovered a complex dissolution process simultaneously involving multiple ions initiated at the corner sites of the crystal. Our simulations indicated a difference in the dissolution rates of sodium and chlorine. While sodiums readily became partially solvated, chlorines more frequently transitioned into the fully solvated state leading to an overall greater dissolution rate for Cl. We determined that this difference arises due to faster water mediated elongations of individual ionic bonds to Na, but a significantly slower process for the last bond in comparison to Cl. In an attempt to investigate this phenomenon further, we performed metadynamics based free energy simulations on a surface slab presenting corner sites similar to those in cubic crystals, aiming to extract the dissolution free energy profile of corner ions. In qualitative agreement with the nanocrystal simulations, this revealed a shallower first free energy minimum for Na, but no statistically significant difference in the corresponding barriers and inconclusive results for the latter stage. Finally, simulations of smaller NaCl crystals illustrated how dissolution proceeds beyond the point of crystal lattice collapse, highlighting the strength of solvated ion interactions.
通过对不同的氯化钠纳米晶体以及浸入水中的表面系统进行从头算分子动力学（AIMD），对氯化钠的溶解过程进行了系统研究。我们发现了一个复杂的溶解过程，该过程同时涉及多个离子，起始于晶体的角位点。我们的模拟表明钠和氯的溶解速率存在差异。虽然钠很容易部分溶剂化，但氯更频繁地转变为完全溶剂化状态，导致氯的整体溶解速率更高。我们确定这种差异是由于水介导的与钠的单个离子键伸长更快，但与氯相比，最后一个键的过程明显更慢。为了进一步研究这一现象，我们在一个呈现出与立方晶体中角位点相似的表面平板上进行了基于元动力学的自由能模拟，旨在提取角离子的溶解自由能分布。与纳米晶体模拟定性一致，这表明钠的第一个自由能最小值更浅，但在相应的势垒方面没有统计学上的显著差异，后期结果也不确定。最后，较小氯化钠晶体的模拟说明了溶解如何在晶格坍塌点之后继续进行，突出了溶剂化离子相互作用的强度。
