Wick Collin D, Xantheas Sotiris S
Department of Chemistry, Louisiana Tech University, Ruston, 71270, USA.
J Phys Chem B. 2009 Apr 2;113(13):4141-6. doi: 10.1021/jp806782r.
Molecular dynamics simulations with polarizable interaction potentials were carried out to understand the solvation structure of chloride and iodide anions in bulk and interfacial water, showing qualitative similarities between the first solvation shell structures at the interface and bulk. For the more polarizable iodide, its solvation structure was found to be more anisotropic than chloride, and this trend persisted at both the interface and in the bulk. The anisotropy of the solvation structure correlated with polarizability, but it was also found to inversely correlate with anion size. When polarizability was reduced to near zero, a very small anisotropy in the water solvation structure around the ion still persisted. Polarizable anions were found to have on average an induced dipole in the bulk that was significantly larger than zero. This induced dipole resulted in the water hydrogen atoms having stronger interactions with the anions on one side of them, in which the dipole was pointing. In contrast, the other side of the anions, in which the induced dipole was pointing away from, had fewer water molecules present and, for the case of iodide, was rather devoid of water molecules all together at both the interface and in the bulk. This region formed a small cavity in the bulk, whereas at the air-water interface it was simply part of the air interface. In the bulk, this small cavity may be viewed as somewhat hydrophobic, and the need for the extinction of this cavity may be one of the major driving forces for the more polarizable anions to reside at the air-water interface.
采用具有极化相互作用势的分子动力学模拟来理解氯离子和碘离子在本体水和界面水中的溶剂化结构，结果表明界面处和本体中第一溶剂化层结构存在定性相似性。对于极化率更高的碘离子，发现其溶剂化结构比氯离子更具各向异性，且这种趋势在界面处和本体中均持续存在。溶剂化结构的各向异性与极化率相关，但也发现与阴离子大小呈负相关。当极化率降低到接近零时，离子周围水溶剂化结构中仍存在非常小的各向异性。发现可极化阴离子在本体中平均具有一个显著大于零的诱导偶极。这个诱导偶极导致水分子的氢原子在其偶极指向的一侧与阴离子有更强的相互作用。相反，阴离子的另一侧，即诱导偶极背离的一侧，存在的水分子较少，对于碘离子而言，在界面处和本体中该侧几乎完全没有水分子。在本体中，这个小空腔可被视为有点疏水，消除这个空腔的需求可能是更可极化阴离子驻留在气 - 水界面的主要驱动力之一。
