Caratzoulas Stavros, Vlachos Dionisis G, Tsapatsis Michael
Department of Chemical Engineering, University of Delaware, Newark, 19716, USA.
J Am Chem Soc. 2006 Jan 18;128(2):596-606. doi: 10.1021/ja0561136.
We have undertaken explicit solvent molecular dynamics simulations to investigate the preferential stabilization of the silicate octamer Si(8)O(20)(8-) over the hexamer Si(6)O(15)(6-) in relation with the ability of tetramethylammonium (TMA) to form an adsorption layer around these cage-like polyions. We have found that the hexamer cannot support such a layer and as a result is vulnerable to hydrolysis. The dynamics of TMA desorption off the surface of the hexamer is investigated in connection with the solvent dynamics. We have studied the energetics of this preferential stabilization by calculating the relative change in the free energies of formation between the complexes Si(8)O(20)(8-).8TMA and Si(6)O(15)(6-).6TMA and found the former to be more stable by 70 kcal/mol. We also find that the energetics are consistent with experimental data, suggesting that the hexamer is a long-lived metastable species. Furthermore, we have studied the solvent structure and dynamics in the vicinity of both the bare polyions and their complexes with TMA. We have found that, as anticipated, both the octamer and the hexamer participate in hydrogen bonds with the water molecules, regardless of whether a TMA adsorption layer exists or not. In fact, we find that the presence of a TMA adsorption layer has a rather profound effect on the stability of these hydrogen bonds-it increases their lifetime by at least a factor of 2 relative to that of the hydrogen bonds between water and the bare polyions.
我们进行了显式溶剂分子动力学模拟，以研究与四甲基铵（TMA）在这些笼状聚离子周围形成吸附层的能力相关的，硅酸八聚体Si(8)O(20)(8-)相对于六聚体Si(6)O(15)(6-)的优先稳定性。我们发现六聚体无法支撑这样的一层，因此容易水解。结合溶剂动力学研究了TMA从六聚体表面解吸的动力学。我们通过计算配合物Si(8)O(20)(8-).8TMA和Si(6)O(15)(6-).6TMA形成自由能的相对变化，研究了这种优先稳定性的能量学，发现前者比后者更稳定70千卡/摩尔。我们还发现能量学与实验数据一致，表明六聚体是一种长寿命的亚稳物种。此外，我们研究了裸露聚离子及其与TMA配合物附近的溶剂结构和动力学。我们发现，正如预期的那样，无论是否存在TMA吸附层，八聚体和六聚体都与水分子形成氢键。事实上，我们发现TMA吸附层的存在对这些氢键的稳定性有相当深远的影响——相对于水与裸露聚离子之间的氢键，它使氢键的寿命增加了至少2倍。
