Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139-4307, USA.
J Chem Phys. 2012 Aug 14;137(6):064706. doi: 10.1063/1.4742854.
Hydroxyl surface density in porous silica drops down to nearly zero when the pH of the confined aqueous solution is greater than 10.5. To study such extreme conditions, we developed a model of slit silica nanopores where all the hydrogen atoms of the hydroxylated surface are removed and the negative charge of the resulting oxygen dangling bonds is compensated by Ca(2+) counterions. We employed grand canonical Monte Carlo and molecular dynamics simulations to address how the Ca(2+) counterions affect the thermodynamics, structure, and dynamics of confined water. While most of the Ca(2+) counterions arrange themselves according to the so-called "Stern layer," no diffuse layer is observed. The presence of Ca(2+) counterions affects the pore filling for strong confinement where the surface effects are large. At full loading, no significant changes are observed in the layering of the first two adsorbed water layers compared to nanopores with fully hydroxylated surfaces. However, the water structure and water orientational ordering with respect to the surface is much more disturbed. Due to the super hydrophilicity of the Ca(2+)-silica nanopores, water dynamics is slowed down and vicinal water molecules stick to the pore surface over longer times than in the case of hydroxylated silica surfaces. These findings, which suggest the breakdown of the linear Poisson-Boltzmann theory, provide important information about the properties of nanoconfined electrolytes upon extreme conditions where the surface charge and ion concentration are large.
当受限水溶液的 pH 值大于 10.5 时，多孔硅中的羟基表面密度降至接近零。为了研究这种极端条件，我们开发了一种狭缝硅纳米孔模型，其中羟基化表面的所有氢原子都被去除，而产生的氧悬空键的负电荷由 Ca(2+) 抗衡离子补偿。我们采用巨正则蒙特卡罗和分子动力学模拟来研究 Ca(2+) 抗衡离子如何影响受限水的热力学、结构和动力学。虽然大多数 Ca(2+) 抗衡离子根据所谓的“斯特恩层”排列，但没有观察到扩散层。Ca(2+) 抗衡离子的存在会影响强受限情况下的孔隙填充，因为表面效应很大。在完全加载时，与完全羟基化表面的纳米孔相比，第一层和第二层吸附水层的分层没有明显变化。然而，水结构和相对于表面的水取向有序性受到的干扰更大。由于 Ca(2+)-硅纳米孔的超亲水性，水动力学被减缓，相邻水分子在孔表面上停留的时间比在羟基化硅表面上更长。这些发现表明线性泊松-玻尔兹曼理论的失效，为表面电荷和离子浓度较大的极端条件下纳米受限电解质的性质提供了重要信息。
