Department of Physics, College of Mathematics and Physics, Chengdu University of Technology, Chengdu 610059, China.
Laboratoire Charles Coulomb (L2C), University of Montpellier and CNRS, F-34095 Montpellier, France.
J Chem Phys. 2023 Jun 28;158(24). doi: 10.1063/5.0155497.
Using large-scale molecular dynamics simulations, we investigate the surface properties of lithium, sodium, and potassium silicate glasses containing 25 mol % of alkali oxide. The comparison of two types of surfaces, a melt-formed surface (MS) and a fracture surface (FS), demonstrates that the influence of the alkali modifier on the surface properties depends strongly on the nature of the surface. The FS exhibits a monotonic increase of modifier concentration with increasing alkali size while the MS shows a saturation of alkali concentration when going from Na to K glasses, indicating the presence of competing mechanisms that influence the properties of a MS. For the FS, we find that larger alkali ions reduce the concentration of under-coordinated Si atoms and increase the fraction of two-membered rings, implying an enhanced chemical reactivity of the surface. For both types of surfaces, the roughness is found to increase with alkali size, with the effect being more pronounced for the FS than for the MS. The height-height correlation functions of the surfaces show a scaling behavior that is independent of the alkali species considered: The ones for the MS are compatible with the prediction of the frozen capillary wave theory while the ones for the FS show a logarithmic growth, i.e., on the nanoscale these surfaces are not self-affine fractals. The influence of the modifier on the surface properties are rationalized in terms of the interplay between multiple factors involving the size of the ions, bond strength, and charge balance on the surface.
使用大规模分子动力学模拟，我们研究了含有 25mol% 碱金属氧化物的锂、钠和钾硅酸盐玻璃的表面性质。两种类型表面（熔体形成表面 (MS) 和断裂表面 (FS)）的比较表明，碱改性剂对表面性质的影响强烈依赖于表面的性质。FS 表现出随着碱金属尺寸的增加，改性剂浓度单调增加，而 MS 则表明从 Na 到 K 玻璃时碱浓度饱和，表明存在竞争机制影响 MS 的性质。对于 FS，我们发现较大的碱离子减少了配位不足的 Si 原子的浓度，并增加了二聚环的分数，这意味着表面的化学反应性增强。对于这两种类型的表面，粗糙度都随着碱金属尺寸的增加而增加，FS 的效果比 MS 更明显。表面的高度-高度相关函数表现出与所考虑的碱物种无关的标度行为：MS 的那些与冻结毛细波理论的预测兼容，而 FS 的那些则表现出对数增长，即在纳米尺度上，这些表面不是自仿射分形的。改性剂对表面性质的影响可以根据涉及离子大小、键强度和表面电荷平衡的多种因素之间的相互作用来合理化。
