Geochemistry Department, Sandia National Laboratories , Albuquerque, New Mexico 87185, United States.
Science-Based Material Modeling Department, Sandia National Laboratories , Livermore, California 94551, United States.
Langmuir. 2017 Apr 18;33(15):3882-3891. doi: 10.1021/acs.langmuir.7b00041. Epub 2017 Apr 4.
Surface energies of silicates influence crack propagation during brittle fracture and decrease with surface relaxation caused by annealing and hydroxylation. Molecular-level simulations are particularly suited for the investigation of surface processes. In this work, classical MD simulations of silica surfaces are performed with two force fields (ClayFF and ReaxFF) to investigate the effect of force field reactivity on surface structure and energy as a function of surface hydroxylation. An unhydroxylated fracture surface energy of 5.1 J/m is calculated with the ClayFF force field, and 2.0 J/m is calculated for the ReaxFF force field. The ClayFF surface energies are consistent with the experimental results from double cantilever beam fracture tests (4.5 J/m), whereas ReaxFF underestimated these surface energies. Surface relaxation via annealing and hydroxylation was performed by creating a low-energy equilibrium surface. Annealing condensed neighboring siloxane bonds increased the surface connectivity, and decreased the surface energies by 0.2 J/m for ClayFF and 0.8 J/m for ReaxFF. Posthydroxylation surface energies decreased further to 4.6 J/m with the ClayFF force field and to 0.2 J/m with the ReaxFF force field. Experimental equilibrium surface energies are ∼0.35 J/m, consistent with the ReaxFF force field. Although neither force field was capable of replicating both the fracture and equilibrium surface energies reported from experiment, each was consistent with one of these conditions. Therefore, future computational investigations that rely on accurate surface energy values should consider the surface state of the system and select the appropriate force field.
硅酸盐的表面能会影响脆性断裂过程中的裂纹扩展，并随着退火和羟基化引起的表面弛豫而降低。分子水平模拟特别适合于研究表面过程。在这项工作中，使用两种力场（ClayFF 和 ReaxFF）对二氧化硅表面进行了经典的 MD 模拟，以研究力场反应性对表面结构和能量的影响，作为表面羟基化的函数。ClayFF 力场计算出未羟基化的断裂表面能为 5.1 J/m，而 ReaxFF 力场计算出的表面能为 2.0 J/m。ClayFF 的表面能与双悬臂梁断裂试验的实验结果（4.5 J/m）一致，而 ReaxFF 则低估了这些表面能。通过创建低能量平衡表面来进行退火和羟基化导致的表面弛豫。退火使相邻的硅氧烷键凝聚，增加了表面的连接性，并使 ClayFF 的表面能降低了 0.2 J/m，使 ReaxFF 的表面能降低了 0.8 J/m。羟基化后的表面能进一步降低，ClayFF 力场的表面能降至 4.6 J/m，ReaxFF 力场的表面能降至 0.2 J/m。实验平衡表面能约为 0.35 J/m，与 ReaxFF 力场一致。虽然两种力场都不能复制实验报告的断裂和平衡表面能，但它们都与其中一种条件一致。因此，未来依赖于准确表面能值的计算研究应该考虑系统的表面状态，并选择合适的力场。
