Adsorption-Induced Surface Stresses of the Water/Quartz Interface: Ab Initio Molecular Dynamics Study

Adsorption-induced deformation is expansion or contraction of a solid due to adsorption on its surface. This phenomenon is important for a wide range of applications, from chemomechanical sensors to methane recovery from geological formations. The strain of the solid is driven by the change of the surface stress due to adsorption. Using ab initio molecular dynamics, we calculate the surface stresses for the dry α-quartz surfaces, and investigate how these stresses change when the surfaces are exposed to water. We find that the nonhydroxylated surface shows small and approximately isotropic changes in stress, while the hydroxylated surface, which interacts more strongly with the polar water molecules, shows larger and qualitatively anisotropic (opposite sign in xx and yy) surface stress changes. All of these changes are several times larger than the surface tension of water itself. The anisotropy and possibility of positive surface stress change can explain experimentally observed surface area contraction due to adsorption.吸附诱导形变（Adsorption-induced deformation）指固体因表面吸附作用而产生膨胀或收缩的现象。该现象的应用场景十分广泛，从化学机械传感器到地质储层甲烷开采均有涉及。固体的应变由吸附过程引发的表面应力变化所驱动。本研究采用从头算分子动力学（ab initio molecular dynamics）方法，计算了干燥α-石英表面的表面应力，并探究了该类表面与水接触时的应力变化规律。研究结果表明：非羟基化表面的应力变化幅度较小且近似各向同性；而与极性水分子相互作用更强的羟基化表面，其应力变化幅度更大，且呈现出明显的各向异性（xx与yy方向应力变化符号相反）。上述所有应力变化的幅度均较纯水的表面张力高出数倍。这种各向异性特征以及正表面应力变化的可能性，可对实验中观测到的吸附诱导表面积收缩现象给出合理解释。