Sandia National Laboratories, MS 1415 and 1322, Albuquerque, New Mexico 87185, USA.
J Am Chem Soc. 2009 Dec 30;131(51):18358-65. doi: 10.1021/ja906190t.
Understanding the acid-base behavior of silica surfaces is critical for many nanoscience and bionano interface applications. Silanol groups (SiOH) on silica surfaces exhibit two acidity constants-one as acidic as vinegar-but their structural basis remains controversial. The atomic details of the more acidic silanol site govern not just the overall surface charge density at near neutral solution pH but also how ions and biomolecules interact with and bind to silica immersed in water. Using ab initio molecular dynamics simulations and multiple representative crystalline silica surfaces, we determine the deprotonation free energies of silanol groups with different structural motifs. We show that previously proposed motifs related to chemical connectivity or intersilanol hydrogen bonds do not yield high acidity. Instead, a plausible candiate for pK(a) = 4.5 silanol groups may be found in locally strained or defected regions with sparse silanol coverage. In the process, irreversible ring-opening reactions of strained silica trimer rings in contact with liquid water are observed.
了解硅表面的酸碱行为对于许多纳米科学和生物纳米界面应用至关重要。硅醇基团（SiOH）在硅表面表现出两个酸度常数 - 一个像醋一样酸性 - 但其结构基础仍存在争议。更酸性的硅醇位点的原子细节不仅决定了在近中性溶液 pH 下的整体表面电荷密度，还决定了离子和生物分子如何与沉浸在水中的二氧化硅相互作用和结合。使用从头算分子动力学模拟和多种代表性的结晶二氧化硅表面，我们确定了具有不同结构基序的硅醇基团的去质子化自由能。我们表明，先前提出的与化学连接性或intersilanol 氢键相关的基序并不能产生高酸度。相反，在具有稀疏硅醇覆盖的局部应变或有缺陷区域中可能找到 pK(a) = 4.5 的硅醇基团的合理候选者。在此过程中，观察到与液态水接触的应变硅三聚物环的不可逆开环反应。
