Department of Chemical Engineering, University of California , Davis, California 95616, United States.
Department of Chemistry and Chemical Engineering, Shanghai Jiao Tong University , Shanghai 200240, China.
Langmuir. 2016 Jul 19;32(28):7045-55. doi: 10.1021/acs.langmuir.6b00934. Epub 2016 Jul 8.
We perform reactive molecular dynamics simulations of monolayer formation by silanes on hydroxylated silica substrates. Solutions composed of alkylmethoxysilanes or alkylhydroxysilanes in hexane are placed in contact with a hydroxylated silica surface and simulated using a reactive force field (ReaxFF). In particular, we have modeled the deposition of butyl-, octyl-, and dodecyltrimethoxysilane to observe the dependence of alkylsilyl chain length on monolayer formation. We additionally modeled silanization using dodecyltrihydroxysilane, which allows for the comparison of two grafting mechanisms of alkoxysilanes: (1) direct condensation of alkoxysilane with surface-bound silanols and (2) a two-step hydrolysis-condensation mechanism. To emulate an infinite reservoir of reactive solution far away from the substrate, we have developed a method in which new precursor molecules are periodically added to a region of the simulation box located away from the surface. It is determined that the contact angle of alkyl tails bound to the surface is dependent on their grafting density. During the early stages of grafting alkoxy- and hydroxysilanes to the substrate, a preference is shown for silanes to condense with silanols further from the substrate surface and also close to neighboring surface-bound silanols. The kinetics of silica silanization by hydroxysilanes was observed to be much faster than for methoxysilanes. However, the as-deposited hydroxysilane monolayers show similar morphological characteristics to those formed by methoxysilanes.
我们进行了硅烷在羟基化硅基底上形成单层的反应分子动力学模拟。将包含烷氧基硅烷或烷醇硅烷的正己烷溶液与羟基化硅表面接触，并使用反应力场 (ReaxFF) 进行模拟。特别是，我们模拟了丁基、辛基和十二烷基三甲氧基硅烷的沉积，以观察烷基硅烷基链长度对单层形成的依赖性。我们还使用十二烷基三羟硅烷模拟了硅烷化，这允许比较两种烷氧基硅烷的接枝机制：（1）烷氧基硅烷与表面结合的硅醇的直接缩合和（2）水解-缩合的两步机制。为了模拟远离基底的反应性溶液的无限储库，我们开发了一种方法，即定期向远离表面的模拟盒区域添加新的前体分子。结果表明，与表面结合的烷基尾部的接触角取决于其接枝密度。在将烷氧基和羟基硅烷接枝到基底的早期阶段，硅烷与离基底表面更远且靠近相邻表面结合的硅醇缩合的趋势更强。观察到羟基硅烷对二氧化硅的硅烷化动力学比甲氧基硅烷快得多。然而，沉积的羟基硅烷单层表现出与甲氧基硅烷形成的相似的形态特征。
