硅基双层复合自组装分子膜结构特性及其润湿行为的分子动力学模拟

Molecular dynamics simulation of structural properties and wetting behavior of silicon based dual composite self-assembled monolayers

用分子动力学方法模拟了单晶硅(Si)表面N-3-(三甲氧基硅烷基)丙基乙二胺(DA)-月桂酰氯(LA)(DA-LA)双层复合自组装分子膜(SAMs)的结构特性,得到膜层中DA和LA分子的最佳覆盖率及分布情况。进一步讨论了水滴在DA-LA双层复合SAMs表面的润湿过程,通过接触角和径向分布函数等参量对其润湿行为进行了分析。研究表明:DA分子在Si上覆盖率为50%、LA分子在DA自组装单分子膜(DA SAM)上接枝率为100%时,分子膜呈有序排布,体系能量最低,从分子角度揭示了Si表面覆盖致密SAMs的形成机制。当取最佳覆盖率体系进行润湿机制模拟时,DA-LA双层复合SAMs表面水滴接触角与实验值相似,表现出良好的疏水性。而DA SAM表面由于DA分子短而稀疏,暴露出底层更亲水的羟基分子,从而导致所得接触角较实验偏小;经测量及计算得出,羟基化Si表面自由能最高,表现出较强的亲水性;DA表面次之;DA-LA表面自由能最低,表现出良好的疏水性。进一步分析发现:羟基化Si表面、DA SAM表面与水滴间存在氢键,加强了表面的亲水性,而DA-LA双层复合SAMs表面与水滴间只存在弱范德华力,有利于表面呈现疏水性。

The structural properties of N-[3-（Trimethoxysilyl）propyl]ethylenediamine（DA）-Lauroyl Chloricle（LA）（DA-LA）dual composite self-assembled monolayers（SAMs）,which was prepared on silicon（Si）substrates,were investigated by molecular dynamics simulation.The optimal coverage rate and arrangement of DA and LA molecules were obtained.The wetting process of water droplet on the surface of DA-LA dual composite SAMs was further discussed,and the wetting mechanism was analyzed by contact angle and radial distribution function.The investigation shows that the system energy is lowest when the coverage rate of DA on Si surface is 50%and the coverage rate of LA bond DA is 100%.In the lowest energy system,molecules of DA-LA dual composite SAMs on Si surface are arranged orderly,it reveals the formation mechanism of dense SAMs on Si surface by molecular method.The wetting mechanism simulation was analyzed in the optimal coverage rate system.The contact angles of water droplets on the DA-LA dual composite SAMs surface are similar to the experiment results,and the surface shows good hydrophobicity.Meanwhile,the contact angles of water droplet on the DA SAM are lower than the experiment results,because the DA coverage rate is low and the chain is short,and the hydrophilic hydroxyl molecules are exposed to the upper layer.By means of measuring and calculating,the surface free energy of hydroxylated Si is high-est,followed by DA surface,and the surface of DA-LA is lowest.Further analysis shows that the hydrogen bond exists between the hydroxy Si surface,DA SAM surface and the water droplet,which strengthens the surface hydrophilicity;however,the weak van Edward force exists between the DA-LA dual composite SAMs surface and the water droplet,which enhances the surface hydrophobicity.