Molecular Simulation On The Preparation Process And The Surface Wettability Of APTES SAM

3-Aminopropyltriethoxysilane (APTES) self-assembled monolayer (SAM) has found wide range of applications in fundamental research and engineering field due to its unique surface properties from NH2. However, there are many controversies over the preparation and properties of APTES SAM in the literature, and the self-assembly mechanism as well as the internal structure of APTES SAM are still not clear. In this paper, the forming process and surface wettability of APTES SAM were studied through the molecular simulation by using the Studio Materials 6.1, and the the self-assembly mechanism, the possible internal structure and its influence on the surface wettability of APTES SAM were revealed, which is significant for the preparation and the research of APTES SAM.By means of quantum mechanical calculation, the mechanism and the order of the precedence of hydrolysis of three ethoxy of APTES molecules was revealed, which showed that, after the physical adsorption of the APTES molecule on the substrate surface, the No 3 ethoxy group of APTES molecule was firstly occurring hydrolysis, and the freshly generated silanol group occurred the polycondensation reaction immediately with the hydroxyl groups on the substrate surface and the APTES molecule was covalently bonded and fixed to the substrate surface, then the No 2 and No 1 ethoxy group occurred the hydrolysis in turn subsequently. All the reactions involved in the hydrolysis process of APTES molecule are spontaneous reactions. On this basis, the possibly existed structure models of APTES SAMs with different hydrolysis degree were constructed by molecular simulation technique, and the surface wettabilities of the different APTES SAMs were studied. Our results showed that, the theoretical initial contact angles of the APTES SAMs corresponding to one, two, three ethoxyl group(s) hydrolysis were 60°±3°,50°± 3°, and 40°± 3°, respectively, and the theoretical forward contact angle of the APTES SAM with two ethoxyl groups hydrolysis was 46°±3°. The theoretical simulation results of the APTES SAM with two ethoxy hydrolysis is consistent with the experimental results given by Zeng et al., which confirms that the APTES molecules in the SAM were not fully hydrolyzed, averagely only one to two ethoxyl groups were hydrolyzed for each APTES molecule, it was not possible to prepare a highly organized APTES SAM under the current reaction conditions.The influence of the protonation of amino group on APTES under the action of the acid catalyst on SAM formation and construction of the homogeneous mixed SAM by using the steric hindrance formed by the unhydrolyzed ethoxyl groups of APTES molecules in the self-assembling process were also studied experimentally and molecular simulatively, and the special effects of the protonation of amino group on above processes were revealed.