Molecular Dynamics Simulations of the First Steps of the Formation of Polysiloxane Layers at a Zinc Oxide Surface

Three different dissolved silane molecules adsorbed at a polar ZnO surface (000&1macr;) are studied by means of constant temperature molecular dynamics simulations. The adsorbed single silane molecules exhibit a different behavior depending on the chemical nature of their tail. For octyltrihydroxysilane molecules with their rather unpolar tail an orthogonal orientation at the polar metal oxide surface is statistically favored with all three polar hydroxide groups of the head being in contact with the polar ZnO surface and the unpolar tail remaining in the isopropanol phase. On the contrary, due to their highly polar tail, aminopropyltrihydroxysilane molecules show a more or less parallel orientation at the surface. Apart from some minor fluctuations two hydroxide groups as well as the amino group of the tail are in contact with the surface. The behavior of the thiolpropyltrihydroxysilane molecules is somehow located in between resulting in parallel as well as orthogonal orientations of the molecule at the surface. Though many of the results obtained for single adsorbed silane molecules can also be transferred to adsorbed silane molecules within whole layers a remarkable difference appears: Now even for aminopropyltrihydroxysilane molecules a mixture of parallel and orthogonal alignment of the molecules can be observed whereas some of the octyltrihydroxysilane molecules also show a parallel orientation.