Construction of the homogeneously mixed SAM composed of octyltriethoxysilane and octadecyltrichlorosilane by taking advantage of the molecular steric restriction

By taking advantage of the steric restriction offered by the C8TES molecules, a homogeneously mixed self-assembled monolayer (SAM) composed of octyltriethoxysilane (C8TES) and octadecyltrichlorosilane (OTS) was constructed by means of the stepwise method on Si(1 0 0) substrate. The kinetic formation processes of the pure and mixed SAMs were investigated by the advancing contact angle measurements, and the topographies of the sample surfaces were characterized by AFM. The experimental results showed that by coadsorption method, it was impossible to construct the mixed SAMs for C8TES and OTS system, since the difference of the chemical activities between C8TES and OTS molecules could not be simply balanced through the tailoring of the concentration ratios in the coadsorption solutions. However, by taking advantage of the steric restriction caused by the three ethoxyl groups on the heads of C8TES molecule as well as the difference of the chemical activities between C8TES and OTS molecules, it was possible to construct both the homogeneously and phase-separated mixed C8TES/OTS SAMs by means of the stepwise method. The molecular steric analysis and AFM scanning experiments by AFM cantilever probes with different spring constants also indicated that, the structure inside the pure C8TES SAM should be in a loose and unassembled state, which resulted in an ultra-low surface contact angle, ∼20°, on the C8TES SAM surface.