Controlling the Synergetic Effects in (3-Aminopropyl) Trimethoxysilane and (3-Mercaptopropyl) Trimethoxysilane Coadsorption on Stainless Steel Surfaces

A versatile and economic method of preparing covalently-bound and uniform bifunctional silane monolayers on stainless steel is presented. Stainless steel is first electrochemically hydroxylated to enable the formation of a bifunctional overlayer via simultaneous liquid-phase deposition of two organofunctional silanes: (3-aminopropyl)trimethoxysilane (APS) and (3-mercaptopropyl)trimethoxysilane (MPS). The chemical composition, in-depth distribution, molecular orientation and chemical bonds in APS, MPS and APS/MPS layers over a range of APS/MPS mixing ratios are studied with synchrotron radiation mediated photoelectron spectroscopy (SR-PES), conventional X-ray photoelectron spectroscopy (XPS) and energy filtered X-ray photoemission electron microscopy (EF-XPEEM). Inelastic electron energy-loss background (IEEB) analysis is employed to determine the surface morphology of the silanized samples. Coadsorption is shown to produce a covalently-bound and highly ordered monolayer with a controllable MPS surface concentration within APS matrix. The results show evidence of strong synergistic effects during simultaneous adsorption of MPS and APS from liquid phase. While the uptake of MPS alone is low, the coadsorption of MPS and APS strongly enhances both the uptake of MPS and ordering in the APS/MPS overlayer. Results from PES, EF-XPEEM and IEEB analysis reveal that the surface is predominantly covered by a well-ordered APS/MPS monolayer with only slight degree of clustering. Clustering is attributed to different hydrolysis rates in solution and structural irregularities on the substrate. Our results conclusively invalidate the assumption that APS/MPS ratio in a deposited overlayer should correlate linearly with the mixing ratio in solution. The reported insights into the chemical bonds, molecular orientation and morphology in APS/MPS overlayers facilitate site-selective coupling of functional molecules to amino and thiol groups with controllable spatial distribution and, in general, knowledge-based development of novel surface functionalities for stainless steel and other metal (alloy) oxides. (C) 2014 Elsevier B.V. All rights reserved.