Polarized Grazing Angle FTIR Study of Molecular Orientation and Bonding of Thioglycolates at the Metal Interface

Abstract

The mercaptoesters have been recognized as an effective coupling agent for bonding metals using acrylate and epoxy adhesives with enhanced bonding strength and improved durability. Here, we report the results of our investigation of the interaction of thioester coupling agents with a gold surface using a grazing angle FTIR (GAIFTIR) spectroscopic technique. We investigated the interaction of alkyl functional thioesters (octylthioglycolate, dodecylthioglycolate and stearylthioglycolate) coupling agents with the metal at the interface region. The formation of strong chemical bonding is proven to be the key feature of this application rather than wetting or hydrogen bonding for conventional adhesive applications. Based on our results we have concluded that the deposited layers exist as thioester bonded to the gold surface through the sulfur-metal bond and that the polarization results indicate that only those thioesters containing C 12 and C 18 , hydrocarbon tail chains self-assemble readily and close pack on the surface. For dodecyl thioglycolate, the C 12 alkyl chains are densely packed and oriented perpendicular to the substrate surface. For the stearyl thioglycolate, the C 18 alkyl tail chains are also closely packed and oriented, but the chains appear to tilt from the normal surface. However, the carbonyl group of the C 18 , thioester appears to be parallel to the surface. For short-chain mercaptoesters, such as octyl thioglycolate, the alkyl chains do not self-assemble or orient on the gold surface. The molecular orientation of the coupling agent at the interface region has also been shown to play an important role in determining the interaction of the adhesive with the coupling agent layers. Results of the interfacial shear strength promotion on mild steel by the mercaptoester coupling agents are also reported.