Self-Assembled Monolayers from Triethoxysilanes: Influence of Water Content and Chemical Structure on Formation Kinetics and Morphology

The condensation of silane molecules onto oxidic surfaces, known as self-assembled monolayers (SAMs), is a key methodology to control surface properties in a simple and versatile fashion. SAM formation, however, can be sensitive to subtle changes in the reaction conditions, which can lead to varying macroscopic properties. Here, we use attenuated total internal reflection infrared (ATR-IR) spectroscopy and quartz crystal microbalance with dissipation (QCM-D) as in situ methods to investigate SAM formation pathways, which we complement with X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle measurements as ex situ methods to assess the final composition, morphology, and functionality of the formed SAMs. We first spectroscopically resolve and quantify differences in the reaction pathway in dry and wet reaction conditions, corroborating the existing literature. Second, we investigate the formation of binary SAMs using n-butyltrimethoxysilane and 1-(3-triethoxysilylpropyl)-2-imidazoline silane as a model system. We show that imidazoline silane is preferentially incorporated into the SAM at low concentrations and that mixed SAMs exhibit much increased surface roughness compared to the pure silane surfaces. The in situ investigation reveals that the SAM deposition of imidazoline-based SAMs is not self-limiting and continuously adds deposited mass on the surface. ATR-IR spectroscopy suggests multiple interactions of the imidazoline moiety with the surface, which enable the polymerization of silane moieties into solution and thus rationalize the increased mass deposition and surface roughness. Our study demonstrates the potential of combining in situ and ex situ characterization methods to reveal differences in reaction pathways that can rationalize surprising macroscopic property changes observed in the formation of functional SAMs.