Electrostatic Induced Molecular Tilting in Self-Assembled Monolayers of n-Octadecylamine on Mica

Self-assembled monolayers of n-octadecylamine on mica (ODA/mica SAMs) have been investigated by atomic force microscopy (AFM) and by attenuated total reflectance infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Topographic data characterizes a stable configuration with the alkyl skeleton tilted approximate to 46 degrees from the surface normal that is rationalized according to a well established structural alkyl chain packing model. Extended contact with air increases molecular tilting up to approximate to 58 degrees. ATR-FTIR and XPS reveal the presence of protonated amino groups within the monolayer and its increment upon exposure to air. The transition between both tilted states is explained assuming the protonation reaction as the driving force and introducing a model to evaluate an electrostatic repulsions term in the overall cohesive energy balance of the system. ODA molecules in the self-assembled monolayer respond to their spontaneous protonation by atmospheric water by tilting as a mechanism to relax the repulsions between -NH3+ heads.