Structure and chemical state of octadecylamine self-assembled monolayers on mica

Structural and chemical data on n-octadecylamine self-assembled monolayers on mica (ODA/mica SAMs) have been obtained from ATR-FTIR and XPS spectroscopies. The analysis of the methylene modes concludes that alkylamine molecules are arranged in a rigid and well-ordered packing. Besides, the magnitude of the splitting of the methylene scissoring deflection is consistent with a molecular tilted configuration within the self-assembled layer, as already reported from topographic AFM data. Molecular dynamics simulations have supported this conclusion. XPS has revealed the presence of an important fraction of protonated amino groups (-NH₃⁺) even in freshly prepared ODA/mica SAMs in air at RT. Two sources of protonation are proposed: (i) the acid-base reaction of (-NH₂) end groups with the water adlayer on the surface of hydrophilic mica and (ii) the formation of an alkylamonium alkylcarbamate by a fast reaction with the atmospheric CO₂ dissolved in such water adlayer. Though the water induced amino protonation is hypothetical, the presence of carbamate is univocally confirmed by ATR-FTIR. Upon extended contact with air (ripening) the conformational ordering in ODA/mica SAMs is slightly improved. Besides, further amino group protonation takes place with no additional carbamate formation. The process is described by a tentative mechanism in which protons are transferred from water molecules at the edges of SAMs islands to the inside. On the other side, carbamation is hindered by the steric effect of CO₂ molecules trying to penetrate the close packed structure of octadecylamine molecules.