Measurements of the thickness compressibility of an N- octadecyltriethoxysilane monolayer self-assembled on mica

The surface forces apparatus technique and the Johnson-Kendall-Roberts theory were used to study the elastic properties of an n- octadecyltriethoxysilane self-assembled monolayer (OTE-SAM) on both untreated and plasma-treated mica. Our aim was to measure the thickness compressibilities of OTE monolayers on untreated and plasma-treated mica and to estimate their surface densities and phase-states from the film compressibility. The compressibility moduli of OTE are (0.96 ± 0.02) × 10 ⁸ N/m ² on untreated mica and (1.24 ± 0.06) × 108 N/m ² on plasma-treated mica. This work suggests that the OTE phase-state is pseudocrystalline. In addition, the results from the compressibility measurements in water vapor suggest that the OTE-SAM on both untreated and plasma-treated mica is not homogeneous but rather contains both crystalline polymerized OTE domains and somewhat hydrophilic gaseous regions.