Growth and stability of a self-assembled monolayer on plasma-treated mica

Increasing the range of surfaces that can be studied using the surface forces apparatus, e.g., for friction measurements, requires chemical modification of the surface of mica, which may be achieved by grafting self-assembled monolayers (SAMs) onto plasma-modified mica. In order to gain a better idea of the grafting conditions leading to good-quality SAMs on plasma-activated mica, we focus, in the present work, on the early stages of octadecyltriethoxysilane (OTE) deposition. We use atomic force microscopy to study the morphologies of incomplete monolayers obtained at different grafting temperatures in the range 10-30 degrees C, and for different immersion times in dilute solutions of OTE. We observe that OTE molecules deposited on plasma-treated mica form stable and robust layers, the morphology of which are markedly affected by the grafting temperature: submonolayers deposited at temperatures below 18 degrees C exhibit micrometer-sized islands of well-packed molecules, whereas much smaller ordered domains, coexisting with a "liquid-expanded" phase, are observed at room temperature and above. We use water contact angle measurements to probe the quality of the SAMs, and find in particular that the most hydrophobic SAMs, presenting large contact angles and low hysteresis, are obtained for deposition temperature below 18 degrees C.