The effect of humidity on the stability of an octadecyltriethoxysilane monolayer self-assembled on untreated and plasma-treated mica

We investigated the stability of an octadecyltriethoxysilane (OTE) monolayer self-assembled on plasmatreated and untreated mica using a surface forces apparatus by measuring the thickness of the water layer that is adsorbed from vapor. The OTE monolayers are initially highly hydrophobic, but contact angle hysteresis indicates that water interacts favorably with the monolayers on prolonged exposure. Defects in the monolayer most likely make it possible for the water to reach the hydrophilic region between the silane headgroups and the mica. This explains why there has been very little success in measuring hydrophobic forces between OTE-coated mica surfaces. Hydrophobic forces between OTE surfaces have been successfully measured only with silica as the substrate. Even though these monolayers are not suitable for studies of the so-called hydrophobic force, they are ideal for studies designed to probe the interactions between the silane headgroups and mica. For a given relative humidity, the water film thickness is always less if the surfaces are plasma-treated before the monolayer is deposited. In the untreated case, water penetrates into the hydrophilic region between the monolayer and the mica. This does not occur in the plasma-treated case because the monolayer is more firmly anchored, most likely through covalent bonding.