The Effect of Humidity on the Stability of an Octadecyltriethoxysilane Monolayer Self-Assembled on Untreated and Plasma-Treated Mica

Abstract

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.

In vitro stability study of organosilane self-assemble monolayers and multilayers

“…On the other hand, hydrophobic coatings are typically more effective in preventing water penetration, and thus demonstrate better stability in aqueous environments. Kim et al found that water penetration even made octadecyltriethoxysilane (CH 3 (CH 2 ) 17 Si(OCH 2 CH 3 ) 3 ) SAM on mica, which is very hydrophobic, unstable in aqueous media [37].…”

Silicon Based Nanocoatings on Metal Alloys and Their Role in Surface Engineering

“…Studies carried out on the effect of humidity on the stability of the Octadecyltriethoxysilane SAMs on plasma treated and untreated mica showed that for a given relative humidity, the water film thickness was always less for a plasma treated mica surface. Plasma treatment is thought to anchor the SAM through covalent bonding thus eliminating water adsorption between SAM and mica [239]. Recently Chandekar et al compared the thermal stability of alkanethiol SAMs on gold and alkylsilane SAMs on hydrated silicon surfaces [240].…”

Optimization of mica surface hydroxylation in water vapor plasma monitored by optical emission spectroscopy

“…They served as anchoring points for further chemical functionalization, e.g. coupling via chloroor ethoxysilane chemistry to form stable hydrophobic monolayers [11][12][13]. Several research groups continued along this line and used a water vapor plasma activation procedure to functionalize mica surfaces with alkylsilanes [10,14], amino-and ethylene oxide groups [15], polystyrenes [16], polymer brushes [17] and proteins [18] as well as for patterning of mica [19].…”