The formation of hydrophobic films on silica with alcohols

The formation of organic films on silica via reaction with primary alcohols was investigated. Hydrophobic films with an advancing water contact angle of 109° and a receding angle of 96° were prepared using 1-dodecanol by heating oxidized silicon wafers at 120° for 2.5 h. Atomic force microscope (AFM) imaging showed the silica surface was smooth after film formation. X-ray reflectivity and ellipsometry both showed that these films were thinner than the extended length of dodecanol, demonstrating that monolayer or submonolayer coverage was achieved. X-ray photoelectron spectroscopy showed that the appropriate stochiometry was achieved, and that it was necessary to exclude oxygen during reaction, otherwise the films became oxidized and produced lower contact angles. Film formation with a variety of primary alcohols was examined. The thickness of the films could be increased by increasing the length of the alcohol from 1-octanol to 1-dodecanol to 1-eicosanol, and the water contact angle could be reduced by reaction with an alcohol containing ω-NH2. The stability of the films in water was similar to that of octadecyltrichlorosilane. Compared to silane films, films formed from alcohols are useful because of the wide variety of alcohols available, but the reaction does require more concentrated solutions, higher temperatures (120 °C), and longer reaction times than those required for silanes.