MOISTURE ABSORPTION CHARACTERISTICS OF ORGANOSILOXANE SELF-ASSEMBLED MONOLAYERS

Monolayer films derived from octadecyltrichlorosilane (OTS) and dimethyloctadecylchlorosilane (DMODCS) were prepared on silicon dioxide surfaces under molecular self-assembly conditions. The hydration state of the thermal oxide surface was found to greatly influence the degree of surface coverage and packing of the monolayers. For OTS on hydrated thermal oxide, high-quality, close-packed monolayers were formed, as evident from contact angle, ellipsometry, and ATR-IR data. For OTS on dry oxide and for DMODCS on both dry and hydrated oxide surfaces, the monolayer coverage was significantly less. Spectroscopic adsorption isotherms for water on the OTS, DMODCS, and oxide surfaces were measured with a specially constructed attenuated total reflectance infrared (ATR-IR) vapor phase exposure test cell. This IR cell permits the detection of submonolayer amounts of water at low surface area oxide interfaces for the first time. Water adsorption on the unsilanized oxide was increased by prehydration of the surface. For the uncured silanized OTS surfaces, moisture adsorption to the surface was actually increased relative to either dry or partially hydrated unsilanized oxide surfaces, even though the outer surface was highly hydrophobic. Curing at 150-degrees-C of the tightly packed OTS monolayer was found to significantly decrease moisture adsorption. In contrast to the behavior observed for OTS, DMODCS treatment lowered the amount of moisture adsorbed to the surface even in the uncured state. These results suggest that water absorbed by the organosiloxane monolayers is bound to hydroxyl groups in the interfacial region.