Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis

Abstract

Chemically grafted monolayers of trialkylsilanes were prepared by reaction of (primarily) alkyldimethylchlorosilanes with silicon wafers under three conditions: in the vapor phase at elevated temperature (60−70 °C), in toluene in the presence of ethyldiisopropylamine (EDIPA) at room temperature, in toluene/EDIPA at 60−70 °C. It was determined that reactions at the solution−solid interface are very slow in the later stages of the reaction and that long reaction times are necessary to achieve maximum bonding density. The bonding density is determined and can be controlled by the reaction conditions. The highest carbon content on the surface (assessed by X-ray photoelectron spectroscopy) as well as the highest contact angles were obtained using vapor phase reactions. A series of nine H(CH2)nMe2Si− surfaces was prepared with n = 1, 2, 3, 4, 8, 10, 12, 18, and 22. Water contact angles (θA/θR = ∼105°/∼94°) are independent of chain length, indicating that these surfaces project disordered methyl groups toward the probe fluid and that water does not penetrate the monolayers. Hydrophobization is achieved topologically: the monolayers prevent water from penetrating and interacting with residual silanols. n-Hexadecane and methylene iodide contact angles decrease with increasing chain length for this series, indicating that these probe fluids penetrate the monolayers and interact with methylene groups. These chemically grafted monolayers differ in structure from those prepared by self-assembly in that the distance between molecules is significantly greater and that all molecules are covalently attached to the substrate. The contact angle hysteresis for these surfaces is a function of alkyl group structure and bonding density: mobile surfaces with flexible chains or rotational mobility and rigid surfaces that pack well exhibit low hysteresis, whereas rigid surfaces that cannot pack well exhibit high hysteresis. We argue that molecular level topography (roughness and rigidity) is responsible for the observed hysteresis.