The Partially Degraded Hydrophilic Silane Pattern and Its Application in Studying the Structures of Long Chain Alkane Films

We developed a protocol to fabricate hydrophilic patterns over an octadecyltrichlorosilane (OTS) film surface with an atomic force microscope (AFM). Through a local probe oxidation under a 100% humidity environment, the OTS was converted into a hydrophilic, carboxylic acid-terminated surface (OTSpd). The OTSpd pattern grew with the voltage dwell time applied on the conducting AFM probe. Eighty nanometer to submillimeter sized OTSpd patterns could be fabricated with a single scanning probe. The OTSpd patterns were used to study the spreading of long chain alkanes. Hexatriacontane (C(36)H(74)) was dip-coated on an OTSpd pattern. Subsequently, an additional hydrophilic OTSpd region was fabricated surrounding the coated C(36)H(74). The alkane spread over this newly created region when heated above its melting point. After cooling to room temperature, the shape and structures of the solidified alkane patterns were characterized. On the methyl-terminated, low-energy surface, the alkane molecules stood directly on the surface. In contrast, on the hydrophilic, high-energy surface, the alkane formed seaweed-shaped patterns after spreading. On the OTSpd surface, the alkane molecules initially adsorbed on the hydrophilic surface with their alkyl chains parallel to the surface. Additional alkane molecules stood vertically or tilted on top of the parallel layer, forming the seaweed-shaped layer. The seaweed patterns were previously thought to consist of only vertically standing alkane molecules. We found that three additional tilted phases existed in the seaweed-shaped structures.