Aggregation Structure and Surface Properties of Immobilized Fluoroalkylsilanes and Their Mixed Monolayers

Stable fluoroalkyl, alkyl and mixed fluoroalkyl/alkyl monolayers were prepared on solid substrates by the Langmuir method. The n-octadecyltrichlorosilane (OTS), 18-nonadecenylrichlorosilane (NTS) and [3-(perfluorododecyl)propyloxy]triethoxysilane (FDOPTES) in monolayers were in a crystalline state at 293 K, as by electron diffraction (ED) studies. In contrast, the n-dodecyltrichlorosilane (DDTS) and [2-(perfluorooctyl)ethyl] -trichlorosilane (FOETS) monolayers with shorter hydrophobic chains were in an amorphous state at 293 K. Molecular images of the crystalline OTS and FDOPTES monolayers were acquired with high-resolution atomic force microscopy (AFM). The direct observation of the molecular arrangement of the crystalline OTS and FDOPTES was successfully achieved The (10) spacings for these monolayers corresponded well to the cross section area of the alkyl and fluoroalkyl chains. It was revealed from the contact angle measurements that the magnitudes of surface free energies of the fluoroalkylsilane monolayers were higher than of the alkylsilane monolayers. The surface free energy of the crystalline fluoroalkylsilane monolayer (FDOPTES monolayer) was higher than that of the amorphous monolayers due to high orientation of the fluoromethyl end groups at the monolayer surface. AFM observation revealed that the (crystalline OTS/amorphous FOETS) mixed monolayer was in a phase-separated state due to the crystallization of the OTS molecules. The (crystalline NTS/amorphous FOETS) mixed monolayer also showed a phase separation. By oxidation of the NTS phase in the (NTS/FOETS) mixed monolayer, a phase-separated structure with large suface energy gap can be constructed. The surface properties of the mixed monolayer was evaluated based on lateral force microscopy (LFM) and adhesion force measurement. Literatures cited.