Boundary lubrication and surface mobility of mixed alkylsilane self-assembled monolayers

We use a combination of experiment and theory to study the boundary lubrication characteristics of mixed self-assembled monolayers (SAMs) formed by tethering alkylsilanes with different chain lengths to planar substrates. The structure of mixed SAMs is characterized using atomic force microscopy, infrared spectroscopy, and hexadecane contact angle measurements. Lateral force microscopy is used to quantify the load and velocity dependence of friction force for one-component (pure) and two-component (mixed) SAMs. We find that if two-component SAMs and one-component SAMs are created with comparable packing density, the two-component systems possess lower friction coefficients. The friction force measured on silicon surfaces grafted with mixed SAMs increases linearly with the logarithm of sliding velocity over a broad range, whereas for pure SAMs, the friction force first increases linearly with the logarithm of velocity and then reaches a plateau. This plateau is believed to arise from the viscoelasticity of the tethered molecules. An analysis of the friction versus velocity data using a simple thermally activated Eyring model for a viscoelastic thin film shows that the lower friction coefficients of mixed SAMs are a consequence of higher mobilities of tethered molecules in the monolayer. This analysis yields characteristic relaxation times of 10(-4) to 10(-5) s for molecules in pure SAMs and relaxation times of 10(-6) s in mixed SAMs. The estimated stress activation volume is in better accord with a surface flow mechanism where either a single molecule or possibly a subsection of a molecule participates in stress-activated motion during sliding contacts.