Friction of Methyl-Branched Fatty Acid Layers at Low and High Sliding Speeds.

The friction of methyl-branched fatty acids was studied with a surface forces apparatus (SFA) with attachments for low-speed (μm/s) and high-speed (cm/s) sliding. The systems studied were isooleic acid, a mixture of unsaturated C-18 isomers with one methyl group located along the chain, and isostearic acid, which consisted of saturated C-18 isomers with one methyl branch. The methyl group was expected to disrupt the close packing of the molecules in the pure substance and when adsorbed on a surface. The friction forces and friction coefficients were compared with results for stearic and oleic acid. Isostearic acid showed low friction at low speeds. At high speeds, a peak was found in the friction coefficient of both isostearic and isooleic acid layers, whereas a gradual increase in friction coefficient with speed was seen for oleic acid, suggesting different relaxation times of the structures formed on the surfaces.