Friction and adhesion of mixed self-assembled monolayers studied by chemical force microscopy

Using a scanning force microscope with tips functionalised by deposition of a self-assembled monolayer (chemical force microscopy), the frictional and adhesive characteristics of mixed self-assembled monolayers, formed by the co-adsorption of methyl and carboxylic acid terminated alkanethiols onto gold surfaces, have been measured. A very close correlation has been found between the friction coefficient measurements and the cosines of the advancing water contact angles of the mixed monolayers, indicating that the friction force is in this case proportional to the surface energy. The adhesion force was also found to increase with the proportion of acid-terminated adsorbates in the monolayer, but the correlation was much less close than was the case for the friction force measurements. The degree of uncertainty in the friction coefficient determinations was significantly smaller than that in the adhesion force measurements, and the magnitude of the change was much greater on increasing the mole fraction of the polar adsorbate from 0 to 1.