A comparative AFM study of the structural and frictional properties of mixed and single component films of alkanethiols on Au(1 1 1)

A comparative study of the frictional and structural properties of single component films of alkanethiols, CnH2n+1SH (n=12,16) and mixed films is presented. Islands instead of a complete monolayer are used to simultaneously measure film compression and friction under applied load. For single component films, the friction and film height change following a stepwise behavior. A simple geometrical model involving interlocking of the alkyl chains at specific molecular tilt angles explains the observations. At low loads the friction of single component films increases monotonously with the applied load. In contrast, in mixed films of C12 and C16 the friction force increases at a high rate first and then at the same rate as in single component films. We propose that the initial fast increase in friction is due to an energy dissipation mechanism based in the formation of gauche defects at the protruding ends of the long thiol molecules.