Frictional properties of two alkanethiol self assembled monolayers in sliding contact: odd-even effects.

Using molecular dynamics simulation, we have investigated the structural effects on the frictional properties of self assembled monolayers (SAM) of n-alkanethiols [CH(3)(CH(2))(n-1)SH, n = 12-15] in SAM-SAM contacts attached on Au (111) substrates. We have observed an odd-even effect where friction coefficient for SAM-SAM contacts with n = odd showed consistently higher values than those with n = even. This odd-even effect is independent of the sliding velocity and the relative tilt directions of the SAMs, and persists over a much higher pressure range than that reported before for SAM-Au contacts [L. Ramin and A. Jabbarzadeh, Langmuir 28, 4102-4112 (2012)]. For odd systems higher gauche defects were shown to be the possible source of high friction coefficient. Under the same load and shear rates (comparable sliding velocities), SAM-SAM contacts show mostly higher friction compared to SAM-Au contacts. For SAM-SAM contacts, a more significant increase of friction occurs at higher shear rates due to a shift in the tilt orientation angle. We show SAM-SAM contacts with misaligned relative tilt orientation angle (∼45°-90°) have considerably lower friction compared with those whose tilt orientation angles are almost aligned in the opposite directions and parallel to the shear.