Odd–Even Effects in the Friction of Self-Assembled Monolayers of Phenyl-Terminated Alkanethiols in Contacts of Different Adhesion Strengths

We have studied the frictional properties of self-assembled monolayers (SAMs) of phenyl-terminated alkanethiols, C6H5(CH2) n SH (n = 13–16) on template-stripped gold. The friction force was measured with atomic force microscopy (AFM), and the magnitude of the adhesion was controlled by immersing the sliding contact in ethanol (giving low adhesion) or dry N2 gas (giving enhanced adhesion relative to ethanol). We observed a linear friction force as a function of load (F = μL) in the systems with low adhesion and a non-linear friction force when the adhesion was higher. The non-linear behavior in the adhesive systems appeared to be area-dependent (F = S c A) and was compared to contact areas calculated using the extended Thin-Coating Contact Mechanics (TCCM) model. In ethanol, the coefficient of friction μ was found to be systematically higher for odd values of n (i.e., for the monolayers in which the terminal phenyl group was oriented closer to the surface normal).