The chemical effects on the adhesion and friction between alkanethiol self-assembled monolayers (SAMs)
on gold are studied using molecular dynamics simulations. Alkanethiol SAMs (S(CH2)
n
X) are investigated
as a function of the terminal group (X = CH3, OH, COOH) and chain length n. The adhesive force is
calculated as a function of the separation between two SAMs with the same terminal group. The maximum
attraction for methyl-terminated SAMs is much weaker than for the OH- and COOH-terminated SAMs
which form interlayer hydrogen bonds. The COOH-terminated SAMs have the strongest attraction. For
the COOH- and OH-terminated SAMs, the adhesion−separation curves depend strongly on n. In both
cases the change in the position of the attractive minimum is large as a function of n. The magnitude of
the attractive minimum varies little for OH-terminated SAMs, and the variation is also small for COOH-terminated SAMs once the odd:even effect is considered. The source of the n dependence is structural
changes within the SAMs. For separations within the attractive region of the adhesion curve, the interlayer
hydrogen bond interactions pull the chains to a more upright tilt angle than the equilibrium, single-monolayer tilt angle. Longer chains have more upright tilt angles for separations in the attractive region.
For COOH SAMs with n ≤ 13 the maximum adhesion is significantly larger for odd n in comparison to
even n − 1. The difference between even and odd cases decreases with n, and for n > 13 the difference
is smaller than our uncertainty. Friction simulations between two SAMs with the same n show that the
shear stress is significantly larger for X = COOH than for CH3.
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