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	<p>Frontiers in Tribology at the Atomic Scale</p>
	<p>Mark Stevens, Sandia National Laboratories</p>
	<p>Title: Chemical Effects on the Adhesion and Friction between Alkanethiol Monolayers: Molecular Dynamics Simulations</p>
	<p>Abstract:</p>
	<p>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)nX) are investigated as a function of terminal group (X= CH3, CF3, OH and COOH) and chain length <em>n</em>. The adhesive force is calculated as a function of the separation between two SAMs with the same terminal group. The maximum attraction is strong for the OH and COOH terminated SAMs which form interlayer hydrogen bonds, and the adhesion-separation curves depend strongly on <em>n</em>. 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 <em>n</em> dependence are 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 <em>n</em> ≤ 13 the maximum adhesion is significantly larger for odd <em>n</em> in comparison to even <em>n</em> – 1. The difference between even and odd cases decreases with <em>n</em>, and for <em>n</em> > 13 the difference is smaller than our uncertainty. Friction simulations between two SAMs with the same <em>n</em> show that the shear stress is significantly larger for X=COOH than for CH3. In agreement with experiment, the friction for CF3 terminated alkanethiols is larger than that for CH3 terminated SAMs in spite of the lower surface energy of the former. </p>
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