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

Abstract

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.

The Origins of Enhanced and Retarded Crystallization in Nanocomposite Polymers

“…The same cut-off was applied for polymer–polymer and particle–polymer interactions. The intramolecular interactions for the polymer included stretching, angle, and torsional potentials, and their parameters can be found in our earlier works [20,21]. The cubic gold nanoparticles with a face centred cubic FCC lattice size of 0.408 nm were cut from a slab to the closest desired size.…”

Unravelling the effects of size, volume fraction and shape of nanoparticle additives on crystallization of nanocomposite polymers

“…A united atom molecular model is used in this work and the details can be found in our earlier work. 22,30 In this model, the CH 2 and CH 3 groups of atoms on the linear hexacontane molecules are treated as single interaction sites. Intramolecular interactions are governed through bond stretching and torsional and angle bending potentials.…”

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…In this context, NEMD simulations have been used to investigate the friction behaviour of monolayers of carboxylic/perfluorocarboxylic acids [245][246][247] and alkylsilanes [248][249][250][251] on silica surfaces. The tribological behaviours of model self-assembled monolayers (SAMs) such as thiols on gold surfaces have also been widely studied with NEMD [252][253][254]. These simulations have revealed the effect of head group and tail group of the nanoscale structure and friction behaviour of the monolayers.…”