Examining the frictional forces between mixed hydrophobic – hydrophilic alkylsilane monolayers

Abstract

Monolayers presenting methyl-terminated (hydrophobic) and hydroxyl-terminated (hydrophilic) surfaces on silica have been studied by molecular dynamics simulation and the effects of hydrogen bonding, chain length, and chain mixing on the frictional properties determined. The hydroxyl-terminated monolayers were found to show large adhesion zones as a result of strong interfacial interlayer hydrogen bonds; the interfacial sliding forces observed in the hydroxyl-terminated monolayers being one order of magnitude higher than the interfacial forces for the hydrophobic surfaces at the characteristic point of zero-load. Mixed hydroxyl- and methyl-terminated monolayers of equal length were found to exhibit intermediate shear stress values between those observed for pure monolayers, with the magnitude of the shear stress depending on the surface content of the hydroxyl-terminated chains. For mixed monolayers of unequal chain lengths, at high loads a maximum in the magnitude of the shear stress as a function of the length of the methyl-terminated chain was observed due to the creation of a buffer zone between the hydroxyl-terminated chains that produces strong hydrogen-bonding interactions. The effect of a constant normal load or constant separation simulation ensemble on the results has also been studied and in general found to have minimal influence on the observed behavior, although some differences are observed for the shear stress at intermediate normal loads due to the formation of stronger hydrogen bond networks at constant load compared to constant separation.

Derivation of coarse-grained potentials via multistate iterative Boltzmann inversion

“…These were performed in the same manner as the bulk simulations at 298 K, but with the first bead of each chain held stationary such that a 2D hexagonally arranged periodic array with density 4.10 chains per nm 2 (gel-like) and 3.79 chains per nm 2 (fluidlike) was achieved; these were chosen to match state points commonly used in alkylsilane monolayer simulations and experiments. 56 The third model used is a CG representation of alkanes. In all cases, a 3-to-1 CG model (i.e., each CG bead represents 3 UA carbon groups) was used to simulate bulk fluid and monolayer systems of alkanes.…”

Molecular adsorption, self-assembly, and friction in lubricants

“…Equilibrium and non-equilibrium molecular dynamics (MD) simulations have been used extensively to examine the structure, dynamics, and tribological properties of lubricants [5,6]. Examples include pure polymer melts and hydrocarbons [7,8,9,10,11,12,13,14], silanes [15], fatty acids and amines [16,17,18,19,20,21], glycerin [22], glycerides [23], zinc dialkyldithiophosphates [24], MoS 2 [25,26], room-temperature ionic liquids [27,28], and carbon nanoparticles [29]. Fig.…”

Structure and Friction of Stearic Acid and Oleic Acid Films Adsorbed on Iron Oxide Surfaces in Squalane

“…Stearic acid [octadecanoic acid, CH 3 (CH 2 ) 16 COOH] and oleic acid [(9Z)-octadec-9-enoic acid, CH 3 (CH 2 ) 7 (CH) 2 (CH 2 ) 7 COOH] have been widely studied in a variety of tribological contexts.…”