Examining the frictional forces between mixed hydrophobic-hydrophilic alkylsilane monolayers

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.