Effect of water on structural and frictional properties of self assembled monolayers.

Self-assembled monolayers (SAMs) of n-alkanethiols [(CH3(CH2)(n-1), n = 14, 15] on Au(111) in the presence of water have been simulated by molecular dynamics simulation. The behavior and effects of compression on structural characteristics and water penetration into monolayers under different ranges of normal pressures have been investigated. Frictional properties of hydrated SAM systems under various sliding velocities, and loading conditions are examined to explore correlation between the amount of water penetration and friction. Simulations for one odd and one even SAM (C14 and C15) systems have revealed interesting odd-even effects in water penetration and frictional properties. We have also compared the frictional and structural properties of hydrated systems to that of dry SAM-Au (one surface of gold is covered by SAM) and SAM-SAM (both gold substrates are covered by SAM) contacts. The results reveal that the even hydrated SAM (C14) shows lower friction coefficient compared with the odd hydrated SAM (C15). We found the presence of water reduces the friction only at lower pressures; and at higher pressures, dry SAM-Au contacts offer lower friction. It was interesting to see that the lubricity effect of water was much stronger for the odd system and persisted to slightly higher pressures (300 MPa for the even SAM and 700 MPa for the odd SAM). At higher pressures, for both odd and even systems, the presence of water increased the friction. We also found that at low sliding velocities and higher pressures apparent water viscosity was enhanced by up to 3 orders of magnitude, indicating possible solidification.