Influence of Salt on the Aqueous Lubrication Properties of End-Grafted, Ethylene Glycol-Based Self-Assembled Monolayers

We have investigated the influence of a high-concentration salt solution (1 M NaCl) on the aqueous lubrication properties of ethylene glycol-based molecules, namely, α-methoxy-ω-mercaptopoly(ethylene glycol) (MW 5000 Da) and α-methoxy-ω-mercaptoheptakis(ethylene glycol) (MW 356 Da), which have been end-grafted onto polycrystalline gold surfaces at high surface density. Macroscopic-scale, yet nondestructive, pin-on-disk tribometry experiments revealed that a high concentration of sodium chloride is deleterious to the aqueous lubricating properties of both films under low-sliding-speed conditions. This behavior was observed to be closely associated with the more collapsed conformation of surface-grafted poly(ethylene glycol) polymer chains in concentrated salt solutions, as confirmed by quartz-crystal microbalance measurements.