Frictional properties of surfactant-coated rod-shaped nanoparticles in dry and humid dodecane

We have investigated the effects of humidity (water content or activity from 0 to ∼0.98) on the frictional properties of surfactant-coated ZnS nanoparticles of various shapes, specifically, nanorods and nanowires, dispersed in an organic solvent (dodecane). The friction coefficients were found to be sensitive to even trace amounts of water, increasing logarithmically with time after the systems were exposed to humid air, doubling after 2-4 h of exposure time to air of relative humidity ∼ 98%. We also show that increasing the humidity caused noticeable effects on the interactions of the nanoparticles, increasing their adhesion and aggregation through capillary forces. These effects should be considered in the design of organic solvents containing nanoparticles with physisorbed surfactants, for example, lube oils with nanoparticles additives, particularly those exposed to atmospheric conditions.