Normal and shear forces between surfaces bearing phosphocholinated polystyrene nanoparticles

Extending earlier studies where phosphocholinated brushes and phosphocholine-exposing phosphatidylcholine lipid bilayer vesicles (PC-liposomes) were shown to be very efficient boundary lubricants in aqueous media by virtue of the highly-hydrated phophocholine groups, we examined interactions between surfaces bearing phosphocholinated polystyrene nanoparticles (pc-PS-NPs). We synthesized such particles by incorporation of alkyl chains terminated with phosphocholine groups in the PS-NP, with the hydrophilic phosphocholines exposed at the NP surface. These were then allowed to adsorb onto mica surfaces, and the normal and shear interactions between them were examined in a surface force balance. On moderate compressions (contact pressures > 9 atm) the pc-PS-NPs were squeezed out, leaving a single layer between the surfaces. Shear of the surfaces revealed a large frictional dissipation, with a friction coefficient μ ≈ 0.2, in contrast to our expectations that such phosphocholinated NPs would provide highly lubricating analogues of PC-liposomes (for which μ ≈ 10⁻³ – 10⁻⁴). This is attributed to a number of factors, including the inherent roughness of the NP multilayers at low compressions and the relatively low areal density of the phosphocholine groups on the PS-NP surfaces. In particular, at higher compressions, sliding results in energy-dissipating breaking and reforming of phosphocholine/mica (dipole/charge) bonds at the mica surfaces (and thus high friction), because of bridging of the surfaces by a single layer of pc-PS-NPs.