  Boundary lubrication is associated with two sliding molecularly thin
lubricated film-coated surfaces, where the energy dissipation occurs at the
slip-plane between lubricated films. The hydration lubrication paradigm, which
accounts for ultralow friction in aqueous media, has been extended to various
systems, with phosphatidylcholine (PC) lipids recognized as extremely efficient
lubrication elements due to their high hydration level. In this work, we extend
a previous study (Lin et al., Langmuir 35 (2019) 6048-6054), where a charged
lipid-poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) conjugate was
prepared, to the very different case of a neutral lipid-PMPC) conjugate. This
neutral molecule stabilizes the liposomes by attaching highly water-soluble
PMPC to the surface of liposomes with its lipid moieties incorporated in the
lipid bilayers. Such neutral polyphosphocholinated liposomes provide a surface
lubricity which is well within the superlubrication regime (coefficient of
friction = ca. 10-3 or even lower). In contrast, negatively charged
lipid/polyphosphocholine conjugates modified liposomes were unable to adsorb on
negatively-charged (mica) surfaces. Our method provides stable liposomes that
can adsorb on negatively charged surfaces and provide superlubricity.
