Do crowded phospholipid monolayers remain fluid?

Phase separation in mixed lipid layers is used here to mimic crowding at interfaces, providing insights into the dynamics of (and within) phospholipid monolayers. The interface is fully fluid at temperatures above the liquid-condensed (LC) phase melting point for the range of accessible interfacial pressures. Upon cooling, temperature-induced phase separation occurs. We moreover control the solid-like LC phase fraction by tuning the lipid composition between dipalmitoyl-phosphatidylcholine (DPPC) and dioleoyl-phosphatidylcholine (DOPC). Interfacial rheology reveals a strong viscoelastic response in DPPC monolayers at physiologically relevant pressures and sub-30∘C temperatures. Mixed monolayers exhibit viscosity scaling with increasing LC phase fraction, behaving somewhat as a suspension of discs. However, we observe weak aggregation and deviations from hard-disc models. This suggests that phase coexistence preserves the effective interface fluidity by mitigating hydrodynamic jamming; this could possibly also be a mechanism governing membrane dynamics in crowded states.