The membrane fluidity of phospholipid bilayers plays a central role in many biological processes, e.g. for cell homeostasis. These membranes not only compartmentalize the cytoplasm and various organelles in eukaryotic cells, but they are also relevant for cell signaling where fine-tuning of the membrane fluidity is a necessity. Furthermore, the formation of metastatic cancer cells relies on an increase in membrane fluidity, which represents just one of many diseases related with membrane fluidity disfunction. Phospholipid monolayers—one leaflet of the lipid bilayer—have a relevance of their own as they coat the alveoli in the lung. Therefore, phospholipids are used in pulmonary lung surfactant replacements to prevent lung collapse in premature infants (neonatal respiratory distress syndrome) as well as in adults. We investigate monolayers of various phospholipids at the buffer—oil interface, controlling temperature and surface pressure relevant for cell membranes. The linear viscoelastic regime is probed with oscillatory interfacial shear rheology by the interfacial needle stress rheometer (ISR), which is a macroscopic technique. The lipid mixtures under investigation consist of phosphatidylcholines with saturated (palmitoyl) and unsaturated (oleoyl) fatty acids leading to different melting temperatures of the hydrophobic tails. At sufficiently high surface pressures, the saturated lipids pack efficiently and form liquid condensed phases in a continuous liquid expanded matrix. The area fraction of liquid condensed phase is measured simultaneously to the rheology by a custom fluorescent microscope mounted on the ISR. It is concluded that the interfacial viscosity does not diverge with area fraction compared to a Brownian hard disk system and the interface maintains its fluidity even at very large area fractions of the liquid condensed phase.

Crowded phospholipid monolayers are fluid as measured by interfacial rheology

ACS Spring 2022

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