(19h) Interfacial Rheology of Phospholipid Monolayers

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. The formation of metastatic cancer cells relies on an increase in membrane fluidity, which represents just one of many diseases related with membrane fluidity dysfunction.

Phospholipid monolayers—one leaflet of the lipid bilayer—have an relevance of their own as they coat the alveoli and prevent lung collapse. 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 (acute respiratory distress syndrome). A detailed analysis of the mechanical properties of phospholipid monolayers in relevant conditions contributes to a sound understanding of the mechanical properties of phospholipid bilayers.

We investigate monolayers of various phospholipids at the water—air and buffer—oil interface, controlling temperature and surface pressure. Their linear viscoelastic regime is probed with oscillatory interfacial shear rheology by the interfacial needle stress rheometer (ISR), which is a macroscopic technique.[1]

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. They can undergo various shape instabilities depending on their size and balance of attractive and repulsive interactions between the lipids. The latter can be tuned by choosing the adjacent bulk phases accordingly, therefore resulting in different rheological properties. The phase behavior is imaged simultaneously to the rheological measurement by a fluorescent microscope mounted on the ISR.

[1] D. Renggli, A. Alicke, R. H. Ewoldt, and J. Vermant, “Operating windows for oscillatory interfacial shear rheology,” Journal of Rheology 64, 141-160 (2020) https://doi.org/10.1122/1.5130620