Surface Rheological Properties and Microstructures of DPPC/POPC Monolayers

In this paper, analysis of π-A isotherm curves, oscillatory dilatational deformation, and microstructure imaging with atomic force microscopy (AFM) has revealed complex behaviors of lipid monolayers. The π-A isotherm hysteresis analysis opens a new discourse on the physical properties of lipid monolayers. Isocycle curves of lipid mixtures elucidate how changes in the molecular packing affect the dynamics of the monolayer. Notably, DPPC monolayers exhibit the lowest hysteresis energy, reflecting high mechanical reversibility during compression-expansion cycles. In contrast, the introduction of POPC (i.e., as molar fractions x_POPC = 0, 0.25, 0.5, 0.75, and 1) into the lipid mixture disrupts the tight packing and increases the hysteresis. AFM images highlight the emergence of lipid domains (LE-LC phase transition and LC phase), which complement the observations from π-A isotherms at surface pressures of 5 and 24 mN/m. Furthermore, examination of interfacial dilatational rheology through barrier oscillation provides quantitative information about monolayer deformation. Our findings reveal that the lipid mixture, surface pressure, frequency, and amplitude sweeping influence the elastic and viscoelastic behavior of the lipid monolayer. Additionally, Lissajous plots from amplitude sweeping effectively distinguish the linear and nonlinear behavior of the monolayer.