Mechanical Properties of DPPC–POPE Mixed Langmuir Monolayers

Abstract

The mechanical properties of lipid monolayers and their responses to shear and compression stresses play an important role in processes such as breathing and eye blinking. We studied the mechanical properties of Langmuir monolayers of a model mixture, composed of an unsaturated lipid, 1-palmitoyl-2-oleoyl-sn-glycero-phosphoethanolamine (POPE), and a saturated lipid, 1,2-dipalmitoyl-sn-glycero-phosphocholine (DPPC). We performed isothermal compressions and sinusoidal shear deformations of these mixed monolayers. Also, the different phases were observed with Brewster angle microscopy. We found that the mechanical behavior is affected by the miscibility of both lipids. In the two-phase region, the compression elastic modulus increases with the amount of the LC phase but does not follow the predictions of a simple effective medium model. The discrepancies arise from the fact that, upon compression, the domains grow at a rate faster than the compression rate but not fast enough to reach thermodynamic equilibrium. Before reaching the LC phase, domain percolation is observed and compression and shear moduli become equal to those of the pure LC phase. Most of the monolayers behave as viscoelastic fluids at the frequencies investigated. A minimum in the compression modulus and shear viscosity was observed for mixtures close to equimolar composition, with the minimum being accompanied by a change in domain shapes.

Seaweed and Dendritic Growth in Unsaturated Fatty Acid Monolayers

“… is the associated molecular area increase. Using the compression modulus κ a linear dependency of from can be found [ 14 , 26 ]: Solving Equation (A3) for yields: The calculation is applied to the data of the isotherms of erucic acid shown in Figure 1 . For the specified experimental conditions (pH, temperature), the molecular area at the phase transition is The compression modulus κ was calculated numerically according to Equation (A3).…”

“… is the associated molecular area increase. Using the compression modulus κ a linear dependency of from can be found [ 14 , 26 ]: …”

“…They predicted two distinctly different growth instability classes: (i) seaweed domains that grow by surface diffusion in the LE phase or (ii) dendritic domains whose growth is determined by the adjective flow beneath the lipid monolayer. The agreement was qualitative, but a quantitative comparison failed [ 7 , 14 ]. According to theory, seaweed growth should not occur because the surface viscosity (the viscosity in the lipid monolayer) is a few orders of magnitude too high [ 14 , 15 ].…”

“…The agreement was qualitative, but a quantitative comparison failed [ 7 , 14 ]. According to theory, seaweed growth should not occur because the surface viscosity (the viscosity in the lipid monolayer) is a few orders of magnitude too high [ 14 , 15 ]. We cannot resolve this issue, but to better understand the different growth instabilities, we quantify the shapes of seaweed and dendritic domains as a function of (i) the compression speed of the monolayer and (ii) the supersaturation in the LE phase.…”

A Sebum-Mimetic Lipid Monolayer and Its Interaction with (Bio)Surfactants

“…The only difference between the two parameters is the rate and direction of the monolayer dilation (oscillatory movement with f = 0.1 Hz and relative amplitude of 2% for E', and linear movement with the rate of 7 mm/min for Cs −1 ). Cs −1 , being registered under conditions closer to equilibrium, is especially helpful to identify the monolayer compression state: gaseous, G (Cs −1 < 12.5 mN/m), liquid expanded, LE (12.5 mN/m < Cs −1 < 50 mN/m), liquid condensed, LC (50 mN/m < Cs −1 < 250 mN/m and solid, S (Cs −1 > 250 mN/m) [26][27][28]. In the first stage of compression, a phase transition from gaseous (G) to liquid expanded (LE1) state takes place at π = 9 mN/m, where the compression modulus, C s −1 reaches 42 mN/m.…”