Surface Viscosity Of Monolayers Research Articles

Molecular View of Phase Coexistence in Model Membranes

We used computer simulations to investigate phase transformations in lipid monolayers. This is important for understanding lipid-lipid interactions underlying lateral organization in biological membranes, and the role of phase coexistence in the regulation of surface tension by lung surfactant. Molecular dynamics simulations with the coarse-grained force field MARTINI were employed to achieve large length (∼80 nm in lateral dimension) and time (tens of microseconds) scales. Lipid mixtures containing saturated and unsaturated lipids and cholesterol were investigated under varying surface tension and temperature. We reproduced compositional lipid de-mixing and transformation into liquid-expanded (LE) and liquid-condensed (LC) phases, and into liquid-ordered (Lo) and liquid-disordered (Ld) phases. Transformation proceeded via either nucleation and growth, or spinodal decomposition, with distinct coarsening kinetics. Nucleation rate and growth exponents were calculated. Partial lipid areas and phase composition showed a different dependence on surface tension. The domain boundary length increased and the line tension decreased with reducing surface tension. Domains of Lo phase manifested spontaneous curvature at low surface tensions. The surface viscosity of monolayers with phase coexistence increased due to domain re-organization under shear. In the Lo/Ld mixture, strong compositional fluctuations were observed at higher temperatures. Monolayer collapse occurred in the disordered phase (LE or Ld), which then transferred into bilayer folds and monolayer-bilayer connection. Domains of coexisting phased either increased or reduced monolayer stability. We also investigated lipid bilayers of the same composition. Decreasing surface tension in monolayers and temperature in bilayers had similar effects on the properties of coexisting phases.

Interactions between polymers and surfactants

A surfactant film (at the water/air interface, or in a bilayer) is exposed to a solution of a neutral, flexible, polymer. Depending on the interactions, and on the Langmuir pressure II of the pure surfactant film, the authors expected to find three types of behavior: (I) the polymer does not absorb; (II) the polymer absorbs and mixes with the surfactant; (III) the polymer absorbs but segregates from the surfactant. Their interest here is in case II. They predict that (a) bilayers become rigid; (b) bilayers, exposed to polymer on one side only, tend to bend strongly; (c) the surface viscosity of monolayers or bilayers is considerably increased; soap films or foams, which usually drain by turbulent (two-dimensional) flows, may be stabilized in case II.

The surface viscosity of mixed monolayers

The theory ofEyring of viscous flow, allows to describe the newtonian surface viscosity of a monolayer containing only one active component. This theory is extended to mixed monolayers with miscible components, allowing to calculate the surface viscosity of a surface mixture if the flow parameters of the single components are known. This theory is experimentally illustrated for the systems senegin/digitonin and cholesterol/phospholipids. The results of the last system are discussed in terms of the property of cholesterol being a liquifier in cell membranes. Next the surface viscosities of dilute cholesterol monolayers penetrated by digitonin are given. In this situation no agreement between theory and experiment is observed, indicating an interaction between these components as already known from other experimental techniques. This increased surface viscosity may be related to the strong hemolytic action of digitonin.