Stiffness of lipid monolayers with phase coexistence

CARUSO, B.; MANGIAROTTI, A.; WILKE, N.

LANGMUIR

AMER CHEMICAL SOC

Lugar: Washington; Año: 2013 vol. 29 p. 10807 - 10816

0743-7463

The surface dilational modulus -or compressibility modulus- has been previously studied formonolayers composed of pure materials, where a jump in this modulus was related with the onsetof percolation as a result of the establishment of a connected structure at the molecular level.In this work, we focused on monolayers composed of two components of low lateralmiscibility. Our aim was to investigate the compressibility of mixed monolayers at pressures andcompositions in the two-phase region of the phase diagram, in order to analyze the effect of themechanical properties of each phase on the stiffness of the composite. In nine different systemswith distinct molecular dipoles and charges, the stiffness of each phase and the texture at the planeof the monolayer were studied. In this way, we were able to analyze the general compressibility oftwo-phase lipid monolayers, regardless of the properties of their constituent parts.The results are discussed in the light of the following two hypotheses: first, the stiffness ofthe composite could be dominated by the stiffness of each phase as a weighted sum according tothe percentage of each phase area, regardless of the distribution of the phases in the plane of themonolayer. Alternatively, the stiffness of the composite could be dominated by the mechanicalproperties of the continuous phase. Our results were better explained by this latter proposal as in allthe analyzed mixtures it was found that the mechanical properties of the percolating phase were thedetermining factors. The value of the compression modulus was closer to the value of theconnected phase than to that of the dispersed phase, indicating that the bidimensional compositesdisplayed mechanical properties that were related to the properties of each phases in a rathercomplex manner.