On The Properties Of Surfactant Monolayers At Low Surface Tensions

Abstract

The properties of surfactant monolayers at the air/water interface depend strongly on the monolayer surface density. As the density increases, the monolayers undergo transitions from gas to liquid and condensed phases, and transform from 2D to 3D geometry as their stability limit is reached. For a given surfactant, the higher the surface density, the smaller is the monolayer area, and lower is the resulting surface tension at the interface. We found that for selected lipid mixtures and lung surfactant extracts, the monolayer surface tension - area isotherms deviate from the expected dependence. For these mixtures, the captive bubble surfactometer measurements show that at low surface tensions (< 20 mN/m) the reduction of surface tension is accompanied by an increase rather than a decrease of monolayer area. We used a combination of experimental techniques, theoretical models and computer simulations to investigate the properties of monolayers of varying composition at low surface tensions. We hypothesize that the observed effect originates from monolayer 2D-3D transformations. Monolayer wrinkling in particular leads to a decrease of monolayer apparent area and lowers the total surface tension.