Behavior of gramicidin A-ethyl nonadecanoate mixed Langmuir monolayers spread at the air-water interface

The behavior of the binary-mixed Langmuir monolayers of gramicidin A (GA) and ethyl nonadecanoate (EN) spread at the air-water interface was investigated. Miscibility and interactions between the components were studied on the basis of the analysis of the surface pressure-average area per molecule (π-A) isotherms and Brewster angle microscopy (BAM) images. Plot of A against mole fraction of gramicidin A (XGA) shows positive deviations from the ideal behavior at π < 19 mN m−1 which suggests miscibility between GA and EP at the working temperatures (15, 20 and 25 °C). The strongest deviation occurs at the ca. 2:1 GA:EN mixed film composition (XGA ≈ 0.66). However, the system follows the additivity law at surface pressures above 19 mN m−1. The BAM technique was applied for direct visualisation of the binary monolayer and show that, only when XGA ≥ 0.3, both components are immiscible at the beginning of the collapse, where the ester molecules are squeezed out from the monolayer to form a three-dimensional solid phase.