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.