The influence of inorganic ions on the properties of nonionic Langmuir monolayers

This paper deals with the characteristics of ethyl palmitate (EP) Langmuir monolayers spread on water and aqueous solutions containing Na+ and Ca2+. No relevant differences due to the presence of ions were observed in the surface pressure–area (π–A) isotherms at 15, 20 and 25 °C. Brewster angle microscopy (BAM) was applied for a direct visualization of monolayers morphology. Circular and compact domains of liquid expanded (LE)–liquid condensed (LC) transition become dendritic shaped upon temperature increase. On ion-containing subphases, the temperature governs the average LC domain size along the LE–LC transition. During the monolayer collapse, numerous microdomains of high refractive index appear, indicating the transformation of the monolayer into three-dimensional (3D) structures through a nucleation process. For monolayers spread on subphases containing Ca2+, independently on temperature, these nuclei are formed at π below the collapse pressure (πc), on domains fusion. The resistance of monolayer to molecular aggregation depends both on the presence of ions and temperature.