Component separation in spread sodium stearoyl lactylate (SSL) monolayers induced by high surface pressure

Insoluble monolayers of commercially available sodium stearoyl lactylate (SSL) spread at the air/water interface were studied by recording pressure–area (Π–A) isotherms simultaneously with the morphology via Brewster angle microscopy (BAM). Whereas a strong hysteresis on the Π–A isotherms was observed for the first compression–expansion cycle, the second and further cycles show a strongly reduced hysteresis. The BAM images show significant changes in the surface textures during compression/expansion reflecting 2D-heterogeneities in the monolayer. The most probable explanation of these phenomena can be given when taken into account the composition of the SSL sample consisting of two components of different interfacial activity—so-called SSL1 and SSL2. The less surface-active component was assumed to be pushed out of the monolayer at higher surface pressures. Additional compression experiments performed separately on the systems enriched by either of these compounds, respectively, confirm the explanation.