Phase behavior and morphology of equimolar mixed cationic–anionic surfactant monolayers at the air/water interface: Isotherm and Brewster angle microscopy analysis

The phase behavior and morphological characteristics of monolayers composed of equimolar mixed cationic–anionic surfactants at the air/water interface were investigated by measurements of surface pressure–area per alkyl chain (π–Aπ–A) and surface potential–area per alkyl chain (ΔV–AΔV–A) isotherms with Brewster angle microscope (BAM) observations. Cationic single-alkyl ammonium bromides and anionic sodium single-alkyl sulfates with alkyl chain length ranging from C12 to C16 were used to form mixed surfactant monolayers on the water subphase at 21 °C by a co-spreading approach. The results demonstrated that when the monolayers were at states with larger areas per alkyl chain during the monolayer compression process, the ΔV–AΔV–A isotherms were generally more sensitive than the π–Aπ–A isotherms to the molecular orientation variations. For the mixed monolayer components with longer alkyl chains, a close-packed monolayer with condensed monolayer characteristics resulted apparently due to the stronger dispersion interaction between the molecules. BAM images also revealed that with the increase in the alkyl chain length of the surfactants in the mixed monolayers, the condensed/collapse phase formation of the monolayers during the interface compression stage became pronounced. In addition, the variations in the condensed monolayer morphology of the equimolar mixed cationic–anionic surfactants were closely related to the alkyl chain lengths of the components.

Graphical abstractAlkyl chain length-dependent morphology of mixed cationic–anionic surfactant monolayers at the air/water interface.Figure optionsDownload full-size imageDownload as PowerPoint slide