Π-A isotherms for anionic, cationic and mixed anionic–cationic surfactants

Adsorption of dodecyltrimethylammonium bromide (DTMABr) and sodium octyl sulfonate (NaOS) from single component solutions and from solutions of their mixtures has been investigated. Π-A isotherms of adsorption at the air/water and the decane/water interfaces are presented. The mean molecular area of surfactants in the monomolecular films is shown, versus the bulk concentration of the particular surfactant. It has been discovered that the adsorption films at the air/water surface are more condensed than at the decane/water interface. The molecular area larger for DTMABr than for NaOS is a result of the additional Van der Waals attractive forces between decane molecules and methyl groups in DTMABr molecule. When mixtures of surfactants are concerned, it has been found that the higher the content of NaOS in the mixture, the more condensed are the adsorption films at the air/water interface.