Adsorption characteristics of sodium dodecylsulfate and cetylpyridinium chloride at air/water, air/formamide and air/water-formamide interfaces

摘要

The adsorption model developed by Kralchevsky and co-workers, the Kralchevsky-Danov-Broze-Mehreteab (KDBM) model, has been applied to fit the surface tension data in the submicellar region of sodium dodecylsulfate (SDS) and cetylpyridinium chloride (CPC) in water, water+50wt.% formamide (FA) and FA media. The KDBM model satisfactorily fits the surface tension data and from the fitting surface density of surfactant monomer (Γ _1), surface density of counterion (Γ _2) and electrostatic potential at the air-solution interface were evaluated. The value of Γ _1 decreases as the FA content in the medium increases due to decreasing solvophobicity, but the value of Γ _2 does not follow this trend. For both SDS and CPC the maximum value of the counterion binding constant (Γ _2/Γ _1) is almost same (about 0.8) in water and FA, but much low (about 0.3-0.5) in 50% FA. In all the three media, on increasing the concentration of SDS or CPC the surface potential passes through a maximum in the low concentration region.