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

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 + 50 wt.% formamide (FA) and FA media. The KDBM model satisfactorily fits the surface tension data and from the fitting surface density of surfactant monomer (Γ1), surface density of counterion (Γ2) and electrostatic potential at the air–solution interface were evaluated. The value of Γ1 decreases as the FA content in the medium increases due to decreasing solvophobicity, but the value of Γ2 does not follow this trend. For both SDS and CPC the maximum value of the counterion binding constant (Γ2/Γ1) 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.