Competitive Adsorption of Perfluorocarbon and Hydrocarbon Surfactants at the Air-Water Interface

The competitive adsorption of perfluorocarbon and hydrocarbon surfactants was studied below and above the critical micelle concentration at the air-water interface. The adsorption experiments were carried out on two mixed surfactant systems; lithium telradecylsulfate (LTS)-lithium perfluorooctylsulfonate (LFOS) and ammonium dodecylsulfate (ADS)-ammonium perfluorononanate (AFNO). The characteristics of the present study were to elucidate the compositions of surfactant film adsorbed from the micellar solution in which the uniformly mixed micelles did not form. The surface tensions vs. concentration (C_T) curves showed two break points corresponding to CMC₁ and CMC₂ for the mixed surfactant solutions. The surface excess of hydrocarbon surfactants was measured by a radiotracer method using ³H-labelled compounds, while the surface excess of perfluorocarbon surfactant was estimated from the Gibbs adsorption equation in the region CMC₁≤CC_T<CMC₂ as well as C_T<CMC₁. The surface compositions obtained below and above the CMC₁ were compared with the micelle compositions, which were estimated from Shinoda's micelle rfiixing theory. Consequently, it was found that LTS was surface-active more than LFOS below the CMC₁ and was gradually replaced by LFOS with increasing total concentration of mixed surfactants above the CMC₁. The surface adsorbed film came to be almost covered with LFOS above the CMC₂. On the other hand, AFNO was surface-active much more than ADS. Hence, it was found that adsorption isotherms of AFNO indicated a maximum value with increasing adsorption of ADS below the CMC₁, and that the azeotropic phenomenon in adsorption was observed at the mixing mole fraction of 0.4. Therefore, the surface compositions of AFNO-ADS remained at a given molar ratio even above the CMC₁.