The total-reflection X-ray absorption fine structure (XAFS) method previously employed for the adsorption of dodecyltrimethylammonium bromide (DTAB) at the air/water interface was applied to that in the presence of NaBr. The surface concentration of the bromide ions ΓBX of DTAB and NaBr was evaluated by using the Br K-edge absorption jump values of the total-reflection XAFS spectra and was compared to the corresponding value ΓBH estimated from the dependence of surface tension on the bulk concentrations of DTAB m1 and NaBr m2. The ΓBX values trace almost perfectly the ΓBH versus m1 curve up to a concentration near the critical micelle concentration (cmc) and deviate gradually above the concentration. This behavior is basically similar to that of the single DTAB system and ensures that the XAFS method is also applicable to the DTAB system, even in the presence of NaBr. In addition, this method was extended to the single nonionic amphiphile with covalently bonded bromine, and the surface concentrations of 6-bromo-1-hexanol (BrC6OH), Γ1X and Γ1H, were evaluated and compared with each other. It was found that the Γ1X value almost perfectly traces the Γ1H versus m1 curve, even at high surface concentrations. The excellent coincidence confirmed that the total-reflection XAFS method can be applied to the nonionic amphiphile system as well as a cationic surfactant with or without an added salt system. Finally, the difference between the ΓBX and ΓBH values observed in the DTAB with and without an added salt system is briefly described.
All Science Journal Classification (ASJC) codes
- General Materials Science
- Condensed Matter Physics
- Surfaces and Interfaces