Preferential adsorption of cationic surfactant mixture studied by bromide ion selective total-reflection XAFS measurement

The total-reflection XAFS measurement possessing bromide ion selectivity at the interfacial region was applied to the adsorbed film of hexadecyltrimethylammonium chloride (HTAC) and dodecyltrimethylammonium bromide (DTAB) mixture. The surface compositions X_j^H of individual ions j (j = HTA⁺, Cl^-, DTA⁺, and Br ^-) were evaluated by combining the surface excess concentration of Br^- estimated from the XAFS with the surface composition of the respective surfactants from the surface tension results. It is clearly shown that HTA⁺ and Br^- are preferentially adsorbed to DTA ⁺ and Cl^- at the air/water interface. The preferential adsorption was estimated numerically in terms of activity coefficient f _i±^H,p of component i and excess Gibbs energy of adsorption ĝ_pr^H,E. Then, the magnitude of ĝ_pr^H,E was compared with that of ĝ_in^H,E attributable to intrinsic interaction between ions.