Anion-specificity at water–air interface probed by total reflection X-ray fluorescence (TRXF)

Accumulation of bromide ions in the Stern part of the electrical double layer (EDL) near a Gibbs monolayer of cationic surfactant, cetyltrimethylammonium bromide (CTABr), at the water–air interface was probed by the total reflection X-ray fluorescence (TRXF). A combination of TRXF with surface tension data allowed for the first time to determine the adsorbed amounts of bromide ions in the Stern and diffuse parts of EDL. TRXF and X-ray reflectivity (XR) were employed to probe the competitive adsorption of other anions present in the aqueous solution. The Gibbs monolayers of CTABr were studied in the absence and presence of different anions from the Hofmeister series (Br−, NO3−, F−, Cl−, H2PO4−, ClO4−, and I−) with the aim to compare their affinity to the interface positively charged by adsorption of surface-active CTA+ cation. For studying the effect of anions on adsorption of CTABr, the concentration of the latter was kept constant (10−4 M), and that of NaX (X− = Br−, NO3−, F−, Cl−, H2PO4−, ClO4−, and I−) was varied between 10−2 M and 10−5 M. The results clearly show the competition between anions for the positively charged aqueous–air interface, with F− being the least “surface active” and ClO4− and I− – the most “surface active” anions, in full agreement with the Hofmeister series.