Ion Pairing and Adsorption of Azo Dye/C₁₆TAB Surfactants at the Air-Water Interface

Mixed layers of 6-hydroxy-5-[(4-sulfophenyl)azo]-2-naphthalenesulfonate (Sunset Yellow, SSY) and cetyltrimethylammonium bromide (C₁₆TAB) at the air-water interface were studied using vibrational sum-frequency generation (SFG) and dynamic surface tension measurements. In the bulk, addition of C₁₆TAB to SSY aqueous solution causes substantial changes in UV/vis absorption spectra, which originate from strong electrostatic interactions between the anionic SSY azo dye with the cationic C₁₆TAB surfactant. These interactions are a driving force for the formation of SSY/C₁₆TAB ion pairs. The latter are found to be highly surface active while free SSY molecules show no surface activity. Dynamic SFG as well as surface tension measurements at low SSY concentrations reveal that free C₁₆TAB surfactants adsorb at the air-water interface on time scales <1 s where they initially form the dominating surface species, but on longer time scales free C₁₆TAB is exchanged by SSY/C₁₆TAB ion pairs. This causes a dramatic reduction of the surface tension to 35 mN/m but also in foam stability. These changes are accompanied by a substantial loss in SFG intensity from O-H stretching bands around 3200 and 3450 cm^-1, which we relate to a decrease in surface charging due to adsorption of ion pairs with no or negligible net charges. For SSY/C₁₆TAB molar ratios >0.5, the O-H bands in SFG spectra are reduced to very low intensities and are indicative to electrically neutral SSY/C₁₆TAB ion pairs. This conclusion is corroborated by an analysis of macroscopic foams, which become highly instable in the presence of neutral SSY/C₁₆TAB ion pairs. From an analysis of SFG spectra of air-water interfaces, we show that the electrostatic repulsion forces inside the ubiquitous foam films are reduced and thus remove the major stabilization mechanism within macroscopic foam.