Abstract
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.
Highlights
Azo dyes are widely used in many applications such as coloring additives in food products.[1]
UV/vis spectra of bulk solutions were recorded for different Sunset Yellow/ C16TAB molar ratios, where the SSY concentration was kept constant while the C16TAB concentration was varied
At a C16TAB concentration fixed to 120 μM, foam capacities decrease with increasing concentrations of Sunset Yellow in the bulk solution (Figure 4a)
Summary
Azo dyes are widely used in many applications such as coloring additives in food products.[1] Every year, >105 tons of dye are produced of which roughly 10% end up in wastewater, where they can cause serious environmental problems.[2] As a consequence, the removal of azo dyes from aqueous solutions is mandatory in terms of environmental protection. The chemistry of dye treatment in wastewater is a challenging task, as azo dyes are very stable against degradation using light, heat, oxidizing agents, etc.[2,3] Common methods for dye removal are relying on approaches such as coagulation, flocculation, absorptive bubble separation, and foam fractionation.[4] Obviously, the interfacial properties of azo dyes are of great importance and in case the dye is not surface active itself, additives are used in order to transfer the dye to the air−water interface. Molecular interactions between surface active additives and azo dyes are an important research topic.[5−13]
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