Pre-micellar interaction or direct monomer to micelle transition for zwitterionic sulfobetaine surfactant in water? A comparative fluorescence study with cationic surfactant

Abstract

Interfacial properties of zwitterionic sulfobetaine surfactant (e.g. SB-12, SB-16) micelles are remarkably similar with those of cationic alkyl-ammonium surfactant (e.g. CTAB, DTAB) micelles and both interact strongly with an anionic fluorophore 8-hydroxypyrene-1,3,6-trisulphonate (HPTS). Does the similarity of their interaction extents into the pre-micellar region? At low concentrations (below the critical micelle concentration or cmc) of the cationic surfactants, fluorescence intensity of HPTS strongly quenches indicating HPTS-surfactant association. However, no net fluorescence quenching was observed for the sulfobetaine surfactants at any concentration. HPTS concentration was varied from 0.8 μM to 80 μM to assess any possibility of HPTS-induced pre-micellar association. The ratio of the protonated and deprotonated emission intensities varies quite differently with the concentrations of sulfobetaine surfactant, particularly, at the micellar transition region, for different initial concentrations of HPTS. This HPTS concentration-dependent behavior apparently indicates HPTS-surfactant association below cmc, however, the observed changes can be satisfactorily justified by micelle-water partitioning of HPTS without invoking pre-micellar interactions. Furthermore, fluorescence anisotropy measurements of MPTS (methyl derivative of HPTS) revealed no significant change in the fluorescence anisotropy decay below cmc, supporting the absence of pre-micellar interaction. This study along with our previous work may provide new insights on the general response of an anion towards zwitterionic surfactants at different concentration ranges.