Excimer—monomer emission in alkylbenzenesulfonate dispersions: Effect of the surfactant structure on aggregation

Abstract

Aqueous micellar dispersions of several alkylbenzenesulfonates (xφC12 and xφC16) where x = 1, 2, 3 ... and indicates the position of the phenyl group (φ) along the alkyl chain were prepared and examined for fluorescence emission from the benzene residues. Quantitative measurements of the intensities of the monomer and excimer contributions to the fluorescence spectrum as a function of surfactant concentration, when normalized to unit absorbance showed two linear regimes. The break point between the two occurred at a critical micellar concentration (CMC) defined by conductivity and surface tension measurements. Evidence for premicellar aggregates, at surfactant concentrations well below the CMC, was demonstrated. The CMC was related to a transition in the range of aggregation size distribution. The aggregation number was shown to depend on the surfactant structure. For a series of isomers, the aggregation number decreases when the sulfonate benzene group is moved from one end to the center of the aliphatic alkyl chain. This tendency was related to the (hydrophobic/lipophilic) balance of the surfactant.