Excimer-Monomer Emission in Alkylbenzenesulfonate Dispersions: Effect of the Surfactant Structure on Aggregation M. AOUDIA, 1 W. H. WADE, 2 AND M. A. J. RODGERS 3 Department of Chemistry, The University of Texas at Austin, Austin, Texas 78712 Received July 18, 1990; accepted February 7, 1991 Aqueous micellar dispersions of several alkylbenzenesulfonates (x4~C 12 and x~C 16) where x = 1, 2, 3. • • and indicates the position of the phenyl group (~) along the alkyl chain were prepared and examined for fluorescence emission from the benzene residues. Quantitative measurements of the in- tensities 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. © 1991 Academic Press, Inc. INTRODUCTION Many physical properties of aqueous sur- factant solutions when plotted against surfac- tant concentration exhibit a characteristic transition over a range of concentration. This transition is generally related to the formation of aggregates arising from the intrinsic am- phiphilic (hydrophobic/hydrophilic) nature of the surfactant species. The transition region has been used to define the critical micellar concentration (CMC) below which monomers are present and above which multimolecular aggregates (micelles) preferentially form. Various techniques have been used to deter- mine CMC values for different surfactants ( 1, 2). However, one should note that a manifes- tation of aggregation behavior has been re- ported in surfactant solutions below the critical 1 Present address: The University of Blida, B d Abdellah EI-Aichi, BP 270 Blida, Algeria. 2 To whom correspondence should be addressed. 3 Present address: Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403. micellar concentration (3, 4). Also, another concept of critical micellar concentration, Coat, was proposed (5) and was defined as the concentration at which a transition in the shape of the micellar size distribution occurs. This Co,it was shown to be a close lower bound to the conventional CMC. In this work, we amplify an earlier study (6) detailing a simple technique for determin- ing CMC values ofsurfactants containing flu- orescing groups as part of the molecular ar- chitecture. It is based on the intrinsic excimer- monomer emission of the surfactant and it is applied to alkylbenzenesulfonates which have been shown previously to exhibit excimer- monomer emission (7). It is also applied to a typical commercial petroleum sulfonate (TRS 10-80) which is one of a family of surfactants used in chemical flooding oil recovery and is obtained by direct sulfonation of a large variety of aromatic petroleum feedstocks (8). CMC values determined by this method are com- pared with those previously determined by other techniques (9-12). Also, from flu- orescence quenching measurements, micellar Journal of Colloid and Interface Science, Vol. 145, No. 2, September 1991 493 0021-9797/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.