PREMICELLAR AGGREGATES OF ROSE-BENGAL WITH CATIONIC AND ZWITTERIONIC SURFACTANTS

Transition from non-micellar to micellar Rose Bengal (RE) may involve little known species which are usually called dye-rich-micelles or pre micellar aggregates (PAs). We have studied the PAs formed between RE a nd a series of surfactants with a zwitterionic alkylsulfobetaine funct ionality, SB-X, where X = 8, 10, 12, 14 or 16 carbon atoms in the alky l chain. We have compared these aggregates to those formed between RE and the cationic surfactant cetylpyridinium chloride. In most of the P As studied, the formation of aggregates broadened and red-shifted ( si milar to 10 nm) Rose Bengal's absorption and fluorescence, and decreas ed the intensity compared to the absorption and fluorescence bands of free RE in water. Such properties are characteristic of J-aggregates i n which the RE molecules are loosely organized in a 'head-to-tail' fas hion. The formation of all PAs containing RE was increased by the addi tion of salts producing water-soluble aggregates. Addition of salts su ch as KCI, Na2SO4 or MgCl2 (0.2-1.5 M) to the PAs formed exclusively b etween RE and SB-12 induced the formation of novel J-aggregates in whi ch the dye absorption band was very narrow, red-shifted by 37 nm, and almost three times more intense. The novel PAs are stable in solution and even co-existed with RE in SB-12 micelles at high salt concentrati ons. They were not formed by eosine or salts having large polarizable anions such as ClO4- or I-. They also were not formed with other surfa ctants, except when SB-IO was used instead of SB-12 in MgSO4 solution. The spectral properties of RE in novel J-PAs indicate a higher level of molecular organization, which may lead to free exciton transitions between the dye molecules as a result of strong dipole-dipole coupling . Our results also confirm that the formation of hydrophobic ion pairs between the RE dianion and the cationic center in surfactant molecule s initiates premicellar aggregation (Bilski et al., J. Phys. Chem., 95 (1991) 5784). Some of the resultant salt-induced PAs must be highly s tructured, which strongly affects the spectral properties of Rose Beng al. (C) 1997 Elsevier Science S.A.