Premicellar aggregates of Rose Bengal with cationic and zwitterionic surfactants

Abstract

Transition from non-micellar to micellar Rose Bengal (RB) may involve little known species which are usually called dye-rich-micelles or premicellar aggregates (PAs). We have studied the PAs formed between RB and a series of surfactants with a zwitterionic alkylsulfobetaine functionality, SB-X, where X = 8, 10, 12, 14 or 16 carbon atoms in the alkyl chain. We have compared these aggregates to those formed between RB and the cationic surfactant cetylpyridinium chloride. In most of the PAs studied, the formation of aggregates broadened and redshifted (∼10 nm) Rose Bengal's absorption and fluorescence, and decreased the intensity compared to the absorption and fluorescence bands of free RB in water. Such properties are characteristic of J-aggregates in which the RB molecules are loosely organized in a ‘head-to-tail’ fashion. The formation of all PAs containing RB was increased by the addition of salts producting water-soluble aggregates. Addition of salts such as KCl, Na2SO4 or MgCl2 (0.2–1.5 M) to the PAs formed exclusively between RB and SB-12 induced the formation of novel J-aggregates in which 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 RB in SB-12 micelles at high salt concentrations. They were not formed by eosine or salts having large polarizable anions such as ClO4− or I−. They also were not formed with other surfactants, except when SB-10 was used instead of SB-12 in MgSO4 solution. The spectral properties of RB 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 RB dianion and the cationic center in surfactant molecules initiates premicellar aggregation (Bilski et al., J. Phys. Chem., 95 (1991) 5784). Some of the resultant salt-induced PAs must be highly structured, which strongly affects the spectral properties of Rose Bengal.