UV-vis spectral studies of interactions between methylene blue and cationic surfactant (alkyltrimethylammonium...

The study of dye aggregation is a dynamic research field and understanding its mechanism is crucial for advancing biomolecular interactions, diagnostics, photodynamic therapy and lasing technology. Methylene blue (MB) is a cationic dye and its aggregation behaviour in water is a topic of interest. However, the study of the aggregation behaviour of MB in aqueous surfactant solutions is scarce. The present study investigates the effect of cationic surfactant on the aggregation behaviour of MB in aqueous solution. The basic design of the study involved recording UV-vis absorption spectra of MB in water and in aqueous solutions of a homologous series of alkyltrimethylammonium bromide surfactants (octyltrimethylammonium bromide (C₈TAB), decyltrimethylammonium bromide (C₁₀TAB), dodecyltrimethylammonium bromide (C₁₂TAB), tetradecyltrimethylammonium bromide (C₁₄TAB) and hexadecyltrimethylammonium bromide (C₁₆TAB)) at room temperature over the wavelength range of 560-700 nm. The dimer dissociation constant (K_D) values for MB in water and aqueous C₈TAB-C₁₆TAB solutions were determined by applying the methodology developed by Terdale and Tantray. The K_D of MB in pure water is found to be 3.9000 × 10^-4 M, which decreased to 1.4000-2.1300 × 10^-4 M upon addition of cationic surfactants (C₈TAB-C₁₆TAB). The absorption spectra were resolved into monomer and dimer contributions, and the dimer spectra were further separated into H- and J-aggregate bands, where H-aggregates were found to predominate as indicated by their higher band intensity. Molecular exciton theory was then applied to determine the geometry, twist angle, and interaction energy. The derived geometric parameters and interaction energies provide quantitative insight into H- and J-aggregation of MB in micellar media. The results are rationalized by dye-surfactant interactions that modulate the extent of aggregation of MB dye in aqueous surfactant solutions.