Determination of micellar microenvironment of pinacyanol by visible spectroscopy

The interaction of pinacyanol, a cationic cyanine dye (PIN), with Triton X-100, sodium dodecyl sulfate, and alkyltrimethylammonium bromide micelles was studied by visible spectrophotometry. PIN exists as a monomer and in aggregated form (dimers, trimers, and higher aggregates) in aqueous solution, but dimers and higher aggregates were split into monomers in the presence of surfactants (or on addition of organic solvents). The interaction of PIN with any kind of micelles produced a bathochromic shift of all of the spectral bands and increased the most red-shifted band. The deconvolution of each experimental spectrum in three bands allowed the calculation of the molar absorptivity of any band. The extent of the binding of the dye to micelles can be measured by the association constant. The highest value of this was achieved with anionic, then with nonionic, and, finally, with cationic micelles. In the latter, the higher the length of the hydrophobic chain, the higher the association constant. By comparing the wavelength shifts in micellar systems with those observed in organic solvents, it was possible to assign a kind of microenvironment of PIN in the micelle.