A spectroscopy study of the interaction of pinacyanol with n-dodecyltrimethylammonium bromide micelles

The interaction of pinacyanol, a cyanine dye (PIN), with n-dodecyltrimethyl ammonium bromide (DTAB) at micellar concentration range was studied by visi ble spectrophotometry. As PIN is present in aggregate and nonaggregate form s, we aimed to determine the dimerization constant (K-D = 35,000 M-1) by no nlinear regression fitting of the experimental spectra and then by the reso lution of such spectra in terms of pure monomer and dimer states. Each stat e was deconvoluted into three Gaussian bands, These functions fitted the sp ectral data. Interaction of PIN with micellar DTAB produced a bathochromic shift of all the spectral bands and increased the most red-shifted band. Sp ectral data showed that the dye is bound into the micelle in the monomeric form. On the other hand, micellization reduced the dimerization process. Th e binding of the dye to micelles is defined by an association constant (K-A = 2160 M-1). In such binding, one is the maximum number of PIN molecules t hat each micelle can accommodate. The association constant allowed us to ca lculate the fraction of micellized PIN, from which we deduced the spectrum of the dye into the micelle. This spectrum was consistent with that of the monomer in a medium with a lower dielectric constant.