Spectroscopy of rhodamine 6G solubilized in complexes of anionic polymers with cationic surfactant

Abstract

Absorption and emission spectra of rhodamine 6G (R6G) are reported in mixed solutions of dodecyltrimethylammonium bromide (DTAB) with various polyelectrolytes, including the sodium salts of poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), poly(styrenesulfonic acid) (PSS), polygalacturonic acid (pectate), and the alternating copolymers of maleic acid with ethylene (PMA-E) and styrene (PMA-St). In the systems of PAA, PMA, PMA-E and PMA-St, in the absence of surfactant, the R6G absorption spectrum has a strong dimer band and exhibits remarkable quenching of the emission spectrum indicating self-association of R6G in the polyion domain. Upon addition of DTAB, the dimer band disappears, the monomer band increases and the emission intensity recovers at a longer wavelength than that for the monomer band in aqueous solution. These observations are explained by the self-association of R6G in the polyion solutions, followed by the solubilization of R6G in the monomeric form in polyion-DTA complexes. In the case of PSS a red-shift of the R6G monomer band is observed in the absorption spectrum of R6G in solution of PSS alone, and the dimer band is absent. In this case DTAB addition does not affect the monomer band position and induces only a small dimer band maximum at high DTAB concentration, indicating territorial site-binding of R6G and DTA. In the pectate system, only minor changes in absorption and emission spectra are observed in the solution of pectate alone, and addition of DTAB leads to full recovery of the free solution spectra, indicating a weak interaction of R6G with pectate. These results are discussed in terms of different competitive, cooperative binding processes of R6G and DTAB by the various polyions.