Interaction of 9-Aminoacridinium and 10-Methylacridinium Cations with Polyelectrolytes, with Emphasis on the Bound-dye Spectra

Visible and UV absorption spectra of two dyes, 9-aminoacridinium (AA) and 10-methylacridinium (MeA) cations, were measured at 25 °C in the presence of three different polyelectrolytes—sodium salts of poly(p-styrenesulfonate) (NaPSS), polyphosphate (NaPP), and polyacrylate (NaPA)—for polymer residue-to-dye ratios from zero to near unity. The optical titration curves of AA-polyanion systems showed a slightly sigmoidal curvature. The method of extended principal component analysis was applied to the experimental spectra of the AA–NaPSS, –NaPP, –NaPA and MeA–NaPSS systems. Only two absorbing component dyes, i.e., bound and unbound, were verified to exist in each system. The equilibrium constant of the binding between dye and polyanion, the pure spectrum of bound dye, and the adjustable parameter α, by which the sigmoidal titration curve is characterized, were determined. In each of the above systems, the bound-dye spectrum behaves in opposition to the common notion of “metachromasy,” showing a bathochromic shift without any new band. The spectra of MeA changed only moderately by the addition of NaPSS. The ultrafiltration of the MeA–NaPP and –NaPA solutions confirmed that most of the MeA cations remain unbound.