Dye-polyanion complexes

The property of polyanions to give metachromatic reactions with basic dyes80 (see Section XI) has been widely exploited for the titration of glycosaminoglycans. Because of its high charge-density, heparin displays large shifts in the visible spectrum of the dye upon complex-formation, and can conveniently be titrated (most commonly, with Methylene Blue or Azure A), even in the presence of minor proportions of other glycosaminoglycans. It should be realized that quantitative responses with metachromatic dyes are different for different pure heparin... 

Electrostatic self-assembly was combined with supramolecular chemistry to obtain inclusion complexes of a polymeric nonlinear optical (NLO) active dye and modified [3-cyclodextrin with induced chromophore orientation [37], The polyanion is a N,N-diallyl-aniline and sodium-2-acrylamido-2-methylpropanesulfonate copolymer functionalized with pendant azo group. The modified /i-cyclodextrin oligo-cation was obtained by treatment of hcptakis(6-dco y-6-iodo-/i-cyclodcxtrin) with excess pyridine. A linear polyamine, chitosan, was also combined with the polyanion, for comparison. Films were deposited on glass slides by dipping them alternatively in aqueous solutions of the cation and the polyanion. UV-visible spectra indicate dye aggregation and suggest the formation of an inclusion complex of the dye with the cyclodextrin, thus isolating the chromophores. 

Polymer-polymer complexation is generally detected via conductometric or potentiometric titrations. Colloid titration represents an inverse-system where a polymer with known characteristics, such as potassium poly (vinylalcohol-sulfate) or poly(diallyldimethylam-moniumchloride), are used to quantify the concentration of polycation or polyanion, hence relying on a 1 1 stoichiometry. Using the cationic dye, tol-uidine blue, as an indicator, a metachromatic end point is detected. Both methods are volumetric. 

In connection with studies concerning the conformational state of hydrophobic polyelectrolytes, the mode of interaction of dyes (and related compounds) with such polymers has been investigated. This matter appears to be quite complex but some aspects are to be pointed out. First, the extent of dye binding is generally related to the actual conformation of polymers chains as compared to more extended conformational states, tightly coiled polyanions can bind much more dye.15-19 This is not only observed for charged dyea-polyelectrolytes... 

Figure 1 shows typical absorption spectra of CMC 4M-A0 complexes. The concentration of AO is kept constant and the S/D ratio is increased. The characteristic monomer absorption peak at 492 nm is replaced by an absorption at 456-460 nm, depending on the S/D ratio of the complex. The short wavelength peak is ascribed to formation of dye aggregates, on the CMC at low S/D ratios. The reappearance of the 492 nm peak is evident at high S/D ratios as the dye becomes diluted on the polyanion. 

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