Interaction of poly (N-vinyl 2-pyrrolidone) with a bisazo dye in aqueous solution

The binding mechanism of 6,6-[(3,3-dimethyl[1,1-biphenyl]-4,-4 diyl) bis(azo)] bis [4-amino-5-hydroxy-1,3-naphthalene disulfonic acid] tetra sodium salt, usually called Evan blue (EB), to poly (N-vinyl 2-pyrrolidone) (PVP) in aqueous solution has been studied spectrophotometrically at pH 9.2 in phosphate buffer in the presence and absence of NaCl. lambda(max) = 606 nm for EB solutions was observed and then by the addition of PVP red shifted to 636 nm. Analysis of results by Scatchard and Hill methods showed a mixed cooperativity mechanism at different [PVP]. Thus at high [PVP] there is positive cooperativity in the conversion of weak to strong complex and negative cooperativity at low [PVP]. The U-shaped Scatchard plot at fixed [PVP] and varying [dye] further supplements the operation of positive and negative cooperativity.