Spectral Studies on the Cooperative Binding Mechanism of Evans Blue to Poly(N-vinyl-2-pyrrolidone)

Abstract

Abstract The binding of Evans Blue (EB) to poly(N-vinyl-2-pyrrolidone) (PVP) was studied at two pH’s 7.2 and 11.2 spectrophotometrically by absorbance change method. At pH 7.2, the absorption spectrum of EB changed unusually on adding PVP and this indicated the formation of two types of spectroscopically distinguishable PVP–EB complexes. At very low [PVP], a weak complex was formed which got converted into a strong complex at high [PVP]. Positive cooperativity was found to be operative in this conversion and in the strong complex at higher [PVP]. Scatchard, Hill, and Schwarz methods were employed to analyze this cooperativity and to determine the binding parameters. The analysis revealed: (i) the binding constants for the weak and strong complexes were of the order of 104 and 105 dm3 mol−1 respectively; (ii) positive cooperativity at higher [PVP] was due to the interaction of neighboring binding sites on different polymer chains. At pH 11.2, only one type of complex was formed with higher affinity. But the binding behavior remained the same as at pH 7.2.