Adsorption of an Azo Dye on Polyaniline/ Niobium Substrate

In this paper the adsorption process of an azo dye on nanostructured polyaniline films electrodeposited on niobium has been investigated using electrochemical impedance spectroscopy. The electrochemical polymerization has been carried such as to obtain PANI films with a highly branched structure and a high specific surface. Based on the impedance results it has been observed that dye concentration affects the adsorption mechanism. The surface adsorption influences the overall mechanism and it could take place simultaneously with the intraparticle diffusion. The relaxation time could estimate the approach of the system to steady state (equilibrium). In diluted dye solution the relaxation time is higher and the intraparticle diffusion could play an important role. Five adsorption isotherms have been used for the mathematical description of adsorption equilibrium data. All the isotherms describe well the adsorption phenomena except Frumkin isotherm. In our case the Langmuir model gave a better fit for concentrated dye solution (> 3x10(-5) mol L-1) and diffused control Langmuir model for diluted dye solution (< 2x10(-5) mol L-1). The high correlation coefficient and low standard deviation value gave the best fit for Langmuir isotherm. The change of Gibbs energy has been also evaluated for the adsorption of dye onto polyaniline/niobium substrate. The negative Delta G(ads)degrees value (-36.38 kJ mol(-1)) and high value of K-ads (33705.67 mol(-1)) obtained in our measurements indicate that adsorption process is spontaneous and a stable adsorbed dye layer on the polyaniline surface is formed. The adsorption takes place via a combination of physical and chemical adsorption but mainly due to physical adsorption.