Effect of chemical parameters on the interaction between cationic dyes and poly(acrylic acid)

• Polyacrylic acid forms 2:1 complexes with methylene blue and toluidine blue.
• Added NaCl and cationic surfactant induced reversal metachromasy.
• Dye–polyelectrolyte interaction is spontaneous and mainly electrostatic.

Being polymers and electrolytes, polyelectrolytes are widely used in many applications. In perspective of dye removal from textile effluents, they have been used in coagulation–flocculation and to a lesser extent in polyelectrolyte enhanced ultrafiltration. To optimize the abovementioned process we need to study in detail the interaction of dye with polyelectrolyte in conditions near the industry reality. In this context we investigate herein the effect of chemicals additives, NaCl, surfactant, pH on the behavior of two model cationic dyes, methylene blue (MB) and toluidine blue (TB), with polyacrylic acid polymer (PAA). Using spectrophotometric method it was found that the poly(acrylic acid) induced metachromasy in the dyes resulting by the blue shift of the absorbance confirming the formation of dye polymer complexes. The stoichiometry of MB–PAA and TB–PAA complexes determined by the ratio method was found to be 2:1. The stability constant of the complexes at 298 K was found to be 5332 for MB–PAA and 4358 dm3 mol−1 for TB–PAA and the amount of additives inducing total destabilization of the complex were determined.

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