Interactions between sodium polyacrylate and mixed surfactants of polyoxyethylene tert-octyl phenyl ether and dodecyltrimethylammonium bromide

The interactions between sodium polyacrylate (PANa) and mixed surfactants of polyoxyethylene tert-octyl phenyl ether (TX100) and dodecyltrimethylammonium bromide (DTAB) in 40 mM NaBr aqueous solutions were studied using isothermal titration calorimetry (ITC). It was found that whether DTAB was titrated into PANa/TX100 or TX100/DTAB was titrated into PANa, three endothermic peaks were detected; representing three processes: (1) binding of DTAB monomers to PANa chains through electrostatic interactions, (2) polymer-induced micellization, and (3) cross-linking of polymer chains. The interaction mechanism was interpreted using a thermodynamic model, and it was found that when the molar ratio of bound DTAB to the carboxylate group of the polymer (C-DTAB(poly)/C-poly) reached about 0.5, the polymer-induced micellization occurred; while when C-DTAB(poly)/C-poly reached about 1, indicating complete neutralization of the electrostatic charges on the polymer chains, the cross-linking of the polymer chains started and precipitation was observed; finally, as C-DTAB(poly)/C-poly reached 1.3, the precipitate was redissolved slowly due to reversion of the charge ratio of the polymer chains.