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Electrokinetic Behavior of Polystyrene Latexes with Different Surface Groups: Effect of Heat Treatment

Abstract In this work we have exhaustively studied the effect of the heat treatment on polystyrene latexes with different functional groups on their surface. Four surfactant-free latexes with different chemical surface groups (sulfate, sulfonate, aldehyde, and carboxyl) were prepared. The sulfate and carboxyl latexes were synthesized by using styrene as monomer and potasium persulfate and 4,4′-azobis(4-cyanopentanoic acid) (ACPA) as initiators, respectively. The sulfonate and aldehyde latexes were prepared by copolymerization of styrene/sodium styrene sulfonate and styrene/acrolein, respectively. After the heat treatment the latexes were analyzed by TEM, IR spectroscopy, conductimetric and potentiometric titration, and the adsorption of a nonionic surfactant. The results were compared with those obtained with the original latexes in order to detect any change in the particle size, polymer composition, amount and type of surface groups, and hydrophobic character of the latexes. The treated sulfate and aldehyde latexes showed some changes in the type and amount of the surface charge and in their hydrophobic character. Once the surface characterization of the treated latexes was completed, all the samples were characterized from the electrokinetic point of view, by measuring their electrophoretic mobilities as a function of pH and electrolyte concentration. The comparison with the untreated latexes confirmed that the sulfate and aldehyde latexes suffered a change in the type of the surface groups, which provoked a decrease in their electrophoretic mobility at acid pH. The polystyrene latexes showed a maximum in their electrophoretic mobility, which was less pronounced and shifted toward low electrolyte concentration for the treated sulfate and aldehyde latexes. The sulfonate and carboxyl latexes showed similar surface characteristics and electrokinetic behavior before and after the heat treatment.