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

reason a maximum appears in the electrophoretic mobility In this work we have exhaustively studied the effect of the heat at an electrolyte concentration instead of the expected contintreatment on polystyrene latexes with different functional groups uous decrease in mobility or in zeta potential with increasing on their surface. Four surfactant-free latexes with different chemielectrolyte concentration. Yet, this point is still unclear. cal surface groups (sulfate, sulfonate, aldehyde, and carboxyl) One of the more accepted mechanisms that explain this were prepared. The sulfate and carboxyl latexes were synthesized anomalous behavior, albeit qualitatively, is the ‘‘hairy by using styrene as monomer and potasium persulfate and 4,4*layer’’ model (1–3). This model postulates that on the surazobis(4-cyanopentanoic acid) (ACPA) as initiators, respectively. face of the particles there is a layer of flexible polymer The sulfonate and aldehyde latexes were prepared by copolymerization of styrene/sodium styrene sulfonate and styrene/acrolein, chains that have terminal ionic groups. These chains are respectively. After the heat treatment the latexes were analyzed extended into solution for a distance which varies depending by TEM, IR spectroscopy, conductimetric and potentiometric tion the electrolyte concentrations. At low concentrations the tration, and the adsorption of a nonionic surfactant. The results extended chains reduce the electrophoretic mobility by diswere compared with those obtained with the original latexes in placing the shear plane farther than if all the charge was on order to detect any change in the particle size, polymer composithe particle surface. This shifting allows an increase of the tion, amount and type of surface groups, and hydrophobic characsurface conduction inside the shear plane and therefore a ter of the latexes. The treated sulfate and aldehyde latexes showed decrease in the z potential. As the electrolyte concentration some changes in the type and amount of the surface charge and increases, the chains collapse over the surface, the distance in their hydrophobic character. Once the surface characterization between the shear plane and the surface being reduced; the of the treated latexes was completed, all the samples were characterized from the electrokinetic point of view, by measuring their reduction in that distance could also yield the diminution of electrophoretic mobilities as a function of pH and electrolyte conthe surface conductance, and then the mobility increases. At centration. The comparison with the untreated latexes confirmed higher concentrations, classical behavior is observed, since that the sulfate and aldehyde latexes suffered a change in the the double-layer compression provokes the complete coltype of the surface groups, which provoked a decrease in their lapse of the chains over the surface. One of the largest probelectrophoretic mobility at acid pH. The polystyrene latexes lems of this model is that it only describes the electrokinetic showed a maximum in their electrophoretic mobility, which was behavior for electrolyte concentrations greater than 10 M. less pronounced and shifted toward low electrolyte concentration According to this model, the latex particles should be for the treated sulfate and aldehyde latexes. The sulfonate and carboxyl latexes showed similar surface characteristics and electrosusceptible to a heat treatment above the glass transition kinetic behavior before and after the heat treatment. q 1996 Academic temperature of the polymer (Tg) , at which the softening of