THE PREPARATION AND SURFACE CHARACTERIZATION OF AN IDEAL MODEL COLLOID

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Abstract
Monodisperse polystyrene latexes were prepared with and without emulsifier using persulfate initiator. The latexes were ion-exchanged with purified Dowex 50W(H+)-Dowex 1(0H−) mixed resin to remove solute electrolyte and adsorbed emulsifier and to convert acidic surface groups to the H+ form. Conductometric titration showed that all latexes contained only strong-acid surface groups, presumably the sulfate endgroups of the polymer chains introduced by the persulfate initiator. The sulfate-stabilized latexes (H+ form) were hydrolyzed to the hydroxyl form after 2–8 weeks at room temperature or 48–120 hours at 363°K. The rate of the acid-catalyzed hydrolysis is enhanced by increasing the glass contact surface area, by adding Pyrex glass beads. Conductometric titration of the hydrolyzed latexes after ion exchange showed no titratable groups, indicating that the latexes were stabilized by hydroxyl groups. The latexes were oxidized to the carboxyl form by heating the hydroxyl-stabilized latex with persulfate and traces of heavy metal ions or the sulfate-stabilized latex in contact with Pyrex glass beads. For example, latex 520′ prepared without emulsifier using persulfate initiator and bicarbonate buffer contained only strong-acid surface groups after cleaning, no titratable groups after hydrolysis, and only carboxyl groups in the same number as the original sulfate groups after oxidation. Thus the three forms of the latex provide ideal model colloids, rigid monodisperse spheres stabilized with the same number of chemically bound surface groups of three types —strong-acid sulfate, weak- acid carboxyl, and uncharged hydroxyl.