THE PREPARATION AND SURFACE CHARACTERIZATION OF POLYSTYRENE LATEXES AS MODEL COLLOID

The purpose of the present investigation was to prepare a model colloid with one type of surface group. Several conventional polystyrene latexes were prepared, cleaned by a mixed bed of anionic and cationic resins, and their surfaces characterized by conductometric titration. All latexes carried a strong-acid group; no weak-acids were detected.;Polystyrene latexes carrying only hydroxyl groups have been prepared by hydrolyzing the sulfate groups on aging the ion-exchanged latexes at room temperature. The hydrolysis step was acid-catalyzed, and occurred only after ion-exchange to the acid form. To inhibit the hydrolysis, the H('+)-counterions were ion-exchanged to the Na('+)-form. Heating the ion-exchanged latexes appreciably enhanced hydrolysis, also increasing the glass-contact surface area through the introduction of Pyrex glass beads, led to a slight enhancement in hydrolysis.;The latex in the hydroxyl form was oxidized into the carboxyl form by means of potassium persulfate oxidant. A new and alternate procedure included heating the ion-exchanged sulfate form of the latex in the presence of Pyrex glass beads.;Polystyrene latex 520', whose recipe contained water, styrene, potassium persulfate, and sodium bicarbonate, initially carried only sulfate groups; these were first hydrolyzed to the hydroxyl form and then later oxidized fully to the carboxyl form. The latex was monodisperse, stable, and exhibited the same size and molecular weight in all three forms. Dissolving the latex in dioxane-water mixture, followed by ion-exchange and titration, showed the presence of buried sulfate groups.;The electrophoretic mobility behavior of latex 520' was studied as a function of pH. All three forms of the latex displayed the same trends, showing no changes at intermediate pH ranges, while at lower pH the mobility decreased, with the hydroxyl and carboxyl forms showing a charge-reversal behavior. In alkaline pH, the mobility first increased slightly before decreasing at higher pH. These trends were explained in terms of ion adsorption and acid-base interactions.;At neutral pH, all three forms of the latex exhibited, approximately, the same zeta potential, ca. -60 mV; the presence of that potential on the latex in the hydroxyl form suggested that the charge on colloidal dispersions could originate from the preferential adsorption of hydroxyl ions or form contact electrification. Calculations based on the DLVO Theory showed that the stability of the different forms of latex 520' increased in the following order: sulfate > carboxyl >> hydroxyl.;Argon gas adsorption measurements gave surface areas that were in good agreement with those obtained from electron microscopy. The complete reversibility of the inert-gas adsorption indicated that the polymer surface was nonporous. The adsorption of water and 2-propanol vapors showed that the surface was mostly hydrophobic with only few polar sites. The sulfate group displayed the strongest interaction of functional groups with water and 2-propanol; a possible clustering of 2-propanol molecules on the sulfate group was suggested. The results showed a good correlation between gas adsorption and conductometric titration techniques.