Heterocoagulation of chrysotile with polytyrene latex / Heterocoagulação entre crisotila e latex de poliestireno

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

Chrysotile is a magnesium silicate with fibrous habit and ideal formula Mg6Si4O10(OH)8, having 1:1 layered structure of tetrahedral tridimite (silicate) with octahedral brucite (magnesium hydroxide). The layers are curved and rolled, resulting in cylindrical fibrils with brucite in the external surface and tridimite in the hollow internal surface. The fibrils aggregate giving rise to bundles, or fibers, which have a positive zeta-potential in aqueous medium. This results from the Mg occurrence after dissociation of the surface hydroxyl groups. Despite its positive zeta potential, the surface of chrysotile attaches colloidal particles of negative or positive nature. Aiming to understand this phenomenon, polystyrene latex particles, with negative (PS-) and positive (PS+) surface charge, were synthesized in aqueous solution, in the absence of surfactants, and used in experiments of heterocoagulation with chrysotile. The aggregation of the fibers was controled through mechanical action or improved hidrofobicity by metilation of the surface using Si(CH3)2Cl2. The zeta potential of chrysotile was modified by washing with HCl or adding NaOH or sodium silicate aqueous solution. The amount of polystyrene particles was varied and its surface charge adjusted adding cetyl trimethylammonium bromide cationic surfactant (CTAB). Results of microscopic analysis (MEV) showed that the formation of heteroaggregates occurs preferentially on disaggregated fibers, or fibrils, which are the most flexible. Also, an expressive quantity of homoaggregates of latex with approximately 10 particles/mg of chrysotile are formed. According with the results of optical density of the supernatants, procedures that reduce the disaggregation and the zeta-potential of chrysotile decrease the initial rate of formation of heteroaggregates with particles of PS-. The adsorption of CTAB inhibits the adhesion of PS- particles and does not interfere in the adhesion of PS+ particles on chrysotile. The heterocoagulation of PS- latex is completely reverted with the addition of sodium silicate to the dispersion. In aqueous suspension, sodium silicate removes impurities of the chrysotile surface, as detected by the increase in the optical density of the dispersion. Particles of PS+ latex practically will not attach on chrysotile washed with sodium silicate. The results indicate that the adhesion of negative particles on chrysotile occurs preferentially at positive surface sites, while the adhesion of positive particles occurs at the negative surface sites, probably caused by the pre-adhesion of impurities on the chrysotile when washing with water. The surface density of positive sites is more significant in flexible and disaggregated fibers, on which the heterocoagulation occurs preferentially. The homoaggregation of the latex particles seems to be dependent mainly on the mobility of the preformed heteroaggregates, which, under stirring, collide among themselves with enough mechanical energy to cause the adhesion between the latex particles

ASSUNTO(S)

heterogoagulação dlvo theory heterocoagulation estabilidade coloidal teoria dlvo colloidal stability

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