EFFECT OF CATIONIC POLYELECTROLYTES ON THE DISSOLUTION OF MAGNETITE IN THIOGLYCOLIC ACID-SOLUTIONS

The influence of cationic polyelectrolytes, polyethyleneimine (PEI) an d polyvinylbenzyltrimethylammonium chloride (PVBTA-Cl), on the kinetic s of dissolution of an iron oxide (magnetite, Fe3O4) by thioglycolic a cid (HTG), is presented. It was found that both polyelectrolytes accel erate the dissolution reaction at pH values below ca. 4, whereas an in hibition is observed at higher pH values. These findings are at varian ce with the results obtained in the case of anionic polyelectrolytes, in which the reaction is inhibited over the whole pH range, The effect s of PEI concentration, of the addition of inert salt to the PEI-HTG-F e3O4 system and of temperature on both systems were also studied. The main effects of PEI are the enhancement of the acidity of the oxide su rface, and the decrease of the available surface sites for HTG complex ation; the effect of PBVTA-Cl is more modest, due to the higher rigidi ty of the polyelectrolyte. Modeling of the experimental results in PEI media leads to a kinetic scheme similar to that describing the reacti on in the absence of PEI, with a pK(a)(s), shifted by -2; this shift a lters the pH dependence of the surface speciation, thus modifying the rate-pH profiles. For PVBTA-Cl, the shift derived from the data is -1.