OPTIMIZATION OF LONG-TERM STABILITY OF MAGNETIC FLUIDS FROM MAGNETITEAND SYNTHETIC POLYELECTROLYTES

Nanometer-sized suspensions of magnetite (Fe3O4) stabilized with polym eric surfactants, principally poly(methacrylic acid) (PMAA), were prep ared by precipitation or sonication and studied by a variety of techni ques. The long-term stability of the ferrofluids made with PMAA was op timal at pH 7 (close to the measured isoelectric point of the magnetit e, 5.9) and improved at all pH values with increasing surfactant conce ntration. With extended times of sonication the molecular weight and p olydispersity of the PMAA decreased, although magnetite in the ferrofl uid was not oxidized (X-ray). Particle diameters of 9-38 nm were measu red by saturation magnetization, electron microscopy, and dynamic ligh t scattering, Centrifugation at 8,000 rpm removed the magnetite from t he suspensions together with roughly one molecule of PMAA for each mag netite particle in the fluid, with the average number increasing with higher concentrations and average molecular weights of the polymer. (C ) 1996 Academic Press, Inc.