Optimization of Long-Term Stability of Magnetic Fluids from Magnetite and Synthetic Polyelectrolytes

• Published in: Journal of colloid and interface science 1996-12, Vol.184 (2), p.519-526
• Main Authors: Mendenhall, George David, Geng, Yunpeng, Hwang, James
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Exact sciences and technology; ferrofluid; General and physical chemistry; Physical and chemical studies. Granulometry. Electrokinetic phenomena; poly(methacrylic acid); ultrasound
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1006/jcis.1996.0647

Nanometer-sized suspensions of magnetite (Fe3O4) stabilized with polymeric surfactants, principally poly(methacrylic acid) (PMAA), were prepared by precipitation or sonication and studied by a variety of techniques. The long-term stability of the ferrofluids made with PMAA was optimal at pH 7 (close to the measured isoelectric point of the magnetite, 5.9) and improved at all pH values with increasing surfactant concentration. With extended times of sonication the molecular weight and polydispersity of the PMAA decreased, although magnetite in the ferrofluid was not oxidized (X-ray). Particle diameters of 9–38 nm were measured by saturation magnetization, electron microscopy, and dynamic light scattering. Centrifugation at 8,000 rpm removed the magnetite from the suspensions together with roughly one molecule of PMAA for each magnetite particle in the fluid, with the average number increasing with higher concentrations and average molecular weights of the polymer.