RESUMO
Magnetite nanoparticles were synthesized by adding ammonium hydroxide to an iron chloride solution. An unsaturated oleate surfactant was adsorbed on the magnetic particles, and a nonionic Span 20 surfactant was applied onto the oleate-adsorbed particles to form a bilayer structure. The bilayer nanoparticles formed stable dispersions with isoparaffin as the liquid base. The experimental parameters were determined at each concentration to prepare isoparaffin-based ferrofluids with concentrations of 200, 300, 400 and 500 mg/mL; these were characterized by density, dispersion, magnetization and viscosity. The density of the fluids increased in proportion to the concentration from 0.93 g/mL to 1.22 g/mL, whereas the dispersion stability decreased from 97% to 69% with increasing ferrofluid concentration. The saturation magnetization of the ferrofluids depended upon the content of particles in the fluid, with values of 17.8 to 42.2 mT at the concentrations of 200 to 500 mg/mL, respectively. The fluid viscosity increased exponentially with the concentration increase in the same range, from 5.1 cP to 53.7 cP at 20 degrees C and from 3.2 cP to 25.6 cP at 40 degrees C.