Preparation of dilution-stable aqueous magnetic fluids

As part of its program in advancing mineral technology, the Bureau of Mines, U.S. Department of the Interior, has investigated the production of water-base magnetic fluids that are stable toward dilution, and therefore fit for mineral and material beneficiation. Extension of the Bureau developed peptization technique for kerosine-base magnetic fluids to the preparation of the aqueous variety was possible when oleic acid was replaced by dodecylamine. The fluids thus prepared were, however, not stable toward dilution. Irreversible flocculation followed the addition of excessive quantities of water, such as would be encountered in mineral beneficiation operations. When dodecanoic (lauric) acid was used instead of dodecylamine as the dispersing agent, the resulting aqueous magnetic fluid was found to be more stable toward dilution with water. In this second generation water-base magnetic fluid, the dodecanate anionic dispersing agent is superior to the commonly used dodecylamine cationic dispersing agent. Chemisorption of dodecanate anion on the magnetite surface proper by chemically bonding to surface iron atoms is contrasted to the mere adsorption of a cationic surfactant on the solution side of the electric double layer. Expansion of the double layer by dilution and the accompanying decrease in dodecylamine bulk concentration can detach the cationic dispersing agent from the particles of old generation magnetic fluids and nullify their electrostatic charge. By contrast, the firmly held anionic dispersing agent will continue to adhere to the magnetic particles while augmenting their electrostatic charge. Thus, while the former fluids may irreversibly flocculate, the new generation fluids will resist flocculation upon dilution.