STUDIES OF THE DOUBLE SURFACTANT LAYER STABILIZATION OF WATER-BASED MAGNETIC FLUIDS

A comparison of the ability of C6C18 carboxylic acids to stabilize aqueous magnetite fluids is made. Micelle formation prevented complete dispersion of magnetite by stearic and myristic acids, but was overcome by introducing the acid in several portions. Improvements in fluid preparation are reported, achieved by perchloric acid treatment of surfactant-stabilized magnetite produced in basic solution, followed by redispersion of the particles in dilute base. The quantity of surfactant in the primary layer has been determined for C10C18 acids, and the particle surface area occupied per molecule of surfactant found to range between ca. 21 and 38 Å2. For decanoic and myristic acids the groups pack efficiently and form a "condensed" film over the surface of the particles. Ammonium and monomethylammonium salts of the same acids were used in attempts to form the secondary layers. Ammonium salts produced dispersion in all cases except Sarkosyl-"O," whereas MeNH3+ salts of oleic and myristic acids were successful. Quantitative data on the relative amounts of surfactant in each layer are presented and discussed. © 1991.