MAGNETIC NANOPARTICLES PRODUCED IN SPONTANEOUS CATIONIC-ANIONIC VESICLES - ROOM-TEMPERATURE SYNTHESIS AND CHARACTERIZATION

Unilamellar vesicles formed spontaneously by forming solutions of sing le-tailed cationic and anionic surfactants [cetyltrimethylammonium bro mide (CTAB) and dodecylbenzenesulfonic acid (HDBS), respectively] have been used as reactors for the direct, room temperature synthesis of n anometer-sized magnetic particles within their internal cores, A micel lar anionic surfactant solution was titrated slowly into a micellar ca tionic surfactant solution containing ferrous chloride, forming defect free unilamellar vesicles that contained the reactant ferrous ions, C TAB/HDBS molar ratios of 13/7 and 6/4 and ferrous chloride concentrati ons of 0.1 and 0.05 M were used. Extravesicular ferrous ions were repl aced with sodium ions by passing this suspension through a gel permeat ion chromatography column saturated with isotonic sodium chloride solu tion. Addition of sodium hydroxide to the extravesicular region caused hydroxyl ions to permeate into the vesicles and react with the availa ble ferrous ions to form the product. Powder X-ray diffraction of the particulate sample showed the intense peaks of magnetite or gamma-ferr ite. Bragg peak broadening was characteristic of crystallites of rough ly the same diameter as particles measured by transmission electron mi croscopy, indicating that each particle was monocrystalline. All of th e intense peaks were distinctly asymmetric. Magnetization measurements showed that the particles were superparamagnetic. Using the Langevin function folded with the particle size distribution to model the magne tization of the sample in response to an applied magnetic field yielde d ''magnetic'' diameter distributions that were consistently smaller t han particle size distributions obtained from transmission electron mi croscopy. The magnetic size is consistent with the existence of a magn etically anomalous surface layer on each particle, as has been reporte d with other ferrites. The asymmetric diffraction peaks may be explain ed with a model that permits surface relaxation, and it is possible th at the two phenomena are linked. (C) 1995 Academic Press, Inc.