Convenient preparation methods for magnetic colloids

doi:10.1016/S0304-8853(99)00080-3

Journal of Magnetism and Magnetic Materials

Volume 201, Issues 1–3, July 1999, Pages 31-33

https://doi.org/10.1016/S0304-8853(99)00080-3 Get rights and content

Abstract

We present convenient laboratory-scale preparation methods for maghemite grafted with fatty acids as well as maghemite–silica core–shell colloids.

Introduction

Ferrofluids with a wide variety of magnetic materials and solvents are currently available. Oleic acid stabilized magnetite in an organic solvent, by now a classical system, is still widely used. Basically, two methods are known to graft magnetite with oleic acid: long-term grinding of bulk magnetite in the presence of pure surfactant and grafting of surfactants onto chemically precipitated magnetite in an alkaline aqueous solution. The latter method, described by Khalafalla and Reimers [1], is still frequently used to synthesize organophilic magnetite. Here we present a new and convenient laboratory-scale preparation method for this type of fatty acid stabilized ferrofluid. Unlike Khalafalla's method, heating is not required and the grafting step takes only a few minutes to complete.

A second type of colloid we investigated are the silica spheres with iron oxide cores [2], [3]. They are prepared by hydrolyzing tetraethoxysilane (TES) which subsequently polymerizes as silica on the surface of the magnetite particles. Since this reaction is carried out in an alkaline ethanol/water mixture, the point of zero charge (PZC) of magnetite must be lowered before these particles can be dispersed in the reaction mixture. Originally, the PZC was lowered by precipitating a very thin silica layer from a metastable sodium silicate solution [2]. However, the PZC can also be lowered by adsorption of citric acid. This makes the tricky precipitation step obsolete, but still allows further silica growth using TES.

Section snippets

Preparation of maghemite

Magnetite was precipitated by dissolving 2.08 g FeCl₂ anh. and 5.22 g FeCl₃ anh. in 380 ml demineralized water and adding 20 ml of 25% NH₃ to this solution while stirring vigorously [4]. After sedimenting the precipitate with a permanent magnet, the supernatant was removed by decantation. 40 ml 2 M HNO₃ was then added to the black sediment and the mixture was stirred for 5 min. The oxidation to maghemite was completed by adding 60 ml 0.35 M Fe(NO₃)₃ to the mixture and stirring it at its boiling

Grafting maghemite with oleic acid

A typical experiment was conducted as follows. 2 ml FF was diluted by adding 50 ml demineralized water. The sol was flocculated by adding a few drops of 25% NH₃, and sedimented using a permanent magnet. After washing with 50 ml water, 100 ml water was added to the precipitate. Under mild mechanical stirring, 2 ml oleic acid was added. Within a few minutes, all magnetic material was transferred to the oil phase. The black oil droplets were separated from the colorless water phase and washed three

Silica deposition from a TES solution

The PZC of FF was lowered by adding a 0.01 M citric acid solution [4] to 45 ml FF until flocculation was visible. The precipitate was redispersed by increasing the pH to 7 with tetramethylammonium hydroxide. Of the resulting dispersion 1 ml was added to a mixture of 75 ml ethanol, 20 ml water and 5 ml 25% NH₃. While stirring, 0.1 ml TES was added, and after a day an additional amount of 0.2 ml TES was added. TEM micrographs show particles similar to those in earlier work [2], [3]. The average particle

Acknowledgments

The following persons are thanked: Marjan Versluijs for performing X-ray measurements, Peter de Peijnder for recording and helping to interpret the DRIFT spectrum and Paul van Ekeren for the TGA measurement.

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