SELF-STABILIZED AQUEOUS
FERROFLUIDS PROPERTIES
AND
CHARACTERISTICS.
Ziolo R. F.
Introduction.
Ferrofluids are very stable colloidal dispersion of ultra-fine particles of a magnetic materials, such as magnetite, in a liquid, which may be chosen to conform to a particular application. A stabilizer or surfactant is usually added at the time of preparation to prevent aggregation of the nanoscale particulate. As a result of their composition, magnetic fluids possess a unique combination of fluidity and the capability to interact with a magnetic field. For magnetic carrier applications, ferrofluids may be thought of as being bi-functional materials with both a magnetic particle and magnetic fluid component.
Perspective.
S. S. Papell made the first ferrofluids by grinding in
a ball mill in the presence of oleic acid (surfactant) and kerosen (USA Patent
№
3215572, 1965). The preparation required many
months of milling and commercial ferrofluids soon followed. Their development
and eventual understanding were pioneered by such researches as R. E.
Rosenszweig, then at Exxon Corporation, and K. Raj, then at Ferrofluidics
Corporation.
The early preparations followed a «top down» strategy starting with the grinding of micron or sub-micron sized
magnetite in the presence of a hydrocarbon and a small amount of surfactant.
The grinding usually lasted in excess of six months. Rapid recombination of the
nascent nanoscale particles probably contributed to the very long grinding
times.
Significant advances in the preparation of
ferrofluids, and thus in the understanding and creation of new ferrofluids,
came when researchers realized that magnetic fluids could be made from a «bottom up» rather than «top down» strategy. Thus, precipitation reactions to produce
magnetic nanoparticles in the presence of a stabilizing agent led directly to
the formation of ferrofluids and eliminated the need to mill. The new approach
decreased the preparation time from months to days.
The technological development of the hydrocarbon
ferrofluids outpaced that of the water based ferrofluids, partly as a result of
practical need, and partly as a result of the extraordinary stability of the
former and instability of the latter. In non-polar carrier liquids, such as the
hydrocarbons, the particulate is stabilized by steric repulsion.
It wasn’t until ionic stabilization was introduced by
R. Massart that water based fluids took hold [1]. The electrostatic repulsion
was achieved by the preferential adsorption of ions of one type on the magnetic
particles and resulted in a relatively stable water-based fluid.
In general, practical ferrofluids are black or very dark brown liquids and are not very suitable for optical applications. Some of the more important uses of ferrofluids today are for heat exchange and mechanical damping in loudspeakers, as seals in hard disk drives and as vacuum seals in general.
To date, applications of ferrofluids in the biomedical
and biotech communities have been driven mainly by the diverse and specific
needs of those communities. Many excellent examples of applications can be
found in the published proceedings of former conferences such as the present
one, Scientific and Clinical Applications
of Magnetic Carriers, and the International Conference on Magnetic Fluids.
Althoung ferrofluids involve nanoscale materials,
their community and technologies evolved long before the emergence of
nanoscience and nanotechnology. In an academic sense, the study of ferrofluids
would qualify as a subset of the later and present a research area rich in
challenges for the newcomer. Interdisciplinary research on ferrofluids as
nanomaterials could lead to more «user-friendly» ferrofluid
designs that may in turn help drive the biomedical and biotechnical applications.
Two such series of water-based ferrofluids will be discussed in the present
talk along with their properties, characteristics and unusual effects.
Sulfonated
polystyrene as matrix.
One series of ferrofluids is formed by a technique of
matrix milling a nanocomposite containing maghemite, g - Fe2O3, in water or a water
miscible solvent. In this case, a solid matrix, a DVB cross-linked sulfonated
polysterene, is used to synthesize the nanoparticles and keep them isolated to
prevent aggregation [2]. The composite is then milled for a few hours to yield
the ferrofluid.
In order to increase the magnetization (M) of the
ferrofluids after centrifugation, the fluids are subjected to ultrafiltration,
which effectively increases the volume fraction of the maghemite. Volum
reductions of between 80 % and 96 % result in stable ferrofluids containing
between 15 % and 55 % by weight solids, as determined by oven drying the fluids
at 110°C.
Analysis of the solids showed that the iron oxide to
polymer ratio was not constant. The mass ratio increased from 1:1 for the
pre-concentrated ferrofluids to about 6:1 for the most concentrated
ferrofluids, demonstrating that only a portion of the original polymer was
necessary for colloidal stabilization of the nanocrystalline g-Fe2O3. Moreover, there was no
apparent effect on the stability of the ferrofluids over this mass ratio range,
unlike that reported for aqueous ferrofluids of magnetite, Fe3O4,
stabilized with sodium oleate. In the latter case, deviation on either side of
the optimal magnetite-to-stabilizer ratio, 7:5, resulted in a drastic reduction
of either the magnetic properties of the fluid or its stability.
Capillary electrophoretic analysis (see below)
established that ultrafiltration served not only to concentrate the ferrofluids
by carrier loss but also to purify them by removal of non-bonded or degraded
polymer with both losses contributing to the increase in magnetization.
Seta potential determinations on the particulate in
the diluted ferrofluids yelded values of from – 65 to – 85 mV. Solution studies
of the hydrodynamic particle size by quasi-elastic light scattering techniques
and by SAXS measurements suggested particles of a very narrow size distribution
with diameters of either 47 or 100 nm. The particle size analyses and general
behavior of the ferrofluids suggest a solvent swollen, three-dimensional
polymer network stabilizing the nanocrystalline g-Fe2O3. The behavior of the
particles appears to mimic the comb-like grafted hydro gels that show rapid
de-swelling response to temperature changes or to gels as described earlier by
Y. Li and T. Tanaka. For the most part, the present ferrofluids, which are
water-0based, are stabilized by steric repulsion.
Magnetic and
optical properties.
Room temperature magnetization values (M) for the
present ferrofluids range from < 2 to 52 kA/m, depending on the preparative
history of the sample and the applied field. In general, the magnetization of
the fluids increases proportionally with the mass loading of iron oxide. A
representative ferrofluid at 300K in the present series is nearly saturated
with a magnetization 35.8 kA/m in an applied field of 800 kA/m. The magnetic
remanence and coercitivy are zero, consistent with superparamagnetic behavior
and the nanoscale dimension of the particles. Below 50 K, a small hysteresis
loop appears. At 10K, the ferrofluid has a sizable coercive field of 22 kA/m.
The hysteresis loop appears symmetric about the center for both the
field-cooled and zero field cooled cases. Superparamagnetic behavior was
observed for all of the ferrofluids in the present series, as well as for the
solid nanocomposite parent.
One of the magnetically strongest ferrofluids in the
present group is a very viscous fluid with a saturation magnetization of 52.3
kA/m in an applied field of 800 kOe. Magnetization versus applied field curves
at 300 K showed virtually no hysteresis consistent with superparamagnetic
behavior. The fluid contained 53 % by weight solids and had a density of 1.55 g
/ cm3.
In the proximity of a one Tesla permanent magnet, the
ferrofluids with an M of about 12 kA/m and higher represents the surface structure
of a ferrofluid in the presence of a magnetic field. The surface instability
results from the interaction of magnetic, gravitational and viscous forces and
the surface free energy of the ferrofluid. Spike heights in the strongest
ferrofluids can reach several centimeters.
Optical data on bulk ferrofluids has been quite
limited. Magnetite-base ferrofluids, for example, are black and opaque except
in very thin layers. Optical data for the present ferrofluids were identical
to those reported for the solid nanocomposite [2], consistent with our
observation of no detected physical or chemical change in the crystallite after
milling. As a result, the ferrofluids have a color similar to that of the solid
composite parent and appear amber or amber red in color except for the
strongest fluids. Because of their relative transparency, the ferrofluids may
be used as an optical diagnostic tool in fluid dynamic and fluid mechanical
studies. For example, use of the present fluids aboard MIRE has enabled the
first observation and study of convection currents inside of a suspended drop
of fourfold in microgravity experiments using simple optical diagnostic
techniques. In the presence of an applied field, the stronger ferrofluids
exhibit optical birefringence.
Changing the alkali metal hydroxide used in the
preparation of the starting material can significantly change the color of the
ferrofluids. The use of Li, K, Rb, Cs and ammonium hydroxides instead of NaOH
causes the ferrofluids to go from lighter to darker red through a significant
shift in the optical absorption edge of the nanocrystalline g-Fe2O3. The role of the cation
in changing the optical properties of the mesocsopic oxide appears to be more
complex that a simple doping phenomenon since K, Rb and Cs cations presumably
do not fit into a normal g-Fe2O3 lattice. The observed shifts supports the
possible effects of the pressure hypothesis as suggested earlier [2] involving
different size cations located between the sulfonated resin and the g-Fe2O3 particles.
The optical transparency of the nanocristalline g-Fe2O3 along with the relatively
high initial permeability, magnetization and stability of the ferrofluids also
enables the existence of machine-usable colored magnetic inks at room
temperature. To our knowledge, nanotechnology has been the only approach to
successfully provide simultaneous color and magnetism in the same material at
room temperature.
Alginate as
matrix.
Alginate is a naturally occurring polysaccharide that finds heavy use in the pharmaceutical and food industries. The availability of magnetic forms of the polysaccharide can offer new avenues for materials management and control.
In the form of alginic acid, alginat forms cross-linked gels in the presence of various cations. Magnetic forms of alginate have been made by incorporating micron scale iron oxides into the calcium cross-linked gel. Magnetic gels are the subjects of current research for applications in medical diagnostics, drug delivery and cell sorting systems.
We have produced alginate based ferrofluids by first forming the magnetic gel [3]. This was accomplished by using Fe as the cross-linking ion and using it as the reaction center for the in situ formation of nanocrystalline iron oxides.
Two mm diameter Fe (II) cross-linked beads were formed by dropping an aqueous solution of sodium alginate at room temperature through a 17-gauge stainless steel needle into a degassed solution of iron (II) chloride. Excess ferrous ions were removed by washing with 1:1 MeOH / water solutions under nitrogen. The beads, which were kept in the MeOH / water mix and under nitrogen, were then treated with a sodium hydroxide solution to effect the conversion of Fe (II) ion to g-Fe2O3. This two-step synthesis was repeated up to four times to increase the loading of iron oxide in the gel. This technique led to gels that on a dry basis contained between 10 and 50 % iron.
Aqueous alginate ferrofluids were then formed by oxidative depolymerization of the beads in water with air bubbled through the suspension to evaporate the methanol, which acts as an inhibitor to the depolymaerization of the alginate.
The physical properties of the alginate ferrofluids are similar to those of the sulfonated polystyrene fluids. Gels obtained after five consecutive loading cycles and then dehydrated have a room temperature saturation magnetization of 300 kA/m at 20 kOe. Magnetization curves of M applied field and temperature (4.2 K to 300 K) suggest superparamagnetic behavior consistent with the small size of the iron oxide.
Analysis.
The effective characterization of water-based ferrofluids is as important as their design. Conventional methods of analyzing these materials include transmission electron microscopy and quasi-elastic light scattering techniques for particle size. Microelectrophoresis is also used and yields data on the average electrophoretic mobility and the zeta-potential of the colloid. None of these methods allows for the fast detection of impurities. Methodology reported by Morneau at al. [4] on the application of capillary electrophoresis (CE) to the analysis and characterization of magnetic fluids will also be discussed. In the study. CE was coupled with diode array UV-visible detection to provide a new, effecient and sensitive method to characterize water-based ferrofluids in terms of surface charge, electrophoretic mobility and the level of purity. The method has also proved useful in quality control of ferrrofluid production.
References.
1.
Charles S. W. // Magnetic Fluids (Ferrofluids) in Magnetic
Properties of Fine Particles (eds Dormann J. L. and Fiorani D.) North-Holland
Elsevier, P. 267 – 374.
2.
Ziolo R et al. // Science, 33 (1992), 1471 – 1477.
3.
Kroll E. et al. // Chemistry of Materials, 8 (1996), 1594 –
1596.
4.
Morneau A. et al. // Colloids and Surfaces A: Physicochemical and
Engineering Aspects, 154 (1999), 295 – 301.
Acknowledgement
I would like to acknowledge the contributions of my colleagues and students to the understanding and development of these ferrofluids.