Langmuir 1994,10, 92-99
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Magnetic Silica Dispersions: Preparation and Stability of Surface-Modified Silica Particles with a Magnetic Core Albert P. Philipse,’ Michel P. B. van Bruggen, and Chellapah Pathmamanoharan Van’t Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Received July 28, 1993. In Final Form: October 12, 199P Preparation and properties are discussed of a novel magnetic dispersion, containing surface-modified silica colloids with a core of single-domain magnetite particles. The underlying idea is to tailor the silica shell thickness and surface properties such that the colloids are stable spheres with isotropicinteractions, whereas an externalmagnetic field producesweak dipolar attractionsand consequently reversible anisotropic structures. The influence of the shell thickness is analyzed in terms of a dipolar-sphere potential. Preparation of magnetite cores, silica growth, and surface modifications with an alcohol and a silane are described in detail. Particle properties are investigated,among other things, with SAXS,light scattering, and electron microscopy. The major conclusion is that the preparation route yields stable, nonaggregated magnetic silicaparticleswith a shape and internal structurewhich is mainly determined by small magnetite clusters in the starting ferrofluid. 1. Introduction
Ferrofluids are dispersions of single-domain magnetic partic1es.l The anisotropic magnetic dipolar attraction in these fluids- is the characteristic difference with dispersions of, for example, silica spheres in which only isotropic interactions are p r e ~ e n t In . ~this communication we report on the synthesis of magnetic silica colloidswhich combine properties of ferrofluids and nonmagnetic sphere dispersions. We aim at monodisperse silica spheres with one single-domain magnetic core. Such spheres have well-defined (magnetic)interactions and the combination of a magnetic dipole and a nonmagnetic “screening”silicashell allows interesting experiments. For example, the shell thickness could be chosen such that in zero magnetic field we essentially have an isotropic sphere dispersion,whereas in an external fieldthe particles reversiblyform anisotropic structures by attractionswhich are only of order kT,even in a saturating field. This idea is further explained in section 2 on interactions between spheres with a dipolar core. It turns out that this explanation is also relevant to evaluate the synthesis results. The specific choice for a silica shell has been made because of well-known chemical silica-surface modification6p7 which can be straightforwardly applied to the particles in this study and which allow further manipulation of particle interactions. The choice of magnetite particles as prepared by Massart’s method* for the magnetic core mainly results from the simplicity of this method and the fact that it produces small single-domain ~~~~
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Abstract published in Advance ACS Abstracts, December 1, 1993. (1) The most up to date review and extensive bibliography is: Cabuil, V. L.,Bacri, J. C., Perzynski,R., E&. Proceedinpofthe sixth International Conference on Magnetic Fluids. J. Magn. Magn. Mater. 1993, 122. (2) Hayter, J. B.; Pynn, R. Phys. Rev. Lett. 1982, 4, 1103. (3) Hayter, J. B. J. Chem. SOC.,Faraday Trans. 1991,87,403. (4) Scholten, P. C. In Thermomechanics of Magnetic Fluids; Berkowky, B., Ed.; Hemisphere Publishing Corporation: Washington, 1978. (5) Pusey, P. N. In Liquids, Freezing and Glass Transition; Hansen, J. P., Levesque, D., Zinn-Jutin, J., Eds.; Elsevier: Amsterdam, 1991;pp 763-942. (6) Van Helden, A. K.; Jansen, J. W.; Vrij, A. J. Colloid Interface Sci. 1981, 81, 354. (7) (a) Philipse, A. P.; Vrij, A. J. Chem. Phys. 1988, 88, 6459. (b) Philipse, A. P.; Vrij, A. J . Colloid Interface Sci. 1989, 128, 121. (8)Massart, R. I E E T r a n s . M a g n . 1981, MAG-17, 1247.
(superparamagnetic) particles. The magnetite synthesis of Sugimoto et al.? for example, does not have the latter advantage. To sketch the background of our work, we make a comparison between an “ordinary” ferrofluid, and one consisting of magnetite particles embedded in surfacemodified silica. A first point of interest is the isoelectric point which for magnetite is a t pH 7. This makes the stability of aqueous ferrofluids in the pH range 6-10 quite problematic, requiring for example the attachment of ligands to the particles to lower the pzc.l0 We apply a first thin silica coating on the magnetite particles in water. This lowers the isoelectricpoint to pH 3, which increases the stability near neutral pH, and the stability in ethanolwater mixtures in which further silica growth takes place with the Stober process.ll Homola et a1.12 report on the coating of yFezO3 particles in water with Ludox silica particles. We employ silica precipitation from an aqueous sodium silicate solution which gives better control of the homogeneity and thickness of the first silica coating. A second point of interest is the surface modification. Particles in magnetic fluids must be stabilized against aggregation by surface charge, and-in particular in nonaqueous solvents-by an organic surface layer.13 Of the variety of surface layers we mention lauric acid,14oleic acid,l5po1y(vinylamine),l6and so-called double surfactant layers.“ The organic groups are not covalently bound to the magnetite particles. Thus one always has to be aware of desorbtion of organic groups. Once a silica layer is present around magnetite, surface silanolgroupscan react with alcohols (in our case octadecyl alcohol) or silane coupling agents to produce stable dispersions in nonaqueous solvents without the risk of desorbtion of the now chemically bounded surface layer.
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(9) Sugimoto, T.;Matijevic, E. J. Colloid Interface Sci. 1980,74,227. (10) Bacri, J.; Perzynski, R.; Salin, D.; Cabuil, V.; Massart, R. J. Magn. Magn. Mater. 1990, 85, 27. (11) SGber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968,26, 62.
(12) Homola, A. M.; Lorenz, M. R.; Mastrangelo, C. J.; Tilburg, T. L. IEE Trans. Magn. MAG-22 1986,716. (13) Scholten, P. C.;Felius, J. A. J. Magn. Magn. Mater. 1990,85,107. (14) De Cuyper, M.; Joniau, M. Langnuir 1991, 7,647. (15) Raj, K.; Moskowitz, R. J. Magn. Magn. Mater. 1990,85, 233. (16) Patton, W. F.; Kim, J.; Jacobson, B. S. Biochim. Biophys. Acta 1986, 816, 83. (17) Woodina. -. A.:. Kilner.. M.:. Lambrick. D. J. Colloid Interface Sci. 1992, 149, 98.
0743-746319412410-0092$04.5010 0 1994 American Chemical Society
Magnetic Silica
Langmuir, Vol. 10, No. 1, 1994 93
Incorporation of magnetite has only been performed for organic host particles, for example consisting of polystyrene, polyacrylamide,18Jg or agarose.20 These “magneticbeads” have biological applications18Jgand are also used for studies on rheologyZ1and structure evolution in magneticdispersions.22Extensive reviews on magnetic beadsl*Jg do not mention inorganic host particles. It remains to be seen whether, apart from the model studies on magnetic spheres we have in mind, other applications can be found for such inorganic magnetic beads. 2. Interactions between Spheres with a Dipolar Core The silica shell surrounding a magnetite core prevents the cores from approaching each other within a distance u. We are interested in the situation in which magnetic core-coreattractions are negligibleat this distance, except in an external magnetic field where they are just strong enough to cause chain formation of spheres. Thus the field changes reversibly an isotropic (hard-sphere) dispersion into a system with anisotropic dipolar attractions. We explore here the conditions for this situation, in particular with respect to the thickness of the silica layer which can be varied in the synthesis procedure described in section 3. Consider a magnetic core of diameter d which is fully magnetized in one direction, giving rise to a permanent dipole moment fi which for core 1is given by
fil = a n d ? / 6
(1)
with M the magnitude of the volume saturation magnetization of the core material. The magnetic interaction energy for two dipoles at a distance r is in SI units4 V(F) =
-4npor3 fi1&
3
(fi,-i)(fi,+) 4npg5
Suppose in the latter case the magnetic core is centered in a silica sphere with diameter u. For identical cores with head-to-tail oriented dipoles the pair potential for the spheres is in view of eqs 1 and 3
0 Ir < u V(r)= m; = Vm,(r) = -ad6/r3; r 1u whereas for freely rotating dipoles we have
V(r) = m; = P(r)
(5)
0 Ir < u
= -a2d’2/6kTr6; r I u (6) with a material-dependentparameter a = W n 1 7 2 m (Note that eq 6 is a kind of Lennard-Jones potential.) We neglect here two other contributions to the pair potentials (5)and (6). First, an organic surface layer or an electrical double layer is present on the silicasurfacegiving rise to a repulsion of finite range. For the present analysis, however, the detailed form of the repulsion is unimportant. The diameter u can be seen in this respect as an “effective” hard sphere diameter. Secondly, there will be a van der Waals attraction between the silica layers. For bare magnetite particles without silica this attraction is usually weaker than magnetic attractions: but for the silica-magnetite spheres this need not be the case. The van der Waals attraction between two spheres at small center-to-center distances r = u V,(r) = -uA/24(r
- a)
(7)
( A is the Hamaker constant) increases with the diameter
u, whereas the magnetic contact attractions at r = u become
(2)
where po is the permeability of a vacuum. The attraction is maximal for parallel dipoles in a head-to-tail configuration, viz. 3 (3) Vm=W = -rlr2/2~ccor Such configurations will occur if a strong external field orients the dipoles. In absence of a field the dipoles oscillate thermally, because of Brownian particle rotations and, if particles are small enough, superparamagnetic oscillationswithin the crystal structure of the core itself. Then eq 2 has to be averaged over all dipole-dipole orientations, which weakens the range and strength of the attraction. The result in the limit of weak interactions is4
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V(r)= -v2,,(r)/6kT; V
