Stability of Cobalt Ferrite Colloidal Particles. Effect of pHand Applied Magnetic Fields

J. de Vicente, A. V. Delgado, R. C. Plaza, J. D. G. Duran, andF. Gonzalez-Caballero*

Departamento de Fsica Aplicada, Facultad de Ciencias, Universidad de Granada,18071-Granada, Spain

Received March 7, 2000. In Final Form: June 21, 2000

An experimental investigation is described on the stability of cobalt ferrite colloidal spheres, by analyzingthe time variation of the optical absorbance of the suspensions as a function of pH and magnetic fieldstrength. Structural and chemical analysis of the particles suggest that they are composed of a mixedcobalt-iron ferrite and magnetite, with some excess oxygen, probably coming from adsorbed water. Inorder to consider all posible particle-particle interactions that might be responsible for the observedbehavior, the classical DLVO theory was extended to include magnetic dipole attractions. The electricdouble layer of the particles was characterized by electrophoresis, and it was found that the ferrite colloidshave an isoelectric point (pHiep, or pH of zero zeta potential, ) of =6.5. This is confirmed by stabilitymeasurements: the absolute value of the initial slope of the absorbance-time curves shows a pronouncedmaximum around pH 7. Concerning the effect of a uniform magnetic field (applied in the direction of thegravitational field), the most significant feature found was that above =1 mT, and for particle concentrationslarger than =0.7 g/L, the suspensions appear more stable the stronger the applied field. Potential energycalculations, while explaining the lower stability of the suspensions around pHiep, show that increasingmagnetic fields decrease indeed the potential barrier between the particles, but not enough to ensureirreversible aggregation. It is hence suggested that the observed stability behavior is due to a long-rangestructuration of the dispersed particles that form long chainlike aggregates extending almost to the wholevolume of the suspension. This may explain that the optical absorbance takes a longer time to decreasein the presence of a magnetic field applied in vertical direction, and also that the final fall in turbidityoccurs at a faster rate than in the absence of the field.

1. Introduction

Colloidal particles are increasingly finding their wayinto many different fields of technology.1-3 One of suchfields is the preparation of ceramic magnets starting fromconcentrated slurries of ceramic particles in the colloidalrange. However, because of the existence of strongmagnetic interactions between magnetized particles, suchinteractions may considerably affect both the rheology4-6and the colloidal stability7-9 of the suspensions. In fact,a rich variety of physical phenomena have been reportedfor the disperse systems that are known as magnetic fluidsin a wide sense, including suspensions of permanentmagnets (ferrofluids or magnetic fluids) or of magnetizableparticles (magnetorheological fluids). In this work, we willfocus on the tendency of the latter to aggregate eitherreversibly or irreversibly in the presence and in theabsence of a magnetic field. Since the formation ofaggregates can affect both the light scattering and

sedimentation rate of the suspensions, our investigationhas dealt with the time evolution of the turbidity of thesystems, as a measure of the changes in both properties.

With the aim of making this study as quantitative aspossible, we decided to use magnetic particles withspherical shape and narrow size distribution. The worksof Matijevic have demonstrated that this can be achievedfor many different inorganic materials, and also magneticones.10-12 In the present contribution, cobalt ferriteparticles will be used as the object of the study; in additionto their intrinsic physical interest, they have been proposedas substitutes of magnetite whenever high electricalresistivity might be required, together with a response toapplied magnetic fields.13 Further, these ferrimagneticparticles can be prepared with the criteria mentionedabove concerning homogeneity in both size and shape.12

Our aim in this work is to experimentally determinethe stability of the ferrite particles in suspension and tofind an explanation for the observed behavior in terms ofthe calculation of the total interaction energy betweenthe particles. The latter will include not only the magneticcontribution but also electrical and van der Waalsinteractions. The study will thus require to characterizenot only the magnetic moment of the particles but alsotheir surface potential. The effects of pH and magneticfield on both the experimental determinations and thecalculations of interaction energies will be described anddiscussed.

* Corresponding author. Fax: +34-958-243214. E-mail:fgonzale@ugr.es.

(1) Williams, R. A. Colloid and Surface Engineering: Applicationsin the Process Industries; Butterworth-Heinemann Ltd.: Oxford, UK,1992.

(2) McKay, R. Technological Applications of Dispersions; MarcelDekker: New York, 1994.

(3) Pugh, R.; Bergstrom, L. Surface and Colloid Chemistry inAdvanced Ceramics Processing; Marcel Dekker: New York, 1994.

(4) Tang, X.; Conrad, H. J. Rheol. 1996, 40, 6.(5) Volkova, O.; Cutillas, S.; Bossis, G. Phys. Rev. Lett. 1999, 82, 233.(6) Felt, D. W.; Hagenbuchle, M.; Liu, J.; Richard, J. J. Intelligent

Mater. Sys. Str. 1996, 7, 589.(7) Tombacz, E.; Ma, C.; Busch, K. W.; Busch, M. A. Colloid Polym.

Sci. 1991, 269, 278.(8) Janssen, J. J. M.; Baltussen, J. J. M.; van Gelder, A. P.; Perenboom,

J. A. A. J. J. Phys. D: Appl. Phys. 1990, 23, 1455.(9) Wang, H.; Wen, C. S. J. Colloid Interface Sci. 1999, 213, 606.

(10) Matijevic, E. Annu. Rev. Mater. Sci. 1985, 15, 483.(11) Sugimoto, T.; Matijevic, E. J. Colloid Interface Sci. 1980, 74,

227.(12) Tamura, H.; Matijevic, E. J. Colloid Interface Sci. 1982, 90, 100.(13) Geoffroy, O.; Porteseil, J. L. In Magnetisme; Tremolet de

Lacheisserie E., Ed.; Presses Universitaires de Grenoble: Grenoble,France, 1999.

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10.1021/la0003490 CCC: $19.00 2000 American Chemical SocietyPublished on Web 09/13/2000

2. Experimental Section

2.1. Materials. All chemicals used were purchased from eitherMerck (Germany) or Panreac (Spain) with analytical quality andwere not further purified. Water used in the preparation of thesolutions and suspensions was of Milli-Q quality (Milli-QAcademic, Millipore, France).

2.2. Methods. Particle Preparation. Ferrite particles wereprepared following the coprecipitation method described byTamura and Matijevic.12 The method involves the coprecipitationof iron(II) and cobalt(II) hydroxides that, upon heating at 90 Cin an oxidizing aqueous medium, yield the ferrite particles. Thefirst stage of the synthesis is the preparation of an aqueoussolution containing 1.25 10-1 M FeSO47H2O, 7.5 10-2 MCo(NO3)26H2O, 0.2 M KNO3, and 0.1 M KOH. Water waspreviously purged with pure N2 for at least 30 min to avoid thepresence of dissolved oxygen as much as possible. This is neededto prevent the oxidation of ferrous ions to ferric ones prior to thenecessary formation of ferrous hydroxide. Not only was waterpurged with nitrogen, but this gas was also bubbled through thestarting solution mentioned, in which KOH provides the hydroxylions required for the formation of the hydroxides, and NO3- ionsfrom the dissolved KNO3 will be responsible for the controlledoxidation of Fe2+ to Fe3+, required for the obtention of theiron(III) oxide that is also present in the ferrite.

This starting solution is placed in 50 cm3 stoppered test tubes.Each tube will contain 20 cm3 of the mixture, and they areimmersed in an oil bath (Memmert, Germany) preheated at 90C and kept at such temperature for 24 h. The suspensions thusobtained are repeatedly cleaned of undesired material (such asgoethite, R-FeOOH) by magnetic sedimentation. Due to the notvery hard hysteresis cycle of the particles (we will discuss thisbelow), the separation of nonmagnetic particles can be achievedby placing the tubes between the poles of a permanent magnet(B ) 0.42 T) and the material not adhered to the wall in contactwith the pole is pipetted off, and substituted with water. Theprocess is considered finished when the supernatant thusobtained is transparent and has low electric conductivity (

The question now remains of whether the structure ofthe particles is homogeneous or rather there is an excesssurface concentration of any of the metal ions. Thispossibility was explored by XPS data. The results obtainedfrom XPS revealed that the mass percents of the elementswere 3.43% Co, 31.74% Fe, and 64.83% O, which corre-sponds to the empirical formula CoFe9.8O69.6 The highoxygen content could be explained due to adsorption ofwater molecules on the surface.

3.2. Magnetic Properties. Figure 3 shows the hys-teresis loop of the powder of synthetic spheres. As observed,a saturation magnetizationMs of 2.7105 A/m is obtained;this value can be compared to experimental determina-tions15 on pure CoOFe2O3, for which Ms ) 3.94 105 A/m.

Figure 3 demonstrates that the synthesized ferrite isa not very hard magnetic material, because of its smallcoercive field Hc ) 0.275 105 A/m (=346 Oe) and its highsaturation magnetization. Furthermore, its remanentmagnetization of 0.82 105 A/m is quite low in comparisonto that of hard magnetic materials.

3.3. Electrophoretic Mobility and Zeta Potential.In Figure 4 we show the zeta potential (deduced from theelectrophoretic mobility, e, using OBrien and Whitestheory, see ref 16) of ferrite spheres, as a function of pHand in the presence of different NaNO3 concentrations.Whatever the latter, all the curves show an isoelectricpoint (pHiep) around pH ) 6.5, a value that is in goodagreement with previously reported data on these col-loids.12 In view of the data presented in Figure 4, it canbe said that pH can be considered as the variable of interestfor the electrical characterization of the cobalt ferritesurface.

3.4. Stability of the Suspensions. Effect of pH andIonic Strength. All our inferences on the stability of cobaltferrite particles will be based on the characteristics of theevolution with time of the optical absorbance (A) of thesuspensions in the different conditions studied. Figure 5is an example: it displays the absorbance of ferritesuspensions as a function of time for constant ionicstrength and different pH values. All suspensions con-tained 2 g/L of particles and 10-3 M NaNO3 suspensions.Because of the density and diameter of the particles, thereis an overall trend of A to decrease with time, due to thedisappearance of the solids from the illuminated regionas settling proceeds. Nevertheless, this trend changes withpH, this being a clear indication of the different sedi-mentation velocities of the particles depending on their

state of aggregation. In particular, it is clearly observedthat the absorbance decreases at a faster rate (mainly, atshort times after the beginning of the experiment) whenthe pH is closer to the pHiep of the particles.

The overall behavior is better appreciated if the initialslope of the A/A0-t curves (A0 is the suspension absorbancefor t ) 0) is plotted as a function of the quantity of interest.Thus, Figure 6 shows [d(A/A0)/dt]t)0 vs pH for two ionicstrengths, namely, 10-3 and 10-2 M NaNO3. It is clearlyseen that, whatever the ionic strength, the suspensionstability is most sensitive to pH: the sedimentation rateincreases (d(A/A0)/dt decreases) as we approach pH )7-7.5 from the acid side, and rapidly decreases as we getinto the basic pH range. In other words, the suspensionsare more stable the further their pH is from neutrality;this was to be expected in view of the data presented in

(15) Tremolet de Lacheisserie, E. Magnetisme; Presses Universitairesde Grenoble: Grenoble, France, 1999.

(16) OBrien, R. W.; White, L. R. J. Chem. Soc., Faraday Trans. 21978, 74, 1607.

Figure 3. Hysteresis cycle of the magnetization of cobalt ferritepowder, at room temperature (293.0 ( 0.2 K).

Figure 4. Zeta potential () of cobalt ferrite particles as afunction of pH in different NaNO3 concentrations.

Figure 5. Optical absorbance A (expressed as % of initialabsorbance, A0) of cobalt ferrite particles as a function of timefor different pH values. Ionic strength: 10-3 M NaNO3.

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Figure 4 and the fact that the zeta potential of the particlesis zero in the vicinity of pH 7. Nevertheless, we will supportthis conclusion on interaction energy calculations below.

3.5. Effect of Applied Magnetic Fields. MagneticField Strength. From the magnetic character of theparticles, a significant effect of a uniform applied magneticfield on the sedimentation rate of the particles wasexpected. Our first experiments were performed byallowing the particles settle with the field constantlyapplied: Figure 7 shows A versus time with and withouta field (parallel to the gravitational field) of 2.5 mT. Thesuspensions had a particle concentration of 1 g/L withoutadded electrolyte. The interesting result here is that thesuspensions are clearly more stable in the presence of thefield: the initial slope is smaller when B is applied. We

suggest that the field magnetizes the particles and thatthesubsequentattractionbetweenthe individualmagneticmoments brings about the formation of large flocculioccupying most of the suspension volume. This explainsthe slow absorbance changes. After sufficiently long time,the flocculi break and sediment rapidly as they are formedby many particles. This might be the reason for the rapidfall in absorbance at longer times in the presence of B.

The experiments were repeated with different fieldstrengths (applied in the vertical direction) as shown inFigure 8, where the initial absorbance slope is plotted vsB. It can be seen that the suspensions are more unstableas the field is increased from 0 to 1 mT; higher fieldsinvert that trend and [d(A/A0)/dt]t)0 approaches zero forB ) 2.5 mT. This behavior appears to be consistent withthe explanation given to the results in Figure 7: the fieldinduces magnetic moments in the particles, but only if Bis high enough the magnetic attraction will be sufficientlystrong to form large aggregates. At low B, the range of theinteraction will be shorter, this giving rise to the formationof smaller aggregates that settle rapidly.

In order to confirm the hypothesis of formation of along-range structure of the particles in the presence ofthe magnetic field, we took a series of pictures of thesuspensions as a function of time. Figure 9 illustrateshow significant the effect of B is on the sedimentationpattern: as observed, large aggregates remain in suspen-sion in the presence of the field, for a time long enoughfor all particles to settle if no field is applied. The verticalorientation of the aggregates is not clearly seen in thepictures.

A thermodynamic study of this phenomenon has beendescribed in ref 17, where it was shown that thermo-dynamic instability and breakdown of the suspension intoseparated phases can occur for certain values of both theapplied field strength and particle concentration. In fact,in the region of instability, arbitrarily small fluctuationsin concentration may grow to form large particle ac-cumulations. This may be the process responsible for ourstability data.

(17) Blums, E.; Cebers, A.; Maiorov, M. M. Magnetic Fluids, Walterde Gruyter & Co: Berlin, 1996.

Figure 6. Initial slope of A/A0-t plots for cobalt ferritesuspensions as a function of pH for different ionic strengths.

Figure 7. Absorbance of cobalt ferrite suspensions in wateras a function of time with and without a constant field B ) 2.5mT parallel to the gravitational field.

Figure 8. Initial slope of A/A0 versus time for ferrite particlesdispersed in water, as a function of magnetic field strength.

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Particle Concentration. Given that both the absorbanceand the number of pair interactions (both magnetic andnonmagnetic) between the particles depend on the con-centration of solids, this was another interesting quantityto analyze. Thus, Figure 10 shows the absorbance as afunction of time for suspensions of cobalt ferrite particlesof different concentrations, with and without applied field(vertical B ) 2.5 mT). Note that the initial absorbance,A0, is roughly proportional to the concentration of solids,as expected. But it is also worth mentioning that A0 is, forany concentration >0.5 g/L, larger when the field isapplied. This is most likely due to the magnetic flocculationinduced by the field: the extinction cross section of thecomplex aggregates formed must be larger than that ofprimary particles,18 hence the larger absorbance measuredin the presence of the magnetic field. For low particleconcentrations, on the contrary, the situation is reversed,and A0 is slightly lower for B * 0 than for B ) 0:apparently, in the case of very low concentrations,interparticle distances are so large that dipole-dipoleinteractions do not produce flocculation. The initial slopesof the curves shown in Figure 10 are plotted as a functionof particle concentration, c, in Figure 11. This graphdemonstrates that the effect of the field on the stabilityis only appreciable for c g 0.5 g/L. For lower concentrations,the aggregates formed have a relatively small volume andsimply settle as large particles. Only when the numberof particles in suspension is sufficiently high can largeaggregates, extending to most of the experimental volume,be formed. Effect of Magnetic Field Direction. Let us now consider

how these arguments apply for a magnetic field appliedhorizontally. Figure 12 shows the time evolution of theabsorbance of the suspensions of ferrite particles in water,

(18) Bijsterbosch, B. H. In Solid/Liquid Dispersions; Tadros, Th. F.,Ed.; Academic Press: London, 1987; p 91.

Figure 9. Snapshots of settling suspensions of ferrite spheres in water with (bottom) and without (up) application of a verticalmagnetic field B ) 2.5 mT. Time interval between the first and the last picture =3 min.

Figure 10. Absorbance of cobalt ferrite suspensions as afunction of time with and without vertically applied magneticfield for the particle concentrations indicated. No addedelectrolite.

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for different particle concentrations (ranging between 0.7and 1.2 g/L), with and without field. Two essentialdifferences are found when these results are compared tothose in Figure 10 (vertical field): the sedimentation ofthe particles is faster in the presence of the horizontalfield, and, furthermore, no significant differences areobserved between the initial absorbances measured withand without H. The reasoning given above, concerningthe structuring effect of the magnetic field, can also beapplied in this case. Thus, roughly horizontal particlechains will also form in horizontal fields, but it is unlikelythat they extend between opposite walls of the cuvette.Hence they will sediment under gravity at a faster ratethan in the absence of the horizontal field, and this willprovoke a faster decay of absorbance with time when His applied. Furthermore, the two opposite effects ofenhancedsedimentationandaggregationmayexplain that

the initial absorbance is similar for H ) 0 and H * 0.Figure13 illustrates, with a schematic picture, the possibleinternal structuration of the suspensions when a vertical(Figure 13a) or horizontal (Figure 13b) magnetic field isapplied.

Interaction Energy between Particles. As mentionedabove, the observed stability of the suspensions should beexplained in terms of both magnetic and nonmagneticinteractions between them. The former can be expresedin terms of a potential energy function, VM, given by19

where M is the magnetization of the material, a the radiusof the particles, s the distance between the surfaces of twointeracting particles, and 0 is the permeability of thevacuum. Equation 1 is obtained from the expression forthe potential energy of interaction between two magneticdipoles of strengths mb i (i ) 1, 2) separated by a distancerb19

under the simplifying assumptions that both magneticmoments are parallel, of the same strength, and orientedhead-to-tail. The common value of the magnetic momentswas calculated from the magnetization multiplied by theparticle volume. Since the experiments were performedat low magnetic fields, the magnetization will be consid-ered proportional to H, using ) 0.94, as determinedfrom experimental measurements.

The electrostatic repulsion between the electric doublelayers of the particles can be calculated from the followingpotential:20

where it is assumed that the particle has a constant andmoderate surface potential and that the diffuse layerpotential can be identified with the electrokinetic or zetapotential, . In eq 3, r is the relative dielectric constantof the liquid, 0 is the permittivity of the vacuum, and is the reciprocal Debye length.

(19) Rosensweig, R. E. Ferrohydrodynamics; Cambridge UniversityPress: Cambridge, UK, 1985.

(20) Hunter, R. J. Foundations of Colloid Science; Clarendon Press:Oxford, UK, 1987; Vol. I

Figure 11. Initial slopes of the curves of Figure 10, normalizedby the t ) 0 absorbance in each case.

Figure 12. Same as Figure 10, but for a horizontally appliedmagnetic field.

Figure 13. Schematic view of possible internal structures ofthe suspensions for vertical (a) and horizontal (b) magneticfields.

VM ) -89

0M2a3

(sa + 2)3

(1)

VM )04[mb 1mb 2r3 - 3(mb 1rb)(mb 2rb)r5 ] (2)

VEL ) 2r0a2 ln[1 + e-s] (3)

Stability of Cobalt Ferrite Colloidal Particles Langmuir, Vol. 16, No. 21, 2000 7959

Finally, the van der Waals atraction between the samespheres can be obtained from21

where A is the Hamaker constant, whose value wasestimated to be 9 10-21 J, based on a thermodynamicstudy of the ferrite/solution interface.22

Figure 14 illustrates the dependence with distance ofeach of the three contributions (eqs 1, 3, and 4) to the totalinteraction energy, for particles suspended in water ( )9 mV). As observed, the attractive magnetic interactionis stronger than van der Waals attraction, except at veryshort distances of approach, because of the finite valueattained by VM when s ) 0. Figure 14 also indicates that,in spite of the superposition of VM and VLW attractions,the overall interaction, VT, is dominated by electric doublelayer repulsions, explainable by the large double layerthickness of the particles dispersed in water.

Similar calculations were performed when the particleswere dispersed in NaNO3 solutions of different pH values.The results for the case of [NaNO3] ) 10-2 M are displayedin Figure 15 (similar trends, not shown for brevity, werefound for 10-3 M NaNO3 solutions), for the case B ) 0. Asobserved, the potential energy curves are in reasonablequalitative agreement with the turbidity data of Figures5 and 6: as expected, the poorer stability of the suspensionsclose to the isoelectric point corresponds to the loweringof the potential barrier preventing particle aggregation.It is hence more interesting to consider the effect of appliedmagnetic fields on the interaction potential, as shown inFigure 16, for ferrite spheres dispersed in water. It canbe seen that the application of increasing magnetic fieldsreduces the height of the potential energy barrier, thus

favoring particle aggregation, in agreement with theresults in Figures 7-12.

Note that, in fact, the potential maximum is only =8kTfor B ) 2.5 mT. But most interestingly, the inset in Figure16 shows that the application of fields above 0.5 mT bringsabout a shallow secondary minimum in the potentialenergy for long separations between the particles. Thisappears to be coherent with the settling behavior withincreasing B shown in Figure 8, where as mentioned, itcan be seen that a minimum field of about =1 mT is neededto achieve structuration of the suspension. The fact thatthe secondary minimum is not deep might explain the

(21) Gregory, J. J. Colloid Interface Sci. 1966, 22, 342.(22) de Vicente, J.; Duran, J. D. G.; Gonzalez-Caballero, F.; Delgado,

A. V., to be published.

Figure 14. Potential energy of interaction between the ferritespheres dispersed in water, as a function of distance betweensurfaces, s. VLW, van der Waals; VM, magnetic; VEL, electricdouble layer; VT, total energy.

VLW ) (- A6)[ 2a2s(4a + s) + 2a2(2a + s)2 + lns(4a + s)(2a + s)2] (4)

Figure 15. Interaction potential as a function of the distances between ferrite particles, for different pH values and no appliedmagnetic field.

Figure 16. Total interaction potential between the ferrimag-netic colloidal particles in water as a function of distance fordifferent applied magnetic fields. Inset: Detail of the longdistance behavior of VT.

7960 Langmuir, Vol. 16, No. 21, 2000 de Vicente et al.

observation that the suspensions are readily redispersedby mild shaking of the cuvette.

Acknowledgment. Financial support for this workby CICYT, Spain, under Project MAT 98-0940, and the

assistance of Prof. D. Martn-Ramos in X-ray diffractionexperiments are gratefully acknowledged.

LA0003490

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