Effects of magnetic fields on stability of nonmagnetic ultrafine colloidal particles

Effects of Magnetic Fields on Stability of Nonmagnetic Ultrafine Colloidal Particles KO HIGASHITANI, 1 KEISUKE OKUHARA,

AND S H I N T A R O H A T A D E *

Department of Applied Chemistry, Kyushu Institute of Technology, Sensuicho, Tobata, Kitakyushu 804, Japan, and * Biotechnology and Water Treatment Department, TO TO Ltd., Nakashima, Kokurakita, Kitakyushu 802, Japan Received September 9, 1991; accepted December 10, 1991 Effects of exposure to a magnetic field on the stability of nonmagnetic colloidal particles, such as ultrafine polystyrene latex and SiO2 particles in electrolyte solutions, were examined by measuring the rapid coagulation rate constant on a low-angle light-scattering apparatus. It was found that the rapid coagulation rate does depend on the magnetic flux density and the duration of magnetic exposure, even though the magnetic flux density is not high, and that the degree of the magnetic effect depends on the particle size and ions in the medium. It is especially interesting to find that the magnetic effect remains for at least 143 h after the magnetic exposure is completed. It is postulated that these effects are mainly attributable to some alteration of the structure of water molecules and ions adsorbed on the particle surface with the magnetic exposure. © 1992AcademicPress,Inc. INTRODUCTION It has been reported that exposure o f materials to magnetic fields gives rise to mysterious p h e n o m e n a , even t h o u g h the magnetic flux density is not high and the exposure time is not long ( 1-5 ). For example, exposure to a magnetic field o f a few kilogauss reduces the a m o u n t o f scale deposited on a pipeline surface, suppresses the corrosion of metal surface, accelerates the solidification o f cement, makes drinking water tasty, accelerates the growth o f plants, and so on. These effects are curious, but the experiments reported so far are rather qualitative and the results are n o t necessarily reproducible. It seems plausible to assume that the stability o f colloidal particles, especially ultrafine particles, is affected by the magnetic field, since the magnetic field influences the scale deposition. Systematic investigations on this problem are very few. In this study effects o f magnetic fields on the stability o f n o n m a g n e t i c colloidal particles are investigated quantitatively; that is, the effects o f the magnetic field i To whom correspondence should be addressed.

on the rapid coagulation rate o f ultrafine colloidal particles in electrolyte solutions are examined, measuring the coagulation rate under various experimental conditions in a low-angle light-scattering apparatus with a stopped-flow test cell. EXPERIMENTS Electrolytes Electrolytes employed here were KC1, NaC1, CaCI2, BaCI2, MgCI2, LaC13, and A1C13 o f reagent grade. Their solutions were prepared by dissolving the electrolytes into water purified by both distillation and reverse osmosis. The solutions were further purified by filtering with a millipore filter (pore size 0.45 # m ) before the coagulation experiment. Particles The colloidal particles e m p l o y e d in this experiment were the polystyrene latex (PSL) and silica particles listed in Table I. P S L particles of diameter (Dp) 200 n m were synthesized and purified by the m e t h o d reported elsewhere (6), but the other particles were obtained from the 125 0021-9797/92 $5.00

Journal of Colloid and IntetJhce Science,. Vol. 152, No. I, August 1992

Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.

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HIGASHITANI, O K U H A R A , A N D H A T A D E TABLE I Colloidal Particles

Particle

Density (g/cm3)

DiameterDp (SD) (rim)

PSL SiO2

1.05 2.20

38 (0.0075), a'a'e 200 b'd 5 f 15,c'e 50 c'd

The Dow Chemical Co. b Prepared in our laboratory. c Nissan Chemical Industries, Ltd. a The size and monodispersity were examined, using electron microscopy. e The size and monodispersity were examined, using photon correlation spectroscopy (Ohtsuka DSL700).

magnetic field, they are described as magnetized in this paper.

Determination of Coagulation Rate Constant A low-angle light-scattering method developed by Lips and Willis (7) was employed to determine the coagulation rate constant K. According to the Rayleigh-Gans-Debye theory combined with the Smoluchowski kinetic theory of coagulation, the variation of the intensity of light scattered by suspended particles at a low angle with time t, l ( t ) , is given as follows: [I(t) - I(O)]/I(O) = 2KN0t.

[1]

manufacturers and used as is. The data on commercial particles in the table were obtained from the manufacturers. The size, shape, and monodispersity of these particles were confirmed by electron microscopy and photon correlation spectroscopy, as described in Table I. Most experiments were carried out using 38-nm PSL particles. Dispersions of these particles were considered to be stable and precoagulation due to the magnetic exposure before mixing was regarded as negligible, because the intensity of scattered light at the beginning of coagulation fell within experimental error among experimental runs, as far as the same size and concentration of particles were employed. The initial concentration of particles, No, was evaluated by measuring the dry weight of particles in suspensions. No = 4.63 × 1011 cm -3 was employed in most experiments.

This equation indicates that the value of K can be experimentally determined from the slope of the relation I ( t ) vs t, if N0 is known. The low-angle light-scattering apparatus employed is illustrated schematically in Fig. 1. Laser light of 632.8 nm wavelength was projected into the scattering cell E after the intensity was adjusted by the filter D. Colloidal and electrolyte solutions at 25°C were placed in syringes N and O, respectively, and injected into the cell through a mixing cell H. The intensity of the light scattered at a low angle was detected by the detector F and recorded by the recorder G. A scattering angle of 6.1 ° was employed in this experiment. The temperature of the cell and the flow tubes was maintained at 25 + 0.1 °C by the temperature controller M. Further information on this experimental apparatus and the determination of Kis given elsewhere (8).

Magnetic Field

Experimental Procedure

A static magnetic field was generated by an electromagnet. It was found that the magnetic flux density was uniform in the space within which the sample was placed, and that the flux density in the space ranged up to 5.6 kG. The colloidal a n d / o r electrolyte solutions were exposed to the magnetic field of a given flux density for a given time before the coagulation experiment. When solutions are exposed to a

The experimental procedure is shown schematically in Fig. 2. Colloidal and electrolyte solutions of given conditions were prepared and then exposed separately to a magnetic field of a given flux density for a given period. In most cases, the solutions were mixed in the test cell of the light scattering apparatus immediately after the magnetic exposure was completed; the variation of the intensity of

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127

MAGNETIC EFFECT ON STABILITY Feedback

I

B K

L

Air Out

Out

H FIG. 1. Schematic drawing of experimental apparatus. A: power source; B: He-Ne laser; C: mirror; D: filter; E: test cell; F: detector; G: recorder; H: mixing cell; I: light source monitor; J: water bath; K: slit; L: lens; M: temperature controller; N: syringe for colloids; O: syringe for electrolytes.

scattered fight was then measured to determine the value of K. But, when the duration of the magnetic effect after exposure was examined, the solutions were left standing for a given period in the temperature-controlled room at 25 +_ 2°C before they were mixed in the test cell. The results given in the following section were obtained using the former procedure, unless specified.

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RESULTS AND DISCUSSION

Figures 3, 4, and 5 show the comparison of the value o f K vs the concentration Ce of electrolytes of three different valences between magnetized and nonmagnetized solutions, respectively. Here 38-nm PSL particles, the magnetic flux density B = 4.45 kG, and the exposure time t¢ = 10 min were employed. There exist slow and rapid coagulation regions, as expected by the DLVO theory, but it is found that the data in the slow coagulation region are not necessarily reproducible. Hence

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FIG. 3. Dependence of coagulation rate constants of magnetized ( e ) and nonmagnetized (O) solutions on KC1 concentration. B = 4.45 kG, te = 10 min, 38-nm PSL particles, No = 4.63 × 101~ cm -3.

JourlTal ~?['Colloid and lnte~'[hce Science,

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HIGASHITANI, OKUHARA, AND HATADE 15

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FIG. 4. Dependence of coagulation rate constants of magnetized ( • ) and nonmagnetized (0) solutions on CaCI2 concentration. B = 4.45 kG, t~ = l0 min, 38-nm PSL particles, No = 4.63 X 10 ~ cm-3.

FIG. 6. Dependence of the reduced coagulation rate constant on the magnetic flux density for monovalent cations. (©) 2 mol/liter NaC1, ( • ) 2 mol/liter KC1, te = 10 min, 38-rim PSL particles, No = 4.63 × 1011 cm-3.

a t t e n t i o n is p a i d o n l y to the c o a g u l a t i o n rate c o n s t a n t in the r a p i d c o a g u l a t i o n region, KR, in this study. It is clear that the m a g n i t u d e o f KR for m a g n e t i z e d solutions is always s m a l l e r t h a n t h a t for n o n m a g n e t i z e d solutions. This r e d u c t i o n o f the KR value is c o n f i r m e d also b y the d a t a o b t a i n e d for the solutions o f NaC1, BaCI2, MgC12, a n d A1C13. These results indicate t h a t the e x p o s u r e o f n o n m a g n e t i c colloidal particles to a m a g n e t i c field does affect their stability in such a w a y t h a t r a p i d c o a g u l a t i o n b e t w e e n particles is suppressed. Hereafter, the m a g n e t i c effect is e v a l u a t e d using the r e d u c e d

rate o f r a p i d c o a g u l a t i o n o f a m a g n e t i z e d solution as c o m p a r e d to that o f a n o n m a g n e t i z e d solution, KRm/KR, whose deviation from u n i t y indicates the m a g n i t u d e o f the m a g n e t i c effect. Figures 6, 7, a n d 8 show the d e p e n d e n c e o f KRm/KR on B for three different valences. Here te is t a k e n to be 10 m i n . I n the cases o f electrolytes o f m o n o v a l e n t a n d d i v a l e n t cations, the m a g n e t i c effect increases g r a d u a l l y with increasing m a g n e t i c flux density a n d b e c o m e s n e a r l y c o n s t a n t at B >~ 4 kG. But, in the cases o f electrolytes o f t r i v a l e n t cations, the m a g netic field has no effect at B ~ 2 kG, a l t h o u g h

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Vol.

152, No.

1, A u g u s t

4

6

B(kG)

Ce(mol/I)

Journal of Colloid and Interface Science

2

1992

FIG. 7. Dependence of the reduced coagulation rate constant on the magnetic flux density for divalent cations. (©) 0.1 mol/liter CaC12, ( • ) 0.1 re•l/liter BaC12, ([]) 0.1 mol/liter MgCI2, t~ - 10 min, 38-nm PSL particles, No = 4.63 × 1011 cm -3.

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B(kG) FIG. 8. Dependence of the reduced coagulation rate constant on the magnetic flux density for trivalent cations. (©) 10-3 re•l/liter AICI3,( • ) 10-3 re•l/liter LaC13, te = 10 rain, 38-nm PSL particles, No = 4.63 × 10 n cm-3.

FIG. 10. Dependence of the reduced coagulation rate constant on the exposure time for divalent cations. (©) 0.1 mol/liter CaCI2,(•) 0.1 re•l/liter BaClz, (E3)0.1 mol/ liter MgCI2,B = 4.45 kG, 38-nm PSL particles, No = 4.63 X l• n cm-3.

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c o n s t a n t at a b o u t 0.90 at B >~ 4 kG. These data indicate that the m a x i m u m effect due to the m a g n e t i c field is o b t a i n a b l e w h e n B is greater t h a n 4 kG. Figures 9, 10, a n d 11 show the d e p e n d e n c e Of KRm/KR o n te for electrolytes o f three different valences, respectively. I n the cases of m o n o v a l e n t electrolytes, n o effect o f m a g n e t i c field was detected at te ~< 8 m i n , while the constant effect appears if re is greater t h a n 10 rain. I n the cases of divalent a n d trivalent electrolytes, the value of KRm/Ks decreases gradually a n d t h e n b e c o m e s c o n s t a n t at te >~ 10 m i n . These data indicate that the m a x i m u m magI

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netic effect is o b t a i n e d at te >/ 10 m i n , irrespective o f electrolytes. I n the preceding experiments, colloidal a n d electrolyte solutions were m i x e d i m m e d i a t e l y after the solutions were exposed to a m a g n e t i c field. Here the solutions were left s t a n d i n g for a given period ts after the exposure, a n d t h e n they were mixed. Figure 12 shows the dependence o f KRm/KR o n ts. T h e reference value KR was o b t a i n e d i n exactly the same way as in the m e a s u r e m e n t of KRm except for the exposure to the m a g n e t i c field. This result indicates that the m a g n e t i c effect r e m a i n s for at

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te(min) FIG. 9. Dependence of the reduced coagulation rate constant on the exposure time for monovalent cations. (O) 2 mol/liter NaCI, (e) 2 mol/liter KC1,B = 4.45 kG, 38-nm PSL particles, No = 4.63 × 10 n cm-3.

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te(min) FIG. 11. Dependence of the reduced coagulation rate constant on the exposure time for trivalent cations. ((3) 10 3re•l/liter A1CI3,(e) 10-3 mol/liter LaCI3, B = 4.45 kG, 38-nm PSL particles, No = 4.63 × 10 n cm-3.

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HIGASHITANI,

I

OKUHARA,

AND

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least 143 h after the magnetic exposure. We call this the m e m o r y effect of a magnetic field. This m e m o r y effect is important. Once a colloidal solution is exposed to a magnetic field upstream in a flow system, for example, the magnetic effect remains all the way downstream. In order to determine the mechanism of the reduction of KR by the magnetic field, values of KRm/KR are compared for Sample 1, in which both the colloidal and electrolyte solutions were magnetized, Sample 2, in which

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FIG. 13. Comparison among reduced coagulation rate constants for various experimentalconditions. Sample 1: Both the particleand electrolytesolutionsare magnetized. Sample 2: Only the electrolyte solution is magnetized. Sample 3: Only the particle solution is magnetized. 0.1 tool/liter CaC12, B = 4.45 kG, te = 10 rain, 38-rim PSL particles, No = 4.63 × 10 a~cm-3. Journal of Colloid and lnteiface Science, Vol. 152, N o . 1, A u g u s t 1992

FIG. 14. Dependence of the reduced coagulation rate constant on the particle size. (©) PSL particles, (•) silica particles, 0.1 mol/liter CaC12,B = 4.45 kG, & = 10 min.

only the electrolyte solution was magnetized, and Sample 3, in which only the colloidal solution was magnetized. A typical comparison is shown in Fig. 13. It is clear that the magnetization of the colloidal solution contributes more to the reduction of KRm/KR than does that of the electrolyte solution, and that the sum of the KRm/KR values of Samples 2 and 3 is approximately equal to that of Sample 1. It is known that water molecules, ions, and hydrated ions in aqueous solutions are adsorbed on the particle surface and the thickness is of the order of a few n m (9). The greater contribution of colloidal solutions to the reduction of KRm/KR seems to imply that the magnetic field alters some structure of the layer adsorbed on the particle surface. If this hypothesis is correct, the effect of a magnetic field on the value OfKRm/KR is expected to increase as the particle size decreases, because the relative thickness of the adsorbed layer becomes large with decreasing particle size. Figure 14 shows the dependence of KRm/KR on the particle size D p . It is clear that the value of K R m / KR decreases with decreasing particle size, as expected, and no magnetic effect is observed at D p = 200 nm. It had been reported that the rapid coagulation rate constant deviates from the value predicted by the Smoluchowski theory at Dp < 100 nm, and that the reduction is attrib-

MAGNETIG EFFECT ON STABILITY utable to the adsorbed layer on the particle surface (8). It is interesting to note that the onset particle size from which the value of K ~ m / K R deviates from unity coincides approximately with the particle size mentioned above. This coincidence of the onset particle size between both experiments, as well as of the results given in Figs. 13 and 14, supports the above hypothesis that the reduction of K R m / K R is attributable mainly to some alteration of the adsorbed layer with magnetic exposure, such as the change of the configuration of molecules in the adsorbed layer. CONCLUSIONS The following conclusions are drawn for the effects of a magnetic field on the stability of nonmagnetic ultrafine colloidal particles. 1. The exposure of colloidal particles in electrolyte solutions to a magnetic field reduces their rapid coagulation rate, if the magnetic flux density is greater than 4 k G and the exposure time is greater than 10 min. 2. The degree of the magnetic effect on the stability of colloidal solutions increases with decreasing particle size when the particle size is smaller than about 200 nm. 3. Colloidal solutions exposed to a mag-

131

netic field maintain the magnetic effect on stability for at least 143 h. 4. It is suggested that a magnetic field m a y alter the structure of water molecules and ions adsorbed on the particle surface. ACKNOWLEDGMENT The authors thank Nissan Chemical Industries, Ltd., for providing a supply of silica particles. REFERENCES 1. Dushkin, S. S., and levstratov, V. N., in "Magnetic Water Treatment in Chemical Undertaking." Khimiya, Moscow, 1986. 2. Yamaoka, K., Sugimoto, S., Kimura, T., Akiyama, R., and Kobayashi, R., J. Jpn. Geotherm. Energy Assoc. 25, 31 (1988). 3. Nakashima, K., and Yamamoto, H., J. Toyota Natl. Tech. Coll. 20, 67 (1987). 4. Chiba, A., and Ogawa, T., Nippon Kagaku Kaishi, 357 (1988). 5. Kaneko, T., and Takatsuji, M., Solid State Phys. 17, 530 (1982). 6. Higashitani,K., and Matsuno, Y., J. Chem. Eng. Jpn. 12, 460 (1979). 7. Lips, A., and Willis, E., J. Chem. Soc. Faraday Trans. 1 69, 1226 (1973). 8. Higashitani,K., Kondo, M., and Hatade, S., J. Colloid Interface Sci. 142, 204 ( 1991). 9. Israelachvili,J, N., in "Intermolecular and Surface Force." Academic Press, New York/London, 1985.

Journal of Colloid and Interface Science. Vol. 152, No. 1, August 1992