Reprinted from

COLLOIDSANDSURFACESAN I~.K)URNAL

A: PHYSICOCHEMICAL ANDENGINEERING ASPECTS

Colloids and SurfacesA: Physicochemical and Engineering Aspects 117 (1996) 37-44

ESR investigations on the stabilization of alumina dispersions byAerosol-aT in different solvents

S. Krishnakumar, P. Somasundaran *

Langmuir Center for Colloids and Interfaces, Henry Krumb School, Columbia University, New York, NY 10027, USA

ELSEVIER

COLLOIDS AND SURFACESA: PHYSICOCHEMICAL AND ENGINEERING ASPECTS

AN INTERNATIONAL JOURNAL DEVOTED TO THE PRINCIPLES AND APPLICATIONS OFCOLLOID AND INTERFACE SCIENCE

Editors-in-ChiefP. Somasundaran, 911 S.W. Mudd Bldg, School of Engineering and Applied Science, ColumbiaUniversity, New York, NY 10027, USA. (Tel: (212) 854-2926; Fax: (212) 854-8362)Th.F. Tadros, c/o Elsevier Editorial Services, Mayfield House, 256 Banbury Road, OxfordOX2 7DH, UK

Co-EditorsD.N. Furlong, CSIRO, Division of Chemicals and Polymers, Private Bag 10, Clayton, Vic. 3169,Australia (Tel: (3) 542-2618; Fax: (3) 542-2515)T.A. Hatton, Massachusetts Institute of Technology, Department of Chemical Engineering,25 Ames Street, 66-350, Cambridge MA 02139, USA (Tel: (617) 253-4588; Fax: (617) 253-8723)H. Mt>hwald, Max-Planck-lnstitut fOr Kolloid- und Grenzflachenforschung, Instituteil Berlin-Adlershof, Rudower Chaussee 5, Geb 9-9, 012489 Berlin, Germany (Tel: 30 6392-3100;Fax: 30 6392-3102)

Founding EditorE.D. Goddard, 349 Pleasant Lane, Haworth, NJ 07641, USAEditorial BoardS. Ardizzone (Milan, Italy) P. Luckham (London, UK)M. Aronson (Edgewater, NJ. USA) RA Mackay (Potsdam, NY, USA)J.C. Berg (Seattle. WA. USA) C.A. Miller (Houston. TX, USA)A.M. Cazabat (Paris, France) R. Miller (Berlin, Germany)N.V. Churaev (Moscow. Russia) I.D. Morrison (Webster, NY, USA)J. Clint (Hull, UK) B. Moudgil (Gainesville, FL, USA)K. De Kruif (Ede, The Netherlands) K. Papadopoulos (New Orleans, LA, USA)S.S. Dukhin (Kiev, Ukraine) R.M. Pashley (Canberra, Australia)G.H. Findenegg (Berlin, Germany) B.A. Pethica (Parsippany, NJ, USA)T.W. Healy (Parkville, Australia) D.C. Prieve (Pittsburgh, PA, USA)K. Higashitani (Kyoto, Japan) C.J. Radke (Berkeley, CA. USA)R. Hilfiker (Basel, Switzerland) R. Rajagopalan (Houston, TX, USA)K. Holmberg (Stockholm, Sweden) J. Sjoblom (Bergen, Norway)J. Israelachvili (Santa Barbara, CA, USA) C. Solans (Barcelona. Spain)E.W. Kaler (Newark, DE, USA) J.K. Thomas (Notre Dame. IN, USA)T. Kunitake (Fukuoka, Japan) B.V. Toshev (Sofia, Bulgaria)K. Kurihara (Nagoya, Japan) F.M. Winnik (Hamilton, Ont., Canada)R.Y. Lochhead (Szeged, Hungary)Scope of the JournalColloids and Surfaces A: Physicochemical and Engineering Aspects is an international journal devoted to the science ofthe fundamentals. engineering fundamentals, and applications of colloidal and interfacial phenomena and proces~es.The journal aims at publishing research papers of high quality and lasting value. In addition, the journal contains criticalreview papers by acclaimed experts. brief notes. letters, book reviews, and announcements.Basic areas of interest include the followin~: theory and experiments on fluid interfaces; adsorption; surface aspects ofcatalysis; dispersion preparation, characterization and stability; aerosols, foams and emulsions; surface forces; micellesand microemulsions; light scattering and spectroscopy; detergency and wetting; thin films, liquid membranes andbilayers; surfactant science; polymer colloids; rheology of colloidal and disperse systems; electrical phenomena ininterfacial and disperse systems. These and related areas are rich and broadly applicable to many industrial, biologicaland agricultural systems.Of interest are applications of colloidal and interfacial phenomena in the following areas: separation processes; materialsprocessing; biological systems (see also companion publication Col/oids and Surfaces B: Biointerfaces); environmentaland aquatic systems; minerals extraction and metallurgy; paper and pulp production; coal cleaning and processing; oilrecovery; household products and cosmetics; pharmaceutical preparations; agricultural, soil and food engineering;chemical and mechanical engineering.

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COLLOIDSANDSURFACES A

ELSEVIERColloids and Surfaces

A: Physicochemical and Engineering Aspects 117 (1996) 37-44

ESR investigations on the stabilization of alumina dispersions byAerosol-OT in different solvents

S. Krishnakumar, P. Somasundaran *

Langmuir Center for Colloids and Interfaces, Henry Krumb School, Columbia University, New York, NYlOO27. USA

Received 23 August 1995; accepted 29 March 1996

-Abstract

Dispersions in organic liquids are used to produce a number of products, such as high-performance ceramics,magnetic tapes, inks, paints and pigments. In this study, the stabilization of colloidal alumina by an anionic surfactant,Aerosol-OT (AOT), in solvents of different polarity was investigated. It was found that in the absence of a dispersant,as the solvent polarity is increased the dispersion stability goes through a maximum. The presence of AOT alters thestability of dispersions in low and high dielectric constant solvents, but not those in moderately polar liquids. Thelatter is shown to be mainly due to the lower adsorption of AOT on alumina from these solvents. Changes insurfactant orientation at the solid-liquid interface in different solvents were also monitored by electron spin resonance(ESR) spectroscopy using adsorbed and chemically bonded 7-doxylstearic acid probes. The results clearly show thatsurfactant orientation and packing at the interface vary with solvent polarity and a compact adsorbed layer is requiredto obtain enhanced stability in the presence of surfactants.

Keywords: Aerosol-OT; Alumina dispersion stabilization; ESR

1. Introduction

Dispersions of colloidal particles in liquids areof paramount importance to several industries,such as ceramics and minerals, waste water treat-ment, paints, inks, pigments and oil recovery[1-3]. Recent studies have suggested that inceramic processing, the packing and distributionof particles throughout the green body affect theporosity and microstructure and control the reli-ability of the final product [4]. The developmentof liquid toner systems in reprographic technol-ogy and "ultra-structure" processing for high-performance ceramics in organic liquids have madeit important to have a proper understanding of

dispersion phenomena in non-aqueous solvents[5].

The process of dispersing solid particles in aliquid can be considered as consisting of thefollowing three stages: (a) wetting of the particlesby the medium, (b) breakage of the loosely boundaggregates into smaller aggregates and (c) thehomogeneous dispersion of these small aggregatesin the liquid medium and prevention of theirreagglomeration [6]. The first two stages refer ingeneral to the dispersibility of the powder and thelast stage determines the stability of the dispersion.The efficiency of the wetting process is dependenton the solid-liquid interfacial free energy and canbe quantitatively described in terms of the Young'sequation [7]. In the case of polar surfaces, wettingcan be related to the dielectric constant and thesurface tension of the dispersing medium [8].* Corresponding author.

~27-7757/96/S15.00 C 1996 Elsevier Science B. V. AU rights reservedPH 80927-7757(96)03674-6

38 S. Krlshnakumar. P. Somasundaran/Col/oids Surfaces A: Physicochem. £IIg. Aspects / /7 ( /996) 37-44

changes in its environment. This technique hasbeen applied in the past to study biological mem-branes [18] and membrane mimetic systems suchas micelles and reverse micelles [19-21].Information on the micropolarity and microviscos-ity of the probe surrounding can be obtained byanalyzing the ESR spectra and used to deducestructural information regarding its environment.This technique has been successfully employed tothe study of adsorbed polymers as well as surfac-tants [22,23]. By using nitroxide probes with theparamagnetic probe attached at various locationsalong the stearic acid chain, Chandar et al. [17]were able to quantify the changes in microviscosityin adsorbed dodecylsulfate layers in alumina dis-persions in aqueous media. Malbrel andSomasundaran [24] were able to show using ESRthat adsorption of water caused conformationalchanges in the adsorbed Aerosol-OT layer at thealumina-cyclohexane interface. This observationwas consistent with changes observed in the sta-bility of the dispersion.

In this paper, we discuss the stability of aluminaparticles in various organic liquids in the absenceand presence of anionic Aerosol-OT, a commonlyused dispersant in non-aqueous solvents.Adsorption of the surfactant from different liquidswas also monitored. Using ESR spectroscopy, westudied the changes in surfactant orientation andpacking at the solid-liquid interface in differentsolvents.

Interparticle forces influence the powder disper-sion process at all stages. The role of variousinterparticle forces in controlling dispersion sta-bility has been well established. It is accepted thatthe total potential energy of interaction between apair of particles can be described as a summationof the (i) electrostatic attractive/repulsive forces,(ii) the attractive London and van der Waalsdispersion forces and (iii) the interaction betweenadsorbed layers on the particle surfaces, referredto as "steric" forces [9,10]:

~=~I+v.+~t (1)

In the absence of a third component, such as asurfactant, polymer or salt, the stability of thecolloidal dispersion will depend only on the van derWaals forces among the various components. Inthe simplest situation of uncharged particles inhydrocarbons of low dielectric constants, only thevan der Waals forces need to be considered.However, the ~1 term will become significant inaqueous media and in other polar solvents wherethe particles can develop significant charge on theirsurfaces through interactions with the solvent.

Addition of dispersing agents such as surfactantsand polymers can alter significantly the propertiesof the dispersions [11,12]. These surface modifiersalter not only the electrostatic interaction amongthe particles but also the dispersion and stericinteractions, depending on their chemical natureand adsorption behavior. The modification of thev. and ~t terms depends on the orientation andpacking of the molecules at the solid-liquid inter-face and hence it becomes important to determinethe microstructure of the adsorbed layer in orderto understand fully the dispersion mechanisms.

Several techniques, such as fluorescence, electronspin resonance (ESR), Raman, infrared and nuclearmagnetic resonance (NMR) spectroscopy, havebeen used to study the adsorption phenomena andto correlate changes in the adsorbed layer micro-structure with variations in macroscopic propertiessuch as wettability and hydrophobicity of thedispersions [13-17]. Among these techniques, spinprobing by ESR spectroscopy is a sensitive tech-nique which involves the incorporation of a suit-able paramagnetic moiety into the system underinvestigation and measuring its response to

2. Experimental

2.1. Materials

The solid used in this study was 0.3 JJ.In Lindealumina with a BET surface area of 14 m2 g-l,purchased from Union Carbide. For the ESRexperiments, AKP-15 alumina purchased fromSumitomo Chemical with a particle size of O. 7 ~mand a surface area of 3.6 m2 g -1 was used. The

anionic surfactant sodium bis(2-ethylhexyl)sulfo-succinate [Aerosol-OT (AOT)] was purchasedfrom Fisher Scientific and was purified by solu-bilization in methanol and recrystallization by sol-vent evaporation. The paramagnetic spin probe

S. Krishnakumar. P. Somasundaran/Colloids Surfaces A: Physicochem. Eng. Aspects 117 ( 1996!) 37--44 39

'""'""/"-_"""~"'""""'" 00 H

,0

(a) (b)

Fig. 1. Structures of (a) Acroso1-0T and (b) 7-doxylstearic acid.

7-doxylstearic acid, with the doxyl group attachedat the C- 7 position relative to the carboxylic acidgroup along the stearic acid molecule, waspurchased from Aldrich Chemical and used asreceived (see Fig. 1). The organic solvents usedwere all Fisher Scientific high-purity solvents andused as received. The water used was triply distilledand had a conductivity of less than 10-6 a-I cm -1.

2.2. Methods

with the spin probes were then treated with surfac-tant solutions in different solvents. All ESR spectrawere acquired using a Micronow 8300 X-bandspectrophotometer at a modulation frequency of100 kHz. For isotropic spectra in the fast motionregime, the rotational correlation times were calcu-lated directly from the spectrum using the following

equation:

'B (s) = 6.25 X 10-10 AHo[(10/1_1)1/2

-(10/1+1)1/2], (2)

where AHo=peak-to-peak distance of the centralline in gauss, 10/1_1 = ratio of the central to high-field line peak heights and 10// +1 = ratio of thecentral to low-field peak heights. For anisotropicspectra in the slow regime, the application of theabove equation led to erroneous values of thecorrelation times. In such cases, ' values wereestimated using the following equation [27]:

,=a(l-S)b (3)

where a and b are constants [28] and S = A'JAz'where A~ is half the separation of the outer hyper-fine extrema and Az is the rigid limit value for thesame quantity obtained from a frozen spectrum ofthe sample under consideration.

The alumina was dried by heating at 200°C for6 h and cooling under vacuum. A 1 g amount ofthe dried alumina was mixed with 15 ml of thedispersing liquid for 6 h and then allowed to settle.The dispersions were then allowed to settle in agraduated test-tube and the settling rates estimatedin terms of the rate of descent of the upper interface.The amount of surfactant adsorbed under variousconditions was estimated by measuring the deple-tion of surfactant in the bulk upon contact withthe solid particles. For this purpose, the superna-tant was removed by centrifuging and then ana-lyzed for residual surfactant. The concentration ofAOT in bulk was detennined by a two-phasetitration technique [25].

The ESR probe was incorporated into theadsorbed layer by two methods. In one case, theESR probe was preadsorbed on the alumina fromcyclohexane and the excess solvent removed bydrying. These particles were then treated with thesurfactant solutions in various solvents. In thesecond case, the doxylstearic acid was chemicallybonded to the alumina surface [26]. The solids

3. Results and discussion

3.1. Stability in the absence of surfactant

Fig. 2 shows the settling behavior of bare alu-mina particles in solvents of various polarities. It

40 S. Krishnakumar. P. SomasundaranjCo/loids surf4Ce.f..4.: Physicoc/rem. Eng. Aspects 117 (1996) 37-44

The A values for ceramic substrates and liquidscan also be estimated from the following equation:Q

Q)a)

""a)d0...,

g

..~

0=...5

'3Q)a)

'0Q)

.~-;

E.P.

As(kT) = 113j (6)

where £: is the static dielectric constant of thematerial being considered. A value of £:s=9.3 isassumed for alumina based on reported measure-ments [29] and Aal is estimated to be 70.5 kT fromEq. (6). Similar calculations were performed forthe different liquids considered here and the resultsare shown in Table 1. For a given set of particles,the AS/L/S value is directly proportional to thevan der Waals attractive energy. It can be seenfrom the values in Table 1 that the dispersionbehavior as predicted by changes in the van derWaals interaction are similar to the observed beha-vior. This suggests that the primary cause of theobserved changes in stability with solvent polarityis the change in the attractive interaction. Fig. 3depicts the variation of the attractive potential[Eq. (4)] with interparticle distance for these barealumina particles in different liquids. It can be seenthat at low interparticle separations the attractiveenergy is maximum for cyclohexane (As/L/S =25.5 kT) and minimum for 2-propanol (As/L/S =1.1 kT). However, one should be aware that inpolar liquids alumina is also capable of developingsignificant amount of surface charge, which canenhance the dispersion stability by contributing tothe electrostatic repulsion term.

. ~ 40 ~ 80

Dielectric constant

Fig. 2. Stability of bare alumina particles in various liquids.

3.2. Stability in the presence of surfactant

The presence of an adsorbed layer of surfactantalters the dispersion behavior markedly. Fig. 4shows the settling rates in different liquids at threedifferent levels of surfactant addition. It can beseen that the dispersions in the non-polar solventsare stabilized significantly by the presence of thesurfactant, with the stability increasing withincrease in surfactant concentration. The disper-sions in the highly polar solvents also can be seento undergo changes in their dispersion behavior.In this case, the changes in settling rate are aConsequence of the electrostatic forces in additionto the changes in the dispersion forces. At high

can be observed that the dispersion stability dis-plays a maximum [corresponding to a minimumin the normalized settling rate; the normalizationis applied to account for differences in the densityand viscosity of the solvents and the normalizedsettling rate = observed settling rate x solventviscosityj(particle density - solvent density)] withincrease in solvent polarity. The maximum stabilityin this case is observed in moderately polar solvents(20 <£<45). Bare particles suspended in a liquidmedium are in constant Brownian motion and canflocculate rapidly on collision if the v. term islarger than about 15 kT. Stabilization can usuallybe achieved by decreasing the van der Waalsattractive forces. The potential energy due to thevan der Waals interaction can be calculatedapproximately for two spherical particles sus-pended in a liquid medium using the followingequation:v. = - As/L/S(aj12Ho) for Ho «a (4)

where As/L/S is the combined Hamaker constantfor the solid in the liquid and can be estimatedfrom the equation

As/L/S = As + AL -2(AsAL)1/2 (5)

where As and AL are the Hamaker constants ofthe respective solids and liquids in vacuum.

(£-1~(£ + lt~(£ + 2)tf2

S. Krishnakumar, P. Somosundaran/Co//oids Surfaces A: Physicochem. Eng. Aspects 117 ( 1996) 37-44 41

Table IHamaker constant and dispersion stability for alumina in different solvents

Solvent AL (kT)Dielectric constant ~ (kT) Observed behavior

2.024.3

10.818.324.332.466.780

11.240.485.78994.699

106.3107.5

25.S4.30;81.11.92.53.83.9:

PoorPoor-OKGoodGoodGoodGood-poorGood-poorGood-poor

CyclohexaneChlorofonn2-Butanol2-PropanolEthanolMethanolMethanol-water (40%)Water

!;;iv

>:.:;e'Q)cQ)iij:pc

!Q)

.~u

~ , 10 20 30 ~

Interparticle distance, nm

50 IG

Fig. 3. Variation of van der Waals attractive energy withinterparticle distance for alumina particles in (a) cyclohexane,(b) water, (c) ethanol and (d) 2-propanol.

Dielectric constant

Fig. 4. Stability of alumina particles in various liquids in thepresence of anionic Aerosol-QT.

does not aggregate significantly [19]. This leadsto the inference that the changes in dispersionbehavior are caused only when a substantialamount of surfactant is adsorbed at the solid-liquid interface. The stabilizing effect of the surfac-tant in non-polar solvents can be explained interms of the modification of the attractive inter-actions by the adsorbed surfactant layer. In lowdielectric constant solvents, AOT adsorbs on alu-mina with its polar head group interacting' withthe polar alumina surface and the hydrocarbontail extending out into the bulk. This makes thesurface rather hydrophobic, with a concomitantreduction in the AS/LIS value for the particles.

adsorption densities, the particles exhibit signifi-cantly high zeta potentials (-50 mY) [30] in waterand a very stable dispersion is obtained. whereasthere is a slight increase in settling rate at E = 65,possibly due to charge neutralization. However,dispersions in the moderately polar liquids do notshow any change in their dispersion behavior asmeasured by the settling rate.

The adsorption of the surfactant on aluminafrom the different solvents is shown in Fig. 5. It isclear that the adsorption itself is influenced by thesolvent pol.arity, with less adsorption from thesolvents of intermediate polarity. We found inearlier work that the adsorption of AOT is lessfavourable from solvents (20 < E < 60) in which it

42 S. Krishnakumar. P. SomasWldaranfColloids surfaces A: Physicodzem. Eng. Aspects 117 ( 1996) 37-44

-e'""0e

G0- 1.0>c

>......C~

A

£:0::J 0.5Q,..0.

..,-<

~~

..~a-

t-~0 o.

~

c

Q;Oi

~o O'.~0 20 40 60 80

Dieleclric constant

Fig. 6. Effect of solvent polarity on the rotational correlationtime of adsorbed 7-doxylstearic acid.

Dielectric constant

Fig. 5. Adsorption of Aerosol-OT on alumina from differentsolvents.

3.3. Surfactant orientation studies by ESR

spectroscopy

It is well known that the orientation of theadsorbed surfactant molecules and their packingat the solid-liquid interface are of extreme impor-tance in determining the efficiency of surface modi-fication. From Figs. 4 and 5, we observe that ataround ~ = 20, although there is significant adsorp-tion at a surfactant concentration of 10-3 M, thesuspension stability does not change appreciably.This suggests that in addition to adsorption den-sity, the arrangement of molecules in the adsorbedlayer is important for dispersion stability. In orderto investigate this aspect, we conducted ESRstudies to determine both the surfactant orientationand packing at the alumina interface in varioussolvents.

Fig. 6 shows the changes in mobility of theadsorbed 7-doxylstearic acid in various solvents.The mobility increases with increasing solventpolarity. The doxylstearic acid adsorbs predomi-nantly through interactions of the COOH groupswith the alumina surface although there is alsosome evidence of interaction of the doxyl groupwith the surface. Thus the increase in probe mobil-ity is caused by a weakening of both these inter-

actions. In the solvents where the probe exhibitshigher mobility, some of the probe was found todesorb into the bulk solvent. In order to circum-vent this problem, we conducted similar experi-ments using alumina particles in which the COOHgroup of the spin probe was covalently attachedto the alumina surface. The mobility results usingthese samples are shown in Fig. 7. Surprisingly,there is no significant difference between the mobi-lities measured in the two cases. This suggests thatthe changes in mobility in both cases reflect mainlya weakening of interactions of the NO group withthe surface. Based on this, we propose that the NOgroup on the stearic acid chain moves away fromthe solid surface as the solvent polarity increases.This is possible if the surfactant molecule adoptsan increasingly stretched out/dangling orientationat the interface with only the COOH anchored atthe interface.

Fig. 7 also shows the changes in probe mobilityobserved with the bound probe when the surfaceis treated with a concentrated solution of AOT(30 mM). A comparison of these mobility valueswith those obtained in the absence of the surfactantreveals the mobility changes to be pronouncedonly in the non-polar and the highly polar solvents.Among the non-polar solvents, the mobility incyclohexane does not change much between the

S. Krishnakumar. P. SomasundaranfColloids surfaces A: Physicochem. £IIg. Aspects 117 ( 1996) 37-44 43

10

I

--.:::.:::~.:::.-~;

So. 0,~

JO~t

O.OOI~

medium. The stability in low dielectric solvents iscontrolled primarily by the attractive van derWaals interaction energy and it can be enhancedby choosing solvents with Hamaker constantvalues similar to that of the solid surface. Theenhanced stability of alumina in non-polar solventsin the presence of AOT is attributed to a reductionin the attractive energy due to the presence of theadsorbed surfactant layer. In higher dielectric con-stant solvents (E> 30) capable of ionization of thesurface as well as the solute species, the stabilitydepends on the interplay of the repulsive electro-static potential and the attractive van der Waalsenergy.

The adsorption of the AOT de~nds also on thenature of the solvent, with the adsorption being aminimum in solvents that are moderately polar.ESR studies using 7-doxylstearic acid corroboratethis weak interaction of the surfactant with thesurface in these solvents. In the absence of anycoadsorbed surfactant, the probe mobility, whichreflects the extent of its interaction with the surface,is least in solvents of intermediate polarity. In thenon-polar solvents the probe mobility changessignificantly in the presence of adsorbed surfactantand this is followed by a marked enhancement insuspension stability. This suggests that the changesin stability are related to the fonnation of a wellpacked surfactant layer at the solid-liquid interface.

.I60 :8040

Dielectric constant

Fig. 7. Effect of solvent polarity on the rotational correlationtime of bound 7-doxylstearic acid in the absence and presenceof AOT (30 mM).

two cases. This is because in cyclohexane even inthe absence of adsorbed surfactant the probe (NOgroup) interacts with the surface and is thereforehighly restricted The formation of a dense surfac-tant monolayer at the surface is therefore notreflected in the probe mobility. However, in thecase of chloroform and THF, the addition of thesurfactant causes a drastic reduction in probemobility due to the entrapment of the boundprobe in the adsorbed surfactant layer. Also, thesimilarity in probe mobility values in cyclohexane,chloroform and THF suggests that the AOTadsorbed layer is of similar nature in all threecases. In the moderately polar solvents there is nosignificant change in probe mobility. This isexpected, as very little surfactant adsorbs on thesolid in these cases. Also, the surfactant thatadsorbs is loosely bound and is not sufficient toentrap and immobilize the nitroxide probes. In thehighly polar solvents some immobilization isobserved, consistent with the formation of denselycondensed surfactant aggregates at the interface.

Acknowledgements

The authors acknowledge the financial supportof this work by the National Science Foundation(NSF-CTS-9311940) and MMRRI, New York.They also thank Xiang Yu for helping with thepreparation of the samples with the bound ESRprobe.

References

4. Conclusions

Colloidal stability in a liquid medium dependsconsiderably on the selection of the dispersing

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[30] S. Krishnakumar, unpublished results, ColumbiaUniversity, 1995.

INSTRUCTIONS TO AUTHORS

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