Effect of charged polyelectrolytes on the electrophoretic behavior, stability, and viscoelastic properties of montmorillonite suspensions

Effect of charged polyelectrolytes on the electrophoretic behavior,stability, and viscoelastic properties of montmorillonite suspensionsM. M. Ramos-Tejada, C. Galindo-González, R. Perea, and J. D. G. Durán

Citation: Journal of Rheology (1978-present) 50, 995 (2006); doi: 10.1122/1.2355653 View online: http://dx.doi.org/10.1122/1.2355653 View Table of Contents: http://scitation.aip.org/content/sor/journal/jor2/50/6?ver=pdfcov Published by the The Society of Rheology Articles you may be interested in Rheology of concentrated soft and hard-sphere suspensions J. Rheol. 57, 1195 (2013); 10.1122/1.4808054 Effect of flow history on linear viscoelastic properties and the evolution of the structure ofmultiwalled carbon nanotube suspensions in an epoxy J. Rheol. 55, 153 (2011); 10.1122/1.3523628 Rheological properties of concentrated aqueous silica suspensions: Effects of p H andions content J. Rheol. 47, 1133 (2003); 10.1122/1.1603237 Effect of humic acid adsorption on the rheological properties of sodium montmorillonitesuspensions J. Rheol. 45, 1159 (2001); 10.1122/1.1392297 Effect of Extending Oil on Viscoelastic Behavior of Elastomers J. Rheol. 26, 427 (1982); 10.1122/1.549672

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Effect of charged polyelectrolytes on the electrophoreticbehavior, stability, and viscoelastic properties of

montmorillonite suspensions

M. M. Ramos-Tejada

Department of Physics, Faculty of Experimental Sciences,University of Jaén, 23071 Jaén, Spain

C. Galindo-González

Department of Applied Physics, Faculty of Sciences,University of Granada, 18071 Granada, Spain

R. Perea

Department of Physics, Faculty of Experimental Sciences,University of Jaén, Jaén, 23071, Spain

J. D. G. Durána)

Department of Applied Physics, Faculty of Sciences,University of Granada, 18071 Granada, Spain

(Received 7 September 2005; final revision received 14 August 2006�

Synopsis

his work is devoted to the study of the rheological properties of sodium montmorilloniteuspensions in aqueous media containing polyelectrolytes in solution. Two differentolyelectrolytes are employed: polyacrylic acid �PAA� and polyethyleneimine �PEI�. PAA can bearegative charge, thus acting as a polyanion, while PEI can be considered as a polycation, althoughhe charge of both polymers is strongly dependent on pH of the solution. The rheological behaviorf clay suspensions is essentially determined by the electric potential of the faces and edges of theaminar clay particles. In order to analyze the changes in the interfacial electric potential of clayurfaces, the zeta potential of clay particles was estimated from electrophoresis measurements forifferent solution compositions. The yield stress and the storage modulus of the suspensions wereetermined demonstrating that only in some cases the storage modulus can be correlated with thehanges in electrostatic interactions between particles. In particular, in clay/PEI suspensions ateutral-basic pH the changes in the viscoelastic properties do not match with those in surface-to-urface electrostatic interactions. Different mechanisms are proposed to explain the wide variety ofheological phenomena observed. © 2006 The Society of Rheology. �DOI: 10.1122/1.2355653�

�
Author to whom all correspondence should be addressed; electronic mail: [email protected]
2006 by The Society of Rheology, Inc.995. Rheol. 50�6�, 995-1007 November/December �2006� 0148-6055/2006/50�6�/995/13/$27.00

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. INTRODUCTION

The rheological properties of clay suspensions are closely related with the formationf homo- and heteroaggregates among clay particles in aqueous suspensions, which de-end on both the properties of the heterogeneous surface of the solid particles and thelectrolyte composition. In this work, we will employ homoionic sodium montmorilloniteNaMt� particles characterized by their laminar geometry, with two kinds of surfaces:aces and edges. In aqueous suspension, the negative charge of the face/solution interfacerises from isomorphous substitution of Si and Al atoms in the tetrahedral and octahedralheets of the crystals, while the charge of the edge/solution interface is determined by the

pH of the solution �H+ is the potential-determining ion� and, in consequence, the surfaceharge can be positive or negative �Luckham and Rossi �1999�; Tadros �1996�; Vanlphen �1977��.The degree of homo- �face–face, edge–edge� and heteroaggregation �face–edge� be-

ween the particles is dominated by the pH and ionic strength of the solution �Durán et al.2000�; Lagaly �2005�; Luckham and Rossi �1999��. In addition, the internal structure andhelogical properties of clay suspensions can be radically modified by charged polymersr surfactants adsorbed on the clay/solution interface �Alemdar et al. �2005�; Alince andan de Ven �1993�; Billingham et al. �1997�; López-Durán et al. �2003�; Öztekin et al.2002�; Parazak et al. �1988�; Porubská et al. �2002�; Ramos-Tejada et al. �2001b,003a�; Sjöberg et al. �1999�; Seppänen et al. �2000�; Sondi and Pravdic �2002�; Tom-ácz et al. �1999��.

The present work is devoted to the study of the electrokinetics, colloidal stability, andheology of NaMt suspensions that contain in solution different concentrations of twoolyelectrolytes: polyacyilic acid �PAA� and polyethyleneimine �PEI�. The first one is annionic polyelectrolyte that, depending of the pH of the solution, can bear negativeharge, while PEI behaves as a cationic polymer when their imine groups are protonated.e are mainly concerned with the determination of the role played by the solution

omposition �pH, polyelectrolyte concentration� on the different flocculation/stabilizationechanisms and, consequently, on the whole stability and rheological response of NaMt

uspensions.

I. MATERIALS AND METHODS

. Materials

The NaMt used in this work was obtained from a natural bentonite, extracted from anre deposit placed in the southeastern of the Spanish peninsula �Almería, Spain�, em-loying the homoionization process described in a previous work �Ramos-Tejada et al.2001b��. The specific surface area obtained by the Brunauer-Emmett-Teller �B.E.T.�ultipoint method �54.1 m2/g� and the bulk and surface chemical compositions were

eported in a previous work �Durán et al. �2000��. The NaMt particles have a plate-likeeometry with a length of �2 �m and an aspect ratio �particle equivalent diameter/hickness� of about 14 �Ramos-Tejada et al. �2001a��. The silica and alumina powders,mployed to estimate the zeta potential of clay edges, were from Riedel de Häen, Ger-any with purity above 98%. All chemicals used for fixing the pH and the ionic strengthere of analytical quality, manufactured by Panreac, Spain. De-ionized and filtered water

Milli-Q, Millipore, France� was used to prepare all the suspensions.The polymers used to modify the properties of the NaMt suspensions were PAA and

EI. Both polymers were supplied by Sigma-Aldrich, Germany. The manufacturer speci-
es that the average molecular weights of the polymers are: Mw�1800 �PAA� and Mw

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2000 �PEI�. The carboxylic groups in PAA are weak acids with a pKa=4.5 �Chibowskind Wisniewska �2002�; Gebhardt and Fuerstenau �1983��. On the contrary, the aminoroups in PEI are weak bases with a degree of protonation progressively decreasing uponncreasing pH, e.g., at pH=6.5 about 50% of the amino groups are protonated and at

pH�11 they are uncharged �Alince and van de Ven �1993�; Hostetler and Swanson1974�; Lindquist and Stratton �1976�; Radeva and Petkanchin �1997�; Suh et al. �1994��.

. Experimental methods

. Electrophoretic mobility

The zeta potential �� of the particles was estimated from electrophoretic mobility �ue�ata measured with a Malvern Zeta-Sizer 3000 HS equipment �Malvern, England� at5 °C, using suspensions containing 0.5 g/L solids. Measurements were performed 24 hfter the preparation of the suspensions, and the pH was readjusted immediately beforeeasuring the electrophoretic mobility.

. Rheological measurements

A Bohlin CS-10 controlled-stress rheometer �Bohlin Instruments, England� was usedor rheological measurements. The measuring system was a cup and bob �Bohlin CSS-5�, with a bob 25 mm in diameter and 28.5 mm in height, and a 26 mm diameter cup.ll determinations were performed at 25.0±0.1 °C in NaMt suspensions with a 5% w/v

oncentration. The aqueous medium was a 10−2 M NaCl solution, and the pH was ad-usted to 3, 7, and 9. The added polymer concentration ranged between 0 and 2 g/L PAAr 0 and 20 g/L PEI. Considering that PEI can be strongly adsorbed onto glass surfaces,ll the suspensions and solutions were prepared and stored in polyethylene flasks.

In viscometry or steady-state measurements a shear stress ramp between 0.03 and0 Pa was applied to the suspensions and the shear rate data was recorded 2 s afterpplication of each shear stress value. In dynamic measurements �oscillometry�, a sinu-oidal shear stress of constant frequency �1 Hz� and variable amplitude ��0� was firstpplied to the samples, and the corresponding shear strain was recorded. The viscoelasticinear region �VLR�, i.e., the �0 interval in which the storage modulus G� is approxi-

ately constant, was determined. After that, the storage and loss moduli were determineds a function of frequency in the range 0.01–10 Hz at a constant shear amplitude wellnto the VLR. Because of the thixotropic character of smectite suspensions �Van Olphen1977�� and in order to obtain reproducible results, samples were always presheared for0 s applying a shear stress in the postyield regime, and measurements were taken after80 s of waiting time.

II. RESULTS AND DISCUSSION

. Zeta potential

The zeta potential �� was estimated in all cases by means of the Helmholtz–moluchowski equation �Hunter �1987�� given that the particle size is much larger than

he thickness of the electrical double layer around the particles. Furthermore, theelmholtz–Smoluchowski equation can be considered as a good approximation to the

nterfacial potential if the adsorbed polyelectrolytes have low molecular weight �lowerhan �103 g/mol� �Ramos-Tejada et al. �2003b�; Viota et al. �2005��. In addition, we

ust consider the surface heterogeneity �faces and edges� of the laminar NaMt particles.
lthough the major part of the surface area corresponds to the clay faces, the edges play

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decisive role in determining the internal structure and the whole properties �stability,heology� of the suspensions. For this reason, it is necessary to estimate the � potential ofoth faces and edges.

In Fig. 1, � is represented as a function of the pH of the solution in the absence andresence �50 mg/L� of PAA or PEI. Given that only around 1% of the total surface areaf NaMt particles can be ascribed to edges �Sondi and Pravdic �1998��, the zeta potentialf the face surfaces �Fig. 1�a�� is estimated directly from ue data measured in clayuspensions. In the absence of polyelectrolytes in solution, we can observe the well-nown pH-independent behavior of the � potential since the surface charge of the faces isenerated by isomorphous substitution. A similar behavior is observed for suspensionsith 50 mg/L PAA added. The presence of neutral �the great part of the molecules at

pH�4.5� or negatively charged �for pH�4.5� PAA molecules in solution does not sig-ificantly modify the electrical properties of an interface that previously bore a highegative surface charge. However, although we do not detect electrostatic changes in thenterface, we cannot reject the probable chemisorption of polyanions on the clay faces by

IG. 1. Zeta potential �� of montmorillonite particles as a function of pH in the absence or presence ofolyelectrolytes �polyacrylic acid, PAA, and polyethyleneimine, PEI� in solution. Polymer concentration added0 mg/L; salt concentration 10−2 M NaCl: �a� clay faces and �b� clay edges.

urface complexation �Billingham et al. �1997�; Lagaly �2005�; Ramos-Tejada et al.


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2001b, 2003a�; Sjöberg et al. �1999�; Tombácz et al. �1999�; Zaman et al. �2002��. Thisdsorption is able to provoke important changes in the degree of homo- and heteroag-regation in the suspensions.

A very different trend occurs in the presence of PEI. For pH�11 a significant pro-ortion of the amine groups are protonated and the polymer behaves as a polycation. Theharge reversal in �face observed between pH 3 and 10 �Fig. 1�a�� can be simply explainedy the adsorption of PEI molecules favored by electrostatic interaction between theegative faces and the cationic polymer. Nevertheless, the interaction of PEI moleculesith clay surfaces is not only favored by electrostatic forces, but also by formation of

urface complexes between the silicon and aluminum atoms and the imine groups. Thisossibility would explain the decrease in �� value, as compared with �� in the absence ofEI, observed at pH 11, although at this pH value the protonation of the imine groups beegligible.

The electrokinetic behavior, and the zeta potential, of clay edges must be inferredndirectly because it is not experimentally accessible. However, since the work by Will-ams and Williams �Williams and Williams �1978�� many authors �Heath and Tadros1983�; Luckham and Rossi �1999�� estimate the zeta potential of edge surfaces from ainear combination of the potentials of silica and alumina, assuming that the isoelectric

IG. 2. Zeta potential of clay faces as a function of polymer concentration for the indicated pH values. Ionictrength 10−2 M NaCl: �a� polyacrylic acid and �b� polyethyleneimine.

oint �pH of zero zeta potential� is located at pHiep�6–7. More recently, Tombácz and


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zekeres �2004� estimated the point of zero charge �PZC� of the NaMt edges by means ofotentiometric titration measurements as a weighted combination of the surface chargesf alumina and silica concluding that pHpzc�6.5. In the present work, assuming that

pHiep�edge��7, we estimated the zeta potential of edges in the absence of polyelectro-ytes in solution as a weighted mean of those of SiO2 and Al2O3 �Durán et al. �2000�;amos-Tejada et al. �2001a, 2001b, 2003a��: �edge= �1/3� �silica+ �2/3� �alumina. The sameethod has been used to estimate the zeta potential in the presence of PAA and PEI. Weeasured the zeta potential of silica and alumina in the presence of the polymers �not

hown here for brevity� and we obtained the results shown in Fig. 1�b� for �edge. Thedsorption of PAA renders the surface potential negative ��edge ranges between �−20 and60 mV� in the entire pH range studied, although for neutral-basic pH the proportion ofissociated acrylic groups must be less than 50%. The adsorption of PEI molecules hashe reverse effect, displacing the entire curve in the absence of PEI toward higher �alues.

The effect of polymer concentration, at different pH values, is shown in Figs. 2 �faces�nd 3 �edges�. From Fig. 2�a� is clear that an increase in PAA concentration �above0 mg/L studied in Fig. 1�a�� does not provokes any significant change in �face. On theontrary, the increase in PEI concentration �Fig. 2�b�� produces a charge reversal in clay

FIG. 3. Same as Fig. 2, but for clay edges.

aces for a polymer concentration between �1 and 4 mg/L. This charge reversal is


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avored by electrostatic attraction between the face surface and PEI molecules, althoughhis effect is more pronounced at low pH because of the higher protonation of theolymer molecules.

From Fig. 3 we can deduce the effect of polymer adsorption on �edge. Remember thate assumed that the isolectric point of clay edges is located around pH 7. In conse-uence, for pH�7 the adsorption of anionic polymers will be favored by electrostaticttraction and, similarly, for pH�7 polycations will be strongly attached, explaining theharge reversal in clay edges observed at pH 3 in Fig. 3�a� �PAA�, and at pH 9 in Fig.�b� �PEI�. However, the trends observed at pH 7 and 9 in PAA solutions �Fig. 3�a��, andor pH 3 and 7 in Fig. 3�b� �PEI� can only be justified considering a mechanism ofdsorption via formation of surface coordinative compounds, because the adsorption inuch conditions is not favored by electrostatic attraction.

. Rheology

. Viscometry

IG. 4. Yield stress ��y� of clay/polymer suspensions as a function of the polymer concentration at thendicated pH values. Clay concentration 5% w/v; ionic strength 10−2 M NaCl: �a� PAA and �b� PEI.

Figure 4 shows the results obtained in steady-state experiments. In this figure the yield


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tress ��y� of clay suspensions �5% w/v� is represented as a function of the polymeroncentration �Fig. 4�a� for PAA; Fig. 4�b� for PEI� at pH 3, 7, and 9. The yield stressas estimated, following the procedure suggested by Barnes et al. �1989�, as the shear

tress corresponding to the plateau found in a log–log plot of shear stress �� versus shearate ��.

In the case of clay/PAA suspensions a clearly different behavior is observed at acidnd at neutral-basic pH �Fig. 4�a��. The yield stress found at pH 3 in the absence ofolymer in solution corresponds to the stress applied to break down the internal structuref the clay gel. The curves for pH 7 and 9 in Fig. 4�a� reflect the typical behavior ofeflocculated clay suspensions in which the yield stress is negligible. A very differentehavior is observed when PEI polymer is added to the suspensions �Fig. 4�b��. The yieldtress is practically constant up to �102 mg/L PEI concentration whatever the pH of theuspensions. For higher PEI concentration a slight increase, followed by an abrupt de-rease �pH 3� or a sharp increase �pH 7 and 9�, is observed.

. Oscillometry

In Fig. 5 the storage modulus �G�� of clay/polymer suspensions is represented as a

FIG. 5. Similar to Fig. 4, but for the storage modulus �G�� measured at a frequency of 0.5 Hz.

unction of the polyelectrolyte concentration �Fig. 5�a� for PAA; Fig. 5�b� for PEI�. The


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� values in these plots correspond to those obtained in frequency sweep measurementsor shear stress amplitude values well into the viscoelastic linear region. We have takenhe G� value measured at a frequency of 0.5 Hz. The tendencies for G� in Fig. 5 areonsistent with those displayed in Fig. 4 for yield stress.

However, it is difficult to correlate the changes in � potential with those in the rheo-ogical quantities ��y ,G�� in suspensions with different solid concentrations: 0.05% w/vnd 5% w/v in zeta potential and rheological measurements, respectively. It seems moreonvenient to use an intensive quantity to express the polymer concentration. We proposeo use the polymer mass added per unit surface area of clay particles in suspension. Inddition, in flocculated sols, in which the electrostatic interactions can determine theheological behavior of the suspensions, it has been demonstrated �Hunter �1989, 2001�;ohnson et al. �1998�; Ramos-Tejada et al. �2001b, 2003a�� that the degree of flocculationnd the most relevant rheological quantities ��y ,G�� are correlated with �2, considering2 as a quantity indicative of the strength of electrostatic interaction. Following thispproach, we have calculated �i�� j �i= j: face-to-face and edge-to-edge; i� j face-to-dge� as a function of the polymer mass added per unit area of clay surface. As occurs inhe absence of polymers in solution �Durán et al. �2000��, we found a significant corre-ation between �face��edge and G� in the major part of the suspensions studied. For this

IG. 6. Storage modulus �left Y axis� or product of the indicated zeta potentials �right Y axis� as a function ofhe PAA concentration expressed as polymer mass added per unit surface area of clay particles: �a� pH 3 and �b�

pH 9.

eason, we will only discus the relation between these two quantities.


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In Fig. 6, G� and �face��edge are represented as a function of the specific PAA con-entration �mg PAA/m2 clay� at pH 3 and 9 �the plot at pH=7, not shown, is similar tohat at pH=9�. The results in Fig. 6 demonstrate that the progressive loss of the elasticharacter �at any pH value� of the suspensions is related to changes in the face-to-edgelectrostatic interactions provoked by PAA adsorption.

In Fig. 7 the storage modulus G� and �face��edge are represented as a function of thepecific PEI concentration at pH 3 and pH 9 �the results at pH 7 are similar to those at

pH 9�. At pH 3, the changes in G� seems to be mainly controlled by face-to-edgelectrostatic interactions. The most remarkable fact is that the adsorption of positivelyharged PEI molecules favors at high PEI concentration a strong electrostatic repulsionetween surfaces, which at last provokes a sharp decrease in the storage modulus.

A very different behavior is observed at neutral-basic pH. At pH=9 �Fig. 7�b�� thelight initial decrease of G� �from 0 to 10−2 mg PEI/m2� is followed by an increase of 2ecades in G� �interval from 10−2 to 10 mg PEI/m2�. Simultaneously, we have observed,or the highest PEI concentrations, the formation of loose big flocculi, which span overhe whole volume of the sample. These changes do not seem to be clearly correlated withny tendency of the electrostatic interactions; in fact, G� is larger when the electrostaticnteraction is more repulsive. In this point, we can only propose some reasonable hypoth-ses to explain the result: �i� the well-know mechanism of bridging flocculation does noteem sound in this case because of the low molecular weight of the polymer; �ii� at low

−2 2

FIG. 7. Similar to Fig. 6, but for clay/PEI suspensions.

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n the clay surfaces can provoke the breaking the weak gel formed in the absence ofolymer reducing the elastic response of the suspensions; and �iii� at high PEI concen-ration �10−2–10 mg PEI/m2�, the adsorbed molecules can favor the existence of hydro-hobic interactions between PEI-covered clay particles. Whichever the microscopic in-erpretation, it is clear that the attachment of neutral PEI molecules provokes, at highnough PEI concentration, the formation of big flocculi and a strong �more than 2 de-ades� increment in the storage modulus of the suspensions.

V. CONCLUSIONS

The addition of low molecular weight polyelectrolytes can provokes dramatic changesn the viscoelastic properties of clay suspensions depending on the pH, and the polymerharge and concentration. Polyacrylic acid behaves as a very effective deflocculant agenthatever the pH of the suspensions �in the pH range 3–9�. The elastic character of the

lay suspensions, in the absence of polymer, is strongly reduced as a consequence of thetrong electrostatic repulsions between the PAA-covered clay surfaces.

The addition of a cationic polyelectrolyte �PEI� yields a wide variety of possibilities toontrol the rheological response of the suspensions. At acid pH, the increase in PEIoncentration provokes a progressive breaking of the clay gel �reduction in storage modu-us and yield stress� correlated with the increase in the electrostatic repulsion betweenlay surfaces. At neutral-basic pH the sequence, as PEI concentration increases, consistsf an initial weakening of the clay gel �slight decrease in G� and �y� followed by a sharpncrease in G� and �y accompanied by the formation of big flocculi in the suspensions.his behavior, which cannot be explained by changes in electrostatic interactions, coulde attributed to hydrophobic attractions between the PEI-covered clay particles.

CKNOWLEDGMENTS

Financial support from Ministerio de Educación y Ciencia, Spain, and FEDER funds,U �Project Nos. MAT2005-07746-C02-01 and FIS2005-06860-C02–02�, and Junta dendalucía �Project No. FQM410� are gratefully acknowledged.

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Effect of charged polyelectrolytes on the electrophoretic behavior, stability, and viscoelastic properties of montmorillonite suspensions