Sol–gel transitions of sodium montmorillonite dispersions

Applied Clay Science 16 Ž2000. 201–227 www.elsevier.nlrlocaterapclaysci

Sol–gel transitions of sodium montmorillonite dispersions S. Abend, G. Lagaly

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Institute of Inorganic Chemistry, UniÕersity of Kiel, Olshausenstr. 40-60, D-24098 Kiel, Germany Received 22 April 1999; accepted 16 July 1999

Abstract The flow behavior of sodium montmorillonite dispersions at salt concentrations below the critical coagulation concentration is determined by the influence of the ionic double layers on the mobility of the particles Žsecondary electroviscous effect.. At solid contents above 3% the dispersions became gel-like with the appearance of a yield value and viscoelastic properties. Increasing salt concentration reduced the thickness of the diffuse ionic layers and the immobilization of the particles. As a consequence, the yield value and the viscosity decreased to a minimum at about 2–20 mmolrl NaCl Ždepending on the montmorillonite.. This behavior was virtually independent on the type of salt. Above the electroviscous minimum the values of the flow properties increased with the salt concentration. Again a gel formed because the interaction between the edgesŽy. and facesŽy. and, at somewhat higher salt concentration, between the facesŽy. became attractive. High yield values and storage moduli were observed. The reversible part of the compliance reached 60–70%. The gel-like dispersion showed pronounced thixotropy. At salt concentrations above 400 mmolrl NaCl and solid contents below 2–3% Ždepending on the montmorillonite., viscosity, yield value, storage modulus, and reversible compliance decreased again because the gel transformed into a sediment. The cause is the contraction of the network into distinct particles when the attraction between the silicate layers is too strong. Formation and properties of the attractive gel were influenced by the type of salt. Potassium and cesium ions enhanced the elasticity of the gel. Sulphate anions reduced the yield value and storage modulus. This effect was very strong with diphosphate which liquefied the gel to a sol. The different states of sodium montmorillonite dispersions: sol, repulsive and attractive gel, sediment, are represented

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Corresponding author. Tel.: q49-431-880-3261; Fax: q49-431-880-1608; E-mail: [email protected] 0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 4 0 - X

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in phase diagrams which were constructed on the basis of compliance Žcreeping. measurements. q 2000 Elsevier Science B.V. All rights reserved. Keywords: bentonites; compliance; creeping measurements; gel; liquefaction; montmorillonite; rheology; storage modulus; viscoelastic properties

1. Introduction Many practical applications are based on the properties of dispersed bentonites. Several million tons of bentonites are used in form of colloidal dispersions. Colloidally dispersed bentonites are thickening and thixotropic agents which are added to drilling fluids, paints, emails, pesticide formulations, cosmetical and pharmaceutical preparations. They are used for underground sealing and geotechnical constructions Ž Odom, 1984; Jasmund and Lagaly, 1993, Chap. 9. . The state of the dispersion is important in many applications Ž Lagaly, 1993. . The dispersion can be a sol, i.e., the particles form a stable colloidal dispersion, or is coagulated Žs destabilized by salts., flocculated Ž s destabilized by polymers. or thickened forming a gel. Gelation is exploited to stabilize structure or, in other cases, must be avoided to ensure ease of flow. Colloidal bentonite dispersions are only obtained in the presence of enough amounts of sodium ions, at least, amounts corresponding to the cation exchange capacity. Optimum dispersion is attained when all calcium Ž and other di- and trivalent metal ions. are removed. This requires not only cation exchange but also decomposition and separation of admixed minerals, in particular carbonates and iron oxides, cementing materials like silica, and humic compounds. Carbonates are decomposed by acids, iron oxides are reduced with dithionite and dissolved by complexation with citrate ions. Minerals like phosphates, sulphates, and silicates which act as a source of calcium ions, must be separated by fractionation. This is usually done by separating the - 2-mm fraction or even smaller fractions ŽJasmund and Lagaly, 1993, Chap. 1. . The pretreatment procedure introduced by Mehra and Jackson Ž 1960. was modified for bentonites by Stul and van Leemput Ž 1982. Ž see also Tributh and Lagaly, 1986a,b. and is the prerequisite for preparation of highly dispersed smectites. During industrial activation the raw bentonite is knealed with amounts of soda Ž Na 2 CO 3 P 10H 2 O. corresponding to the CEC or slightly more. The calcium ions are not removed but remain in the system. Only a part of the calcium ions are precipitated as CaCO 3 , a certain amount remains attached to the clay mineral. The degree of colloidal dispersion is distinctly lower than produced by more expensive pretreatments but in most cases high enough to fulfil the technical requirements. An almost complete Ca2qrNaq exchange can be performed in technical scale but is expensive and worthwhile only for special applications.

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A stable colloidal dispersion coagulates when a certain amount of salt is added. The threshold salt concentration is called critical coagulation concentration c K . Flocculation, i.e., destabilization by macromolecules is not considered in this paper. When the solid content of the dispersion is high ŽG 1%., the small aggregates of coagulated particles no longer settle independently to form a more or less dense sediment but aggregate to flocs which connect to fill space. Large amounts of the dispersion medium are enclosed in the network of the particles. This structure shows a certain stability against mechanical forces. For instance, a test tube containing such a dispersion can be turned without the dispersion flowing out. Such a system of dispersed particles is called a gel. Formation of the gel depends on the particle–particle interactions which are governed by type and concentration of the salts, on the solid content and the shape and size of the particles. The objective of the paper is to show how construction of rheological phase diagrams can help understand the behavior of clays. Such diagrams are obtained by more sophisticated rheological measurements like creeping and oscillatory experiments.

2. Materials and methods 2.1. Sodium montmorillonites Four sodium montmorillonites ŽTable 1. were prepared from the raw bentonites Žobtained from Sud-Chemie, Germany. by the dithionite-citrate treat¨ ment, oxidation with H 2 O 2 , sodium saturation, size fractionation Ž- 2 mm., dialysis and freeze-drying ŽStul and van Leemput, 1982; Tributh and Lagaly, 1986a,b.. The layer charge determined by the alkylammonium method varied between 0.28 and 0.33 Ž eqrunit. . 2.2. Sample preparation Freeze-dried sodium montmorillonites were dispersed in water Ž overnight shaking. to prepare stock dispersions. Amounts of 1 ml of these dispersions Table 1 Montmorillonites and the origin of the parent bentonites M40 M40A M48 M50

Origin

Layer charge Žeqrunit.

Wyoming, Greenbond, 1974 Wyoming, Greenbond, 1985 Milos, Greece ŽPrassa Co.., 1992 Ordu, Turkey, 1997

0.28 0.28 0.30 0.33

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were diluted with the same volume of salt solution and again equilibrated by shaking. The ionic strength of the salt solutions Ž NaCl, KCl, CsCl, CaCl 2 , Na 2 SO4 , Na 4 P2 O 7 P 10H 2 O. was varied between 0.01 and 1000 mmolrl. 2.3. Rheological measurements The most familiar, and often the only, instrument used is the Brookfield viscosimeter. This instrument is designed for Newtonian fluids but often misused for non-Newtonian systems. In more powerful devices the yield point and the viscosity were obtained from the flow curves Ž shear stress t against rate of shear g˙ .. Newtonian fluids show a linear relationship between the shear stress and the rate of shear. The viscosity of such a dispersion is the shear stressrshear rate ratio trg˙ . The apparent viscosity of a non-Newtonian fluid is the ratio trg˙ at any rate of shear. The plastic viscosity of a non-Newtonian fluid is obtained from the linear section of the flow curves at high shear rates. Extrapolation of this linear section to g˙ s 0 gives the yield value. Thixotropic and antithixotropic behavior is indicated by a hysteresis loop between the flow curves at increasing and decreasing rates ŽGuven and Pollastro, 1992. . ¨ Creeping and oscillatory experiments prove to be very useful for studies of sol–gel transitions and for characterization of gels. In creeping experiments a

Fig. 1. Creeping experiments: deformation g as a function of time t. During the time t 0 the sample is deformed by applying a constant shear stress t 0 , at t ) t 0 the sample relaxes and deformation decreases to a plateau. J Žcompliance. sg rt 0 , at t - t 0 and t G t 0 . : Deformation; — — —: shear stress; . . . . . . : deformation of a Newtonian viscous fluid Žfor comparison..

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constant stress, t s t 0 , is applied for a certain time t 0 , and the increasing strain Žrelative deformation. g is measured. When the stress is removed at t s t 0 , the deformation decreases to zero Ž elastic systems. or reaches a plateau Ž viscoelastic systems. ŽFig. 1.. The compliance J Ž t . at any time is related to the strain g by J Ž t . s g Ž t . rt 0

Ž1.

The position of the plateau gives the elastic Ž J0 q J R . and viscous Ž J N . contribution of the compliance ŽFig. 1. . In oscillatory experiments the system is subjected to an oscillating deformation. These experiments yield the complex modulus GU , the storage Želastic. modulus GX , and the loss modulus GY : GU s GX q GY

Ž2.

The magnitude of the complex modulus is < GU < s Ž GX 2 q GY 2 .

1r2

s tmrgm

Ž3.

where tm and gm are the maximum amplitudes of stress and deformation ŽBarnes et al., 1989; Macosko, 1994. . Table 2 Sequence of the rheological measurements, T s158C; CSsstress controlled, CR sstrain Žshear rate. controlled measurement Run

Type

Parameters

Results

1

Creeping experiment ŽCS., I

stress Žt ., deformation Žg .

2 3 4

Creeping experiment ŽCS., II Resting Oscillation ŽCS. amplitude sweep

5 6

Resting Oscillation ŽCS. frequency sweep

7 8

Resting Flow curve ŽCS.

M s10 mN m, Žt s 0.407 Pa., t 0 s 50 s M s 0, t s 50 s g˙ s 0, t s120 s M s 5 15 mN m, Žt f 0.2 0.6 Pa., f s1 Hz, t s10 min g˙ s 0, t s120 s M s10 mN m, Žt s 0.407 Pa., f s 0.5 5 Hz, t s15 min g˙ s 0, t s120 s M s 2=10y3 2 mN m, Žt s 0.1 80 Pa., t s120 s g˙ s 0, t s120 s g˙ s 0 1000 sy1, t s120 s g˙ s1000 0 sy1, t s120 s

9 10

Resting Flow curve ŽCR., I

11

Flow curve ŽCR., II

™ ™ ™

™

™

™

™

t, g – t, g, phase angle Ž d . – t, g, d

– t , g , g˙

– t , g , g˙

t , g , g˙

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We used the Physica UDS 200 rheometer which allowed stress or strain-controlled experiments Ž plate–plate geometry, plate diameter 5 cm, gap 0.5 mm. . The rheological properties were measured with the same sample. First, creeping experiments were performed, then oscillatory measurements and, finally, the stress-controlled ŽCS. and strain-controlled Ž CR. flow curves were measured

Fig. 2. Yield value te Ža. and plastic viscosity h Žb. of 2% Žwrw. sodium montmorillonite dispersions at NaCl concentrations 0–100 mmolrl. `: Wyoming M40; v: Wyoming M40A; B: Milos M48; I: Turkey M50.

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ŽTable 2.. Temperature was 158C. In the creeping experiments a constant stress t 0 s 0.4 Pa Žcorresponding to a momentum of 10 mN m. was applied for 50 s. The storage Ž elastic. modulus GX and the loss modulus GY obtained from oscillatory experiments are reliable values only in case the structure of the system is not disrupted during the oscillating deformation. Therefore, the variation of GU as a function of tm was first measured ŽTable 2.. In most cases the system showed the linear viscoelastic behavior up to tm f 2 Pa. In the

Fig. 3. Yield value te Ža. and plastic viscosity h Žb. of 2% sodium montmorillonite dispersions ŽTurkey, M50. in the presence of NaCl ŽB., KCl ŽI., CsCl Ž'. and CaCl 2 Ž`..

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frequency test that followed the moduli GX and GY were measured with increasing frequency Ž 0.5 y5 Hz. at t s 0.4 Pa Žmomentums 10 mN m.. The values of GX given below refer to measurements at an oscillation frequency of 1 Hz. The flow curves were recorded after the oscillatory experiments. The plastic viscosity was obtained from the linear section of the strain-controlled flow curves at g˙ 1000 sy1. Extrapolation of this linear section to g˙ s 0 made the

™

Fig. 4. Yield value te Ža. and plastic viscosity h Žb. of 2% sodium montmorillonite dispersions ŽTurkey, M50. in the presence of NaCl Žv ., Na 2 SO4 ŽI., and Na 4 P2 O 7 Ž'..

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Fig. 5. Yield value te of sodium montmorillonite dispersions ŽTurkey, M50. at different NaCl concentrations as a function of the solid content Ž2–5% wrw.. v: without NaCl; I: 2 mmolrl NaCl added Žminimum of the electroviscous effect.; ': 10 mmolrl NaCl Žat beginning aggregation..

yield value te . The degree of thixotropic or antithixotropic behavior was measured by the area of the hysteresis loop.

Fig. 6. Shear thinning of 4.5% dispersions of sodium montmorillonite ŽWyoming M40A. at different NaCl concentrations A, 1 mmolrl NaCl Žrepulsive gel.; B, 10 mmolrl NaCl Želectroviscous minimum.; C, 400 mmolrl NaCl Žattractive gel..

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3. Results 3.1. Rheological properties of the sols Sodium montmorillonite dispersions with solid contents above 1% developed a yield value te . The 2% dispersion of the four montmorillonites showed yield values around 1 Pa ŽFig. 2a.. At NaCl concentrations between 1 and 10 mmolrl the yield value of the dispersed M48 and M50 decreased to a sharp minimum,

Fig. 7. Variation of the storage modulus GX Žat 1 Hz. with the NaCl concentration. v: 2%; I: 3%; ': 4% sodium montmorillonite. Ža. Turkey M50, Žb. Wyoming M40 A.

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then rose at higher concentrations. The curves for the Wyoming montmorillonites M40 and M40A were similar, the yield values of M40A were somewhat higher and the shallow minimum was shifted to a higher salt concentration. The Žplastic. viscosity Ž Fig. 2b. changed in a similar way Žcf. Bergseth, 1963.. The Wyoming montmorillonites developed a broad minimum, and viscosity hardly changed between 1 and 100 mmolrl NaCl. ŽThe viscosity of water at 158C is 1.14 mPa.. Usually, the type of interlayer cation influences the flow behavior of the montmorillonite dispersions. This was not the case at very low salt concentrations. The decrease of the Bingham yield value te ŽFig. 3a. and the plastic viscosity h ŽFig. 3b. at ionic strengths below 2 mmolrl was independent on the type of interlayer cation. Differences arose at salt concentrations above the minimum. At an ionic strength of about 10 mmolrl, te and h increased in the order NaCl - CaCl 2 - KCl - CsCl. An effect of the anion was expected for sulphate, and, more strongly, phosphate or diphosphate Ž and other oligomeric or polymeric forms. . The yield value te did not change when chloride was replaced by sulphate Ž Fig. 4a. . Slightly different viscosities ŽFig. 4b. were developed at concentrations above the minimum; sulphate showed a weak liquefying action. In contrast, the influence of diphosphate was very strong. Very small amounts of Na 4 P2 O 7 Žionic strength - 0.5 mmolrl, corresponding to a concentration - 0.025

Fig. 8. Storage Ž GX . and loss modulus Ž GY . as a function of the oscillation frequency. 4.5% Žwrw. dispersions of sodium montmorillonite ŽWyoming M40A.. A: 1 mmolrl NaCl Ž`, v; repulsive gel.. B: 10 mmolrl NaCl ŽI, B; electroviscous minimum.. C: 400 mmolrl NaCl Ž^, '; attractive gel.. Full symbols: storage modulus GX ; open symbols: loss modulus GY.

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mmolrl. increased te and h but higher concentrations reduced these values strongly. The yield value increased with the solid content Ž Fig. 5. . The shallow minimum Žat 2 mmolrl NaCl for M50; 20 mmolrl NaCl for M40A. was also observed at higher montmorillonite concentrations. The dispersions of M50 became slightly thixotropic above 3% Ž without NaCl addition. and 4% Ž at 2 mmolrl NaCl.. At 10 mmolrl NaCl the 2–3% dispersions behaved weakly

Fig. 9. Yield value te Ža., storage modulus GX Žb. and hysteresis Žarea of the hysteresis loop. Žc. of 2% sodium montmorillonite dispersions between 10 and 1000 mmolrl NaCl. `: Wyoming M40, v: Wyoming M40A, B: Milos M48, I: Turkey, M50.

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Fig. 9 Žcontinued..

antithixotropic and changed to thixotropic at higher solid contents. The shear thinning of the gels was pronounced Ž Fig. 6. . 3.2. Viscoelastic properties The 2% dispersion of M50 showed viscous flow up to 10 mmolrl NaCl ŽFig. 7.. At 4% solid content the storage modulus was 73 Pa Ž without salt. , then decreased to a sharp minimum at 2 mmolrl NaCl Ž GX s 0.5 Pa., and rose to 200 Pa at 10 mmolrl NaCl. At salt concentrations corresponding to the minimum ŽFig. 8. linear viscoelastic behavior was not observed. The dispersions of Wyoming montmorillonite ŽM40A. showed similar changes but now extending over a broader range of concentrations ŽFig. 7.. The loss moduli also decreased to a minimum at 2 mmolrl NaCl ŽM50. and 20 mmolrl NaCl ŽM40A.. At G 3% solid content GY was distinctly smaller than GX , for instance, 4% dispersion of M50 without NaCl addition: GX s 73 Pa, GY s 25.5 Pa; for M40A: GX s 70 Pa, GY s 8 Pa. 3.3. Aggregation The interactions between the clay mineral particles become attractive at the critical coagulation concentration of about 10 mmolrl NaCl. This caused the yield value te and also the storage modulus GX to increase with the salt concentration. For 2% dispersions of M50 te rose to a high maximum Ž 24 Pa. , then decreased and disappeared at 600 mmolrl NaCl ŽFig. 9a.. The yield value

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Fig. 10. Storage modulus GX of 2% dispersions of sodium montmorillonite M50 in the presence of NaCl Žv ., KCl ŽI. and CsCl Ž'..

of the M48 dispersion reached a maximum of 14 Pa. The increase was very modest for the Wyoming montmorillonites. The storage modulus GX changed in a similar way ŽFig. 9b.. The M50 and M48 dispersions showed much stronger thixotropy than the Wyoming montmorillonites. Maximum thixotropy corresponded to the maximum of the yield value and storage modulus Ž Fig. 9. .

Fig. 11. Yield value of 2% dispersions of sodium montmorillonite M50 in the presence of NaCl Žv ., Na 2 SO4 ŽI. and Na 4 P2 O 7 Ž'..

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Addition of KCl and CsCl instead of NaCl increased the elasticity Ž Please note the different scale of GX in Fig. 9b and Fig. 10. . The decrease of te and GX was shifted to higher salt concentrations. The loss modulus changed in a similar way but the absolute values were smaller by one order of magnitude. The influence of the co-ions Ž Fig. 11. followed the trends indicated by Fig. 4. Sulphate instead of chloride decreased the yield value. A yield value was not created in the presence of diphosphate until the concentration was raised to above 600 mmolrl Na 4 P2 O 7 . The storage modulus changed in a similar way. Increasing the solid content from 2% to 4% Ž M50, 100 mmolrl NaCl. enhanced te from 24 to 100 Pa, GX from 200 to 2000 Pa, and h from 5 to 8 mPa. The thixotropic behavior increased strongly with the solid content. Two points should be noticed in Fig. 12: Ž i. The 2% dispersion showed a weak antithixotropic behavior, that is negative values of the hysteresis area, around 20 mmolrl NaCl. Žii. The thixotropy of the 3% and 4% dispersions increased to a sharp maximum at 400 mmolrl NaCl. 3.4. Sol–gel transitions Determination of the reversible Želastic. part of the compliance, Jrev s 100 Ž J0 q J R .rŽ J0 q J R q J N . Ž%. ŽFig. 1., is used to distinguish sol and gel. A sol shows Jrev s 0. We define a gel as the state with Jrev ) 0. Typical strain g vs. time curves are seen in Fig. 13a. Evidently, gel states were formed at low and high NaCl concentrations Ž Fig. 13b. . Viscous flow Žstrain increasing above 100%. was observed at the minimum of te and h at 10 mmolrl NaCl.

Fig. 12. Hysteresis Žarea of the hysteresis loops. of 2% Žv ., 3% ŽI., and 4% Ž'. sodium montmorillonite dispersions ŽTurkey, M50. in the presence of NaCl.

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Fig. 13. Ža. Strain g vs. time for 4.5% Žwrw. dispersions of sodium montmorillonite ŽWyoming, M40A. at A: 1 mmolrl NaCl Žrepulsive gel.; B: 10 mmolrl NaCl Žminimum of electroviscous effect, viscous flow, g )100%.; C: 400 mmolrl NaCl Žattractive gel.. Žb. Reversible compliance 100 Ž J0 q J R .rŽ J0 q J R q J N . Žsee Fig. 1. as a function of the NaCl concentration. 4% Žwrw. dispersion of sodium montmorillonite M50.

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Above 10 mmolrl NaCl, the reversible compliance ŽFig. 14a. increased to a plateau Ž Jrev s 60–70%.. The position of the plateau did not change at higher solid contents. The reversible part of the compliance disappeared at 500 mmolrl NaCl Ž Fig. 14b.. With KCl a plateau of 60% was reached which extended up to 1000 mmolrl KCl. Addition of 200 mmolrl CsCl increased Jrev to 90%.

Fig. 14. Reversible compliance of 2% sodium montmorillonite dispersions ŽTurkey, M50. at ionic strengths between 10 and 1000 mmolrl. Ža. NaCl Žv ., KCl ŽI., and CsCl Ž'.; Žb. NaCl Žv ., Na 2 SO4 ŽI., Na 4 P2 O 7 Ž'..

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Sulphate addition reduced Jrev to 50%, and, as expected, no reversible part of the compliance was measured in the presence of Na 4 P2 O 7 ŽFig. 14b..

4. Discussion 4.1. Sol and repulsiÕe gel In water ŽpH f 6.5. and in very diluted salt solutions Žbelow 5–10 mmolrl NaCl. sodium montmorillonite forms a stable sol of delaminated particles Ž see, e.g., Fig. 8 in Lagaly, 1989 or Fig. 3.12 in Jasmund and Lagaly, 1993. . The single silicate layers or packets of them are surrounded by the diffuse layers of counter-ions and are repelled from each other by the electrostatic forces Ž DLVO theory, see, e.g., van Olphen, 1977; Guven and Pollastro, 1992; Lagaly et al., ¨ 1997, Chaps. 2 and 3; Quirk and Marcelja, 1997.. Nevertheless, aqueous ˇ dispersions with G 1% montmorillonite showed an yield stress which decreased to a minimum at very small NaCl concentrations Ž Fig. 2a. . The viscosity dropped in a similar way ŽFig. 2b. . This behavior is caused by the Ž secondary. electroviscous effect. At particle concentrations above 1% Ž wrw. the diffuse ionic layers around the silicate layers Ž or thin packets of them. restrict the translational and rotational motion of the particles Ž cf. Fig. 4 in Permien and Lagaly, 1994a. ŽNorrish, 1954; Callaghan and Ottewill, 1974; Rand et al., 1980; Avery and Ramsay, 1986; Sohm and Tadros, 1989; Ramsay and Lindner, 1993; Mourchid et al., 1995; Lott et al., 1996; Kroon et al., 1998. . The consequence of the repulsion between the diffuse ionic layers is a certain degree of parallel orientation ŽFukushima, 1984; Ramsay et al., 1990; Mourchid et al., 1995; Tateyama et al., 1997.. The dispersion stiffens and, above 3–3.5% solid content, becomes gel-like. A viscoelastic region appears Ž Avery and Ramsay, 1986; Ramsay et al., 1990. with a storage modulus of about 70 Pa Ž 4% dispersions, Fig. 7a,b.. This type of gel is called ‘‘repulsive gel’’ Ž not correctly from a language point of view because not the gel is repulsive but the interparticle forces are. . These gels may also be considered as inorganic lyotropic liquid crystals ŽBradfield and Zocher, 1929; Buzagh, ´ 1929; Sonin, 1998.. Addition of salt reduces the extension of the diffuse ionic layer and, therefore, the yield value and viscosity. The thickness of the diffuse layer is expressed by 1rk with 1rk s

ž

´´ 0 RT 2 F 2I

1r2

/

Ž4.

where F is Faraday constant Žs 96,485 C. , ´ is dielectric permittivity, ´ 0 is the permittivity of vacuum Žs 8.85 = 10y12 As Vy1 my1 . . I s Ž 1r2. ÝÕi2 c i is the

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ionic strength Ž Õi , c i s valency and concentration of the ion i .. Thus, the thickness of the diffuse layer only depends on the ionic strength Ž at constant ´ and T .. The minimum values of the viscosity, the yield value, and the moduli were reached at concentrations between 2 and 50 mmolrl NaCl depending on the montmorillonite Ž Fig. 2.. Since the thin shape of the particles, i.e., the aspect ratio, is seen as a critical factor of the appearance of the electroviscous effect ŽAchachi et al., 1998., one understands that the position of the minimum changes with the type of montmorillonite. As the rheological data decrease to a minimum because of the attenuation of the electroviscous effect, the type of cations as counter-ions had no influence ŽFig. 3a,b.. Even divalent ions like calcium made no exception; the values were determined by the ionic strength only. This was also the case when chloride was replaced by sulphate ŽFig. 4a,b.. In contrast, addition of traces of diphosphate increased the yield value and viscosity before these values fell off at slightly higher concentrations. An increase of the density of the negative edge charges by diphosphate adsorption strengthens the restriction of the particle motion and, as a consequence, the yield value and the viscosity are enhanced. Further addition of diphosphate then reduces the electroviscous effect by compression of the diffuse ionic layer, and te and h drop to low values. 4.2. Aggregation and ‘‘attractiÕe’’ gel The critical concentration of sodium chloride needed to coagulate a montmorillonite dispersion Žsolid contents F 0.1%. is very small, about 5–15 mmolrl at pH s 5–7 Žsee Table 1 of Permien and Lagaly, 1994b. and, evidently, not much dependent on the origin of the montmorillonite. Still distinctly smaller are the critical concentrations of calcium chloride Ž 0.4 mmolrl. and aluminum chloride Ž0.08 mmolrl. ŽPenner and Lagaly, 1999. . The type of anion can strongly influence the coagulation process. Oxoanions like sulphate and phosphate rise the critical coagulation concentration Ž Penner, 1998.. The presence of 10y2 mmolrl Na 4 P2 O 7 increased the critical sodium chloride concentration of a 0.025% dispersion of sodium montmorillonite ŽWyoming. from 0.013 molrl to 0.4 molrl; for beidellite ŽUnterrupsroth. from 0.007 to 0.3 molrl ŽFrey and Lagaly, 1979.. Even addition of 10y4 molrl Na 4 P2 O 7 to a 0.025% dispersion of sodium montmorillonite ŽWyoming. increased c K to 0.2 molrl NaCl ŽPermien and Lagaly, 1994b. . The relatively low coagulation concentrations of sodium montmorillonite dispersions compared to other colloidal systems Ž Lagaly et al., 1997, Chap. 3. were first attributed to edgeŽq.rfaceŽy. contacts and formation of cardhouse structures Ž Hofmann and Hausdorf, 1945; Hofmann et al., 1957; van Olphen, 1977; Khandal and Tadros, 1988; Sohm and Tadros, 1989. . However, the edge charge density of montmorillonite particles at pH ) 6 is low and negative

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ŽPeigneur et al., 1975; Anderson and Sposito, 1991. . The relation between the interlayer exchange capacity Ž as determined by the alkylammonium method. and the total exchange capacity shows that a considerable part of exchangeable cations reside at the edges Ž Lagaly, 1981; Lagaly, 1994. . The diffuse layer extending from the basal plane surfaces of the particles spills over into the edge region and can dominate low positive edge charge densities ŽSecor and Radke, 1985; Chang and Sposito, 1994, 1996; Lott et al., 1996. . It follows that salt coagulation of sodium montmorillonite occurs by edgeŽ y.rfaceŽy. contacts ŽPermien and Lagaly, 1994b.. Because of the low charge density at the edges the c K value is very small. In addition, the repulsion between an edge and a face will be smaller than between two faces even if the charge densities are the same ŽPierre, 1992; see also Permien and Lagaly, 1994b.. The repulsive forces are very dependent on the surface potential if this value is low but almost independent at high surface potentials Ž cf. for instance Lagaly, 1986; Permien and Lagaly, 1994b; Lagaly et al., 1997, Chap. 3.3. . A weak increase of the edge surface potential by phosphate adsorption then strongly increases the repulsive force and the critical coagulation concentration. One may be surprised that the increase of the edge charge density by phosphate adsorption can raise the critical coagulation concentration to values as high as several hundreds mmolrl NaCl. Because of the larger contact area between two approaching faces compared with the area between an edge and a face, faceŽy.rfaceŽy. coagulation is promoted when, after phosphate adsorption, the edgeŽy.rfaceŽy. coagulation concentration approximates the value for faceŽy.rfaceŽy. coagulation. Transition from edge-to-face into face-to-face coagulation, in particular at higher solid contents, is promoted by the following effect. When, at the critical salt concentration, the contact between an edgeŽy. and a faceŽy. is attractive, the force depends on the angle of inclination between the two particles and on the thickness of the particles ŽTateyama et al., 1988.. In the case of delaminated montmorillonite particles this interaction is strong enough only when the two silicate layers are in perpendicular orientation. As long as the interaction between the faces is repulsive, the tendency of particles to assume the parallel orientation disrupts these edgeŽy.rfaceŽy. contacts and coagulation occurs when the facerface coagulation condition is reached. Aggregation by NaCl was accompanied by an increasing yield value ŽFigs. 2a, 5 and 9a., viscosity ŽFig. 2b., and storage moduli ŽFigs. 7a,b and 9b.. The flocs connect to fill space and a three-dimensional network of particles forms. Because the particle–particle interaction is attractive, these gels are called ‘‘attractive’’ gels. At low salt concentration the interaction is attractive between edgesŽy. and facesŽy.; at higher salt concentration, it is attractive between the faces too. There is, probably, a continuous transition from the edgerface Žcardhouse. to the facerface structure Žband-type structure.. Keren Ž1989. stressed the impor-

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tance of facerface contacts and discussed that the gel strength could be enhanced by contacts between surface regions of low or even zero charge density which may occur as a consequence of the layer charge heterogeneity. If the forces between the faces become strongly attractive at high salt concentrations, the network is contracted and disrupted, distinct particles form, and the yield value and storage moduli decrease steeply ŽFig. 9a,b.. These values, therefore, show maxima which also correspond to maximum thixotropy. Aggregation of the layers into particles at high salt concentrations was observed by small-angle X-ray scattering Ž Faisandier et al., 1998. . Presently, it is unclear why the maximum values of te and GX of the four montmorillonites are so different Ž Fig. 9a,b.. Different particle sizes may be one reason. A network of smaller particles better resists to mechanical forces than one of larger particles as in the case of the Wyoming montmorillonites. The charge density of the edges and the distribution of the layer charges should also be of influence. Potassium and cesium ions are enriched in the Stern layer to a higher level than sodium ions ŽSchramm and Kwak, 1982; Chan et al., 1984; Bailey et al., 1994.. The electrostatic repulsion is reduced, the interparticle contacts are strengthened and the modulus GX increases ŽFig. 10.. Sulphate ions did not effect the yield value and viscosity at low salt contents ŽFig. 4a,b. but reduced the yield value at higher salt concentrations Ž Fig. 11. , the decrease of the viscosity was modest. Phosphate addition eliminated the yield value. The cause is the increased negative edge charge density by adsorbed diphosphates ions Ž as described above. with the consequence of eliminating the attractive edgeŽy.rfaceŽy. contacts. The beginning increase of the modulus GX at about 60 mmolrl Na 4 P2 O 7 Žs 240 mmolrl Naq. ŽFig. 11. may be considered as the onset of faceŽy.rfaceŽy. attraction. This value corresponds to the critical coagulation concentration of NaCl in the presence of Na 4 P2 O 7 ŽG 200 mmolrl Naq.. A slight increase of the edge charge density by adsorbed sulphate ions also reduced the yield value. However, the weak adsorption of sulphate ions did not change the elasticity of the network, both moduli had the same values as in the presence of NaCl, but the reversible compliance was reduced Ž Fig. 14b.. Wendelbo and Rosenqvist Ž 1987. assumed that, as a consequence of acid rain, adsorption of sulphate ions on the edges of clay minerals could reduce the mechanical stability of a soil. 4.3. Sol–gel transitions Determination of the reversible part of the compliance at different solid contents and NaCl concentrations allows the construction of phase diagrams ŽFig. 15a,b. which show the fields of sol, repulsive gel, attractive gel, and sediment. These different states are also recognized by the flow curves Ž Fig. 16. .

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Fig. 15. Sol–gel diagram for sodium montmorillonite and NaCl. Ža. Turkey M50, Žb. Wyoming M40A.

At low salt concentrations the Ž pseudo. plastic flow curves did not show hysteresis loops. The flow at the electroviscous minimum was almost Newtonian. The attractive gel developed a yield value and, usually, thixotropic behavior Ž Fig. 12..

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Fig. 16. Flow curves Žshear stress t against rate of shear g˙ . for 4.5% Žwrw. dispersions of sodium montmorillonite ŽWyoming M40A.. A: 1 mmolrl NaCl Žrepulsive gel.; B: 10 mmolrl NaCl Želectroviscous minimum.; C: 400 mmolrl NaCl Žattractive gel..

At low solid contents but high ionic strength visual inspection revealed transition of the gel into a sedimenting dispersion. At these conditions flocs form which settle to a sediment. The storage modulus and the reversible compliance become zero, and the viscosity decreases steeply. The cause is the increased facerface attraction which contracts the network to distinct particles. These particles aggregate to flocs and settle to a sediment. When the solid content becomes high ŽG 3% for M50., the gel structure extends to salt concentrations of 1 molrl NaCl, and the system maintains the characteristic gel properties, i.e., a high storage modulus and a reversible compliance of 60–70%. The phase diagram of the sodium montmorillonite from Wyoming ŽM40A. ŽFig. 15b. also shows the four domains. The fields of the repulsive and attractive gel are clearly separated. The boundary between the sol and the attractive gel is shifted to higher NaCl concentrations. The field of flocs at 2% solid content is just detectable at the highest ionic strength. The effect of the alkali chlorides on the colloidal state is illustrated in Fig. 17. A 2% dispersion of sodium montmorillonite ŽM50. with increasing NaCl concentration changes from sol to attractive gel and then to coagulate Ž sediment. . The increased Stern-layer adsorption of potassium and cesium cations reduces the electrostatic repulsion, and the sol–gel transition is shifted to slightly smaller salt concentrations. The stronger attraction between the particles at higher KCl and CsCl concentration stabilizes the particle network which better resists to fragmentation and contraction into distinct particles; the sedimentation domain is

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Fig. 17. Salt effect on the transition sol–gel and gel–sediment. 2% dispersions of sodium montmorillonite M50, I s ionic strength.

eliminated. The influence of the increased negative edge charge density on the sol–gel transition is weak for sulphate but very marked for diphosphate and represents the well-known liquefying effect of this anion. 5. Conclusion The phase diagrams of sodium montmorillonite dispersions show distinct fields of sol, repulsive gel, attractive gel, and sediment. The gel state is characterized by the appearance of a yield value, a high storage modulus and a reversible compliance of 50–80%. Gel formation at low salt concentration and high solid content is caused by the electroviscous effect, i.e., the extended ionic double layers restrict the mobility of the particles. Salt addition reduces this effect. The attractive gel at solid contents above 1% is formed at such salt concentrations that the edgeŽy.rfaceŽy., edgeŽy.redgeŽy. interaction and, eventually, the faceŽy.rfaceŽy. interaction become attractive. At lower solid contents and high salt concentration the interparticle forces are very strong and the network of silicate layers is condensed, distinct particles form which settle to a sediment. The borderline between gel and sediment depends on the type of counter-ion and co-ion. The phase diagrams are fingerprints of the bentonites. References Achachi, Y., Nakaishi, K., Tamaki, M., 1998. Viscosity of a dilute suspension of sodium montmorillonite in a electrostatically stable condition. J. Colloid Interface Sci. 198, 100–105. Anderson, S.J., Sposito, G., 1991. Cesium adsorption method for measuring accessible structural surface charge. Soil Sci. Soc. Am. J. 55, 1569–1576.

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