The rheological and colloidal properties of bentonite dispersions in the presence of organic compounds IV. Sodium montmorillonite and acids

E LS EV I ER

Applied Clay Science9 (1994) 251-263

The rheological and colloidal properties of bentonite dispersions in the presence of organic compounds IV. Sodium montmorillonite and acids T. Permien, G. Lagaly * Institute of lnorganic Chemistry, Kiel University, Olshausenstrafle 40, D-24098 Kiel, Germany Received 29 March 1994;accepted after revision 1 July 1994

Abstract Addition of acids to sodium montmorillonitedispersions (2 percent w/w) in water influences the flow behaviour (shear stress T* at shear rate ~,= 94.5 s- ~and yield value To) in a characteristic way. At concentrations > 10 -5 mol/1 the flow values decrease to a minimum at 10-3-10 -2 mol/1 acid, then increase sharply. This behaviour is found for many acids, independent on their chemical nature, e.g. hydrochloric, formic, acetic, propionic, glutaric, puromellitic acid. For several aromatic acids less soluble in water (benzoic, salicylic, o-nitrobenzoic, phthalic acid) the flow behaviour could only be measured up to the minimum of the shear stress. In the presence of oxalic and malonic acid the shear stress and yield value remain low up to concentrations of 10- t mol/1. The flow at low acid concentrations (up to the minimum) is determined by the secondary electroviscous effect. Protonation of the edges at higher acid concentrations and formation of edge( + ) / face( - ) card-houses steeply increase viscosity and yield value. Oxalic and malonic acid, the anions of which chelate the aluminum ions at the edges, impede the increase of the flow values. The possible effect of aluminum (and magnesium) ions released from the structure by the attack of the diluted acids is discussed. The aluminum ions are not the decisive factor for the increase of the shear stress. However, at higher concentrations of aluminum ions in more strongly acidic solutions (pH < 3) the card-house is disintegrated and the shear stress decreases sharply. The acids influence the salt (NaC1) stability of the sodium montmorillonite dispersions. Acids which are strong enough to protonate the edges decrease the critical coagulation concentrationbelow 8 mM. Oxalic acid increases this concentration to 39 mM.

1. Introduction One expects that the flow behaviour of montmorillonite is dramatically changed when the clay is contacted with acids. A practical aspect is the behaviour of bentonite in clay liners when the milieu is acidic in the initial steps. * Correspondingauthor. 0169-1317/94/$07.00 © 1994 ElsevierScienceB.V. All fights reserved SSDI0169-13 17(94)00023-9

252

T. Permien, G. Lagaly /Applied Clay Science 9 (1994) 251-263

Protons react in several ways with clay minerals: (i) Adsorption of sufficient amounts of protons at the edges creates positive edge charges and promotes formation of edge( + )/face( - ) contacts and card - house type networks. (ii) The protons replace exchangeable counter-ions in the double layer of the clay particles. They are predominantly adsorbed in the Stern layer (Barshad, 1969). The special role of protons is expressed by surface force measurements between mica surfaces which reveal the absence of structural hydration forces in the presence of protons (Quirk, 1986; Pashley and Quirk, 1989). (iii) The clay minerals are slowly decomposed in acidic medium. The initial step involves the dissolution of the octahedral cations, and, as long as the clay minerals are not too seriously attacked, the enrichment of these cations in the Stern layer (Schwertmann, 1969). Under rude technical conditions bentonites are transformed into bleaching earths. In all cases the interaction with protons reduces the effective negative charge of the particles and the electrostatic repulsion between the particles. Acids not only influence the flow behaviour of clay dispersions by donating protons. They also act as electrolytes (salts) and can change the particle-particle interaction when they (or their anions) are adsorbed on the clay mineral particles. Adsorption of organic acids onto montmorillonite is feasible but weak. Besides electrostatic interactions complex formation between the anion and aluminum ions at the edges can play a significant role (Siffert and Espinasse, 1980). The true amount of acid or acid anion adsorbed is difficult to measure because most of the acid added to a clay is consumed by the attack on the clay mineral structure (Siffert and Espinasse, 1980), A principal difficulty in measuring adsorption of anions is the volume exclusion effect (negative adsorption). The anions as co-ions are repelled from the negative charges of the clay mineral particles (Bolt and Warkentin, 1958; Chan et al., 1984; Quirk and Murray, 1991 ). Therefore, the concentration changes of anions measured in the solution when a clay is brought into contact with salts or acids is the difference of the positive adsorption and the anion exclusion. A further effect must be noted. Many anions are precipitated as insoluble salts when other cations then alkali metal ions are bound at the exchangeable sites.

2. Experimental methods Bentonite from Wyoming (Greenbond, M40) was purified by dissolution of iron oxides with sodium dithionite and sodium citrate solutions and oxidation of organic matter with hydrogen peroxide. The sodium montmorillonite was separated as the < 2/xm fraction of the purified bentonite (Stul and Van Leemput, 1982; Tributh and Lagaly, 1986; Samii and Lagaly, 1987). The sodium montmorillonite was dispersed in water (somewhat above 2 percent w/w) and shaken intensively for 12 hours, then ultrasonicated (38 W, 3 min) and again shaken for 24 hours. Known volumes of these dispersions were mixed with known volumes of diluted acids (Table 1) so that the final dispersion had the desired acid concentration ( 10- 6_ 10 -t or 10° mol/l) and a solid content of 2 percent (w/w). These dispersions were again shaken for 18 hours and immediately measured in the viscosimeter. The measurements were repeated 120 hours later to realize the influence of time-dependent changes.

T. Permien, G. Lagaly /Applied Clay Science 9 (1994)251-263

253

Table 1 Acidity (pKa values) of the organic acids used Acid

pKa

Acid

pKa

oxalic o-nitrobenzoic malonic phthalic salicylic formic

1.46 2.17 2.83 2.96 3.00 3.77

benzoic glutaric pimelic acetic propionic

4.22 4.33 4.47 4.76 4.88

Table 2 The critical coagulationconcentrationof sodiumchloride, CK,in the presence of 5 × 10 - 3 mol/l acid Acid

CK (mmo 1/1)

pH

Acid

CK (mmo 1/I)

pH

hydrochloric puromellitic formic salicylic benzoic glutaric

0.5 0.5 3.5 6.5 8.0 8.0

2.0 2.0 3.0 3.5 3.5 3.5

pimelic water propionic malonic oxalic salicylate

8.0 8.0 13.0 20.0 39.0 41.0

3.5 6.5 4.0 3.0 2.0 6.0

The pH of the dispersions was measured with colourimetric test papers because even visually clear supernatant liquids obtained by centrifugation did not give reproducible results with glass electrodes (Keller and Matlack, 1990). The flow behaviour of the dispersions was measured in a Couette type low-shear viscosimeter (Contraves, Zurich) at 20°C. The non-Newtonian flow of the dispersions is described by the shear stress ~'* at a shear rate ~,= 94.5 s-1 and the yield value To. This value was obtained by extrapolating the linear or almost linear section of the flow curves at the highest shear rates ( ~ 120 s -1) to ~ = 0 . As the shear stress T* of the dispersion medium is constant over the concentration range of the acids, the changes of ~'* describe the changes of the particle-particle interaction. The salt stability of the acidic colloidal sodium montmorillonite dispersions is defined as the critical sodium chloride concentration, cK, required to coagulate the dispersion in the presence of 5 × 10- 3 mol/l acid. The cK values were determined by test tube tests at room temperature (Permien and Lagaly, 1994c). The solid content of the dispersions was 0.025 percent ( w / w ) . The pH value at the point of coagulation is given in Table 2.

3. Results Addition of hydrochloric acid and short chain fatty acids influences the flow properties above 10 -3 mol/l (Fig. 1). The shear stress and the yield value decrease to a shallow minimum at about 10 -2 mol/1, then increase steeply. Hydrochloric acid and the three fatty acids only differ in the concentrations where the steep increase starts. This is a consequence

254

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263 3300 3000 2700

2400 2100 ~o~

1800 1500

"~

1200

.~

600

I~

300 0

900

........

i

10-6

...... 2

,a

10-5

........ 2

i

10-4

........ 2

i

10-3

........ 2

i

10~2

........ 2

i

........ 2

10-1

,

100

2500

o

2000

.EE 1500 O >

2

1000

7,

-o .m

>~

500

2 10-6

2 10-5

2 10-4

2 10-3

acid c o n c e n t r a t i o n

2 10-2

10-1

100

(mote/L)

Fig. 1. Shear stress (at @= 94.5 s L) and yield value of sodium montmorillonitedispersions (2 percent w/w) in the presence of hydrochloric ( 1). formic (2), acetic (3) and propionic (4) acid. of the different pH values produced by the differently strong acids (pK~ values see Table 1 ). The curves almost superimpose when the shear stress is plotted against the pH value (Fig. 2a). Dicarboxylic acids H O O C - ( C H 2 ) , - C O O H behave in a similar way. The steep increase beyond the minimum is clearly seen for glutaric acid (n = 3) and probably occurs for pimelic acid (n = 5). Due to the lower solubility of this acid the measurement could only be extended to 2 × 10 -2 mol/1. For the shorter chain acids (malonic acid, n = 1, and oxalic acid, n = 0) the steep increase is absent (or would occur at much higher concentrations). The different behaviour of oxalic acid is clearly seen in Fig. 2. The shear stress remains low down to pH = 1 but increases for glutaric acid and the three short chain fatty acids. The aromatic carboxylic acids exert the same influence as the acids mentioned before (Fig. 4). A minimum and a very steep increase at 10 -3 m o l / l are observed for benzene

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

255

3300 3000

O n

3

2700

S

(a)

2400

1

21 O0

to to

2

1 800

i._

1 500

03

1 200

k_

O

900

rto

600 300 I

0

6

,

I

,

I

5

,

I

4

,

I

3

i

2

pH 2000

(b)

O

O..

e

to to O) I..

to k_

O

40~o ~ . 6

1200

.IS tO

7

5

4

5~ ½

'

I

pH Fig. 2. Shear stress (at @=94.5 s - ' ) of the sodium montmorillonite dispersion versus the pH value of the dispersion. (a) 1 = hydrochloric, 2 = formic, 3 = acetic, 4 = propionic acid. (b) 5 = oxalic, 6 = glutaric acid.

tetracarboxylic acid. Only the low concentration branch of the minimum is seen for the other, less soluble acids.

¢OOH

The aromatic carboxylic acids

COOH

NO 2

benzoic acid

salicylic acid

o-nitrobenzoic acid

256

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

2000 1800

no

1600 1400

In 03 (D t-,4--'

1200

03

800

I,_

600

(3 c" 03

3

1000

400 200 0

........

i

10 - 6

........ 2

1

I 0 -5

........ 2

i

1 0 -4,

........ 2

i

10 -3

........ 2

,

I0 -2

........ 2

I0

-I

1000

900

/

8OO 0 rl

700

'~

600

D

500

>

_

4 0 0

300

2O0 >"

IOO o

. . . . . . . .

J

. . . . . .

..i

2 10 -6

.

.......i

2 1 0 -5

.

. . . . . . .

2 1 0 -4

-.

2 1 0 -3

,

2 10 -2

•

10-I

acid concentration (mote/L) Fig. 3. Shear stress (at ~= 94.5 s ~) and yield value of the sodium montmorillonite dispersion in the presence of dicarboxylic acids. 1 = oxalic, 2 = malonic, 3 = glutaric, 4 = pimelic acid.

phthalic acid

benzene-l, 2,4,5-tetracarboxylic acid (puromel litic acid)

The dispersions were measured 18 hours after the addition of the acid but the attack of the clay mineral proceeds slowly further on. For instance, a few minutes after adding hydrochloric acid (concentration in the dispersion 6 × 10 - 3 mol / 1) the pH of the dispersion is 2.0; 18 hours later 2.75 and after 5 days 3.5. In the presence of the weaker organic acids

T. Permien, G. Lagaly /Applied Clay Science 9 (1994) 251-263

257

12O0 A

1000

800

L--

L-

GO0 400

0 tI/)

200 0

i

. . . . . .

,

. . . . .

2

10 -o

iiii

f

10 -.5

i

l i t

2

...I

1 0 -4

. . . . . . . .

2

t

. . . . . .

2

10-3

400 360 320 280 "~

240

5

200 (~1

> -o

160

2

,20

80 40 i

0 I 0-0

2

10-5

2

1 0 -4

2

10-3

2

10-2

acid concentration (mote/L) Fig. 4. Shear stress (at "~= 94.5 s - 1) and yield value of the sodium montraorilionite dispersion in the presence of aromatic acids. 1 = benzoic, 2 = salicylic, 3 = o-nitrobenzoic, 4 = phthalic, 5 = puromellitic acid.

the pH remains nearly constant (e.g. for 0.1M acetic, oxalic, and glutaric acid, pH = 3.5, 1.5 and 2.7). The flow behaviour does not significantly change within several days. As shown for hydrochloric and glutaric acid (Fig. 5), the shear stress decreases somewhat at acid concentrations < 10-3 mol/l and its increase at higher concentrations becomes steeper. The same changes were observed with the other acids. A dispersion of sodium montmorillonite in water is coagulated by a small amount of sodium chloride. The critical coagulation concentration is cK = 8 mmol/1 (atpH = 6.5, room temperature, solid content 0.025 percent w/w). Many acids influence the colloid stability (Table 2). There is a group of acids (hydrochloric, puromellitic, formic and salicylic acid) which reduce the cK, other acids (propionic, malonic and oxalic acid) increase the critical NaC1 concentration.

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

258

5000 2700

Q

2400 2100; 1800 1500 1200

o n

E

900

v

600 300

tO in

0 10-6

2

10 -5

2. . . . . . . . . . 2 . . . . . . . . . .2 .

10 -4

10-3

K_

O3

2500

tO rbO

b

2000

1500

1000

500

0

........

t

.

, , ..... i

2 10-6

........

t

2 10-5

........

i

2 10-4

........

i

2 10-3

........

,

2 10-2

10-1

acid concentration (mote/L) Fig. 5. Time-dependent changes of the shear stress (at ~,= 94.5 s ~) of the sodium montmorillonite dispersion between 18 hours (O) and 5 days ( n ) . (a) Hydrochloric acid, (b) glutaric acid.

4. Discussion Low concentrations ( < 10 3 mol/l) of all acids influence the flow of the sodium montmorillonite dispersions in the same way. The values of ~'* and % decrease with increasing acid concentration and reveal that contacts between protonated edges and the negative faces [edge( + )/face( - ) contacts] and formation of card-house structures are not important at these acid concentrations. Rather, the curves are very similar to the curves for sodium montmorillonite dispersed in sodium chloride solutions (Fig. 6). At lower concentrations, the acids simply act as electrolytes. For the same reason the differences between hydrochloric acid, formic, acetic and propionic acid are modest when the shear stress is plotted against the pH value (Fig. 2a). The flow behaviour of sodium montmorillonite in water and sodium chloride solution was discussed before (Brandenburg and Lagaly, 1988; Permien and Lagaly, 1994a, Permien and Lagaly, 1994b). The relatively high shear stress and yield value in water which decrease

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

259

1100

1 000 900 El rl

E U'I

800 700

600 .500 400 300

01

200 1 O0

0 10-6

10 - s

10 . 4

10 -3

10 . 2

10 - t

NaCI concentration ( m o t e / L ] Fig. 6. The influence of sodium chloride on the shear stress (at y = 94.5 s - ~) (Q) and the yield value ( • ) of 2 percent (w / w) sodium montmorillonite dispersions.

with increasing electrolyte concentration are caused by the secondary electroviscous effect (Norrish, 1954; Callaghan and Ottewill, 1974; Rand et al., 1980; Gtiven, 1992). The total volume which would be occupied by the undisturbed double layers around the non-interacting particles is larger than the volume of the dispersion medium really present. The particles are under a geometrical constraint and cannot move or rotate fully independently from each other (fig. 4 in Permien and Lagaly, 1994a). The arrangement of the particles is not random but a certain parallel orientation is preferential due to the plate-like shape (Fukushima, 1984; Vali and Bachmann, 1988; Ramsay and Lindner, 1993). Mechanical force, i.e., a shear stress, is required to disarrange the particles, and the dispersion gains a certain viscosity and yield value. Increasing salt concentration decreases the thickness of the diffuse ionic layers. The geometrical constraint is alleviated, the shear stress is diminished and the yield value is reduced or disappears. When the sodium chloride concentration is further enhanced (to about 0.1 mol/1), the interaction between the faces of the clay mineral particles becomes attractive and ~-* and % increase sharply. At hydrochloric and formic acid concentrations between 10-3 and 10-2 mol/l the interaction between the faces is still repulsive. The thickening of the dispersion results from the interaction between the protonated edges and the negative faces and, very likely, formation of card-house structures. Because of the lower dissociation constant (higher pKa) of acetic and propionic acid compared to hydrochloric and formic acid (Table 1) the steep increase is shifted to higher total concentrations of these acids (Fig. 1). Fig. 2 shows that pH < 4 is required for the stiffening of the dispersion. Rand et al. (1980) also reported a strong increase of the extrapolated shear stress of a sodium montmorillonite dispersion (0.74 percent w/w) in 10-3 mol/l NaCI at pH < 4 and concluded that, above pH = 4, there is no evidence for edge-face coagulation (see also Chen et al., 1990). The strong increase of the shear stress is also seen in the paper reported a few years ago (Brandenburg and Lagaly, 1988). Due to the different experimental conditions, in particular the long-lasting pH adjustment, the increase occurs at a somewhat higher pH value.

T. Permien, G. Lagaly /Applied Clay Science 9 (1994)251-263

260

3000 A

2700

E

2400 2100

03 03 (1)

1800

U3

1200

L o O) ¢-.

600

03

300

1500

900

0 10-6

.

2

.

.

.

,3

.

.

.

.

i

10-5

,

,

2

3

,

,

....

r

1 0 -4

,

,

2

2,

,

,

....

l

1 0 -3

,

,

2

3

aluminum concentration (mole/L) Fig. 7. Influence of aluminum ions on the shear stress of the montmorillonite dispersion. 1 = without salt, 2 = with 2 × 10 -3 mol/1 sodium chloride, 3 = w i t h 10 -3 mol/I sodium oxalate.

The attack of the clay mineral particles at higher acid concentration leads to the liberation of octahedral cations. The aluminum ions should have the strongest effect on the colloidal stability and flow behaviour. Aluminum ions as trivalent cations or in form of their oligomeric and polymeric complexes are highly effective coagulating agents. Aluminum concentrations in the dispersion medium of about 10-55-10 -4 mol/1 (rule of Schulze-Hardy, see also Matijevic, 1973) make the particle-particle interaction attractive. Even if most of the aluminum ions released are adsorbed in the Stern layer on the particle faces, the consequence is the same: the adsorbed aluminum ions strongly decrease the Stern potential, the electrostatic repulsion between the faces is reduced and coagulation is promoted. When the clay dispersion does not consist of single, isolated silicate layers (as in well-prepared sodium montmorillonite dispersions at low solid content), but contains aggregated layers or even discrete particles, a part of the aluminum ions occupy exchangeable sites in the interlayer space and do not contribute to the particle--particle interaction. (This is the cause why H + treated montmorillonite dispersions show temporary stability (Barshad, 1969; Schwertmann, 1969). ) The effect of aluminum ions added is clearly seen in Fig. 7. As expected, the shear stress increases strongly at aluminum concentrations > 10 _ 4 mol/l. Hydrated aluminum ions ( [ AI (OH2) 6] 3÷ ) are proton donators (pKa = 4.8), and the acidity of the solutions corresponds to that of acetic acid (pK, = 4.75). Thus, stiffening of the dispersion by aluminum ions may also be a consequence of the acidity, that is, of the interaction between protonated edges and negative faces. However, the increase of the shear stress occurs at a much lower concentration of aluminum ions ( 10-4-10 - 3 mol/1) than of acetic acid ( > 10- ~ mol/1). Magnesium ions play a similar role but, as divalent cations, are not as effective as aluminum ions or their polymeric species. Chen et al. (1990) discuss the release of magnesium ions by sodium chloride concentrations above 0.02 mol/l and their influence on the flow behaviour within the pH range of 4-10. It is difficult to evaluate to what extent the multivalent cations contribute to the steep increase of the flow parameters. Aluminum ions adsorbed at the faces reduce the repulsion

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

261

between the faces. As the face/face repulsion counteracts the attraction between positive edges and the faces, small amounts of aluminum ions enhance the mechanical stability of the edge( + )/face( - ) network. Higher concentrations of aluminum ions make the face/ face interaction attractive and destabilize the card-house. The shear stress decreases very sharply in strongly acidic medium (fig. 4 in Brandenburg and Lagaly, 1988). This process is assisted by the protons which act in a similar way. Oxalic acid is a much stronger acid than glutaric or pimelic acid (Table 1) and formation of the card-house structure and stiffening of the dispersion is expected at a lower concentration - - which is not observed (Fig. 3). The shear stress ~-" even remains low down to pH = 1.5. Oxalate ions are chelating ligands for aluminum ions. As mentioned above, complex formation of oxalate ions with edge aluminum ions plays an important role (Siffert and Espinasse, 1980). Adsorption of oxalate formed by incomplete oxidation of organic matter in soils was recognized in the IR spectrum (Farmer and Mitchell, 1963). Adsorbed oxalate ions diminuish the positive edge charge density or recharge the edges so that edge( + ) / face( - ) contacts are no longer formed. Even in acidic medium the oxalate anion and not the acid form is the chelating ligand: ÷

\ I / OH 2 f~lt\OH:+

:~+ C204.

---

\ I /Oxc.,PO At\ ,

/I

o--C.-o

-,- 21"!20

OC

\2t °"'

/ I\OH

'

"C20+ .,. HaO

+

---

"°'c,.°/I

\o"C'~o

.,.

Oxalate ions also chelate aluminum ions in the dispersion medium and alleviate their effect on the flow behaviour (Fig. 7). Malonic acid seems to act in a similar way. Longer chain dicarboxylic acids are poorer complexing agents for aluminum ions and are expected to be distinctly less adsorbed than oxalic acid. They act as proton donators which create positive edge charges and edge( + ) / face( - ) networks. The time-dependent changes (Fig. 5) related to the slow attack of the clay mineral by the diluted acids reveal the influence of the released aluminum (and magnesium ions). These ions decrease the thickness of the diffuse ionic layer and reduce the influence of the electroviscous effect. At higher acid concentration the mechanical stability of the edge ( + ) / face( - ) network is somewhat enhanced in spite of the increase of the pH value. As discussed above, the reduced face/face repulsion stabilizes the edge( + ) / f a c e ( - ) network, at least as long as the aluminum concentration is not high enough to break down the card-house. Sodium montmorillonite dispersions in water at pH > 6 are coagulated by sodium chloride when, at the critical coagulation concentration, the interaction between the negatively charged edges and the faces (i.e. e d g e ( - ) / f a c e ( - ) contacts) becomes attractive (Permien and Lagaly, 1994c). Addition of acids which recharge the edges from negative to positive reduces the critical coagulation concentration because edge( + )/face( - ) contacts

262

T. Permien. G. Lagaly / Applied Clay Science 9 (1994) 251-263

are formed. This is found for the stronger acids (hydrochloric, puromellitic, salicylic and formic acid) (Table 2). The pH value at the coagulation point of most acids is < 3.5 but several of the weaker acids do not influence the cK value in spite of the low dispersion pH. The conclusion is that the Hammett surface acidity Ho of the edges is somewhat below Ho = 4.2 ( = pKa of benzoic acid). Titration of dried sodium montmoritlonite with Hammett colour indicators (in benzene!) gives 0.03 mol/g sites with Ho ranging from + 3.3 to + 1.5 and 0.01 mmol/g for Ho from + 1.5 to - 3 . 0 . Only a few sites are very strongly acidic (/4o < - 5 . 6 ) (Benesi, 1957; Salomon and Hawthorne, 1983). The adsorption of oxalate (and probably also of malonate ions) increase the density of the negative edge charges (see scheme above) and the e d g e ( - ) / f a c e ( - ) coagulation requires higher salt concentrations. Protonation of the edges and complexation of edge aluminum ions are two competing processes. Binding of oxalate ions onto the aluminum ions is strong enough to compete successfully with protonation so that even at pH = 2 of the dispersion medium the critical coagulation concentration for sodium chloride is distinctly increased. Salicylic acid may also act as chelating ligand (Huang and Keller, 1972) but protonation is the dominant mechanism because the critical coagulation concentration in the presence of this acid is below 8 mmol/l. An increase of cK only occurs when, by addition of salicylate ions, protonation of the edges is impeded by the higher pH. The cK value (slightly above 8 mmol/l) in the presence of propionic acid is unexpected. The weak acid does not protonate and recharge the edges so that coagulation will occur by edge( - )/face( - ) contacts. One may suppose that the acid like the anionic surfactants causes a modest increase of the Stern potential (Permien and Lagaly, 1994d). As the electrostatic repulsion is highly dependent on the potential at small Stern potentials, even modest changes of the Stern layer adsorption influence the salt stability.

5. Conclusion

The effect of acid addition to sodium montmorilionite dispersions is the same for very different acids. The shear stress and yield value decrease to a minimum at acid concentrations between 10 -3 and 10 2 mol/1, then increase steeply. A few acids which can chelate the aluminum ions at the edges reduce the flow parameters at higher concentrations.

References

Barshad, 1., 1969. Preparationof H saturated montmorillonite.Soil Sci., 108: 38~12. Benesi, H.A., 1957. Acidity of catalyst surfaces. II. Aminetitration using Hammettindicators. J. Phys. Chem., 6 t: 970-973. Bolt, G.H. and Warkentin, B.P., 1958. The negative adsorption of anions by clay suspensions. KolloidZ. Z. Polymere, 156: 41-46. Brandenburg, U. and Lagaly.G., 1988.Rheologicalpropertiesof sodiummontmorillonitedispersions.Appl.Clay Sci., 3: 263-279. Callaghan, I.C. and Ottewill,R.H., 1974. lnterparticleforcesin montmorillonitegels. Disc. FaradaySoc., 57:110118.

T. Permien, G. Lagaly / Applied Clay Science 9 (1994) 251-263

263

Chart, D.Y.C., Pashley, R.M. and Quirk, J.P., 1984. Surface potentials derived from co-ion exclusion measurements on homoionic montmorillonite and illite. Clays Clay Miner., 32: 131-138. Chen, J.S., Cushman, J.H. and Low, P.F., 1990. Rheological behavior of Na-montmorillonite suspensions at low electrolyte concentration. Clays Clay Miner., 38: 57-62. Farmer, V.C. and Mitchell, B.D., 1963. Occurence of oxalates in soil clays following hydrogen peroxide treatment. Soil Sci., 96: 221-229. Fukushima, Y., 1984. X-ray diffraction study of aqueous montmorillonite emulsions (correctly: dispersions!). Clays Clay Miner., 32: 320-326. Giiven, N., 1992. Rheological aspects of aqueous smectite suspensions. In: N. Giiven and R.M. Pollastro (Editors), CMS Workshop Lectures, 4. Boulder, Colorado, pp. 81-126. Huang, W.H. and Keller, W.D., 1972. Geochemical mechanisms for the dissolution, transport, and deposition of aluminum in the zone of weathering. Clays Clay Miner., 20: 69-74. Keller, W.D. and Matlack, K., 1990. The pH of clay suspensions in the field and laboratory, and methods of measurement of their pH. Appl. Clay Sci., 5: 123-133. Matijevic, E., 1973. Colloid stability and complex chemistry. J. Colloid Interface Sci., 43: 217-245. Norrish, K., 1954. The swelling of montmorillonite. Disc. Farad. Soc., 18:120--134. Pashley, R.M. and Quirk, J.P., 1989. Ion exchange and interparticle forces between clay surface. Soil Sci. Soc. Am J., 53: 1660-1667. Permien, T. and Laguly, G., 1994a. The rheological and colloidal properties of bentonites dispersions in the presence of organic compounds. I. Flow behaviour of sodium montmorillouite in water-alcohol. Clay Miner., in press. Permien, T. and Lagaly, G., 1994b. The rheological and colloidal properties of bentonites dispersions in the presence of organic compounds. II. Flow behaviour of Wyoming bentonite in water-alcohol. Clay Miner., in press. Permien, T. and Lagaly, G., 1994c. The rheological and colloidal properties of bentonites dispersions in the presence of organic compounds. IlI. The effect of alcohols on the coagulation of sodium montmorillonite. Colloid Polymer Sci., submitted. Permien, T. and Lagaly, G., 1994d. The theological and colloidal properties of bentonites dispersions in the presence of organic compounds. V. Influence of surfactants on the flow hehaviour and salt stability, Clays Clay Miner., submitted. Quirk, J.P., 1986. Soil permeability in relation to sodicity and salinity. Philos. Trans. R. Soc. London, A 316: 297317. Quirk, J.P. and Murray, R.S., 1991. Towards a model for soil structural behaviour. Aust. J. Soil Res., 29: 829867. Ramsay, J.D.F. and Lindner, P., 1993. Small-angle neutron scattering investigations of the structure of thixotropic dispersions of smectite clay colloid. J. Chem. Soc. Farad. Trans., 89: 4207-4214. Rand, B., Pekenc, E., Goodwin, J.W. and Smith, R.W., 1980. Investigation into the existence of edge-face coagulated structures in Na-montmorilionite suspensions. J. Chem. Soc. Farad. I, 76: 225-235. Salomon, D.H. and Hawthorne, D.G., 1983. Chemistry of Pigments and Fillers. Wiley, New York, pp. 32-38. Samii, A.M. and Lagaly, G., 1987. Adsorption of nuclein bases on smectites. In: L.G, Schultz, H. van Olphen and F.A. Mumpton (Editors), Proc. Int. Clay Conf. Denver 1985. The Clay Minerals Soc., Bloomington, IN, pp. 363-369. Schwertmann, U., 1969. Aggregation of aged hydrogen clays. Proc. Internat. Clay Conf. Tokyo, 1969, Israel University Press, Jerusalem, pp. 683-690. Siffert, B. and Espinasse, P., 1980. Adsorption of organic diacids and sodium polyacrylate onto montmoriilonite. Clays Clay Miner., 28: 381-387. Stul, M.S. and Van Leemput, L., 1982. Particle-size distribution, cation exchange capacity and charge density of deferrated montmorillonite. Clay Miner., 17:209-215. Tributh, H. and Lagaly, G., 1986. Aufbereitung und Identifizierung von Boden- und Lagerst~ttentonen. GIT Fachz. Lab., 30: 524--529, 771-776. Vali, H. and Bachmann, L., 1988. Ultrastructure and flow behaviour of colloidal clay dispersions. J. Colloid Interface Sci., 126:278-291.