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IV. Swelling of Sodium Clays

IV. Swelling of Sodium Clays

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of organic matter in conferring different degrees of stability on soil aggregates in

water. This method has been used to assess the stability of soil aggregates of the

surface soil of a red-brown earth which had experienced varying periods in

pasture and in arable phases (Greenland et al., 1962).




Norrish (1954) and Norrish and Rausell-Colom (1963) measured the X-ray

spacing of oriented flakes of Na-montmorillonite (Wyoming) and single crystals

of Li-vermiculite (Kenya) as a function of NaCl and LiCl concentration in the

absence of a restraining pressure; that is, under conditions of free swelling since

there was no applied suction to restrain the swelling.

The following relationships were obtained between the basal spacing and the

electrolyte concentration, c, for Na-montmorillonite and Li-vermiculite:

d(001) = 11.3 c-0.5





+ 21

+ 22


( c from 0.3 to

( c from 0.15 to 2





This strong dependence of swelling on electrolyte concentration is indicative

of diffuse double layer behavior and especially so as the slope of the lines is

related to c-0.5 that is K - I in diffuse layer theory. The difference between the two

minerals is not due to the saturating cation since the slope of Li- and Namontmorillonite is almost the same (Norrish, 1954).

The intercepts shown in Eqs. (18) and (19) were interpreted as arising from the

aluminosilicate layer thickness (10 A) and the two layers of water (5.5 A)

associated with each of the opposing surfaces; this is, in effect, evidence for the

existence of Stem layers.

Since K - I is 3.04 c - O . ~ for a 1:l electrolyte, the slope of 23.8 [Eq. (19)]

converts to 7.8 K - I which is the separation of the Gouy planes at a given


Norrish and Raussel-Colom (1963) also obtained X-ray spacings for a Livermiculite crystal immersed in 0.03 M LiCl as a function of applied pressure.

Their results are shown in Fig. 4 in which the half-separation of the Gouy planes

$[d(OO1)-21] varies with applied load (pressure). Also shown in the figure is the

expected pressure-distance relationship for reduced Gouy electric potentials of

2, 3, 4, and 6 . If the full crystallographic charge density of 5.8 X 104 esu cm-2

(0.193 Cm-2 or one unit charge per 83 A2) is substituted into Eq. (13) for the

concentration of 0.03 M ,the reduced electric potential calculated is 5.9, corresponding to a surface potential of - 150 mV.

Figure 4 shows that the spacing-pressure relationship is reversible. Over the






decreasing pressure

increasing pressure

1 5A spacing also present

calculated repulsive force

46 \/









h 10'










half distance between Gouy planes




Figure 4 The pressure-distance relationship as measured for Li-vermiculite in 0.03 M LiCl

(Norrish and Rausell-Colom, 1963). The increasing and decreasing pressure points show that the

relationship is reversible. The lines shown are for reduced electric potentials at the Gouy plane of 2,

3, 4, and 6. The points are close to the line Y, = 4 indicating a Gouy potential of about -90 mV.

pressure range of 0.2 to 0.0035 MPa the points are either between Y , = 3 and Y,

= 4 lines or actually on the line Yc = 4 so that -90 mV would be a reasonable

estimate of the surface potential for this Li-vermiculite. From the force-distance

curve for Li-muscovite (0.344 Cm-*), Pashley (1981b) derived a surface potential of - 110 rnV in a 0.03 M LiCl solution.

It was indicated earlier that suction or applied load acts in the same sense as an

attractive force. When the van der Waals attractive pressure is calculated, fore

the actual separations of the surfaces, over the pressure range in Fig. 4 it was

found that it contributes less than 1 % of the pressure needed to balance the

Langmuir repulsion [Eq. (13)]. The applied load is the force which balances the

osmotic swelling pressure.



Viani et af. (1983) measured the X-ray spacing of Na-montmorillonite (Wyoming) in equilibrium with

M NaCl and pressures within the range of 0.7 to

0.03 MPa and obtained results similar to those for Li-vermiculite (0.03 M LiCl),

even though their electrolyte concentration was much less. On the basis of their

measurements and conventional double-layer calculations, they decided that, as

the calculated double-layer pressures were so much smaller than the applied

suction, the repulsive pressure balancing the applied suction must be because of

water structural forces (see also Low, 1987).

The surface separations reported by Viani et al. (1983) varied from 30 to 90 A

and in this respect it is noteworthy that Pashley and Israelachvili (1984) and

McGuiggan and Pashley (1988) considered that the hydration or structural forces

were operative for separations up to 30-40 8, for K- and Na-muscovite which has

a surface density of charge about three times that of Wyoming montmorillonite.

On this basis it would appear that the influence of structural forces in Namontmorillonite is virtually expended at a separation of 33 A [d(OOl) = 43 A] at

which distance and beyond diffuse double-layer behavior governs free swelling

in NaCl solutions of decreasing concentration when no applied pressure is involved (Norrish, 1954).

Aylmore and Quirk (1962) found that at a suction of 1 MPa there was no effect

of NaCl concentration on the swelling of Na-montmorillonite cores in the overconsolidated state; that is, the swelling at that pressure is associated with the

effects of cation hydration. The solution uptake for all concentrations at 1 MPa

suction was less than that for M NaCl at 1 kPa suction when the d(OO1) value was

19 A. At a pressure of 0.1 MPa the effect of electrolyte concentration was

evident. However, the difference in water content between 10-2 M NaCl and

distilled water was hardly discernible. The swelling pressures measured by Viani

et af. (1983) were in the range of 0.03 to 0.7 MPa and were thus in the transition

region between cation hydration forces, responsible for limited crystalline swelling, and diffuse double-layer behavior. This conclusion is supported by the

presence of d(OO1) spacings of 19 A together with larger spacings at pressures of

0.3, 0.5, and 0.7 MPa in their experiments.

Since there is hardly any difference in the swelling of Na-montmorillonite

cores between 10-2 M NaCl and distilled water, the expected swelling pressure at

10-2 M NaCl for Gouy plane separations of 60 and 80 A using a reduced Gouy

potential of YG = 2.75 (-70 mV) was calculated. Pressures of 0.067 and 0.036

MPa were obtained using Eq. (13) which may be compared with the experimental values of 0.1 and 0.05 MPa observed by Viani et al. (1983) using


NaCl. Thus to a considerable extent the swelling of Na-montmorillonite at these

pressures is explicable in terms of diffuse double-layer theory.

The matter at issue here is not the presence or absence of water structural

forces but their magnitude, their range of operation, and their significance relative to DLVO theory. On the matter of relevance to soil behavior, the immediate



surface soil is subject to flooding under irrigation or during intense rain, and

furthermore the suction in the transmission zone during water entry is of the

magnitude of 1 kPa (Bodman and Colman, 1944; Marshall and Stirk, 1949). In

these circumstances the free swelling behavior of a clay [Eqs. (18) and (19)]

would be the appropriate reference and hence the behavior of sodic soils should

be considered in these terms.

For cores of Na-montmorillonite (Wyoming), Aylmore (1960) reported a solution content of almost 4.8 cm3 g-I clay in equilibrium with 0.1 M NaCl at a

suction of 1 kPa. The slope of the relationship between the X-ray spacing and

c-0.5 for montmorillonite is 11.3 [Eq. (18)] and this would indicate a separation

of Gouy planes of 36 A at 0.1 M NaCl. If 2 X 5.5 A is added for the Stem layer

on each surface the actual surface separation is 47 A. From this, using 750 m2g-I

for the interlamellar area (an overestimate), an expected solution content of 1.76

cm3 g-l clay can be calculated. The solution content values, attributable to

double-layer interactions, are appreciably less than the measured solution content. The additional solution is enmeshed in a gel structure and held by capillary

forces. By the adroit use of scanning and transmission electron microscopy,

Tessier (1990, 1994) has revealed the nature of the sponge-like structure of Namontmorillonite gels at small suctions and their collapse with increasing suction

as well as the organization of the aligned lamellae constituting the walls of the


For Na-Wyoming montmorillonite the transition from limited to extensive

crystalline swelling takes place at a concentration of about 0.3 M NaCl and the

flocculation-dispersion transition in a suspension occurs at 0.01 M NaCl. This

illustrates the difference between the primary and secondary minimum and emphasizes that before particles within a soil disperse they first have to be moved

from within the primary minimum.

Studies of the behavior of Na-smectites are important in that they provide a

basis for appreciating more fully the behavior of Ca clays and sodic soils. The

transition from limited to extensive crystalline swelling for montmorillonite (Wyoming) in dilute solutions occurs over the range of 20-60% exchangeable sodium (Shainberg and Otoh, 1968; Shainberg and Kaieserman, 1969; Bar-On et al.,

1970) and at the stage that this process has begun soil containing such a montmorillonite would have long since become intractable.

Slade et al. (1991) investigated the swelling of Na-smectites; the smectites

conformed to the definition of having a 17.6 A spacing in glycerol when Mg

saturated. The smectites examined include the following for which the total

charge and that arising from isomorphous replacement in the tetrahedral layer are

given in parentheses: Wyoming (0.74, 0.08), Otay (1.04, 0.06), Nibost (1.06,

1.02), and Drayton ( I . 14, 0.74). Neither of the beidellites (Drayton and Nibost)

exhibited extensive crystalline swelling in water and in fact their d(O01) spacings

did not exceed 16 A. Their behavior is thus more akin to vermiculites and

15 1


emphasizes the importance of the structural origin of the charge. Otay montmorillonite, with a similar total charge as the beidellites, gives a 19 8, spacing in

0.25 M NaCl at which concentration the Wyoming montmorillonite quasicrystals

have expanded to a d(OO1) value of about 43 8, (Table VI). With increased

surface density of charge the expansion of Otay quasicrystals is delayed to lower

concentrations of NaCl.



Figure 5 shows the effect of NaCl concentration on the swelling of compressed

cores (overconsolidated condition) of Na-Willalooka illite (Alymore and Quirk,

1962); this sample of Willalooka illite has a surface density charge of 0.260





0.0 !







Suction (MPa)

Figure 5 Solution content-suction relationship for Na-Willalooka illite with respect to NaCl

concentration for compressed clay cores in the overconsolidated condition. (After Aylmore and

Quirk, 1962.)



Cm-* (area, 160 m2g- I) and is not interstratified as indicated by the agreement

between the surface areas measured by nitrogen, water vapor, and cetyl pyridinium bromide adsorption (Greenland and Quirk, 1962). The solution contents

measured were the result of the interaction between crystals. The clay cores were

wet slowly in stages and Fig. 5 shows the effect of electrolyte level on wetting to

1 kPa and the subsequent effect of increased suction on the amount of solution

retained by the core. The increase in solution uptake with decreasing NaCl

concentration is in general conformity with diffuse double-layer principles; between concentrations of 0.1 and 0.01 M NaCl there is a doubling of the solution

content. At 1 kPa the distilled water uptake was 8.4 crn3 g-1 and the solution

uptake of 1 M NaCl was 0.48 cm3 g-l. This latter uptake is only a little greater

than the uptake of CaCI, solutions at the same suction, indicating, as expected,

that in 1 M NaCl the crystals are still within the potential minimum. With

increasing suction the effect of electrolyte concentration is progressively diminished, being only slightly evident at 1 MPa suction when the solutions uptake,

for all concentrations, is less than the uptake of 1 M NaCl.

Similar studies of the water uptake at I kPa suction by Na-kaolinites have

revealed that the interaction between these crystals is affected very little by

electrolyte concentration and that their swelling is restricted since the solution

uptake is the same as that of Ca-kaolinites (Aylmore and Quirk, 1966). Rocky

Gully kaolinite from the pallid zone of a laterite was the same clay as was used

by Schofield and Samson (1954). This clay, with a surface area of 36 m2g-',

increased in volume by 3 1% from the dry state to equilibrium with 1 kPa suction.

The water content of the Na-kaolinite (Rocky Gully) in distilled water was 0.36

cm3 H20 g-l. The swelling after pretreatment of the clay with Na tripolyphosphate to nullify the effect of positive edge charge was unaffected.

The dramatic effect of increased pH values on the behavior of kaolinites in

suspension (Schofield and Samson, 1954) contrasts markedly with the absence of

any effect on the swelling of the Na-kaolinite following the nullifying of the edge

charge. These results emphasize the hazard involved in extrapolating from the

behavior of suspensions to that of overconsolidated soils and clay.




The results shown in Table VI reveal that Na- and Ca-montmorillonite (Wyoming) have similar permeabilities at high concentrations of electrolyte. The

permeability of Ca-montmorillonite is sustained in distilled water but the permeability of Na-montmorillonite becomes virtually zero below a concentration of



Table VI

Permeability and X-Ray Spacing for Na- and

Ca-Montmorillonite (Wyoming) in NaCl and CaCI, Solutions


0. I M CaCI,

< 10-5 M CaCI,

I .O M NaCl

0.5 M NaCl

0.3 M NaCl


X-ray spacing

(105 cm sec-1)


I .7


I .8








0.3 M NaCl (Quirk, 1952; see also Quirk, 1968). Mering (1946) considered that

the large macroscopic swelling of Na-montmorillonite was not due to the expansion of the crystals. However, Quirk (1952) concluded that the dramatic decrease

in permeability was due to the onset of extensive crystalline swelling as a result

of the development of diffuse double layers on all the opposing surfaces. This

interpretation was confirmed by X-ray measurements (Table VI) which showed

that the basal spacing jumped from 18.9 in 0.5 M NaCl to 43 A in 0.3 M NaCl.

Furthermore, Norrish and Quirk (1954) also reported that Ca-montmorillonite

maintained a basal spacing of 19 8, even in distilled water; the lamellae of the

montmorillonite remain in a primary potential minimum and are separated by 9 A

of water or about three molecular “layers.”

The swelling (solution content) of compressed cores of Na- and Camontmorillonite (Wyoming) and Ca-illite (Willalooka) in relation to electrolyte

concentration at 1 kPa suction (Aylmore and Quirk, 1962) is shown in Table VII.

Since the material is saturated, the volume of the solution taken up is a measure

of the swelling since the initial porosities were similar. It may be noted that the

solution content of Ca-montmorillonite changes to some extent over a wide

concentration range, whereas those of the Na-montmorillonite increase markedly

as a result of the development of diffuse double layers on opposing lamellae in

dilute solution. The characteristics of the swelling of Na-illite are essentially

similar to Na-montmorillonite in that the solution content attained at a given

suction is greater the more dilute the NaCl solution used to wet the clay (Fig. 5).

Although the swelling of Na-illite and Na-montmorillonite is qualitatively similar

the solution contents attained by the illite are less than those for montmorillonite

because the surface area of the illite is much less, being about 160 m2gg1

compared with 750 m2g-1 for the montmorillonite (Table VII, Fig. 5).

The Ca-illite solution contents (Table VII) are almost constant, differing little

between a concentration of 1 M CaCl, (K-I = 1.76

and distilled water.

Clearly diffuse double-layer theory cannot be used to describe the swelling



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IV. Swelling of Sodium Clays

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