EFFECT OF F R E E Z I N G ON THE S W E L L I N G
OF C L A Y M I N E R A L S
By K. NORRISH and J. A. RAUSELL-COLOM
Division of Soils, C.S.I.R.O., Adelaide, Australia.
[Received 8th February, 1962]
ABSTRACT
It has been known for some time that when a gel of montmorillonite is
freeze-dried the residual clay retains the volume and shape of the gel,
and forms a skeleton of low density and some rigidity. Because there is
no apparent change in the gel structure during freeze-drying, the dry
residues have been studied in the past in an effort to determine the
structure of the gels. However, X-ray diffraction measurements have
shown that, as a result of freeze-drying, the distance between the elementary layers of Na-montmorillonite drops from high spacings (> 30/~)
to zero. The present investigation demonstrates that the main reduction in interlayer spacing takes place on freezing, with a further
small collapse on drying. Na-montmorillonite gels, swollen oriented aggregates of Na-montmorillonite and Na-nontronite, and swollen single
crystals of Li-vermiculite and butylammonium-vermiculite,all behave in
the above manner.
INTRODUCTION
Weiss, Fahn and Hofmann (1952) reported that when a thixotropic gel of Na-montmoriUonite is frozen and the water then sublimated under vacuum (freeze-dried), the solid particles of clay remain as a rigid skeleton of a very low density (0.05 g/ml), occupying
the same volume as the gel. This result was interpreted as direct
evidence of the existence of rigid structures in clay gels, thus providing an explanation of the phenomena associated with thixotropy.
Small angle X-ray diffraction measurements by Norrish (1954)
showed that when Na-montmorillonite is allowed to swell in water
or in dilute solutions of sodium chloride the elementary silicate
layers move apart to distances greater than 30 A. I f the interpretation of Weiss, Fahn and Hofmann (1952) were correct, i.e., if the
water from the swollen montmorillonite could be removed by freezedrying without altering the arrangement of the silicate sheets, there
would be practical advantages in making the X-ray diffraction
measurements on the freeze-dried residues rather than on the gels.
The intensities of diffraction should increase because of reduced
absorption and also because of the extra contrast between the clay
and its medium. Also, it should be possible to extend the diffraction
measurements to higher angles where they are normally confused by
diffraction from water.
10
K. NORRISH AND J. A. RAUSELL-COLOM
Accordingly, low-density structures without loss of volume were
obtained by freeze-drying Na-montmorillonite which had been
swollen in water and in dilute solutions of sodium chloride. When
the diffraction pattern from the freeze-dried residues, originating
from either thixotropic gels or oriented aggregates, was examined,
it was found that the inter-particle distances present in the gels
(>3fl A) were not retained but dropped to zero (basal spacings of
10 A). This was in agreement with the previous work of Call (1953)
--later confirmed by Van Olphen (1956)--who obtained surface
areas, by the standard Brunauer, Emmett and Teller (B.E.T.) method,
not differing significantly from those of the original powdered
montmorillonite. All these observations indicate that although the
gels do not change in volume on freeze-drying, there is collapse in
local regions, with the necessary development of large pores.
The aim of this investigation was to establish how the open
structure of the gel collapses to that present in the freeze-dried
residues.
EXPERIMENTAL
A few grams of bentonite from Wyoming, U.S.A., and of nontronite from Spokane, Washington, U.S.A., were saturated with
Na + by several treatments with a 2 N solution of sodium chloride,
followed by dialysis until the clay was free of C1-. The Na-saturated
clays were then diluted with distilled water to a concentration of
0.1 per cent. and passed through a Sharpies centrifuge to select the
particles of an equivalent spherical diameter of less than 0"lt,.
Thin oriented aggregates were prepared by evaporation of the gel on
a flat glass surface.
A thin flake cleaved from a crystal of vermiculite from Kenya
was cut into pieces of approximately 0-5 • 10 mm, some of which
were treated with a 1 N solution of lithium chloride and others with
a 4 per cent. solution of butylammonium chloride, until saturation
with the corresponding cations was complete.
The following specimens were mounted in thin walled plastic
tubes (0.9 mm int. diam.):
(a) a thixotropic gel (concentration 4.5 per cent.) of Na-montmorillonite;
(b) an oriented aggregate of Na-montmorillonite swollen in 0.1 N
sodium chloride solution;
(c) an oriented aggregate of Na-nontronite in 0-1 N sodium chloride
solution;
(d) a flake of Li-vermiculite in 0-1 N lithium chloride solution;
(e) a flake of butylammonium-vermiculite in water.
After sealing the ends of the tubes to prevent evaporation these
specimens were mounted in a Norelco diffractometer and, diffraction
FREEZE-DRYING OF CLAY MINERALS
11
traces were obtained over the ranges of 20 from 0.5 ~ to 2 ~ (low
angles) and from 3.5 ~ to 10 ~ (high angles). Then, while each specimen was still mounted in the diffractometer, a stream of cold carbon
dioxide was directed at it. The counter was positioned at 1~ during
the cooling process. After some 10 to 20 seconds the specimen froze
suddenly, and simultaneously there was a sharp drop in the diffracted
intensity. The diffraction pattern of the frozen specimen was then
recorded for both angular ranges. Finally, for the two specimens of
vermiculite, the flow of cold carbon dioxide was stopped and the
specimens allowed to thaw. A sudden increase in the intensity of
diffraction at 1~ was observed at the moment of thawing. The
diffraction patterns corresponding to this final stage were also
obtained.
These results are reproduced in Figs. 1 and 2; C o K a radiation
was used for all the traces. For the measurements at low angles slit
apertures were small (1/30 ~ divergent, 0.075 m m receiving), in order
to reduce the background near the direct beam. In the high angle
region the slit apertures were bigger (1 ~ divergent, 0.15 m m receiving),
and therefore the diffracted intensities are not to be compared with
those of the low angle region.
Quantitative measurements of the angular distribution of diffracting particles in each specimen were obtained from the decrease
of intensity that occurred when the specimen was rotated with the
counter kept stationary at a value of 20 where there was significant
diffraction (RauseU-Colom and Norrish, 1962). These measurements are plotted in Fig. 3 as curves of orientation, giving, for each
specimen, the number of diffracting particles at different angles from
the plane of orientation. Arbitrarily, the total number of diffracting
particles at zero degrees of rotation is made equal to unity for all
specimens.
DISCUSSION
The diffraction traces obtained before freezing (see Figs. 1 and 2)
are characteristic of swollen specimens of d a y minerals. The intensities follow the theoretical curve F2.~_~*, but there are deviations
corresponding to the distribution of interlayer distances in the gel
(Norrish, 1954). As these distances are greater thart 40/~, there are
no deviations from F2..'~
-, in the range from 3.5 ~ to 10~ F r o m 0.5 ~
to 2 ~ the diffraction effects are better defined for Li-vermiculite than
for rnontmorillonite and nontronite. Because the interlayer spacings
in the swollen butylammonium-vermiculite are very high (,~540 A,
according to Walker, 1960) sharp diffraction effects cannot be observed on that trace, as they would appear at angles smaller than
0.5 ~ where scatter from the direct beam makes observation impracticable.
*FL ~ represents the product of the square of the modulus of the structure
factor of montmorillonite (its component in the direction of c*) by the function
giving the angular correction.
12
K. NORRISH AND J. A. RAUSELL-COLOM
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HIGH A N G L E
FIG. 1--Diffraction traces from swollen montmorillonite and nontronite specimens
before freezing (
) and after freezing ( - - - - ) : A - - - 4 - 5 ~ Na-montmorillonite in water; B---oriented aggregate of Na-montmorillonite in 0.1 N NaC1
solution; C--oriented aggregate of Na-nontronite in 0.1 N NaC1 solution.
13
FREEZE-DRYING OF CLAY MINERALS
That the open structure of the swollen specimens collapses on
freezing is demonstrated by the disappearance of diffraction effects
at low angles, and by the presence of peaks in the high angle region
(Figs. 1 and 2), revealing the aggregation of the elementary silicate
sheets. The effect is reversible, as, on thawing, the gel returns to
its original state (Fig. 2).
A comparison of the orientation curves of Fig. 3 shows that, for
any one specimen, there is a decrease in orientation on freezing, and
that this effect is also reversible, the original orientation being restored on thawing.
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in 0.I N LiCI solution; E--butylammonium-verrniculite in water.
Observations with a polarizing microscope showed that the
volume of the specimens did not change on freezing. The decrease
in orientation manifested itself by the disappearance of uniform
interference eolours.
It is interesting to speculate on the mechanism of collapse of the
open structure of the gels on freezing. As stated above, no major
14
K~ NORRISH AND J. A. RAUSELL-COLOM
change appears to occur during cooling until the moment that ice is
observed to form. If it is assumed that ice crystals nucleate at
random through the gel, then, as these crystals grow, water will
migrate to their surfaces and at the same time pressures will be exerted
on the silicate layers, causing local collapse before solidification
of all water takes place. If the ice crystals are not highly anisotropic
in shape they will tend to disrupt the preferred orientation of clay
particles*. Fig. 4 shows diagrammatically the structure of the get
before and after freezing, and indicates how the Kartenhaus structures
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ROTATION OF SPECIMEN (degrees)
FIG. 3--Curves of orientation for the specimens of Figs. 1 and 2 before
freezing (
), after freezing ( - - - - ) , and after thawing ( . . . . . .
).
described by Weiss, Fahn and Hofmann (1952) form largely as a
result of freezing. After thawing, diffuse ionic double layers,
which were destroyed on freezing, will be re-established on the
silicate surfaces, and the resulting osmotic repulsion will restore the
clay to its original state.
*It is well known that moderate amounts o f such isotropic particles as q u a r t z
have a disrupting effect on the orientation of clay films, and ice crystals would be
expected to have the same effect.
FREEZE-DRYING OF CLAY MINERALS
15
NI
B
(1
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FIc, 4--Schematic representation of the structure of a clay gel: a--before freezing;
b--after freezing.
N--ice crystalization nuclei; B---edge-to-face bonds.
16
K. NORRISH AND J. A. RAUSELL-COLOM
The spacings of 20-1 A, 18/k and 15.3 A observed for the frozen
montmorillonites (Fig. 1) correspond approximately to 4, 3 and 2
layers of water molecules between the sheets. These spacings,
and the sharpness of the diffraction peaks, which reflects the number
of silicate sheets per crystal, appear to be related to the degree of
orientation in the original gels, i.e., the poorer the orientation the
larger the interlayer spacing and the fewer the number of silicate
layers per crystal. This behaviour is what would be expected if
there are edge-to-face bonds which hinder the movement of individual sheets in the swollen gel. The difference in interlayer
spacings in the frozen vermiculites (Fig. 2) is due to the presence of
the long organic ion in the butylammonium complex.
Only those clays showing interlayer swelling could be studied by
diffraction techniques. However, it seems probable that gels of
other clays would show a similar behaviour, particularly in regions
where there is preferred orientation of crystals, i.e., in the volumes
which have been termed domains (Aylmore and Quirk, 1960).
REFERENCES
AYLMORE,L. A. G., and QUIRK,J. P., 1960. Nature, Lond., 187, 1046.
CALL, F., 1953. Nature, Lond., 172, 126.
NORRtSH, K., 1954. Disc. Faraday Soc., No. 18, p. 120.
RAUSELL-COLOM,J. A., and NORRISH,K., 1962. J. sci. lnstrum., in Press.
VAN OLVHEN,H., 1956. Clays and Clay Minerals (A. Swineford, editor). Nat.
Acad. Sci.--Nat. Res. Counc., Washington, Publ. 456, p. 204.
WALKER, (~. F., 1960. Nature, Lond., 187, 312.
WEISS, A., FAHN, R., and HOFMANN,W., 1952. Naturwissenschaften, 15, 351.