STUDIES I N COLLOIDAL CLAYS. 1' E. A. HAUSER
AND
D. S. LE BEAUZ
Department of Chemical Engineering, Massachusetts Institute Cambridge, Massachusetts
Of
Technology,
Received J u l y 1, 1938
Only in recent years has it been possible to produce for the purpose of further studies well-defined monodisperse fractions of colloidal clays by rational supercentrifugal separation (2, 3). The lack of data concerning, primarily, clay sols of low concentration and varying particle sizes made highly desirable a closer study of freshly prepared natural suspensions of the clay mineral montmorillonite (the major constituent of bentonite) in Concentrations from about 0.06 to 2 per cent by weight. Of special interest was the question whether any signs of structure could be detected in such low concentrations and, if so, how these change with particle size of the suspension and also what effect certain pretreatments of the original suspensions, as, for instance, thermal concentration a t different temperatures, would have on their properties. Changes due to the addition of electrolyte have not been considered in this investigation, but will be discussed separately in a later publication. The particular bentonite used in this work was of the Wyoming type, mined by the American Colloid Co., Chicago, Illinois. (For further details as to location of the deposit, see reference 3.) The analysis of the finest fraction of this bentonite gave the values in table 1. The different particle size fractions were produced according to a method described by Hauser and Reed (2). Six fractions were obtained. The gels produced by supercentrifuging had about 10 per cent dry weight content. They were stored in the condition in which they were scraped from the liner of the centrifuge. The overflow of the finest and final fraction was collected and stored as a sol of 0.46 per cent dry weight. It will be referred to as fraction 7 . The following experiments were carried out with fraction 6 containing particles of an average apparent diameter of 14 mp. Comparative studies Presented a t the Fifteenth Colloid Symposium, held at Cambridge, Massachusetts, June 9-11, 1938. * Present address: Dewey & Almy Chemical Co., Cambridge, Massachusetts. 1031
1032
E . A. HAUSER AXD D. S. LE BEAU
were also niadc xith fraction 4 (average apparent particle diameter = 28 n i p ) and fraction 2 (average apparent particle diameter= 48 m p ) and fraction 7 (This sol contailis only particles of an apparent diameter below 14 nip ) PREPARATION O F STANDARD SOLS
Sols varying in concentration from 0.06 to 2 per cent dry neigllt of Iientonitc 1% ere' made up from fractions 6, 4, and 2, by careful dilution of t h e gels obtained by supercentrifuging with conductivity water, to approuniately the dcaired concentration\ The exact ronceiitrationb of these di+ persioris of knon n apparent particle size were determincd grarinlctrically by eisporalion to constant weight a t 105°C
-~~
~
TABLE 1 Composition of the bentonite _ _ _ _ _ _ ~ ~ - - - - ~
~
Loss r t 105°C SlOZ FezOl
_ _ ~ ~ ~ _ _ per cenl
20 88 66 16 0 80 2 67 2 13
6 58 2 26
CaO
so
--
. S o t determinpd __
3
09 78
Finally, a dispersion of fraction 6 of only 0.077 per cent dry w i g h t T ~ produced for the purpose of studying the influence of temperature on property changes of sols obtained by concentration instead of dilution. This standard dispersion was reconcentrated by means of thermal evaporation a t 75°C. and 95°C. to approximately the same concentrations as obtained by dilution of the original gels. APPARENT SPECIFIC GRAYITT AT 25'c.
All determinations were carried out with a simple type of pycnometer, this having been found most suitable for the work. All precautions essential in obtaining accurate results as, for example, temperature control provided by a thermostat sensitive t o i O.Ol"C., perfect cleanliness, freedom from air bubbles in the sols, etc., were carefully observed. The apparent specific gravity was det,ermined as follows :
w,-
W B
v, - v w VB
= w w =
vL3
= apparent specific gravity
R
1033
STUDIES IN COLLOIDAL CLAYS
where TV, = weight of suspension in pycnometer (25"C.), W B = weight of bentonite present in W ,as known by dry weight determinations, V, = volume of pycnometer (at 25OC.), V ,,, = volume of water in suspension (25'C.), and Vo = apparent volume of bentonite in suspension The apparent specific gravities of seven dispersions of fraction 6, all obtained by dilution varying in per cent dry weight from 0.067 to 1.939 are given in table 2. I t can be seen tha;, starting with the most dilute dispersion which gives a value slightly below the one generally recorded in literature for dry bentonite (2.6-2.7), there is a continuous increase in apparent specific gravity. I t is more pronounced in the lower concentrations than in the TABLE 2 Apparent specific gratiities ( W C . ) of seven dispersions of fraction 6
0 710 1 146
1 506 1 613
2 796 2 845 2.860 2 861
higher ones. From concentrations above 1.5 per cent dryweight the apparent specific gravity approaches an asymptotic value. Since the specific
weight it should be a gravity of a substance is determined by the term volume ' constant, independent of the amount (weight) of the substance used. The actual weight of the bentonite present in any of the dispersions studied being known, the only variable left is its apparent volume, since the determination of the volume of bentonite present in the suspension is an indirect one. The calculated values for the apparent volume turn out t o be smaller than the volume as computed when using the dry specific gravity of bentonite. This can be explained by assuming that the amount of water present in the pycnometer when filled with the suspension is in excess of the theoretical amount needed to fill the pycnometer containing a known volume of bentonite. Therefore, the surplus of the water must be strongly adsorbed and compressed on to the surface of the bentonite particles.
1034
E. A. HAUSER AKD D. S. LE BEAU
As a further consequence of this deduction one would have to expect that the apparent specific gravity will decrease inversely with the particle size of the sols studied. Increasing particle size results in a decrease of available surface area of the disperse phase. The apparent specific gravities3 for fractions 4 and 2, respectively, are recorded in tables 3 and 4 (figure 1). As can be seen, fraction 4-although here, too, an increase in apparent specific gravity with increasing concentration is noticeable-does not
%
CONCENTRATION
FIQ. i.Apparent specific gravities for fractions 2, 4, 6, and 7 a t 25°C.
reach the values of fraction 6. Fraction 2 in turn gives lower apparent specific gravities than fraction 4. However, up t o a concentration of 2 per cent, none of these fractions approaches a constant value, as fraction 6 did. This can be explained by the presence, a t a given concentration, of a smaller surface area available for adsorption in fraction 2 than in 4, and in fraction 4 than in fraction 6. 3 For the determination of the apparent specific gravity, two pycnometers were always used. Each pycnometer was filled and weighed four different times. Each series consisted of f o u r weighings, and the weights checked within 0.0002 g. The apparent specific gravities calculated from the average of each series were checked with a series using the second pycnometer. Even in the most dilute dispersions the differences in weight between the pycnometer filled with the suspension and the pycnometer filled with distilled water were never less than 0.0020 g. With this method the calculated maximum error in the most dilute suspension studied is It 5 per cent. Therefore, given a value of 1.94 for the apparent specific gravity of the most dilute dispersion of fraction 2, the possible deviation can be between 1 8 4 and 2.04. With increasing concentration of the suspension the maximum possible error naturally decreases rapidly.
1035
STUDIES IN COLLOIDAL CLAYS
The figures shown in tables 3 and 4 demonstrate that the most dilute suspensions exhibit a value for the apparent specific gravity which is lower than the one generally accepted for dry bentonite. Similar results were found b y van Bemmelen ( 5 ) when determining the apparent specific gravity of silica gel containing different amounts of water. Mininia in apparent specific gravity could be found with silica gels having a maximum amount of water content, as well as with those cantaining a minimum amount of water, whereas in medium concentrations the apparent specific gravity rose. Since only the range of very dilute suspensions has been studied in this investigation, no second minimum in apparent specific gravity can be expected. However, the asymptotic value obtained in higher concentrations of fraction 6 might point t o this effect in case the concentration were further increased. TABLE 3 AppaTent specific gravities of f r a c t i o n Q at 36'C.
TABLE 4 Apparent specific gravities of fraction 3 at 35%'.
-
CONCENTRATION I N PER CENT DRY WEIQAT
APPARENT SPECIFIC QRAVITIEB
CONCENTRATION I N PER C l N T D R l WEIGHT
APPARENT SPECIFIC QRAVITIES
0.086 0.273 0,548 1.063 1.573 2.033
2.054 2.544 2.718 2.762 2 . a27 2 842
0.094 0.270 0.544 1.035 1.505 2.040
1.941 2.477 2.609 2.731 2.732 2.782
When the dilute suspension of fraction 6 was reconcentrated by el-aporation at 75°C. (table 5 ) and the apparent specific gravities weredetermined, the increase in apparent specific gravity, especially in lorn concentrations, never reached the value of the untreated samples, although it mas still observable. During thermal evaporation partial agglomeration of primary particles takes place. This has been confirmed by ultramicroscopic studies. Such agglomeration increases the effective particle nize of the suspension as well as the apparent volume of the disperse phase, while the available active surface decreases. The stability of a sol which has been obtained by dilution from a stable gel will increase with increasing concentration of tho disperse phase. Less agglomerates will form, i.e., more active surface will be available; moreover the available surface automatically increases with concentration. Therefore, the apparent specific gravity must increase. Reconcentration of the dilute suspension of fraction 6 a t 95OC. (table 6)
1036
E. A. HAUBER AND D. S. LE BEAU
shows a similar decrease in apparent specific gravity in lower concentrations (figure 2). Fraction 7 is a sol containing, as previously mentioned, particles of an average particle size diameter below 14 mp. Since its dry weight content was only 0.46 per cent, it was necessary to concentrate it by evaporation. TABLE 5 A p p a r e n t specific gravities a t 25°C. us fraction 6 , reconcentrated at 75°C. COICESTRATION I S PER CENT D R Y WEIGET
,
0.306 0,512 0.016 1,358
0
1,
APPARENT SPECIFIC ORAVITIIS
CONCEWlT.ATION IN PER CENT D R Y WEIGHT
__ .______ 2.460 2.682
0 289 0 583 1 118
~
APPARENT SPECIFIC GRAVITIES
2 617
' ~
2 696 2 7f4
I
2.716 2.728
~
~
02
TABLE 6 B p p a i e n t specific gravities at $5'12. of fraction 6 , reconcentrated at 96OC.
04
06
I0
08
'4
I2
14
ib
18
20
CONCENTRATION
FIG.2. Apparent specific gravities of fraction 6 a t 2573.
Only one dirpemion was produced by dilution for the specific purpose of studying a concentration lower than 0.46 per cent. The results are given in table 7 , Here., t,oo, an increase in apparent specific gravity with concentration can hc found. As \T-as t o he expected with a system containing extremely fine particles, the miginal dispersion already has a higher apparent specific grak-ity than any of the fractions so far discussed. The sol obtained by dilution exhibits an apparent specific graT,ity corresponding to the one
1037
STUDIES IN COLLOIDAL CLATS
accepted for the dry material. This system is very dilute aiid the particles most probably are monoplates, so that neither appreciable iiiterlaniellar swelling nor adsorption can take place. However, the first of the series of reconcentrated dispersions (0.505 per cent) reveals a sudden and remarkable increase in apparent specific gravity. The apparent specific gravities are extremely high and are still rising at a concent,ration of 1.41 per cent. These sols also reveal a pronounced increase in T k o s i t y and exhihit high yield values. These data substantiate the fact already established by Hauser and Reed (3) that t,he t,endency to forni gels increases with decreasing particle size. The formation of a strong thixotropic gel without the addition of electrolyte at a concentration of only 1.41 per cent dry weight is, t o our knowledge, the lowest figure so far recorded. The above data, if viewed in the light of t,he previous discussion, indicate further that increased adsorption of the dispersing medium on the dispersed particles, TABLE 7 d p p a r e n t specific gravities at 86'C. of fraction,7 C O N C E N T R A T I O I I N P E R CENT D R Y W E I G H T
0.201* 0.460t 0.505t 0.70St: 1,303: 1.4101
1 ~
~
~
I
A P P A R E N T SPECIFIC G R A V I T I E S
2.710 2.911 3,007 3.028 3,042 3.081
* Sol obtained by dilution of the original sol. t Original sol. 1Original sol concentrated at 95°C. as indicated by the abnormal increase in apparent specific gravities (formation of large water hulls), is a predominant factor in the gelation of colloidal clay suspensions of low concentration. DETERMINATIONS O F ABSOLUTE VISCOSITIES
A Hoeppler viscosimeter (4) was used, which permits a quick ticterininat,ion of absolute viscosities over a fairly large range. The method is based on the principle of a ball rolling down the wall of a glass tube iiiclined 10" from the vertical. The ball travels in a guided and predetermined eccentric position through the tube, so that uncontrollable nall effect$ and the influence of turbulent flow are avoided. The determinations of viscosity were carried out a t 25°C. and 40°C. I n the very dilute sols up t o 0.27 per cent dry weight only smnll differences in the viscosities of fractions 2 and 4 can be found. Fraction 6 (table 10) is slightly higher in viscosity than fractions 4 (table 9) and 2
103s
E. A. HAUSER AND D. S. LE BEAU
(table 8), and fraction 7 again is slightly higher than fraction 6 (figure 3), With such low concentrations no great differences were to be expected. owing t o the small number of particles present. With increasing con1
6
5
"
m
z > 4
g
I
>
I
Y
5 3 0
Y)
U m
2
I
0
04
08 12 16 PER CENT $ONCE!4TAATlON
24
20
FIG.3. Viscosities of fractions 2, 4, 6, and 7 a t 25°C. and 40"C., determined in a Hoeppler viscosimeter. .-
TABLE 8 Viscosities o f fraction 9
TABLE 9 Viscosities of fraction 4
I I _
1
CONCENTRATION INPERCENT D R Y WEIGHT i
CONCENTBAT I O N IXI'ER CENT DR' NEIQZT
--___
0.094 0.270 0.544 1.035 1,505 2.040
0.956 1.065
0.696 0.735
1.154
0.884
0.086 0.273 0.348 1.063 1,573 2.033
I
CpAT25-C.
1I
0.968 1.065 1.312 1.840 3.360 6.428
CpAT40'C.
0.717 0.790 0.934 1.356 2.411 4.609
l _ l
centratiori of fraction 2 , the viscosity increases nearly proportionally Only in the Yery high (above 2 per cent) concentrations studied does a slight deviation become n~ticeable.~Fractions 4 and 6 deviate rapidly The scale used In the reproduced curve does not show this deviation
1039
STUDIEE I N COLLOIDAL CLAPS
from the curve of fraction 2 above a concentration of 0.27 per cent. The increase in viscosity becomes more and more pronounced. This cnn be easily explained by the following considerations. The amount of particles present in a given concentration of these three different sols increases with their decreasing apparent particle diameter. The particles in sol 4 are packed closer than in sol 2, and in sol 6 closer than in sol 4. Thus the resistance against shear will increase. On this basis it is quite evident that fraction 2, having the largest particles and the smallest overall siirface, TABLE 10 Viscosities of fraction 6 CONCENTRATION I N PER CENT DRY WEKGET
cp AT 25'c.
Cp AT 40' C.
0.067 0.152 0.368 0.710 1.085 1.203 1.506 1.643 1.939
1.002 1.048 1.137 1.469 2.187 2.429 3.929 4.271 7.117
0.715 0.777 0.806 1.056 1.505 1.690 2.724 2.946 5.212
TABLE 11 Viscosities of fraction 7 CONCENTRATION I N PER CENT DRY WEIGHT
0.201 0.460 0.505 0.798 1.302 1.409
c p AT 25"
c.
1.085 1.308 2.253 6.751
cp AT 40" c. 0.744 0.931 1.600 3.698
TABLE 12
TABLE 13
Viscosities of fraction 6 after evaporation at 75°C.
Viscosities of fraction 6 after evaporation at 9 P C .
CONCENTRATION IN PER C E I T DRY WEIGET
0.153 0.306 1.045 1.358
1
1
""
'
1.038 1.093 1.892 2.473
cp A T 40'c 0.756 0.806 1.376
IN PER CENT DBY WEIOHT
0.283 0.582 1.177
AT
25' C.
1.093 1.350 2.116
0.791 0.956 1.489
will begin to show a more rapid increase in viscosity only in concentrations very much higher than those of fractions 6 and 4. The values obtained for the viscosity of the first two sols of fraction 7 (table 11) (the original sol and the one obtained therefrom by dilution) correspond to these considerations. The first sol of fraction 7 obtained by reconcentration shows a very sudden and enormous increase in viscosity, which could also be recorded for the second sol obtained by evaporation and which is in line with the extremely high apparent specific gravities recorded. These are the sols where, as previously mentioned, thixotropy could already be observed a t concentrations as low as 1.4 per cent. If we
1040
E. A . HAC-SER AKD D. S. LE BEAU
FIG.4. Change in viscosity of fraction 6 after concentration, determined a t 25°C. and a t 40°C. in a Hoeppler viscositnetrr 0 07
0O b
0 05
I TIME
0 04
003
0 02
001
0 0
4
E
I2
16
WEIGHT
20
24
28
3
(GRAMS)
FIG.5. Fraction 2 i n a Storiner viscosimeter at 25°C.
STVDIES IN COLLOIDAL CLAPS
1041
assuine that a t a given concentration't he thickness of the adsorbed water layer surrounding a particle would be the same if the apparent particle
WEIGHT (GRAMS)
FIG.6. Fraction 2 in a Stormer viscosimeter at 40'C.
WEIGHT (GRAMS)
Fro. 7. Fraction 4 in a Stormer viscosimeter a t 25'C
diameter is 28 mp, 14 mw, or less, then the ratio of adsorbed water to particle will be greater the smaller the particle. Therefore comparatively more
1042
E. A. HAUSER A S D D . S. LE BEAU
water will be immobilized in a sol of’extremely fine particles than in one of greater particle size.
WEIGH1 $RAMS)
FIG.8. Fraction 1 in a Stormer viscosimeter a t 40°C. 0 01 006
0 0:
0 04 I -
TIME
0 03
0 0;
00
(
WEIGHT (GRAMS)
FIG.9. Fraction 6 in a Stormer viscosimeter a t 25°C.
The viscosity nieasureinents in the Hoeppler viscosimeter for fractions 4. 6, and 7 r e r e discontinued as soon 3.5 the first signs of “structural viscosity” coulcl be detected, since in our opinion even the refined falling-ball
1043
STUDIES IN COLLOIDAL CLAYS
method cannot give accurate and reproducible results in systems of such a nature. The viscosities of fraction 6, reconcentrated at 75°C. and 95"C., show 0.07
006
0 05
0 04 I
m 003
002
001
0 0
4
8
I2
16
20
24
28
3
WEIGHT $RAMS)
FIG. 10. Fraction 6 in a Stormer viscosimeter a t 40°C.
WEIGHT
(GRAMS)
FIG. 11. Fraction 7 in a Stormer viscosimeter a t 25°C.
very little change up to concentrations of 1.17 per cent (tables 12 and 13). In higher concentrations the viscosity does not rise to the same degree as the original (figure 4).
1044
E. A . HAUSER AND D. S. LE BEAU
The determinations of absolute viscosity at 40°C. gave the results to be expected. A general decrease in viscosity is observed.
0.0 7 O.Ob
0 05
004 I
rn 003
0 02
0 01
0 WfIGHT (GRAMS)
FIG. 13. Fraction 6, reconcentrated by evaporation a t 75"C., in a*Stormer viscosimeter at 25°C.
Since it is impossible to make any accurate statements as to the formation of a structure or the development of a yield point when using a falling-
STUDIES IN COLLOIDAL CLAYS
1045
ball type of instrument, it was decided to study the different sols in a Stormer viscosimeter a t 25OC. and 40°C.
WEIGHT (GRAMS)
FIG. 14. Fraction 6, reconcentrated by evaporation a t 75"C., in a Stormer viscosimeter a t 40°C.
FIG.15. Fraction 6, reconcentrated by evaporation at 95"C., in a Stormer viscosimeter a t 25°C.
Fraction 2, which consists of t h e coarsest particles (average apparent particle diameter = 48 mp), exhibits no yield point up to 2.04 per cent
1046
E. A. HAUSER AND D. S. LE BEAU
(figures 5 and 6). Fraction 4 (average apparent particle diameter = 28 mp) reveals a noticeable yield point already a t 1.5 per cent (figures 7 and 8), fraction 6 (average apparent particle diameter = 14 m l ) at 1 per cent (figures 9 and lo), and fraction 7 (average apparent particle diameter less than 14 mp) a t 0.5 per cent (figures 11 and 12). Bt 40°C. all the yield points decrease. The decrease becomes the more pronounced the smaller the particle sizes of the fractions studied. The yield points naturally increase with increasing concentration. No appreciable changes in yield points of fraction 6 have been found when reconcentrating the sols by evaporation (figures 13, 14, 15, 16). The results of viscosity determinations have demonstrated that the viscosity follows the Einstein equation only for the coarsest fraction and in
FIG. 16. Fraction 6, reconcentrated by evaporation a t 95"C1 in a Stormer viscosimeter at 40°C.
extreme dilutions. For the finer particles increasing deviations are observed. Yield points also increase with decreasing particle sizes and increasing concentration. The unexpectedly high viscosities at the concentrations studied cannot be accounted for by simple particle interference. I n addition a high degree of solvation (adsorbed water hulls) is necessary to explain this fact satisfactorily. It seems only logical that such a bond will be weaker the higher the temperatures, thus causing a reduction in viscosity. OPTICAL DEKSITY
It finally was decided to examine the finest fractions for their light transmission, as it was hoped that such measurements might permit some
STUDIES IN COLLOIDAL CLAYS
1047
insight as t o the actual distribution of the particles in the sols. A Hardy color analyzer (1) was used for this purpose. All measurements were recorded with distilled water as a blank.
2
CONCENTRATION
FIG. 17. Optical density of fraction 6
2
CONCENTRATION
FIG. 18. Opticel density of fraction 7
The curves for the optical density of fraction 6 show a very pronounced light absorption in the smaller wave lengths (figure 17). This simply proves that the sol contains a large number of small particles. The curves for the optical density of fraction 7 (figure 18) are of interest
1048
E. A. HAUSER AND D. S. LE BEAU
as they show, when the sol is reconcentrated by evaporation, a sudden increase. These systems have a very marked tendency to form thixotropic gels when being cooled down. Although no final explanation for this phenomenon can yet be offered it seems highly probable that the particles, becoming more and more locked into equilibrium positions, cause an increased absorption or scattering. (Further work t o clarify this effect is in progress.) STREAM DOUBLE REFRACTIOK
Stream double refraction in fine clay fractions has already been reported by Hauser and Reed (loc. cit.). I n the present study it was noticed in fraction 6 (average apparent particle diameter = 14 mb), but seemed to be a function of the concentration of this fraction, since it became noticeable only in concentrations above 1 per cent. Below this concentration no stream double refraction could be noticed, even after prolonged storage. The part of fraction 6 which had been reconcentrated by evaporation a t "-0 13 C. and 95°C. also showed stream double refraction, but only from concentrations above 1.045 and 1.177 per cent, respectively. No stream double refraction could be observed in fraction 7, either a t higher concentration or after long storage. It seems that in these systems stream double refraction is detectable xvith the eye only if particles of a certain range and concentration are present, since with larger particle sizes this phenomenon again becomes less pronounced. However, even the finest fraction in extreme dilution !\ill. if put into motion and placed between crossed nicols, exhibit rtrong double refraction, which proves that the slightest mechanical influence causes some orientation of the submicrons present in the system. This also proves that gelation is not a result of preferential orientation of particles. SUMMARY
The apparent specific gravities of monodisperse montmorillonite fractions of extremely low concentrations and different particle sizes show an
increase with decreasing particle size and increasing concentration. This is explained by the presence of strongly adsorbed water on the surface of the particles. Absolute viscosity determinations of the same systems demonstrate an increase with decreasing particle size and increasing concentration. These results are in line with the changes in apparent specific gravity. The coarsest fractions show no yield point up to concentrations of 2 per cent. The finer fractions reveal a n increase with decreasing particle size and increasing concentration. The yield point as well as the viscosity de-
STUDIES I N COLLOIDAL CLAYS
1049
creases with increasing temperature. Gelation seems to cause a sudden rise in optical density over all wave lengths. Preliminary studies in stream double refraction indicate it to be a function of particle size and concentration. Orientation of the submicrons in a sol, if placed between crossed nicols, is detectable only if the system is put in motion. REFERENCES HARDY, A. C.: J. Optical SOC.Am. 26, 305 (1935). (2) HAUSER, E.A.,AND REED,C. E.: J. Phys. Chem. 40, 1169 (1936). (3) HAWSER, E. A.,AND REED,C. E.: J. Phys. Chem. 41,911 (1937). (4) HOEPPLER, R.: Chem.-Ztg. 67, 62 (1933); Z. tech. Physik 14, 165 (1933). (5) VAN BEMMELEN, J. hl.: Z. anorg. allgem. Chem. 18,98 (1898). (1)
