STUDIES I N THIXOTROPY.
I1
THETHIXOTROPIC BEHAVIOR AND STRUCTURE OF BENTONITE' E. A. HAUSER
. ~ N DC .
E. REED
Department o j Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts Received J u n e
% .?
1937
INTRODUCTION
As the result of extensive work by previous investigators we now possess knowledge of the general influence of such important variables as concentration of disperse phase and electrolyte, temperature, pH, various addi tion agents, etc., upon the thixotropic activity of several systems. There has as yet, however, been no systematic inyestigation of the effect of particle size of disperse phase upon thixotropy in any system, despite the fact that the importance of this variable has been recognized (1, 3, 15). Previous neglect of such an important variable niay be attributed perhaps to the difficulty of obtaining suitable quantities of suspensions of particles of varying average size, particularly xhen the particles exist in the low range of colloidal dimensions. The present authors have described a centrifugal method (6) involving the use of the so-called supercentrifuge, by means of which it has become possible to make particle size fractionations in colloidal systems and measure the particle size distribution in the resulting fractions. Using this technique the authors have been able to prepare a series of particle size fractions of the colloidal clay mineral bentonite and to study the thixotropic properties of the resulting suspensions of particles of varying average particle size. In addition to the notable thixotropy of its aqueous suspensions, bentonite exists naturally over a suitable range of particle sizes to enable the production of a series of particle size fractions. The present n-ork is divided into three parts: I. Productioii of the particle size fractions; 11. Study of the structure of the bentonite particle; and 111. Study of the thixotropic behavior as a function of particle size. Presented at the Fourteenth Colloid Symposium. held at lfinneapolis, Minnesota. June 10-12. 1937. 911
912
E. A. HAUSER AND C. E . REED I.
PRODUCTIOX OF THE PARTICLE SIZE FRACTIONS~
-4lthough the chief constituent of bentonite is montmorillonite ( I 4j, it is mineralogically a heterogeneous substance, and when the properties of different fractions are to be compared and complication in the interpretation of results is to be avoided, great care must be taken that the composition of every fraction finally obtained is the same. The raw natural bentonite was obtained in powdered form and dispersed in distilled water (about 1 per cent concentration) with an electrically driven agitator. Froiii this time on the bentonite remained in suspension and was never allowed to dry. Most of the impurities were in the coarser particle size range and were removed by gravity sedimentation after several months’ storage. A few impurities came out in the particle size range just above the largest fraction actually used in experiments. The remaining suspension of colloidal particles was fed into the supercentrifuge, which consists essentially of a balanced vertical bowl rotated at a high rate of speed by an electric motor ( 6 ) . The suspension flows continuously into the bowl a t the bottom, and as it flows through the bowl the suspended particles are settled out onto the walls. The coarsest particles settle out near the bottom, the finer particles traveling farther up the bowl before they are finally deposited on the walls. The walls of the bowl were fitted closely with a flexible celluloid liner which could be removed when it carried a suitable quantity of sedimented material. When this liner was removed from the bowl the bentonite it held was in the forni of a stiff jelly, which could be scraped off and stored in glass jars. This jelly ranged in appearance from the opaque niuddy yellow color characteristic of the larger particles settled out a t the bottom of the liner to the transparent golden yellow color revealed by the very fine particles a t the top of the liner. In the early stages of the fractionation, it was therefore possible in scraping the jelly-like sediment from the celluloid liner to grade the particle size roughly by color alone. Each fraction scraped off the liner was redispersed in distilled water and rerun through the centrifuge under conditions giving a greater spread of sedimentation. For example, the fine fraction obtained from the first run was re-run with the centrifuge bowl rotating a t a higher rate of speed than previously, so that the relatively large particles of this fine fraction tended to settle out at the bottom of the bowl while the finest particles of all would settle out at the top of the bowl. By a long series of such successive runs and redispersions, the originally The bentonite used was of the Wyoming variety and Jyas mined by the ilmerican Colloid Company. The mine is near Colloid Spur, Wyoming, along the CB&Q Railroad, two miles northeast of Upton, in Weston County, Wyoming. The actual deposit covers part of Section 27, Township 48, north of range 65, Tvest of the 6th P. M.
STVDIES IK THIXOTROPT.
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polydisperse bentonite suspension was separated into six fractions of varying average particle size, five of which were used in the experiments. The resulting suspensions were subjected to electrodialysis to effect the removal of exchangeable metallic ions, after which their particle size distribution curves were measured by the method described previously (6). These hydrogen bentonite suspensions served as stock suspensions of the material used in all later experiments. TABLE 1 Average equivalent splierical diameters o j bentonite fractions I
FRACTION N O
RANGE DIAMETERS IN m p
1 D,,,
DIU,*
1
11 0 13 5 23 0 28 8 48 0
1
2 3
4
I
6
182 23 5 32 6 43 1 165 0
-
I
DN.
I
14 3 20 3 28 1
I
33 8 87 0
I2W
1000
--.-
800
600
0
x 0
N
400
200
0 IS
20
25
30
40
SO
MI
7a
60 90 io0
IS0
200
D i N mr
FIG.1. Bentonite fractions obtained by centrifugnl fractionation
The distribution curyes, as usual, are reported in terms of equivalent spherical diameters and are plotted in figure 1. The log of equivalent spherical diameter is plotted as abscissa, and the ordinate is so adjusted that the area under the curves between any tn-o yalues of D is proportional to the weight per cent of the material existing between these two diameters. For purposes of future plots, the average equivalent spherical diameter of each fraction tabulated in table 1 has been defined as that diameter, D,, , which divides the fraction into txvo equal weights of particles, one group having diameters larger and the other group having diameters smaller than D a V .
9 14
E. A. HhUSER Ah-D C . E. REED
The chemical analyses of the largest and of the smallest particles are compared in table 2, with regard to the important constituents. As will be noticed from table 2, the chemical analyses of both fractions check quite closely. The presence of alkaline and alkaline earth elements calls for some explanation. X sample of fraction 1 was subjected to drastic acid washing with 2 N hydrochloric acid. By this treatment it was found possible to remove practically all the (NazO+&O) and CaO. On the other hand, no hIg0 was removed by this treatment, an indication that the MgO is embedded much more firmly in the lattice than the Na, E(, and Ca. Electrodialysis for an indefinitely long period would probably remove the (NanO+KsO) and CaO removed by acid washing. The amount of these constituents, however, is negligible compared to the quantities of potassium hydroxide eventually added to induce gelation; indeed TABLE 2 Ctienizcal analyses of dzalyzed bentonite Based on weights of clay which had been dried t o constant tveight a t 105OC FRACTION 6: FRACTION 1: LARQEST PARTICLES ISMALLEST PARTICLES ~
Loss a t 105°C... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SiOp. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fen0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..I ~
RpOs* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CaO... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . hlgO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
NasO
+ K?O (as SanO). . . . . . . . . . . . . . . . . . .
so3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I
,I ~
percent
0.00 61 68 4 28 24 68 0.24 2.60 0.21 0 OS
~
I
,
, ~
1
per cent
0.00 61.56 -1.36 24.56 0.20 2.57 0 25 0 085
* Includes A 1 2 0 3 , Tion,111n304,and P203.
the amount is small compared to the equivalent weight of the bentonite, and it will be seen that the equivalent weights of the limiting fractions 1 and 6 are identical within the limit of experimental error. X-ray examination gave identical powder patterns for fraction 1 arid fraction 6. The base-exchange capacity was determined by electronietric titration with sodium hydroxide. The titration curves for the largest and the smallest particles are compared in figure 2. The curves are practically coincident and indicate an exchange capacity of 92 milliequivalents per 100 grams of clay, or an equivalent weight in the usual sense of 1090. hll of the suspensions showed strong dityndallisni upon stirring, evidence of the anisometric shape of the particles. The difference in physical appearance of the five fractions shows up in the cornparatiye spectrophotometric analyses shown in figure 3, where per cent transmission is plotted against wave length of transmitted light.
STCDIES I N THIXOTROPY.
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PH
M I L L I E Q U I V A L E N T S OF NaOH P E R GM. OF C L A Y
FIG.2. Potentiometric titration of fractions 1 and 6 with sodium hydroxide
FIG.3. Light transmission curves. 0.9 per cent hydrogen bentonite suspensions. Thirkness of transmitting layer = 1 cm.
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E. I. HACSER Ah-D C . E. R E E D
Fraction 1 is easily seen to be the most transparent. It is evident that the greatest absorption will take place in the ultra-violet. The fractions appear to be similar except for particle size, and it thus becomes possible to interpret the differences in colloidal behavior upon the basis of this variable without complication. 11.
STUDY O F T H E STRUCTURE O F T H E BEXTONITE PARTICLE
In recent years our knowledge of the structure of clays has undergone rapid advancement. Marshall (12) has given a most enlightening summary of the latest developments in this field. The theory is now held that the cations concerned in base exchange go directly into the crystal lattice of the clay in a manner analogous to zeolites. Among the clays, bentonite is notable for its unusually large exchange capacity as well as for its ability to swell strongly in water. Hofmann, Endell, and Wilni (7) have proposed a layer lattice structure for montmorillonite, the chief constituent of bentonite. The structure proposed consists of superimposed layers of silica and alumina, as shown in figure 4, reproduced from a recent article by Hofmann and Bilke (8). Kater niolecules can enter between the groups of Si-A-Si planes and force them apart, thus causing the swelling. The structure shown is an idealized one, and Marshall (12) has postulated that it is possible for several isomorphous replacements to occur. Thus aluminum is supposed capable of replacing silicon, in which case an exchangeable cation would go along with the aluminum to neutralize what would otherwise be a net negative charge on the lattice. In addition, it is possible that the broken oxygen bonds at the edges of the particles can hold metals and hydrogen, and it is possible that some of the oxygen atoms linked to silicon can react with water to form hydroxyl groups, in which hydrogen is replaceable. While the structure proposed by Hofmann et al. may undergo modification in the light of future research, its importance lies in its emphasis of the very open porous structure possessed by what we shall continue to call the bentonite particle. The following nieasureinents made on the particle size fractions of bentonite should throw additional light on the structure of bentonite in suspension. Figure 5 shows a plot of pH versus the log weight per cent concentration of hydrogen bentonite. Measurements were made on fractions 1, 3, aiid 6 by means of a Beckmann glass electrode. No difficulty was experienced in reproducing the results, which were checked on a Leeds and Northrup glass electrode. It is seen that the pH-concentration relation is practically the same for all three fractions. The data are well represented by the equation
STUDIES IS THIXOTROPY.
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917
where U H + is the activity of hydrogen ion and W the weight per cent concentration of hydrogen bentonite based on evaporation t o dryness at 105°C. The analogous relation for a n-eak acid such as acetic acid n-ould show a W
i
n H,O
-*
b=YA
H,O. (AI,O,, Fe,O,, 3 4MgO).4 S O ,
+ n H,O
FIG.4. 1Iontmorillonite
PH
WT.
BENTONITE
FIG.5. Hydrogen-Lon activity in hydrogen hentonlte suspenslons a t 23-24°C.
exponent of around 0.5, whereas a strong acid like hydrochloric acid would have a W exponent approaching 1. Marshall and Gupta (13) have also noted what a strong acid hydrogen bentonite appeared to be as a result of hydrogen-ion activity measurements. In view of the fact that the deter-
918
E. A. HAVSER AND C . E. REED
W T % BENTONITE
FIG.6. Conductance d a t a for hydrogen bentonite. iiqueous suspensions a t 25°C.
AVERAGE E Q U I V A L E N T SPHERICAL D I A M E T E R
FIG.7. Effect of particle size on conductance of bentonite solutions. Temperature, 25°C.
STUDIES I N THIXOTROPY.
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919
mination of hydrogen-ion activity is essentially an equilibrium measurement, it was decided to see how the hydrogen-ion in hydrogen bentonite behaved in conductance, which is a non-equilibrium measurement. Conductances were measured by the Kohlrausch method at a frequency of 1000 cycles. L4 check upon the operation of both the glass electrode and the conductance equipment was obtained by measuring hydrogenion activities and conductances of a series of standard hydrochloric acid solutions. The experimental results checked well with those in the International Critical Tables. The results of the conductance measurements are seen in figure 6, where the specific conductance in reciprocal ohms is plotted against the weight per cent of hydrogen bentonite for seviral different particle size fractions. The specific cohductance increases as the concentration of bentonite increases and as the particle size decreases. This increase in specific conductance with a decrease in particle size is emphasized in figure 7. Here specific conductance is plotted against average equivalent spherical di* ameter a t constant concentration. The most interesting and important feature of the conductance results emerges when they are compared with the hydrogen-ion activity measurements. I n the relatively dilute suspensions involved, one is inclined t o feel that the hydrogen ion, if acting normally, should display an equivalent conductance not widely different from its limiting conductance. Since the hydrogen ion ordinarily carries the greater part of the current in systems wherein it is present, it should be possible to calculate the conductance of a hydrogen bentonite suspension with a reasonable degree of accuracy from the value of its hydrogen-ion activity. Assuming that the activity aH’ is equal to the concentration CH+,we have, by the usual definition: 1000 L .IH- =
-e,
where I, = specific Conductivity in mhos and -1 = equivalent conductivity in mhos. Substituting for C” its value in terms of the weight concentration of bentonite and sol.c.ing for L we obtain:
L
=
wo
CH- - 0.00118 *’.\H__ 1000 1000
.IHL
-~
Assuming that in these dilute suspension> = LI~HT
L
0.00118AmH-”0
si
= ___~-__
1000
920
'
E. A. HAUSER AND C. E. REED
At 25°C. = 349.7 (International Critical Tables, Vol. VI., p. 230). The final expression for L a t 25°C. then becomes:
L X lo5 = 41.2W0.*7 This relation is plotted in figure 6 for comparison with the experimentally determined conductance data. The resulting curve shows that values of the specific conductance calculated from activity measurements are in excess of 200 per cent of the highest experimental values realized. Allowance for the conductance of the negatire clay particles would make the calculated conductances even higherS3 A further comparison of the pH and conductance is seen in figure 5, where the heavy broken line represents pH values computed from the
FIG.8. Conductometric titration curves of bentonite fractions a t 25°C
highest experimental specific conductances (fraction 1) plotted against the corresponding concentrations of hydrogen bentonite. Figure 8 shows conductometric titrations for fraction 1 and fraction 3. At the equivalent point it is seen that the specific conductance of sodium bentonite is only about 25 per cent lower than that of hydrogen bentonite. Inasmuch as the equivalent conductance of the sodium ion is only 50, whereas that of hydrogen ion is 350 at 25"C., the reduction in conductance is not as great as might be expected and indications are that the sodium Since n r i t i n g t h e manuscript the authors have been informed t h a t similar discrepancies between conductometric and potentiometric pH have been recorded with AgI sols b y Verwey and Kruyt ( Z . physik. Chem. A167, 159 (1933)) and with gum arabic sols by Pauli and Ripper (Kolloid-Z. 62, 162 (1933)).
STUDIES I N THIXOTROPT.
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ion is inore strongly dissociated from the lattice of the particles than hydrogen ion. Finally, a determination was made of the relative quantity of electrolyte (potassium hydroxide) required to flocculate the various fractions.
Experimental determination of flocculation values I t is very difficult t o reproduce frocculation values for clay suspeiisions accurately, because of the great influence of such factors as method of shaking, method of dilution, etc. Fortunately, in the present experiments we are primarily interested in relative values. By adhering t o a standard procedure in evcry casc, it was possible to obtain reasonably reproducible values which are certainly relatively correct. The experiments were conducted in test tubes at a constant concentration of 0.05 per cent bentonite. The stock suspension mas diluted t o such an extent that upon addition of the desired quantity of standard potassium hydroxide the final concentration of bentonite would amount to just 0.05 per cent. The potassium hydroxide was added from a pipet, the test tube being shaken during the addition. After addition of potassium hydroxide, the test tube was shaken for half a minute and then allowed t o stand for twenty-four hours, at the end of which time the presence or absence of sedimented floes was noticed. The determination of a flocculation value is a trial and error procedure during which a series of suspensions must be made up, treated with different amounts of potassium hydroxide, and observed. The results are reported in table 3, in terms of the milliequivalents of potassium hydroxide per gram of bentonite required to produce flocculation under the standard experimental conditions. d plot of flocculation number against average equivalent spherical diameter (figure 9) shows that over twenty times as much potassium hydroxide is required to flocculate fraction 1 as is required to flocculate fraction 6. The preceding experimental results might be explained on the basis of two theories. -kcording t o the structure proposed by Hofmann et al., dispersion of bentonite in water involves the forcing apart of the ultimate plates of Si-A-Si, which comprise layers of the lattice. There is no theoretical reason why such a process could not result in ultimate particles 10 A.U. thick and of indefinite length and breadth. The fractionation procedure would then be separating particles of substantially the same thickness, but of different lengths and breadths. Since the particles ~ ~ o ube ld very thin in comparison to their other two dimensions, fraction 1 composed of the so-called smallest particles would have practically the same surface as fraction 6, composed of the largest particles. It is easy to understand on this basis why the electrometric titration curves and pH-concentration relation are similar for both large and small fractions. I t is not so easy
922
E. A . HAUSER A S D C . E. REED
to account on this basis for the large difference in quantity of potassium hydroxide required to flocculate the various fractions. Furthermore, all fractions, unresolvable in the ordinary microscope, produce about the same amount of twinkling in the ultramicroscope. If the particles in all fractions were the same thickness, one would expect to find evidence in the ultramicroscope of the greatly increased length and width necessarily possessed by the larger fractions. TABLE 3 Flocculation numbers of bentonite jractions I I
FRACTION NO.
AYERAGE EQUIYALEKT SPHERICAL DIAMETER
FLOCCULATION N O
mp
14.3 20.3 28.1 33.8 87.0
22-26 16-20 9-10 3-4 1-2
Y
E
g2e rz 24 0 1
g 20 I 0 L
16
s 2 12 I 0
8
z z 9 4
54
-? 0'
3 u.
20 40 60 80 100 I20 140 A V E R A G E E Q U I V A L E N T SPHERICAL D I A M E T E R
FIG.9. Flocculation of hydrogen bentonite with sodium hydroxide
The alternative explanation, which n e feel fit+ all of the facts better, isIthe assumption of the very open porous layer structure. From thib point of view the particles in fraction 6 may be thicker as well as longer and broader than the particles in fraction 1. The layers of groups of Si-A-Si planes are, however, far enough apart to enable the passage in and out of water molecules, metallic ions, and hydrogen ions. Thus each portion of the whole mass of the bentonite particle contributesits share of exchangeable cations, and each portion of the mass of the whole
STUDIES I N THIXOTROPY.
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923
structure can furnish hydrogen ions which contribute t o the overall hydrogen-ion activity of the suspension. On the other hand, the true external surface per unit weight must obviously be greater in fraction 1 than in fraction 6, if the particles in fraction 6 are to be thicker than the particles in fraction 1. h smaller external specific surface accounts for the smaller amount of potassium hydroxide required to flocculate fraction 6 in comparison to fraction 1. The large discrepancy between the conductance and activity measurements indicates that the hydrogen ions are bound quite closely to the large negatively charged particles. We prefer to view bentonite as a colloidal electrolyte possessing a large porous multivalent anion which has many hydrogen ions very closely associated with it. The structure of the particle is so open that these ions can register an effect upon activity measurements where they undergo no net displacement from the particle. On the other hand, it is well recognized that it takes work to separate two bodies of opposite charge, and the larger the charge the greater the work. Hence the hydrogen ions might be expected to be more closely associated with the larger particles of greater negative charge than with the smaller particles, an expectation confirmed by the decreage in specific conductance with increase in particle size. 111. STCDY O F THE THIXOTROPIC BEHAVIOR OF B E S T O S I T E AS A FUNC-
TIOK OF PARTICLE SIZE
Thixotropic behavior was studied by measurement of setting times by the inverted-tube method. The method has been used many tinies previously in investigations of thixotropy and is most convenient for obtaining relatire data. Broughton and Squires (2) discussed the invertedtube method in comparison to other methods, and concluded that it was capable of giving consistent results. -411 experiments were conducted in Pyrex tubes of 10.3 to 10.5 nim. internal diameter. The tubes were 100 mni. in length and were in every case filled with exactly 4 cc. of the material to be tested. In all cases the sample t o be tested was prepared directly in the tube by addition of proper amounts of ,the stock suspension of hydrogen bentonite, standard electrolyte, and distilled water. ,211 ingredients were measured in calibrated pipets. -After being filled, the tubes were fused shut with a blast lamp. It was early discovered that the results depended to some extent upon the method of inverting the tube. Thus, if great care were taken in inversion, the setting time obtained would be less than if slight shaking occurred during inversion. For this reason it was deemed wisest to eliminate as far as possible any effects due to the observer’s personal method of inversion, by constructing an instrument capable of performing the inversion in a reproducible manner. Figure 10 shows a scale drawing of
924
E. A . HAUSER AND C. E. R E E D
the instrument finally evolved. Since its action is that of a physical pendulum, it has been called the physical penduluni thixotrometer. The tube to be tested is made secure upon the rotary wheel by the clamp C, located at a distance of 7 j in. from the axis of rotation. Upon release of the lever L, the wheel is caused to rotate through 180" by weight P. When P has traveled through 180" and reached the dotted position shown, the tube held by clamp C has been inverted. The wheel would now normally revert to its initial position, were it not for the action of the sandpaper brakes which engage the surface the instant counterclockwise rotation starts to occur. Since the wheel is traveling with zero velocity a t the end of its 180" path, the sandpaper brakes simply prevent it from accelerating in the opposite direction, and the whole inversion is performed with sub-
FIG. 10. Physical pendulum thixotrometer
stantially no jar being delivered to the tube under test. It is possible to adjust the apparatus to invert tubes of different weights by moving the weight P in or out on its supporting arm. It is essential to place the tube in the same position in clamp C for each test. The practice followed was to have the meniscus of the material in the tube just even with the lower front edge of the clamp. No difficulty was encountered in obtaining reproducible results with this apparatus. The procedure followed in obtaining the s'etting time was one of successive approximation. The particular sample under test was agitated strongly by shaking the test tube violently. Tests showed that 5 seconds' violent shaking was in general sufficient to break down the gel structure completely. The tube was then immersed in a constant-temperature bath controlled to &O.O5"C., and the time noted. After a lapse of time be-
STUDIES IN THIXOTROPY.
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925
lieved to be sufficient, the tube was gently removed from the bath, placed on the inverter, and inverted. If any flow occurred down the wall of the tube, the sample was considered as not set and the above process repeated, the sample being allowed to “set” for a longer period. By trial and error it was possible to find a time interval below which flow would occur and above which there would be no flow. This time interval is called the setting time, and its reciprocal is known as the rate of gelation. By following such a test procedure, the effect of variation in many significant quantities has been established. The electrolyte added was potassium hydroxide, because its action upon hydrogen bentonite is a simple one, resulting in the formation of potassium bentonite and water. In effect only one ion is added to the system instead of two, as in the case of a salt, and interpretation of results is facilitated. Potassium hydroxide was found to effect the formation of stronger gels in shorter times at lower concentrations than either sodium hydroxide or lithium hydroxide. -4fter addition of electrolyte the setting time did not assume a constant value immediately, but tended to fall in value and approach an asymptotic condition about twenty-eight days after preparation. The setting times reported are in most cases those at the end of twenty-eight days. Figure 11 shows the effect of particle size upon the setting time. The concentration of bentonite is 0.85 per cent in every case, and a t a given concentration of potassium hydroxide the time required in forming a gel of given strength increases as the size of the particles increases. Figure 12 shows the large decrease in setting time effected by increase in concentration of bentonite and increase in temperature. The temperature effect appears to be irreversible, in that gels which have reached a welldefined setting time a t 25°C. upon being heat-treated several days a t 40°C. and then lowered again to 25°C. do not exhibit their old setting time at 25°C. but a setting time much nearer the final value reached a t 40°C. In all cases but one to be mentioned below, increase in temperature resulted in a decrease in setting time. It was postulated that setting time might go through a minimum as temperature was increased, but careful measurements up t o 140°C. under many varied conditions failed to reveal anything but a continuous decrease in setting time with increase in temperature. These tests were not carried to higher temperatures, as it was feared that fundamental changes might take place in the bentonite particles in such a manner as to make the results incomparable to those at lower temperatures. Figures 13 and 14 emphasize the effect of particle size by plotting setting time against average equivalent spherical diameter. The present authors have reported the discovery of rheopexy in bentonite (5). This is a phenomenon much more difficult to meawre even
926
E. A . HACSER AXD C . E . R E E D
than thixotropy. The thixotropic setting time is obtained on a system which undergoes violent agitation to break up the structure and is then allowed to set to a gel undisturbed. The rheopectic setting time obtained
FIG.11. Thixotropic setting times of bentonite suspensions a t 25°C. Concentration of bentonite = 0.85 per cent.
S E l T I N G T I M E IN M I N U T E S
FIG.12. Influence of concentration of dispersed phase on thixotropic setting time
on the same system is always much lower, owing to the fact that during the period of set the system is subjected to gentle mechanical action such as tapping. It is not at all certain just what type of motion is most conducive to rheopectic behavior. It was found as fi result of subjecting the
STUDIES IN THIXOTROPY.
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927
tubes to various kinds of mechanical contortions that the following procedure seemed to produce the most rapid gelation: The tube, 100 mm. in length, was grasped 20 mm. from the top between the thumb and third finger, and was then made to oscillate like a pendulum about the point where it was grasped. The amplitude of the oscillations was 15" to 20" on each side of the vertical position, and the frequency employed was about two hundred and fifty complete oscillations per minute. In extreme cases the gels subjected to this treatment during their period of set reached a
FIG.13. Thixotropic setting time of 0.85 per cent bentonite suspensions a t 25'C.
given strength hundreds of times faster than if they were left to set alone. The effect of such treatment in decreasing the setting time of a given gel is truly remarkable. In many instances it was possible to trap small air bubbles within the gels by proper agitation, and to use the motions of these bubbles as telltales for the motions of the various layers within the gel. It was observed that the motion described above caused the small bubbles to vibrate up and down as though they were entrapped in an elastic medium. Furthermore, the bubbles on one side of the tube would be traveling up while those on the other side would be traveling down.
928
E. A . HAUSER A X D C . E. R E E D
The bubbles were observed to vibrate up and down with a frequency equal to the number of complete oscillations of the tube. This bubble motion indicated the niotions of the corresponding layers of gel. It was undeniable that the motion induced a velocity gradient across the tube, resulting in gentle shear between the various layers of gel. At any point in the tube the direction of the gradient, and hence the direction of the shearing stress, reversed with each oscillation of the tube. @
20 40 60 80 100 AVERAGE E Q U I V A L E N T SPHERICAL D I A M E T E R [ ~ J I ]
0
FIG. 14. Effect of particle size on setting times a t different temperatures. per cent bentonite; 59.5 millimoles of potassium hydroxide per liter.
0.85
While it should be possible to construct a machine to simulate mechanically the motion just described, it is questionable whether any additional information obtained from such a machine would assist in the task of obtaining a satisfactory qualitative explanation of rheopexy. Figure 15 shows the rheopectic setting time (determined by the above method) compared with the thixotropic setting time. In general, the type of electrolyte added has notable effects upon the setting time. At a concentration of 0.85 per cent bentonite, there was no
S T r D I E S IZ; THIXOTROPY.
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I1
evidence of gel structure until about 2.5 milliequivalents of potassium hydroxide per’gram of bentonite had been added (over twice the exchange capacity!), whereas at this same concentration of bentonite, gels which exhibit strong thixotropy and rheopexy can be produced upon addition of 0.0049 milliequivalent of tetraethylammonium hydroxide per gram of bent onit e. The pH as such seems to have no particular effect upon thixotropy. Thixotropic and rheopectic gels of 0.85 per cent bentonite may be produced at any pH from 1 to 12. Addition of hydrochloric acid to hydrogen
10 20 30 AVERAGE EQUIVALENT SPHERICAL DIAME’IER
40
IN m.p
Fro. 15. Influence of particle size on setting times. 0.85 per cent bentonite; 76.5 millimoles of potassium hydroxide per liter; temperature, 25°C.
bentonite will produce a thixotropic gel a t a pH of 1, which is not much different from one produced at a p H of 12 by addition of potassium hydroxide or one produced a t a pH of 7 by addition of proper relative quantities of potassium hydroxide and potassium chloride. a n interesting series of gels was produced by addition of dialyzed iron oxide sol (positively charged) to hydrogen bentonite (negatively charged). Addition of 0.0247 g. of ferric oxide per gram of hydrogen bentonite gave a gel which had a thixotropic setting time in excess of one hundred and thirty hours and a rheopectic setting time of two minutes. Additional ferric oxide lowered the thixotropic setting time to a few minutes. Posi-
930
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tively charged aluminum oxide ( h 1 2 0 3 ) sol (dialyzed) gave a similar e f f e ~ t , ~ but a quantity of negatively charged vanadium pentoxide sol‘just sufficient to flocculate the quantity of iron oxide sol added above had no effect. The setting time of the gels produced by addition of ferric oxide sol was greatly increased by an increase in temperature in strong contrast to the effect of temperature upon the potassium bentonite gels. Ultramicroscopic examination of the gelation of these well-defined fractions served to corroborate previous work by Hauser (4),in which the particles were reported to lose their Brownian movement gradually and come to a complete rest. The present observations were carried out with a recently developed General Electric type H3 “capillary mercury vapor lamp” as a light source, thereby greatly increasing resolution and intensity of reflection. If potassium hydroxide equal to 9 niilliequivalents per gram of bentonite is added, it is possible to observe in the ultramicroscope the changes which are responsible for larger scale thixotropic behavior. The individual particles cluster up, forming particle clouds which in turn form a loose structure of secondary clusters intermeshed with patches and channels of freely mobile dispersing medium. The Brownian motion of thc particles forming the clouds stops completely, whereas a few single particles can be seen in free motion in the patches and channels of dispersing medium. If this structure is submitted to any influence producing relatively strong shear, the clouds break up and all the particles are again in individual Brownian motion. If the system is now allowed to remain undisturbed, the clouds will re-form and Brownian action will gradually cease. If the freshly prepared specimen containing electrolyte is subjected to relatively gentle tapping or shear, the formation of the clouds into secondary clusters is more rapid. Under these circumstances, the structure formed by the secondary clusters of clouds is more sharply defined. Presumably we are observing rheopectic behavior upon a microscopic scale. When larger quantities of electrolyte are added, the clouds form more rapidly and are of notably denser structure. An excess of electrolyte produces spontaneous aggregation into individual flocs of relatively high density. Such dense flocs appear brilliantly white in the field. These dense white flocs cannot be noticeably dispersed, and to all intents indicate the presence of the final stage of flocculation of the system. Microscopic research is now being continued with the new equipment, and results will be reported in a later publication. =in important feature of the present inr-estigation of bentonite gels is The formation of a thixotropic gel in a kaolin suspension to which an aluminum eo1 had been added has been described recently by J . Pryce-Jones in a private coni-
munication to one of us. This phenomenon has a close resemblance t o complex coacervation as described by Bungenberg de Jong (Kolloid-Z. 79,228 (1937)).
STUDIES IS THIXOTROPY.
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the unexpectedly low concentrations at which gelation occurred. This is in contrast to Freundlich’s extensive work on the thixotropy of bentonite (3), where it was difficult to obtain structure a t concentrations of bentonite less than 10 per cent by weight. This difference is probably accounted for by the fact that Freundlich allowed his hydrogen bentonite to dry, and made up his gels from the dry powder. Lewis, Squires, and Thompson (10) have shown that the drying of hydrogen bentonite results in irreversible deplasticization. Hydrogen bentonite, once dried, will not swell in water to a degree even approximating the swelling of the metallic bentonites. This may be due to the reaction of some of the hydroxyl groups on adjacent particles, with the elimination of water and the formation of oxygen bridges, which effectively cement many particles into one large particle. For the qualitative detection of structure in very weak gels, the socalled bubble test is very valuable. This test rests upon the fact that a bubble will always rise in a Newtonian liquid, since such a liquid cannot sustain tangential shear. Thus bubbles will always rise in the thickest oil. On the other hand, as soon as a yield point is developed, a finite tangential stress or shear such as exerted by a small bubble beneath the surface of a liquid may be sustained.j If a gel is shaken down to a sol and then allowed to “set,” entrapped bubbles may be observed rising at a slower and slower rate, until eventually they come to a complete stop. At this point the yield point has become great enough to resist the shearing stress produced by the buoyant force. This test has shown that in many specimens agitation has reduced the system to the sol condition, or at least to such a weak gel that it will retain no visible bubbles and will not be cloudy, as it might be expected to be if it were retaining individual bubbles too small to see. In other cases, the bubble test shows that even the most violent agitation will fail to break up the gel completely, the bubbles coming to rest instantly after agitation. In many cases such gels, even though very weak after agitation, will often increase in strength upon “setting,” indicating that a process of construction is proceeding within them. While thixotropy is referred to as a sol-gel transformation, it may well include those cases where the gel cannot be broken down ,completely. In the case of the finest bentonite fraction (fraction 1)’it was possible to use the bubble test to observe structure qualitatively at concentrations of bentonite as low as 0.01 to 0.05 per cent. Needless to say, a strong electrolyte such as potassium chloride must be added to the hydrogen bentonite to observe structure at such low concentrations. Very little has been written concerning the actual structure of gels which J . Pryce-Jones differentiates between t r u e gels and false body systems.
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exhibit strong thixotropy. Freundlich gives a summary of the proposed theories in his monograph on thixotropy (3). It appears that all authors are in agreement that the suspended particles become loosely locked into place to form a structure which will resist shear. Disagreement occurs when attempts are made to theorize as to the mechanism by which the particles become locked into place. The solvation or water hull theory attributes the structure to the formation of quasi-solid hulls of adsorbed water around the particles, the hulls eyentually increasing to such sizes that they coalesce and impart rigidity to the system as a whole. As solvation is generally considered to decrease with an increase in temperature, it is difficult to see how on the basis of this theory one could account for the great decrease in setting time occurring with an increase in temperature. Lewis, Squires, and Thompson (10) have advanced an essentially mechanical picture of the gelation process in clays in which the structure is conceived as being built up of the loose, completely random packing of platy particles. This picture is similar to that of Freundlich, who has pointed out the correlation between loose packing, plasticity, and thixotropy. To account for gel formation at the low concentration of 0.1 per cent, the pure mechanical picture of gelation due to packing must assume ratios of length and breadth to thickness of the individual particles of the general order of 1000-2000 :1. Ultramicroscopic examination lends no support to the assumption of such ratios. The theory of gelation due to mechanical packing might account for the facts of thixotropic behavior in concentrated suspensions like those investigated by Freundlich, but to account for gelation of this type in more dilute suspensions the mechanical theory is unsatisfactory. I n his monograph on thixotropy, Freundlich describes a theory which attributes the formation of a gel of the type under discussion to the constituent particles becoming locked ipto place in equilibrium positions, wherein the force of attraction between particles (assumed to be of the extended van der Waals type described by Iiallman and Willstatter (9)) is just balanced by the force of repulsion due to the mutual repulsion of the diffuse double layer. The presence or absence of electrolyte regulates the effective sphere of action of the repulsive force by regulating the value of the zeta potential, and hence determines whether there is a completely stable suspension, coagulation, or the intermediate stage of thixotropy.6 In order for a gel of a given strength to be formed, a suitable number of the particles must take up their equilibrium positions, and as this process is one which occurs at random, it will take a finite time which we measure *See also t h e recent publications by H. c'. Hamaker (Rec. trav. chim. 66, 1015 (1936); 66, 3, 727 (1937)).
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as the setting time. The more pronounced the Brownian movement of the particles, the greater their chance of slipping into the equilibrium position from which they have no tendency to depart. The smaller the particle and the higher the temperature, the greater is this Brownian movement and hence the greater the speed of gel formation. On the basis of ultramicroscopic examination, it seems most probable that in the formation of a gel the particles are not necessarily distributed uniformly throughout the total volume, but are grouped together in primary clusters, which in turn coalesce to form a network throughout the whole volume interwoven with patches and channels of free dispersing medium. It is possible that rheopexy manifests itself in the presence of any gentle motion which tends to help the primary clusters to aggregate into the secondary network responsible for the rigidity of the structure. Violent mechanical action will break down not only the secondary network but also the primary clusters of particles. We feel that this theory of gel formation and structure is capable of accounting for all known facts, even in the most dilute gels, and is in full accord with the most recent ultramicroscopic observations. SUMMARY
1. Natural bentonite has been subjected to a centrifugal fractionation procedure which has yielded five fractions of varying average equivalent spherical diameter of 14.3 to 87 mp. 2. The exchange capacity and pH-concentration relation of all particle size fractions of hydrogen bentonite are identical within the limit of experimental error. 3. The specific conductance of hydrogen bentonite suspensions increases with a decrease in average equivalent spherical diameter of the suspended particles, but is much less than would be calculated from the hydrogenion activity of the same suspensions. 4. Under corresponding conditions of concentration of bentonite and electrolyte and temperature, the smaller the average equivalent spherical diameter of the suspended particles, the shorter the time required for the formation of a gel of given strength. 5. I n suspensions of the finest particles of bentonite, it is possible to detect evidence of gel structure at concentrations of bentonite less than 0.05 per cent. 6 . Ultramicroscopic observation indicates that gelation consists in the formation of primary clusters of individual particles, followed by secondary aggregation of these clusters into a network intermeshed with channels and patches of free dispersing medium. 7. Discussion of the results has indicated that they lend support to the THE JOURNAL OF PHYSICAL CHEMISTRY, VOL.
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open porous structure of the bentonite particle and the “equilibrium field of force theory” of gel structure. REFERENCES (1) BRADFIELD: J. Phys. (‘hem. 32,208 (1928). (2) BROUGHTON, G . , AND SQUIRES, L.: J. Phys. Chem. 40, 1041 (1936). (3) FREUNDLICH, H . : ActualitCs Scientifiques e t Industrielles, 267, Thixotropy. Hermann & Cie, Paris (1935). (4) HAUSER,E. A . : Kolloid-2. 48, 57 (1929). (5) HAUSER, E. A , , AND REED,C. E . : J. Am. Chem. SOC.68, 1822 (1936). ( 6 ) HAUSER, E. A . , AKD REED,C. E.: J. Phys. Chem. 40, 1169 (1936). (7) HOFMANN, U., ENDELL, K., AND WILM,D. : Z. Krist. 86,238,340 (1933). (8) HOFMANN, U.,AND BILKE,W . : Kolloid-Z. 77,246 (1936). (9) KALLMAN, H . , AND WILLSTATTER, M..: Naturwissenschaften 20, 952 (1932). (10) LEWIS,SQUIRES, A N D THOMPSOS: Trans. Am. Inst. Mining Met. Engrs. 118, 1 (1936). (11) MARSHALL, C. E.: Science Progress 30,422 (1935-36). (12) MARSHALL, C. E.: Z. Krist. 91, 433 (1935). (13) MARSHALL, C . E., ASD GUPTA,R. S.: J. Soc. Chem. Ind. 68,433T (1933). (14) Ross, C. S.,AKD SHANNON, E. V.: J. Am. Ceram. Soc. 9,77 (1926). (15) RUSSELL:Proc. Roy. SOP.(London) A164,554 (1936).
ERRATA The following corrections should be made in the first paper in this series, J. Phys. Chem. 40, 1169 (1936): Page 1179. The ordinatmein figure 4 should be (100)W/T. Page 1179. Rz = 2.17 cni. Page 1180. The ordinate in figure 5 should be f(D)jlO’.
