THE GELATION OF BENTONITE SUSPENSIONS' It has been known

T H E GELATION OF BENTONITE SUSPENSIONS‘ GOEFFREY BROUGHTONS

AND

LOMBARD SQUIRES

Department of Chemica2 Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts Received June 1 1 , 1986

It has been known for some years that certain sols, on standing, form gels which can be reliquefied by shaking. This phenomenon was first discovered by Schalek and Szegvary (13) in 1923 for ferric oxide sols to which a small amount of electrolyte, for example, sodium chloride, had been added. Since then many cases of this isothermal reversible sol-gel transformation, which Freundlich has termed “thixotropy,” have been reported. Bentonite, a clay-like material, probably of volcanic origin and found largely in Wyoming, exhibits thixotropy to a marked degree in aqueous suspensions. Suspensions of suitable concentration will set to a gel in a few seconds, although a number of other factors materially influence this setting time. Particle size undoubtedly plays a great part, since centrifuged suspensions will give dispersions showing thixotropy down to at least a concentration of 1 per cent, whereas ordinary bentonite suspensions require a concentration of over 4 per cent.s The hydrogen-ion concentration of the suspension, as might be expected, was also found by Freundlich, Schmidt, and Lindau (5) to be of great importance. Acidwashed bentonite or electrodialyzed bentonite suspensions do not show thixotropic behavior. In this paper the effects of temperature and concentration upon gelation are described. No attempt has been made to determine the influence of pH, the hydrogen-ion concentration being kept m constant as possible. METHODS OF MEASUREMENT

The investigation of thixotropy is greatly hampered by the lack of an adequate method of measurement. The inverted-tube method first introduced by Schalek and Szegvary (14) has been most commonly used. The sol is introduced into a tube, which is stoppered or preferably sealed 1 Presented before the Thirteenth Colloid Symposium, held a t St. Louis, Missouri, June 11-13, 1936. 2 Fellow of the Salters’ Institute of Industrial Chemistry, London, England. a Centrifuged dispersions will be termed “ultrabentonite” in the following in order to distinguish them from ordinary bentonite suspensions. 1041

1042

GEOFFREY BROUGHTON AND LOMBARD SQUIRES

off. After vigorous shaking the tube is allowed to stand undisturbed in a vertical position for a known time and then inverted. If flow occurs down the wall of the tube, the sol is said to be still liquid. If no flow is apparent then the sol is said to have solidified. By trial and error a time interval can be found below which flow occurs and above which there is no flow. This is called the setting time; its reciprocal is taken aa the rate of gelation. TABLE 1 T h e efect of the amount of material upon the time of setting Ultrabentonite, 2.09 per cent; pH, 7.84; T,27.5"C. T U B E DIAMETDR

mm.

7.5

9.6

I I

AMOUNT OF LIQUID

B l T T I N Q TIME

cc.

seconds

1 1 5 20

405

1 .o 2.0

1485 1485

395

405

DIAMETER

FIG.1. Effect of diameter of tube upon time of setting of ultrabentonite a t 27.5"C.

Theoretically, it is seen that this setting time must depend upon a number of factors,-the strength of the gel formed, its weight, the surface tension of the suspension, etc. Practically, it varies greatly with the diameter of the tube; also great care has to be taken in inverting the tube, since slight shaking may cause flow to occur when none would take place

1043

GELATION OF BENTONITE SUSPENSIONS

with gentle inversion. With care consistent results can be obtained, although these vary with the observer. One would desire a true history of the development of gel structure, i.e., of resistance to initiation of flow, but the inverted-tube method unfortunately only gives one point upon such a curve.4 I n spite of the extensive use of the inverted-tube method, little appears to have been published upon the influence of amount of material enclosed in the tube, the diameter of the tube, etc. As is evident from table 1, within reasonable limits the former has no effect upon the setting time. On t’he other hand, the diameter of the tube has a very marked effect, Z

IN SECONDS

,O

E 2

FIQ.2. Effect of diameter of tube upon setting time a t 27.5”C. 0 , 6.60 per cent bentonite; @, 2.09 per cent ultrabentonite.

as seen from figure 1. Approximately, at least, the relation between tube diameter and setting time for a given suspension and temperature is z = kDn (1) where z is the setting time, D the internal diameter of the tube, and k and n are constants. Figure 2 shows that the diameter of the tube has a greater influence upon ultrabentonite than upon the crude suspensions. Its influence also decreases with temperature. The values of k and n unfortunately vary according to the suspension examined, so that no general correction factor can be given for changing tube diameter (table 2). Furthermore, as will be shown later, the temperature coefficient of the setting time of a given suspension is not independent of the tube in which it is measured. 4 As Freundlich and Rawitaer (4)have stated, “The setting time is characterized neither through a certain variation of tensile strength nor through an abrupt variation in the velocity of transition. I n no respect does it represent a unique point on the solidification curve. For comparison of different sols the setting time is nevertheless very suitable.”

1044


These considerations led to the view that another method of measuring the rate of gelation of thixotropic suspensions would be desirable. Freundlich and Rawitzer (4) measured the tensile strength of a ferric oxide sol by means of a cup suspended by a torsion wire in the sol contained in an outer cylinder, which could be rotated. Pryce-Jones (12) adopted a similar but more elaborate device. Such methods, while useful in showing TABLE 2 V a l u e s of k a n d n f o r different suspensions TDMPERATURE

SVSPENSION

'C.

27.5 27.5 75.0

k

CONCENTRATION

n

per cent

Bentonite Ultrabentonite Ultrabentonite

6.60

0.044

2.09 2.09

0,000038

3.0 7.8 2.1

1.8

020

7

g

024

Y u

60

z

z_ 0 2 0

u 0 w

J

z

2

E

016 40

i

g Oi2 > 008

jL

E 20

0.04

0 0

16 8 STANDING TImE

MINUTES

0.02

004

006

008

01 R L

C Mz

Fia. 3 FIQ.4 FIQ.3. Effect of diameter of viscometer upon time of setting. 0,viscometer I, internal diameter 1.2 em.; 8,viscometer 11, internal diameter 1.5 cm. FIQ.4. Ultrabentonite a t 58°C. Standing time, two hours. Velocity of sphere plotted against the square of its radius.

there is no abrupt transition between sol and gel, require elaborate apparatus and care in operation while still yielding only comparative results. A more natural property to investigate appears to be the viscosity of the sol. In fact, most studies of the gelation of emulsoidal solutions have been made by viscosity measurements. Gelation is a kinetic process, almost certainly consisting of the building-up of a structure, which inter-


1045

feres with the flow of the liquid. Unfortunately most methods of measuring viscosity cause simultaneously the breakdown of this gel structure to an unknown degree. The falling-ball viscometer has, however, the advantage that the sphere continuously passes on to the undisturbed suspension, so that its travel is relatively free from the effects of structure breakdown. This led to a decision to use the falling-ball method in measuring the rate of gelation of bentonite suspensions. The ordinary type of falling-ball viscometer was used (1). The tube had a width of about 12.5 mm., and a glass tap was fused to the lower end to facilitate the removal of the dropped balls. The distance between marks was 13.9 cm. The viscometer was placed in a thermostat, the temperature of which could be kept constant to f 0.25'C. Glass balls about 5 111111. in diameter were first used, since Freundlich (2) has pointed out that the pH, and as a consequence the setting time, of ferric oxide sols is altered by contact with metal. Steel balls about in. in diameter were afterwards used, but no irregularities in behavior were observed. The sol was placed in the viscometer, allowed to come to the temperature of the bath, shaken well, and replaced. After the sol had stood for a known length of time the ball was dropped and the time of fall taken. I n this way a curve such as that shown in figure 3 could be obtained. No attempt was made to convert the times of fall into absolute viscosities, for several reasons. Ultrabentonite belongs to the class of substances showing plastic flow. That is, it possesses a definite yield point and its apparent viscosity is not independent of the shearing force. For a true liquid the curve connecting velocity and the square of the radius of the sphere would be a straight line passing through the origin. This is not the case for ultrabentonite (figure 4). Furthermore, Stokes' law, upon which any calculations would have to be based, is derived for the case of a homogeneous fluid and cannot hold for a gelating sol. Nevertheless, measurements using the same sphere and viscometer give comparative results for the rates of gelation over the whole curve, and not merely for one particular point. Time of fall is linear in standing time, at least during the early stages of gelation and

y=ax+b (2) where y is the time of fall, x the time of standing, and a and b are constants. Granting that the degree of development of structure, Le., gelation, is proportional to the increase in time of fall, it follows that the rate of gelation, as measured with a given ball, is independent of the time of standing. This surprising result shows a marked difference from the case of emulsoidal gel formation. Thus Mardles (lo), for cellulose acetate gels in benzyl alcohol and other solvents, found by the falling-ball method that g - ?lo = aekt (3)

ZO

40

60 EO TEMPERATURE

100'C

FIQ.5

FIQ.7 FIQ.5. Effect of temperature on setting time. 2.09 per cent ultrabentonite; 0 , 2.19 per cent ultrabentonite. FIQ.6. Effect of temperature upon setting time of 2.09 per cent ultrabentonite. 8 , 2 5 " C . ;El,40°C.; @,5OoC.;O1600C. FLQ.7. Effect of temperature upon setting time. @,2.09per cent ultrabentonite; 0 , 2.19 per cent ultrabentonite. Diameter of tube, 9.6 mm. FIQ. 8. Effect of temperature on standing time (5). Time of fall = 50 secs. 0, 2.09 per cent ultrabentonite; 8 , 2.19 per cent ultrabentonite; 8,2.55 per cent ultrabentonite; 0 , ultrabentonite and 15 per cent ethyl alcohol; a, ultrabentonite and 0.5 per cent gelatin. 1046

1047


where q is the ,viscosity at time t, q o is the viscosity at zero time, and a and k are constants. These viscosities are proportional to his times of fall, since they were calculated directly from these, ignoring the objections mentioned above. It will be noticed that the rate of gelation is also independent of the diameter of the viscometer tube. TABLE 3 V a l u e s of A and B f o r two ultrabentonites differently prepared I

I

CONCBINTRATION

CEARACTER OF SURPBINSION

I

B

I

A'

B'

0.021 0.022

1 98 1.95

-----A

pH

p e r cent

Centrifuged twice. . . . . . . . . . . . . . . . . . . 2.09 Centrifuged once... . . . . . . . . . . . . . . . . . 2.19

7.84 8.40

0.0165 3.34 0.0173 3.28

I000

2

m

Y

w

z N

100

m

loo

LO

40TEMPERATURC a ao

zN

im'c

I00

Io0

20

40

LO

80

1OO'C

TEMPERATURE

FIG.9 FIG. 10 FIQ.9. Effect of diameter of tube on the temperature coefficient. 2.55 per cent ultrabentonite. 8,diameter of tube 11.11 mm.; 0 , diameter of tube 12.70 mm. FIG.10. Effect of diameter of tube on the temperature coefficient. 2.09 per cent ultrabentonite. 0 , diameter of tube 9.5 mm.; 0 , diameter of tube 7.5 mm. THE EFFECT OF TEMPERATURE

The effect of temperature upon gelation wtw examined by both methods. Schalek and Szegvary (14) found for ferric oxide sols that if t is the temperature in "C.then log 2 = - At THQ JOURNAL OF PEYUCAL CEBYI~TEY, VOL.

40, NO. 0

+B

(4 1

1048


where A and B are constants and z the setting time. Increase in temperature in bentonite suspensions likewise causes gelation to become much more rapid (figures 5 and 6), and for both methods the results can be expressed by equations of the form of equation 4. In the falling-ball method, if the standing times at various temperatures for a given time of fall are plotted against temperature on semilog paper, a straight line (figure 8) is again obtained or log x =

- A’t + B’

(5) where 1: is the standing time and A’ and B’ are constants, Furthermore, it was found that two ultrabentonites, prepared by different methods and having slightly different compositions, gave the same values of A and

FIQ.11. Effect of concentration on time of setting in a 9-mm. tube. 0 , 55°C.

@, 25°C.;

B in the inverted-tube method, and also gave the same values of A’ and B’ in the falling-ball method, as shown by table 3. The constants A and B , however, are changed if tubes of different diameters are used for the same suspension, as shown by figures 9 and 10. This limits their value for comparative use, as, for suspensions of widely differing concentrations, measurements cannot be made in the same tube. The effect of temperature upon the rate of gelation again presents a distinct difference from the behavior of emulsoid sols. Emulsoid sols, such as that of gelatin, do not gel under any conditions above a certain temperature, whereas bentonite simpensions apparently gel at any temperature. Furthermore, the rate of gelation of ‘gelatin increases on

GELATION O F BENTONITE SUSPENSIONS

1049

lowering the temperature (8). In contradistinction to this, bentonite suspensions, as can be seen from figure 6, gel much more slowly at low temperatures. THE EFFECT O F CONCENTRATION

Concentration has a very great influence upon the setting time of bentonite suspensions. The suspensions referred to in figure 11 were made up by diluting a strong suspension of bentonite and varied slightly in pH from 9.3 to 9.6, the weakest suspension having the highest pH. This change in pH was probably insufficient to affect the setting times materially.

FIQ. 12. Effect of concentration on temperature coefficient. Diameter of tube IXI, 4.99 per cent bentonite; 8 , 5.63 per cent bentonite; @, 5.93 per cent

9 mm.

bentonite; 0 , 6.60 per cent bentonite.

Concentration does not affect the temperature coefficient A in equation 4 ae greatly as does a change in tube diameter (figure 12). DISCUSSION

It is seen that the inverted-tube method is capable of giving comparative results providing care in manipulation is exercised and the diameter of the tube in which the measurements are made is kept constant. The falling-ball method is preferable in that it gives the whole of the gelationtime curve and not simply one point. The results of the two methods appear to be in agreement. The explanation of gelation and thixotropy, as shown by bentonite

1050


suspensions, is difficult. Hauser (6), observing the behavior of very dilute bentonite suspensions under the microscope, found the particles in Brownian motion. Addition of electrolyte caused translation to cease, and still greater quantities of electrolyte caused the cessation of rotary motion. Providing addition of electrolyte had not been too great these changes could be reversed by agitation. Addition of a hydrophilic sol, itself showing Brownian motion, e.g., gum mastic, resulted in cessation of the motion of the gum mastic particles when gelation occurred. There seems no doubt that some sort of structure is built q as the gel forms; the difficulty is to account for gelation at the extremely low concentrations at which it occurs (certainly less than 1 per cent in some cases). For bentonite, two alternative hypotheses have been suggested. The first might be termed the mechanical or ‘ L h o ~ ~ e - ~ f - ~theory, a r d ~ ”the clay particles being visualized as flat plates, so packed in three-dimensional, random orientation, edge touching edge in the gel, that movement is impossible TABLE 4 Effect of a diluent on setting time 2.55 per cent ultrabentonite; 3 cc. used for each experiment DILUENT ADDED

cc.

0 0.125 0.25 0.50 0.75

2

(WATER A D D E D )

( c ~ H ~ oADDED) H

seconds

seconds

165 420 720 1800

165 210 370 335 480

and a solid house-of-cards structure is set up (9). The chief objection is that in order to obtain interference between the particles at the low ooncentrations at which gelation is observed, the particles must be plates, having a length and width much greater than their thickness, the ratio between these quantities being of the order of 100 to 1 or os’er, which seems extremely great. On the other hand, although direct microscopic evidence is lacking, this theory of the bentonite plates being of microscopic thickness and macroscopic length and width was first put forward by Wherry (15) on microscopic grounds. X-ray evidence has also shown that the bentonite clays are made up of silica and gibbsite (A12(0H)e) layers, which are separated by water (11). The spacing due to this separation varies with the water content of the clay, and thus it appears that in a large excess of water, the layers may actually break away from each other and behave as plates of molecular thickness. Other properties of the bentonites, e.g., adsorptive power, are also in agreement with the thickness of the plates being of molecular or colloidal dimensions.

1051


The only other theory to receive serious consideration has been the suggestion that the clay particles adsorb layers of water sufficiently thick to build up quasi-fluid particles occupying enough volume so that interference is sufficient to induce gelation. This hypothesis requires the formation of water hulls around the particles many molecules thick (which seems unlikely). Bentonite gels have also been shown to possess a tensile strength, which is hard to explain on the basis of particles with adsorbed water surfaces touching each other (9). As a general rule solvation decreases with temperature and one would, therefore, expect the water hulls, if such are formed, to decrease in thickness with increasing temperature, thus leading to a decrease and not an increase in rate of gelation. Further evidence against the water-hull hypothesis is also provided by experiments in which alcohol was added to a bentonite suspension (table 4). The setting time of the suspensions remained nearly constant, whereas addition of water in equal amounts by volume caused the setting times to become indefinitely great. Alcohol is usually considered to act as a dehydrating TABLE 5 Values of the temperature coeficient u

h

Temperature interval :

4O-50"C.. .......................................................

4O-50"C......................................................... 40-5O'C.. ......................................................

2.02 1.52 2.22

Calculated value.. ............................................... Value from ordinary chemical rsaction.. ..........................

agent when added to aqueous colloids; thus, on the water-hull theory, one would expect it to be even more effective than water in increasing setting time. The viscosity of an alcohol-water mixture is greater than that of either pure component, hence Brownian motion would be retarded and an increase in this factor could not account for the more rapid gelation. On the other hand, the mechanical theory of gelation seems at &st sight equally unable to account for the rapid increase in rate of gelation with temperature, unless further assumptions are made.. Freundlich has attempted to draw a parallel between the coagulation of hydrophilic sols and the gelation of thixotropic sols. It is true that the rate of change increases with rise of temperature in each case, but the increases are of a different order of magnitude. According to the von Smoluchowski theory of coagulation the time of coagulation should be proportional to q/T, where q is the viscosity of the solvent and T the absolute temperature. This was experimentally confirmed by Freundlich and Basu (3) for the coagulation of a copper oxide sol by sodium sulfate. Some experimental

1052


values of the temperature coefficient, Le., the ratio of the rates of change for temperatures 10°C. apart are compared in tablr 5 with the value which might be expected for aqueous bentonite suspensions. The constants a in equation 2 for the falling-ball method were used. Evidently, the phenomenon of thixotropy in bentonite suspensions is not truly analogous to coagulation. On the mechanical theory the bentonite particles, even immediately after shaking, are in close juxtaposition, although the strength (and hence the apparent Yiscosity) of the gel is small. Each subsequent impact between particles builds up the strength as they become more firmly fixed in position, and thus the apparent viscosity increases. However, as the structure becomes stronger, collisions

4

8 STPMNG

12

lb

TIYE IN

rlvurcs

FIQ.13. Effect of addition of 0.5 per cent gelatin. Q, 25°C.; El, 35'C.; 0 , 45°C.; 0, original sol at 25°C.

of ordinary impact energy have little or no effect, whereas collisions of abnormally great impact energy, of which there will always be some from the distribution theory, have a disproportionate effect in raising the strength. In a coagulation process the particles have a tendency to agglomerate, whereas in the thixotropic gelation of bentonite the particles have a tendency to keep apart and are only forced into contact by continual impacts. After a time, only impacts of exceptional energy are effective in causing increase in gel strength. Temperature causes an increase in these impacts of abnormal energy and hence a quite disproportionate increase in the gel strength. In other words, the gel strength is not proportional to the number of impacts, but depends much more upon their energy content. Hence the influence of temperature is explained by the


1053

great increase in strength brought about by the greater number of high energy impacts. The action of alcohol is difficult to explain; possibly it affects the ionization of the clay particles or the electrical forces acting between them. Gelatin likewise increases the rate of gelation, as can be seen from figure 13; possibly the adsorbed gelatin on the particles allows a structure to be built up more easily. The mechanical theory of thixotropy also receives support from the fact that thixotropic sols, as far as has been at present investigated, show no change in their physical properties upon gelation. No work appears to have been done upon bentonite, but Heyman (7) has shown that if there is any volume change in an iron oxide sol upon gelation it is less than 0.0002 per cent. Other evidence, particularly the viscosity characteristics, has also been cited in a previous paper (9). SUMMARY

Two methods have been developed for measuring the rate of gelation of bentonite suspensions. The first measures the time elapsed after the suspension has been shaken before a tube containing it can be inverted without flow. In the second the velocity of fall of a sphere through the suspension is measured at different times after shaking. Both give results qualitatively in agreement but the latter method is preferred, since it is independent of the diameter of the containing vessel and gives the entire gelation-time curve. The results are found to be in conflict with the water-hull theory of gel structure and to be qualitatively in agreement with the mechanical theory, in which the gel is pictured as made up with bentonite plates of molecular thickness touching in completely random, three-dimensional orientation. REFERENCES (1) See, e.g., BARR:A Monograph of Viscometry. Oxford (1931). (2) FREUNDLICH: Thixotropy. Actualitbs Scientifiques e t Industrielles, No. 267. Plermann & Cie, Paris. (3) FREUNDLICH AND BASU: Z. physik. Chem. 116, 203 (1925). (4) FREUNDLICH AND RAWITZER: Kolloidchem. Beihefte 26, 231 (1927). (5) FREUNDLICH, SCHMIDT, AND LINDAU:Kolloidchem. Beihefte 36, 43 (1932). (6) HAUSER:Kolloid-2. 48, 57 (1929). (7) HEYMANN: Trans. Faraday Soc. 32, 462 (1936). (8) LAMPITTAND MONEY:J. Soc. Chem. Ind. 66, 88T (1936). (9) LEWIS,SQUIRES,AND THOMPSON: Trans. Am. Inst. Mining Met. Engrs. 118, 1 (1936). (10) MARDLES:Trans. Faraday SOC.18, 327 (1923). (11) MARSHALL: Science Progress SO, 422 (1936). (12) PRYCE-JONES: J. Oil Colour Chem. Assoc. 17, 305 (1934). (13) SCHALEK AND SZEQVARY: Kalloid-Z. 32, 318 (1923). (14) SCHALEK AND SZEQVARY: Kolloid-Z. 33, 326 (1923). (15) WHERRY:Am. Mineral. 10, 120 (1925).

THE GELATION OF BENTONITE SUSPENSIONS' It has been known

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