ROLE OF EXCHANGEABLE CATIONS IN VISCOSITY OF CLAY SUSPENSIONS

by V. E. NAS~

Cities Service Research and Development Company, Tulsa, Oklahoma

A B S T R A C T A study was made of the changes in viscosity of Wyoming bentonite suspensions with variations in cation ionization, base saturation, clay concentration, sodium chloride concentration and type of exchangeable cation. There is little change in the viscosity with decreasing cation ionization until a certain threshold value is reached. Below this value the viscosity increases rapidly, indicating that the repulsive potential barrier has been reduced to the order of the kinetic energy of the particles. Unfortunately, a quantitative estimate of the ionization was not possible at this value because of the high salt content of the system.

Exchangeable aluminum was found to act as a strong bonding agent between clay particles in the pH range 4.50-5.50. This was explained on the basis of the formation of multivalent, aluminum-hydroxyl complex ions.

I N T R O D U C T I O N

The in terac t ion of c lay par t ic les in an aqueous med ium is of in te res t from a colloid chemis t ry s t andpo in t as well as in the technica l fields of dr i l l ing m u d technology, soil physics and c lay sedimenta t ion . Recent ly , two mechanisms have been proposed to expla in the associat ion of clays in d i lu te sal t-free suspensions. Van Olphen (1956) considers the in te rac t ion to be be tween the posi t ive edges of one par t ic le and the negat ive surfaces of another . However , M 'Ewen and P r a t t (1957), and M'Ewen and Mould (1957) have concluded f rom opt ical measurements t h a t there is an edge- to-edge associat ion of par- t icles forming flat r ibbons. The work presented here is consis tent wi th e i ther of these views since the measurements reveal only the gross resul t of a par t ic le- to-par t ic le association. I n order to expla in the gela t ion proper t ies of these suspensions i t is necessary to assume tha t associat ion can occur between par t ic les a t cer tain poin ts on their surface bu t no t a t others. I f associat ion could occur between any points on the surface, comple te f locculat ion would result . Conversely, if no associat ion occurred, t he par t ic les would r ema in in a suspended s ta te . I n order to form an inter lacing ne twork of par t ic les t h roughou t the dispersion med ium i t is necessary t h a t the par t ic les be in a s ta te in te rmedia te between dispersion and flocculation. This dual na tu re of the c lay par t ic les m a y be due to a difference in the charge d i s t r ibu t ion of the exchangeable ions a t the edges compared to the p l ana r surfaces resul t ing in different repuls ive potent ia l s . Al te rna te ly , i t m a y be expla ined b y assuming t h a t p a r t of the surface is hydrophi l ic and p a r t hydrophobic . This l a t t e r app roach has been used in this paper . These terms, as used here, refer to the

328

EX~ttANGEABLE (JATIONS IN VISCOSITY OF CLAY SUSPENSIONS 329

sensitivity of the clay surfaces to flocculation by electrolytes. In this meaning of the two terms there is no difference in the two explanations. One may think of a surface with a high repulsive potential as having exchangeable ions that are extended a considerable distance into the dispersion medium. Therefore, the dispersion medium in the double layer, for practical purposes, is asso- ciated with the particle surface and the surface would be hydrophilic, accord- ing to the classical definition of the term. A hydrophilic surface is equivalent to a high repulsive potential, and a hydrophobic surface is equivalent to a low repulsive potential.

The objective of this study was to determine, through the measurement of viscosity, the interaction of montmorillonite clay particles under condi- tions of varied cation ionization, base saturation, clay concentration, sodium chloride concentration and type of exchangeable cation.

A C K N O W L E D G M E N T S

The writer wishes to express his sincere appreciation to the Cities Service Research and Development Company for permission to publish this paper on an investigation conducted at the Production and Exploration Research Laboratory at Tulsa, Oklahoma. Appreciation is also expressed to Dr. B. N. Rolfe for many helpful suggestions throughout the course of this work and for his critical reading of the manuscript.

E X P E R I M E N T A L P R O C E D U R E

Wyoming bentonite obtained from the Baroid Sales Division of the National Lead Company was used throughout this research. The natural clay was suspended in distilled water and the fraction to be studied (< 0.3 ~) was separated with the International no. 2 centrifuge. I t was then converted to the hydrogen form by passing through columns of hydrogen- and hydroxyl- saturated exchange resins (Illco C-211 and Illco A-244, respectively).

Hydrogen-sodium systems and hydrogen-calcium systems were prepared by titrating the hydrogen clay with freshly prepared sodium or calcium hydroxide. The aluminum-saturated clay was prepared by leaching with 1 N A1Cla and removing the excess salt by washing with distilled water until no chloride was detected in the leachate. Hydrogen-aluminum systems and sodium-calcium systems were prepared by mixing the homoionic clays in the desired proportion. Suspensions containing sodium chloride were pre- pared by adding solutions of the salt to clay suspensions of known concentra- tion and cation saturation.

Measurements of the pH were made with the Beckman H2 glass electrode pH-meter. Sodium activities were determined by the clay membrane method of Marshall (Marshall and Bergman, 1942). Conductivity measurements were made with a dip cell using a Leeds and Northrup 60 c/s a.c. galvanometer and Wheatstone bridge. The viscosity was determined with an Ostwald viscometer.

330 SEVENTH NA'rmNAL CO~FEJ~ENC~ ON CLAYS XN~)(JLAY MINERALS

R E S U L T S

Sodium-hyd~vgen Clay Suspensi,l~s Titration with sodium hydroxide.--Thc change in the ionizat ion of sodium

with add i t ion of N a O H to the hydrogen c lay suspension (0.773 percent) is shown in Fig. 1, along wi th the corresponding viscosi ty changes. The mos t obvious fea ture of the ac t iv i ty curve is the sharp increase in sodium ac t iv i ty a t the equivalence po in t along with the corresponding decrease in cat ionic bonding energy# Accompany ing this change in ac t i v i t y there is a decrease

4 0 �84

- 3 . 0

~o

05

0 4

0.3

U ~ e2

, i , , , 1 i , i , , i ,

: ~ j J j i ) p ! ;

I i , i i i i i : : ; :

~ 1 0 0 r

I LJ D

, I i

0 lO ~ 0 50 4 0 5 0 6 0 7 0 8 0 9 0 I 0 0 l lO 120 meQ N o O H pe r I 0 0 g c l a y

FI(~u~s 1.--Clay titration curve for 0. 773 percent Wyoming bentonite (fraction <0.3/~) with sodium hydroxide. A : sodium ion activity (moles/L). B : fraction active. C : mean free bonding energy of sodium. D : relative viscosity of clay suspension.

in viscosi ty. A t first glance, this might be t a k e n as p roof t h a t an increase in ionizat ion resul ts in a decrease in the associat ion between par t ic les wi th a corresponding decrease in viscosi ty. However , i t mus t be remembered t h a t this change occurs af ter the equivalence po in t and does no t represent ioniza- t ion of the surface. I t mere ly represents an add i t ion of sodium hydrox ide

z Using the terminology of Marshall (1950) the cationic mean free bonding (AF)ca~lo. is defined as (~F)cation ~ RT In c/a where c is the total concentration of the cation and a is the measured activity. The fraction active, f, is defined as air and is a measure of the degree of ionization.

EXCHANGEABLE (JATIONS IN VISCOSITY OF CLAY SUSPENSIONS 331

to the external solutions. In fact, we might expect this addition of excess electrolyte to depress the ionization of the adsorbed cations. The fact that the activity measurements do not indicate a depression of ionization and tha t the viscosity decreases constitutes an apparently anomalous reaction. Perhaps the most obvious explanation would be that hydroxyl ions are adsorbed to the clay surface, thereby increasing the exchange capacity of the clay and providing a greater density of surface charge. However, the adsorption of hydroxyl ions seems to be minor, because both the sodium

i , i ~ i , i i i i i i i

to O IOC " = 0 716 meq No * per g -- x = 0 5.58 meq Na § per g f x 8C 0 : 0 716 meq H + per g,

~h 2c~- 4c6C ~ A 05

~- o4

~ 0.3

~ 0.~:

u. 01 ,<~ ~ 0 0

i i L I I I I I I I I I

~7~" ~ c 8o0 ~ J

"~50( 16o0 ~

~40( ~500 ~

10151 1~3 ' 1!5 1 2 ! 0 I 21.5 1 ~.0

CLAY CONCENTRATIOrq- W'~ PER CENT

FIOU]~E 2.--The effect of cl~y concentration on tim ionization of Wyoming bentonite (fraction <0.3/~). A : sodium and hydrogen ion activities (moles/L). B : fraction active. C : cationic mean free bonding energy.

activity and the conductivity increase linearly and at about the same rate as for sodium hydroxide alone. An alternate hypothesis is tha t exchangeable aluminum, which significantly affects the viscosity, becomes ineffective at this point. This possibility will be considered in more detail later.

Effect of clay concentration on viscosity.--Changes in viscosity and cation activity with increasing clay concentrations have been determined for three levels of base saturat ion--approximately 100 percent hydrogen, 75 percent sodium, and 100 percent sodium. The cation activity data are presented in Fig. 2, which shows a steady increase in cation activity and a decrease in

332 SEVENTH ~ATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

cationic bonding energy with increasing clay concentration. The low bonding energy for the dilute clay suspensions containing exchangeable sodium is probably due to Donnan hydrolysis. Dote that with an increase in clay con- centration the degree of ionization (fraction active) increases. This is in contrast to the behavior predictable by the interionic attraction theory of strong electrolytes. This effect was also substantiated by conductivity measurements.

I f particle association is directly proportional to clay ionization, a decrease in reduced viscosity with increased clay concentration is to be expected. The viscosity curves in Fig. 3 indicate, however, that there is a definite enhance- ment in viscosity with increased clay concentration. The viscosity in Fig. 3 is expressed as reduced viscosity (viscosity per unit volume of clay), thereby

i J i i i 1 T i i , J

24 o ; 0 7 1 6 meq N 0 * / g

~,~ X : 0 5 3 8 meq N a * / 9 . b 22 -- 0=0 .716 m e q H * / g .

2C

'o I

m I

/ I I i ~ i I I o , 2 3 4 ~ 6 ~ ~ ~ I'o J',

VOLUME FRACTION OF CLAY, ~ x I0 3

FIGURE 3.--Variation of reduced viscosity [ ~ ~ -- 1/r *with concentration ofWyoming

bentonite (fraction <0.3#) for cation saturatio~ns of approximately (1) 100 percent sodium, (2) 75 percent sodium--25 percent hydrogen, and (3) 100 percent hydrogen.

accounting for the expected change due to increased concentration of clay. The sharp increase in viscosity at the high clay concentrations is probably due to the interference between particles resulting in a greater resistance to orientation.

Some indication of the effect of cation ionization on the viscosity of clay suspensions can be obtained from curves 1 and 3 of Fig. 3. The viscosity of the hydrogen-saturated clay (no. 3) is consistently greater than that of the sodium-saturated clay. Along with this higher viscosity of the H-saturated clay, there is a much smaller cation activity and fraction active (ionization). The difference in viscosity, however, is not as great as one might expect

* r refers to volume fraction of clay in suspension.

EXCHANGEABLE CATIONS IN VISCOSITY OF CLAY SUSPENSIONS 333

from comparison of the fractions active of the two systems. A sixfold decrease in fraction active results in only a twofold increase in viscosity, indicating that factors other than ionization are involved in the interaction of particles.

Effect of sodium chloride on viseosity.--The curves of Fig. 4 show the change in viscosity with increasing sodium chloride concentration for two levels of sodium saturation. From consideration of the effect of electrolytes on the ionic double layer, one would expect the addition of sodium chloride to enhance the association of clay particles and thereby increase the viscosity of the suspension. The data show that this holds only at high concentrations of sodium chloride. In fact, the first additions of sodium chloride cause a decrease in viscosity. This effect may be due to the adsorption of chloride on the clay. Measurement of the chloride activity with an Ag : AgC1 electrode indicates that there is indeed some adsorption of chloride ions. Sodium ion

i l i i l i i [ i i [ i i i i l i i i i i l ]

70 60 5O

1

I

20 - I

I

* = 0 7 1 6 meq N o + / g

X = 0 5 5 8 meg No+/g

j j : J i o i i t I I I [ I i J I I i i i i i i ~ L J

2 ~ ~ , 5 . 6 , 7 8 9 l 2 / / 20 50 4 0 5 0 6 78910

CONCENTRATION No CI ( m e q / I )

FIovm~ 4.--Variation of relative viscosity of Wyoming bentonite suspensions (concen- tration 0.773 percent, fraction <0.3#) with sodium chloride concentration for sodium saturations of approximately (1) 100 percent and (2) 75 percent.

activity measurements on the same material show that the amount of non- ionized sodium is constant up to about 0.01 N NaC1. Hence the sodium associated with the adsorbed chloride is in a completely ionized state, thereby increasing the amount of sodium in the double layer.

The viscosity of the two clay suspensions at various sodium saturations differs at the high concentrations of sodium chloride. This is true even though the sodium activity, chloride activity, and conductivity are similar. This difference may be due to the presence of adsorbed aluminum. As we shall see later, the presence of aluminum significantly affects the interaction of clay particles.

The effect of sodium chloride on viscosity for various degrees of base saturation is shown in Fig. 5 (calcium) and Fig. 6 (sodium). The most signi- ficant feature of these curves is the maximum which occurs at a base content of about 50 meq/100 g and in the pH range, 4.50-5.50. This maximum is common to both the sodium-hydrogen and the calcium-hydrogen systems

334 SEVENTH NATIONAL COIVFERENOE ON CLAYS AND ~ A Y MINERALS

6d

6C

5E

52

4~

o 44

4r 1

28

d

8

�9 I I

i i i i i

F o= O0 NoCI ~= O.O001N NaCI

= 0 .0005N NeCI x= O001N NnCI �9 = O .O05N NoCI �9 = O.OIN NoCt i = 0 05N rloCl

' ' ~ ' o ' ~ ' 50 6 0 70 90 I 0 meq Ca(OH) 2 per IOOg. cloy

F I G ~ 5,--Variation os re]stive viscosity os %u bentordte suspensions (concen- tration 0.663 percent, fraction < 0 . 3 / ~ ) w i t h increasing calcium saturation and sodium chloride concentration.

4 8

4 4

~ 4o

, 3 6

~ as

5 2~

IZ

8

4 L l l I I I I I - I

]0 20 30 40 50 60 70 80 90

meq. NaOH per IOOg clay

0 = 0 .0 NoCl 0 = O,001N NaCI t = 0 .005N NoCI �9 = 0.01N NaCI �9 = O .05N NaCI

D I / f - I

}00 I10 120 130."

FIGURE 6.--Variation of relative viscosity of Wyoming bentonite suspensions (concen- tration 0.663 percent, fraction < 0 . 3 # ) with increasing sodium saturation and sodium chloride concentration. Note: on this scale NaC1 concentrations 0.0001 0. 0005, and 0. 001 fall on the same curve.

EXCHANGEABLE CATIONS IN VISCOSITY OF CLAY SUSPENSIONS 335

and corresponds to 75 percent base saturation. These maxima probably are due to exchangeable aluminum on the clay. The clays were in the hydrogen- saturated form for 30 days prior to use--sufficient time for an appreciable quantity of aluminum to be released from the crystal lattice of the clay.

Effect of exchangeable aluminum on viscosity.---The change in the viscosity of the clay suspension as the A1 : H ratio on the exchange complex increases is shown in Fig. 7. As the aluminum content increases the viscosity increases to a maximum at about 50 percent aluminum saturation. Further increases in aluminum saturation result in a decrease in viscosity. This increase in

s

>=

0 )0 20 30 40 50 60 70 80 90 I00 ALUMINUM SATURATION- PER CENT

FIGUP.~ 7.--Variation of relative viscosity of a suspension of Wyoming bentonite (concentration 0. 778 percent, fraction <0.3F) with replacement of exchangeable hydrogen by aluminum.

aluminum content is accompanied by an obvious increase in the size of the floccnles formed. Apparently there is an optimum degree of flocculation for maximum viscosity.

The titration of the hydrogen-aluminum clays with sodium hydroxide results in a decided change in the suspension viscosity (Fig. 8). In all cases the first addition of sodium hydroxide causes dispersion of the clay with the corresponding decrease in viscosity. Further additions of sodium hydroxide show only a minor effect, with some indication of a maximum.

In Fig. 9 the viscosity curves are presented for the titration of freshly prepared hydrogen-saturated clays with sodium hydroxide. The change in

336 ~EVENTH ~ATIONAL CONFER]~NCE ON CLAYS AND CLAY MINERALS

0 = 50% FI ; 50% Al

0 I0 ~0 50 40 50 60 70 80 O~ LO0 llO 120 150 140

meq NoOH/IOOg. cloy

F [ G ~ E 8 . - - T h e r e l a t i v e v i s c o s i t y o f W y o m i n g b e n t o n l f ~ s u s p e n s i o n s ( c o n c e n t r a t i o n 0 . 778 p e r c e n t , f r a c t i o n < 0 . 3 # ) t i t r a t e d w i t h ~ a O H ; a t f i v e I-~ : A I r a t i o s .

\// . . o o o , o ,

�9 = 00075N N~CL NoCI --~" -

115 4

2 _~

I I I I I l i i L I I I I CO 20 40 60 SO I00 120 140

rneq NQOH IOOg clay FIQURE 9.--Variat ion of relative viscosity of Wyoming bentonite suspensions (concen-

tration 0 .778 percent, fraction < 0 . 3 / ~ ) with increasing sodium saturation and sodium chloride concentration. The clay, prior to titration, was hydrogen saturated.

EXCHANGEABLE CATIONS IN VISCOSITY OF CLAY SUSPENS[ONS

. . . . 2 " I . . . . . . " t ,o o : o s .o c,

I I I / \ / �9 : o . o o s o N NaCl |

~2HI I ~ I �9 ~ O,OO~N No~, "I

oc

337

I i i I I I I I I I I I ' i OO 2 0 4 0 6 0 8 0 I 0 0 120 140

meq, NoON / I O O g cloy

FIeUI~E 1 l . - -Variat ion of relative viscosity of ~Vyoming bentonite suspensions (concen- tration 0.778 percent, fraction <0 .3 /~ ) with increasing sodium saturation and sodium chloride concentration. The clay, prior to titration, contained an exchange- able ion ratio of 50 percent H : 50 percent Al.

0 | I I i I ] I I I I I i 1 1 0 2 0 4 0 6 0 8 0 I 0 0 120 140

m e q NoON / 1 0 0 9 cley

FIeu~E 10.--Variation of relative viscosity of Wyoming bentonit~ suspensions (concen- tration 0.778 percent, fraction < 0 . 3 # ) with increasing sodium saturation and sodium chloride concentration. The clay, prior to titration, contained an exchange- able ion ratio of 75 percent H : 25 percent A1.

338 S~V~NTR ~ATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

viscosity with electrolyte content was obtained by adding sodium chloride to the sodium-hydrogen systems. The contrast between these curves and those presented in Fig. 6 for a hydrogen-saturated clay prepared 30 days prior to use is striking.

The viscosity of hydrogen-saturated clays is very sensitive to electrolytes. Fig. 9 shows that additions of sodium chloride greatly increase the viscosity of the hydrogen-clay. However, after about 25 percent of the hydrogen has been neutralized by sodium hydroxide, the sensitivity to electrolytes becomes relatively small. A small maximum does occur at about 60 meq/100 g of sodium hydroxide, but this is probably due to a small amount of exchangeable aluminum.

i i , i i I i , i i J J i

1 4

O = 0 NoCI

o = 0 . 0 0 2 5 N NoCI

�9 = 0 . 0 0 5 0 N NoCI I~ / t l k ~ , = 0 . 0 0 7 5 N NaCt

IO

11:

I I I I I I I I I I I I I I 0 ~O 4 0 6 0 8 0 I 0 0 120 1 4 0

m e q N o O H / ' l O O q . cloy

FZg'~TRE 12.--Variation of relative viscosity of Wyoming bentonite suspensions (concentration 0. 778 percent fraction <0.3#) with increasing sodium saturation a~a4 sodium chloride concentration. The clay, prior to titration, was aluminum saturated.

The marked effect of exchangeable aluminum on the viscosity of clay suspensions in the presence of NaC1 is shown in Figs. 10, 11 and 12 for varied degrees of aluminum saturation.

Fig. 10, for 25 percent aluminum-75 percent hydrogen clay, shows that the initial effect of adding sodium hydroxide is to reduce the viscosity to a minimum at all levels of sodium chloride concentrations, except for 0.01 N NaC1. At the higher concentration of sodium chloride the viscosity was too high to measure accurately, except when the exchangeable sodium exceeded 60 meq/100 g. Subsequent additions of sodium hydroxide caused the viscosity to increase to a maximum at about 50-60 meq NaOH/100 g and then decrease

EXCHANGEABLE CATIONS IN VISCOSITV OF CLAY SUSPENSIONS 339

to a constant level. The cation exchange capacity of this material is approxi- mately 90 meq/100 g as determined from the titration curve.

As the aluminum content increases to 50 percent saturation (Fig. 11) there are two significant changes in the viscosity relations. First, the hydrogen-aluminum clay is very insensitive to electrolytes. With addition of sodium hydroxide the viscosity decreases to a minimum and then rises to a maximum for the low sodium chloride concentration. For the higher concentration of sodium chloride the viscosity increases directly to a maxi- mum. The second significant difference between these curves and the preceding ones is the presence of a second maximum at about 80-90 meq NaOH/100 g.

The 100 percent aluminum-saturated clay (Fig. 12) shows about the same characteristics as the 50 percent aluminum-saturated clay, except for the decrease in the viscosity. This seemingly is due to greater flocculation and less gelation.

D I S C U S S I O N

It is apparent from a consideration of the viscous behavior of montmoril- lonite suspensions that gel structures are formed under suitable conditions. The type of gel structure is most probably the brush-heap type proposed by yon N~geli in which he pictures the gel as made up of interlaced particles throughout the system and with the dispersion medium being held in the mass by capillary forces. This structure implies that the clay particles are of a heterophilic nature, i.e. the particles contain areas both of a hydrophilic character and of a hydrophobic character. I f the particles were hydrophilic over the entire surface there would be no possibility of particle-to-particle contact and no structure could form. In the opposing situation, in which the surface is entirely hydrophobic, complete contact could take place and dense floceules would form when the stabilizing factors (other than solvation) were removed. In this case no structure would form. For particles of an inter- mediate character, it would be possible for particles to coalesce at certain points on their surface, while at the same time the adsorbed water would prevent complete fioccu]ation and a structure would be formed throughout the suspension.

According to the picture of the gelation mechanism given above, it is obvious that the interaction of clay particles may be altered by any condition that would change the ionization of the hydrophobic part or would change the ratio of the hydrophobic to hydrophilic part. Consider the hydrogen- saturated clay. This is a heterophilic system which forms a stable suspension in the absence of electrolytes. The dual character of this system is brought out by the addition of sodium chloride, which causes an increase in viscosity. The sodium chloride reduces the ionization of the hydrophobic sites suffi- ciently to permit adherence of neighboring particles and some gel structure formation. The fact that complete flocculation does not occur attests to the hydrophflic character of these clay particles. While a small amount of hydro- gen is probably replaced by the sodium chloride, this does not appear to be

340 SEVENTH ~ATIONAL CONFERENCE ON CLAYS AND CLAY MINt,~ir

significant since the viscosity does not decrease as in the case of exchange of hydrogen with sodium hydroxide. The replacement of only a small amount of hydrogen by sodium results in a breakdown of the gel structure and in dispersion of the clay. The addition of sodium to the clay apparently converts the clay from a heterophilic to a hydrophilic system, as indicated by the dispersion of the day even in the presence of sodium chloride.

The calcium-saturated clay behaves somewhat like the hydrogen clay (data not shown) except that the structural formation tendencies are exhi- bited to a much lesser extent. The addition of sodium chloride causes only a slight increase in viscosity compared to the large change for the hydrogen system.

I t is difficult to give a quantitative evaluation of the effect of ionization on the viscosity of clay suspensions. In the cases studied here it was impossible to vary the ionization without also changing some other variable (e.g. hydrogen ion saturation or clay concentration). Only in the comparison of the hydrogen-saturated and the sodium-saturated clay was it possible to show that a decrease in ionization resulted in an increased viscosity. The effect of the addition of sodium chloride on the viscosity is enlightening. There is little change in the viscosity of the clay until a certain threshold value of sodium chloride is reached. At this value the viscosity increases rapidly. Apparently, at this point the ionization, which controls the repulsive energy barrier, is reduced to such an extent that flocculation can occur. However, there are no adequate means of measuring the ionization of the clay at this high salt concentration.

The replacement of hydrogen by aluminum on the clay results in an in- crease in viscosity to a maximum at about a 1 : 1 ratio of aluminum to hydro- gen. As the aluminum replaces hydrogen, the clay apparently becomes more hydrophobic resulting in the formation of compact fleccules at the higher aluminum content. There is seemingly an optimum ratio of hydrophobic to hydrophilic sites as far as viscosity is concerned. The fact that addition of sodium chloride to clay systems containing more than 50 percent exchange- able aluminum does not affect the viscosity indicates that the aluminum is nonionized and the clay is completely flocculated.

The influence of aluminum on the viscosity of the clay suspension may be understood by considering the reaction of A1 a+ with sodium hydroxide. According to Russell (1950, pp. 95-97), the exchangeable aluminum on clays is in sixfold coordination with water molecules. The addition of sodium hydroxide results in replacement of part of the water by hydroxyl ions. This unit appears to be unstable and combines with another similar unit by sharing two hydroxyl ions and losing two water molecules. The over-all reaction would be

2Al(H20)68+ 9- 2 0 H - --> [A12 (OH)2 (H~.O)e] 4+ ~- 6H20 This process could continue as more hydroxyl ions are added forming large

polyvalent aluminum-hydroxyl-water complex ions. Sodium ions would be adsorbed on the clay as the A13+ becomes neutralized. A mechanism thus would be provided whereby clay particles could be bound together through

EXCHANGEABLE CATIONS IN VISCOSITY OF CLAY SUSPENSIONS

a bond of the type :

341

H20 H20 H20 H20

H O/

This bond would account for the maximum in the viscosity curve as the amount of sodium hydroxide increases. As sodium hydroxide is added in excess of that needed for complete neutralization of the AlS+, aluminum hydroxide would form thereby eliminating the bonding force between the clay particles and causing the viscosity to decrease. The viscosity is not high in the absence of sodium chloride, probably owing to the dispersing action of the adsorbed sodium ions. As sodium chloride is added, the sodium ions become less ionized with a decrease in the zeta-potential and with the bonding action of the aluminum ion becoming dominant. This reaction would occur in the pH range, 4.50-5.50, and it does not explain the second maximum which occurs at higher pH values for clays containing greater than 50 percent aluminum saturation. More work is necessary before an explanation can be given.

C O N C L U S I O N S

From a consideration of the results of this study the following conclusions have been drawn :

(1) The viscosity of clay suspensions cannot be related directly to the ionization of the exchangeable cation. Apparently, there is an energy barrier to flocculation, and until the ionization is reduced sufficiently to overcome this energy barrier, the viscosity is practically constant with changing ionization.

(2) The addition of sodium chloride to clay suspensions causes an increase in viscosity only after a certain threshold value is reached. This value depends upon the exchangeable cation. Below this point, addition of NaC1 may cause a decrease in viscosity.

(3) An increase in clay concentration results in an increase in viscosity at a much faster rate than would be expected from consideration of the dispersed phase. Apparently interference between particles becomes dominant at higher concentrations (approximately 2 percent).

(4) The replacement of a small amount (10 percent) of hydrogen or calcium by sodium on the clay is sufficient to reduce the viscosity and disperse the clay suspension, even in the presence of 0.01 N NaC1.

(5) Aluminum ions adsorbed on the clay act as strong bonding agents between clay particles in certain pH ranges. The mechanism proposed for

342

this bond in this paper is of the type :

H20 H~O H20 H20 _\/~..oH......~ L

I \ ~ o H / / \ H~o H~o H~O ~0

SEVEI~ITH ~ATIOI~IAL COI~I~ERENC~E O:bl CLAYS AI~ID CLAY MXNEBALS

Wc~

REFERENCES

Marshall, C. E. (1950) The electrochemistry of the clay minerals in relation to pedology : Fourth Intern. Congr. Soil Sci. Trans. Amsterdam, v. 2, pp. 71-82.

Marshall, C. E. and Bergman, W. E. (1942) The electrochemical properties of mineral membranes. II. Measurements of potassium-ion activities in colloidal clays : J. Phys. Chem., v. 46, pp. 52-61 ; IV. The measurement of ammonium ion activites in col- loidal clays : Ibid., pp. 327-334.

M'Ewen, M. B. and Mould, D. L. (1957) The gelation of montmorillonite. II. The nature of interparticle forces in sols of Wyoming bentonite : Trans. Faraday Soc., v. 53, pp. 548-564.

M'Ewen, M. B. and Prat t , M. I. (1957) The gelation of montmorillonite. I. The formation of a structural framework in sols of Wyoming bentonite : Trans. Faraday Soc., v. 53, pp. 535-547.

Russell, E. J. (1950) Soil Conditions and Plant Growth : Longmans, New York, 635 pp. Van Olphen, H. (1956) Forces between suspended bentonite particles: in Clays and

Clay Minerals, Natl. Acad. Sci Natl. Res. Council, pub. 456, pp. 204-224.