744
W. E. SLABAUGH AND J . L. CULBERTSON SUMMARY
1. The adsorption curves of activated dysprosium ions in an exchange column were determined, both after the initial adsorption and in two different phases of the elution with ammonium citrate solution. 2. The results of the experiment are shown to be in good agreement with predictions made on theoretical grounds by Boyd, hleyers, and Adamson. 3. The general relations between the adsorption and elution curves are discussed. REFERENCE (1) BOYD,G . E., MEYERS,L. S., A N D ADAMSON, A. W.: J. Am. Chem. SOC.69, 2849 (1947); reference is made especially t o p. 2858.
T H E EFFECT OF CERTAIN REAGENTS UPON T H E PROPERTIES OF BENTONITE COLLOIDS. I
INORGANIC SALTS' W. H. SLABAUGH' , ~ N DJ. L. CULBERTSON Department of Chemistry, The Slate College of Washinglon, PuZEman, Washington
Received
JUnE
23, 1960
I. INTRODUCTION
Many workers have investigated the properties of bentonite colloids, but there remain many unanswered questions regarding the mechanism by which various reagents affect the viscosity, stability, and electrical properties of these colloidal systems. In the present work, several relationships between the amount and type of reactant and the properties of the colloidal dispersion have been discovered which appear to be of fundamental importance and may shed some light on this problem. No literature survey of this problem is offered because of recent adequate reviews (3, 8). 11. MATERIALS AND EXPERIMENTAL METHODS
The bentonite used in this study was a typical Wyoming bentonite, 200 mesh, mined by the Baroid Division of the National Lead Company. A suspension of 5 g. per 100 ml. of water was electrodialyzed in a modified Mattson cell, after which its concentration was reduced to 2.2 per cent solids. The resulting sus1 Submitted by W. H. Slabaugh in partial fulfillment of the requirements for the Ph.D. degree, The State College of Washington, June, 1950. 2 Present addresfi: Department of Chemistry, Kansas State College, Manhattan, Kansas.
EFFECT OF REAGENTS ON BENTONITE COLLOIDS I
745
pension was very stable and showed no change in properties during a IO-month period. By means of electrometric titration, using a glass electrode, the electrodialyzed bentonite was found to exhibit a diprotic character, as indicated in figure 1. With both inorganic and organic bases (alkyl and aromatic amines) the primary and secondary neutralization points occurred at approximately 45 and 75 milliequiv. per 100 g. of ovendry bentonite. Particle-size separation by sedimentation gave fractions which showed the same diprotic character and the same endpoints, as would be expected in view of Hauser and Reed's work (4) on the baseexchange capacity of various particle-size fractions of bentonite. Titration
curves of hydrogen bentonite described by Mitra and coworkers (10) and by Marshall (8) do not show the diprotic nature as distinctly as does the present work. A large part of this study is based upon mea\urenient\ of the structural viscosity of the various systems encountered. Following the general suggestions of Green ( 2 ) a rotational viscometer was constructed, the essential parts of which are shown in figure 2. The viscometer \\as operated at 30 R.P.M. for the majority of the single-point determinations of viscosity. This speed produced laminar flow for the systems encountered, and the data fell on the linear portion of the multipoint consistency curve for a Bingham body. Several multipoint consistency analyses were made in this work, but the deductions made from these more complex measurements were essentially identical with those made from single-
746
W. H . SLABAUGH .4ND J. 12. CULBERTSON
point observations. All viscosity measurements were made at 3O.O0C., and the principle of rheological equilibrium suggested by Ambrose and Loomk (1) waa followed. Electrophoresis measurements were made in a Smith and Lisse cell (13). X-ray diffraction spacings were measured with a Geiger-counter x-ray spectrometer.
SUPPORTING
FIBER
FIG.2. Viscometer 111. EXPERIMENTAL RESULTS Ah’D DISCUSSION
In order to study the effect of various reagents upon the colloidal properties of hydrogen bentonite, measured amounts of reagent were added to a suspension of the bentonite in the viscometer. When water mas added the viscosity decreased, as shown in figure 3. The same type of dilution effect was observed with additions of sodium hydroxide, potassium hydroxide, and ammonium hydroxide, wherein the amount of solvent (water) added was equivalent to the amount of pure water added. However, when bentonite was similarly treated with calcium hydroxide and barium hydroxide, a slight decrease in viscosity was followed by a sharp rise, which appeared l o be related to the base-exchange nature of the bentonite. When small amounts of 0.156 M sodium chloride were added to hydrogen bentonite a sharp decrease followed by a rapid rise in viscosity occurred (figure 4). The concentration of bentonite in the system was varied between 0.29 and 1.75 per rent solids. Regardless of the bentonite concentration, the relation between the viscosity and the resultant millimolarity of the system with respect to the electrolyte gave a series of curves exhibiting linear portions which could be
747
EFFECT OF REAGENTS ON BENTONITE COLLOIDS, I
0
20
40 60 UEQ. B I S E
80 100 I20 PER IOOg. BENTONITE
140
160
180
FIG.3. Viscosity of hydrogen bentonite after the addition of various bases
FIG. 4. Viscosity of hydrogen bentonite after the addition of sodium chloride. The concentration of salt is given in millimoles per liter of reaction mixture.
748
W. H. SLABAUGH AND J. L. CULBERTSON
extrapolated to a common point on the salt concentration axis. This quantity of electrolyte is defined as the critical salt concentration (C.S.C.), the full importance of which will be discussed later. When other alkali chlorides were added in similar manner to hydrogen bentonite the classical lyotropic effect on viscosity was demonstrated. Again there were linear portions in the viscosity curves which yield values of the critical salt concentration identical with that found with sodium chloride. These data are summarized in figure 5, which shows only those data obtained with suspensions containing 1.15 per cent solids. There was no noticeable lyotropic series evidenced by the alkaline earth chlorides.
FIG.5 . Viscosity of hydrogen bentonite after the addition of varioua alkali chloride8
The sodium halides also produced a lyotropic series, although the effect of sodium fluoride on the bentonite system appeared to be anomalous, as may be observed in figure 6. The linear portions of the curves yield C.S.C. values ranging from 1.2 to 1.6 millimoles, R result which indicates that the lyotropic effect estends to the C.S.C. value for these d t s . I t is well known that the valences of the cation and the anion play significant parts in the lyotropic effect. Figure 7 shows this effect on the viscosity of the bentonite system. The results of this group of determinations show that the C.S.C. values depend upon the valence. The results of this study are summarized in table 1. There is apparently a significant relationship between the equivalent concentration yalues of the C.S.C. for the sodium salts.
EFFECT O F RE:AGKh’TS OK BENTONITE COLLOIDS, I
MILLIMOLARITY OF
FIO.0. Viscosity
749
SALT
of hydrogen bentonite after the addition of various sodium halides
FIG.7. Viscosity of hydrogen beutonite nfter the addition of salts of varying cation and anion valences.
750
M'. El. SLABAUGH AND J. L. CULRERTSON
TABLE 1 Effect of valence of.added ion upon the critical salt concentralion
i
CPITICAL SALT CONCENTEATION
1
! NuCl.. . . . .:.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MgCIz.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AICla. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NnzSOd . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NaaFe(CN), . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Millimoles
'
Milliequivolents
1.45 0.95 0.65 0.72 0.48
1
1.45 1.90 1.30 1.44 1.44
~
1
~
TABLE 2 Data on the systenis whose viscosities are reported i n jigures 4 , 6 , and 7 YIILIYOUPITY OF SALT
SALT
0
1
0.5
1
1
1.0
1.5
1
2.0
1
2.5
1
3.0
I
4.0
Electrophoretic velocities a t room temperature in em./sec./ v./cm. X 108; 1.15 per cent solids NaCl. . . . . . . . . . . . . . . . . . . KC1 . . . . . . . . . . . . . . . . . . . . RbCl. . . . . . . . . . . . . . . . . . . CSCl . . . . . . . . . . . . . . . . . . . MgCII. . . . . . . . . . . . . . . . . . AlCla. . . . . . . . . . . . . . . . . . . NazSOd.. . . . . . . . . . . . . . . . NaaFe(CN)s. . . . . . . . . . . .
NaCl . . . . . . . . . . . . . . . . . . . KCl .................... CSCl . . . . . . . . . . . . . . . . . . . MgCIz. . . . . . . . . . . . . . . . . . AICla.. . . . . . . . . . . . . . . . . . Na2S0, . . . . . . . . . . . . . NapFe(CN) 6 . . . . . . . .
6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1
5.5 5.7 5.5 4.8 6.0 8.71 5.7 5.41
4.71 5.5 5.5 4.2 I 5.4 4.7 5.3 5.21
1
4.81
:::1
13.0 12.7 12.3 23,2 35.5 23.2 27.1
21.5 20.8 23.3 40.2 61.2 34.8 44,4
5.0
I
1
~
1
3.6 3.8
1
3.2 3.0
~
4.9 5.1
29.0 29.7
I
4.4 3.8 4.7
3.8
4.8 4.7
4.91
_____~ 5.2 5.2 5.2 5.2 5.2 5.2 5.2
4.71 4.8
I
j
65.3 76.8 42.5
34.3 36.9 40.2 67.7 91.3 51.0 69.2
.1.6 4.5
39.3 47.0 85.5 110
1 1 4.1 4.3
4.6 3.5
44.3 58.8 61.2 95.5 115 63. 7 93.5
52.2 69.0 80.3 121.
pH of rhpcrsing medium NaCl CSCl
nwZ AlCls Na2SOd NarFe(CN)s
3.45 3.45 3.90 3. so 4.20 4.00
3.35'
[
3.60, 3.421 3.781 3.681
3.25' 3.251 3.421 3.261 3.691 3.58,
3 25' 3.32 3.191 3.681
1
3.23' 3.15 3.251 3.16 3.60 3.51
3.10 3.20 3.17
3.02 3.02 3.10 3.19 3.60 3.47
~
1
I
2.96 2.95 3.19 3.10
,
The electrophoretic velocities, conductances, and pH values of all systems whose viscosities are reported in figures 4, 5 , and 7 are listed in table 2. These
EFFECT OF REAQENTS ON BENTONITE COLLOIDS. I
751
data are sampled in figure 8, which shows representative data for the system 1.15 per cent hydrogen bentonite and sodium chloride. A more complex analysis of the system was made as follows: prior to the addition of an electrolyte, part of the hydrogen in the hydrogen bentonite was neutralized. Oakley (11) made flocculation studies on bentonite by adding sodium chloridesodium hydroxide solutions to acid bentonite. The present study is primarily concerned with viscosity, and the salt was added after the alkali had been added. The results here are not in disagreement with Oakley's data.
FIG.8. Viscosity, specific couductance, pH, and electrophoretic velocity of hydrogen bentonite to which sodium ehlmidr has been ridded.
Figure 9 gives the viscosity curves T\ hich I\ ere obtained when sodium chloride was added to hydrogen bentonite which had been partially neutralized. The lettered points on the titration curve correspond to the amount of alkali which was added t o the hydrogen bentonite before the salt was added. It is to be noted that as some of the hydrogen ions were neutralized with sodium hydroxide the addition of sodium chloride had less effect on the viscosity of the system. After all the primary hydrogen had been neutralized, the addition of salt produced practically no effect upon the viscosity of the system. The present evidence indicates that salt affects the viscosity of hydrogen bentonite only if primary hydrogen ions are present. If the action of salt is assumed to be primarily
752
W. H. SLABAUGH AXD J. L . CULBERTSON
adsorption and exchange, the exchange activity of the secondary hydrogen ion is not great enough to permit the addition of salt t o produce a noticeable effect upon the viscosity of the system. It is logical to expect those hydrogen ions in the most accessible positions to be neutralized first. This mould include those base-exchange sites on the edgea and exposed layers of the laminar parts of the particles. Since only about 20 per cent of the exchange sites (5) are on the edges of the laminae, it is apparent that the primary neutralization involves these edges and approximately half of
rfter certain
the charges on the layers within the particles. This is indicated in the potentiometric titration curve in figure 10, where A represents the neutralization of the hydrogen ions on the exposed areas of the particle, B the primary neutralization, and C the secondary neutralization. There are a t least two possibilities regarding the nature of these primary and secondary hydrogen ions. First, these ion sites may be adjacent t o each other on the laminae and merely exhibit different ionizing potentials. The neutralization of a particular hydrogen ion may cause an electronic shift in the silicate structure so as to reduce the ionization potential of an adjacent, hydrogen ion, thus inducing a secondary nature to it.
EFFECT OF REAGENTS ON BENTONITE COLLOIDB. I
753
On the other hand, these primary and secondary hydrogens may exist on opposite sides of the otherwise symmetrical layers of the bentonite structure. It is this latter mechanism of ionization which seems to be justified by this study. A fuller proof and discussion of this concept will be offered in connection with the study of the reactions between bentonite and organic reagents. The results shown in figures 4,5 , 6, and 7 indicate that the C.S.C.point is of fundamental significance. Mattson (9) theorized that the total electrolyte concentration is an important factor in the stability of a lyophilic system. The zeta potential becomes a function of the dispersing medium as well BS the particles. These data appear to verify this theory nicely, particularly the data described in figure 4. However, the critical salt concentration indicates that below and
ob
2 0 4 0 60 80 MEO. NaOH PER 1009. BENTONITE
I(
FIQ.10. The three types of hydrogen ions in hydrogen bentonite
above this concentration of total electrolyte the changes in viscometric structure may be primarily due to different mechanisms. It is generally agreed that the addition of electrolyte to hydrophilic colloids involves an alteration of both the adsorbed electrical charges and the solvated layer of suspending medium. If ethyl alcohol, ordinarily considered as a dehydrating agent for the solvated layer on colloidal particles, is added to hydrogen bentonite, there is essentially no more effect on the viscosity of the system than when an equal amount of water is added. This indicates that the early addition of electrolyte is not a dehydrating process, but more likely involves an alteration of the electrical charges which are adsorbed on the surfaces of the bentonite particles. From figures 5 and 6 it appears that the mechanism of electrolyte action prior to the critical salt concentration is independent of the nature of the cation and
754
W. H. SL.4BAUGH AND J. L. CULBERTSON
dependent upon the lyotropic character of the anion. Beyond the C.S.C. point, there is a divergence of the effects of the various electrolytes and these can be traced to several causes. First, there is a difference in adsorbability of the union. Table 3 gives the number of millimoles of sodium halide adsorbed or exchanged n.hen a 0.165 per cent suspension of hydrogen bentonite was made 30 millimolar with respect to the salt. The smaller effect of the iodide on the rheological btivcture than of the bromide and chloride salts may br attributed 1 o the considerable difference in the hydration of these ions. H1:isko (6) reports that ionic mobilities increase in the order F-, C1-, Br-, I-. This would indicate that the iodide ion is less hydrated than the chloride ion, and for this reason the iodide ion would exert less effect on the rheological structurt. which forms after the C.S.C. point than is shown by the chloride ion. 'rhus, the two factors-amount of electrolyte adsorbed and the radii of the hydrated ions which arc adsorbed-apparently determine the effect of the anion of the salt on the hydrogen bentonite
.
TABI,IC 3 Milliniolrs of s o d i u m halidr adsorbed ~~
UT
orchunged
S~CI
i
SuIrr
2.921 0.17
2.88 0.21
~
.. .. .. , pK. ...... . . . . . Frnrtioir of salt adsortmi . . .
Stilt added
. .'
K ~ F
~
15.12 '
1
~
I ~
NaI
~
None
2 . 8 X i 3 . 5 2 0.25
I
TABLE 4 Adsorption erterqirs oj alkali in base-rrchange pruceasea ( a r h i : m r ~m i l s ) I, I ha
___
.
0.55
0.74
!
I< 1 RI,
1.22
~! I
' cs -
.
1.70 ~-
Second, with regard to the cations of the added salt, the curves for pH versus the amount of electrolytr present in figure 8 show no break, that is, the sodium ions can exchange with equal facility during this whole process. However, if the primary hydrogen IS first neuti,alized, the ions in the dispersant can no longer enter into the same type of exchange, and the effect of the electrolyte on the \ iwometric structure nil1 be much less. Finally, as secondary hydrogen ions become neutralized, the addition of electrolyte causes-no viscometric changes in the system. This is explained by the fact that all hydrogen ions on the laminae as well as on the surface of the micelle are now neutralized and the exchange sites are now occupied by sodium ions. Addition of sodium chloride now offers little possibility of further ion exchange, which in turn does not bring about serious changes in structure. The above theory is further substantiated by relating the information in figure 5 to the work of Krishnamoorthy and Overstreet (7) concerning the adborption energies of alkali ions in base-exchange processes, as summarized in table 4. It is proposed that the changes in rheological structure during the addition of electrolyte up to the C.S.C. point are largely due to a reduction of the
755
EFFECT OF REAGENTS OX BENTONITE COLLOIDS. I
zeta potential. Beyond this point the adsorption energy and the ionic radii of the electrolyte ions become significant, and some of the base-exchange sites become populated with the alkali ions. These changes are attributed primarily to the electroviscous effect. Hauser (4) introduced the concept of far-reaching forces to account for the structure of these systems. The present work substantiates this idea. I n the original bentonite system, the particles are highly charged and uniformly dispersed. At concentrations below 0.75 per cent solids these systems tend to settle out, yielding soft, easily redispersed precipitates. T'pon the addition of electrolyte, the charge in the solvated hull is partially neutralized, whereupon the particles may approach each other to a certain extent. These far-reaching forces now come into play wherein the particles form an open, brush-heap structure.
rrmrx
5
X - r n v lattzrc spacings of berLionlte treated wzth uaraous salls ~
A.
A.
LiCl . . . . . . . . . . . . . . . . . . . . . NaCI.. . . . . . . . . . . . . . . . . . . . .
RbCI. . . . . . . . . . . . . . . . . . . CrCI... . . . . . . . . . . . . . . . . . . .
11.9 11.6
~
ION ~
__
RADII OF AYDPATED IONS
1 ION ~
-
~
10 7.8 5.2
1
RADII OF H Y D M T L D
loss
..
2.
A.
L i . .. . . . . . . . . . . . . . . . . . . 1 Na. . . . . . . . . . . . . . . . . . . . . . . K . . . . . . . . . . . ., . . .
11.6
Rb . . . . . . . . . . . . . . . . . . . . . . . .
cs. . . . . . . . . . . . . . . . . . . . . . .
5.1 5.0
This structure accounts for the high viscosity and stability even a t concentrations as low as 0.1 per cent solids. The C.S.C. value sharply defines the transition between the unstable, uniformly dispersed system and the stable, brushheap structure. This transition has been followed with the microscope as well as with coagulation studies. X-ray diffraction determinations of the 001 spacings were made on the bentonite systems which had been treated with the various salts. The liquid suspensions were placed on microscope slides and permitted to dry in the atmosphere a t room conditions. No control over the relative humidity was made, except that all samples were prepared a t the same time and were permitted to reach equilibrium with the air during the same period. The x-ray lattice spacings as a function of the relativo humidity were not studied. Table 5 gives these spacings for those systems which were 7 millimolar with respect to the electrolyte. A comparison with untreated hydrogen bentonite is included in the table. Replacement
756
W. H. SLABAUGH LYD J . L. CULBERTSON
of a portion of the hydrogen ions by the alkali ions caused the spacings to go through a minimum a t the potassium ion. This can be reconciled by bearing the following two factors in mind. First, according to Nachcd and Wood (10) the radii of hydrated alkali ions aa determined by ionic mobility measurements are given in table 6. It is reasonTABLE 7 p H of hydrogen bentonite a n d a l k a l i chloride
1
SALT
PII ov 1.15 PEP CENT
s?jsleins
i
EYDROOEx
xoLm
os
BEWTONITE PLUS SALT
--.,
.
._
None.. . . . . . . . . . . . . . . . . . . . . LiCI.. . . . . . . . . . . . . . . . . . . . . . . . NaCl. . . . . . . . . . . . KC1 . . . . . . . . . . . . . . . . . . . RbCl. . . . . . . . . . . . . . . . . . . . . . . .............. ~
""""I
3.55 3.18 3.43 3.42
1 i:
' '
'
! ~
3.6G 3.42 3.42 3.36 3.32 3.29
I' ~
i ~
1 ~
0.7 A 0.9 1.2 1.2 1.5
ALKALI ION E x c m A N c w
x
1w
11
lB6 1.6 2.3 2.6 2.9
I
2.4
14 0 4
I
12
n
4
W
a
Y0
4
1.8
10
3X W
n W
z
h
0 r > e
1.4
P 2 4
(d
4 -I
(1
1.0
E 6 u
-I
2
0
-
I
v)
8
0.6
4 7 MILLIMOLAR SALT IN
1.15% BENTONITE
FIG.11. Relation of adsorption energies, radii of hydrated ions, x-ray spacings, and amount of alkali ion exchanged on hydrogen bentonite.
able to assume that a t room temperature a fair degree of hydration of these ions will persist on the laminae of the bentonite and that, as a result, the lithium ion will exert a greater effect upon the 001 spacing than the other alkali ions. Second, cesium shows the greatest adsorption energy, and from pH measurements of the hydrogen bentonite plus alkali chloride systems given in table 7, it is apparent that a greater portion of the hydrogen ions in hydrogen bentonite are replaced by the cesium ion than by an equivalent amount of lithium ion. In table 7 the data in column A are based upon pH measurements of the total
EFFECT OF REAGENTS ON BENTONITE COLLOIDS. I
757
system, while column B represents the pH of supernatant liquid obtained by centrifuging. Taking into account the radii of the hydrated ions, the adsorption energy, and the amount of alkali ion exchanged, it is logical to experience a minimum in the 001 spacings at the midpoint of the alkali series. These observations are summarized in figure 11, where the data A from t,able 7 are used. With lithium chloride relatively small amounts of alkali ion are exchanged, but the ionic radius of this cation is so much larger than the radii of the other hydrated ions in this series that the 001 spacing is relatively large. At the other end of the series, considerably larger amounts of cesium ion were exchanged but this factor was partially overcome by the smaller ionic radius of this cation. IV. SUMMARY
Addition of electrolytes to hydrogen bentonite has been shown to produce three effects: 1. Partial neutralization of the adsorbed diffuse layer, resulting in a reduction of the effective diameter of the bentonite micelle. 2. Base-exchange equilibrium between the electrolyte and the hydrogen bentonite, resulting in rheological structure changes. 3. Serious modification of the far-reaching forces which account for a great change in the bulk structure of the bentonite system. The concept of a critical salt concent)rationhas been introduced, the discovery of which proves almost conclusively that the concentration of electrolyte in the dispersing medium plays a decisive role in determining the rheological structure and stability of the bentonite system. A significant relationship between the adsorption energies of the alkali ions in base exchange and the amount of alkali ion exchanged has been found and these factors, when related to the radii of these hydrated ions, lead to an adequate esplanation of the 001 x-ray diffraction spacings. The aid of Mr. James Coleman, supported in part by the State College of Washington Committee on Research, in the construction of the electrophoretic cell is acknowledged. REFERENCES LOOMIS, A . G.: Physics 4, 2ti5-73 (i933). (:REEN, &:WRY: Industrial Rheology and fihcologicd 8trtrctures. ,John lviley and 60118, Inc., New York (1949). (3) H A U ~ ~ E ~E . AR. :, Chem. Revs. 57, 307-321 (1945). (4) HAUBER, E . A . , ASD REED, C. E . : J . Phys. Cheni. 41, 911-34 (1937). ( 5 ) IIENDRTCKK, s. B.,NELSON, R. .4., ANI) ALEXANDER, L. T.:J. Am. Chem. S O ? . 62,
(11 (2)
~ I B R O S E ,11.
A.,
AND
145744 (19.10). (6) HLASKO, M.:Roczniki Chem. 17, 11-19 (193i). (7) I
