IONIC EXCHANGE IN RELATION TO THE STABILITY O F COLLOIDAL SYSTEMS .HANS JENNY AND R. F. REITEMEIER Departments of Soils and o j Chemistry, University of Missouri, Columbia, Missouri Received August 23, 1934 INTRODUCTION
Numerous investigators (5, 7) have demonstrated a close relation between the elektrokinetic potential (zeta potential) of colloidal particles and the amount of electrolyte necessary to flocculate the system. A number of years ago Wiegner (10) called attention to the r61e of ionic exchange in modifying the stability of a suspensoid, and it appeared that the significance of the interchange phenomenon had been greatly underestimated by many students of coagulation processes. The present study attempts to clarify the relation among ionic exchange, zeta potential, and flocculation value for colloidal clay systems and to furnish quantitative correlations among these three magnitudes. EXPERIMENTAL TECHNIQUE
Ionic exchange The technique of ionic exchange experiments has been fully reported previously (3). In principle, natural Putnam clays were electrodialyzed and the resulting H-clays converted into basic clays (Na-clay, Ca-clay, etc.) by addition of known amounts of hydroxides, except in the case of La- and Th-clays and methylene blue clay, which were obtained by leaching of NH4-clay with lanthanum nitrate, thorium chloride, and methylene blue chloride. To these basic clays electrolytes were added at various concentrations, and the exchange adsorption determined by analysis of the supernatant liquid. The amount of electrolyte added to the clay sols is expressed in terms of symmetry concentrations, that is, in multiples of the number of milliequivalents of adsorbed ions in the system. In particular, the magnitude of exchange expressed in percentage for the symmetry concentration equal t o one (8= 1)is called symmetry value. All exchange data used in the present publication are symmetry values and have been reported previously (4). 593 THE JOURNAL OF PHYSICAL CHEMISTRY, VOL.
39, NO. 5
594
HANS JENNY AND R. F. REITEMEIER
Zeta potentials
All measurements of migration velocities were made with an ultramicroscope and an open cataphoresis cell as described by Tuorila (8). The cell was improved by introducing removable electrodes to permit thorough cleaning, which reduced the mean error of measurement to 2 per cent. The cell constructed obeyed Smoluchowski’s equation strictly. Table 1 contains only a single observed migration velocity for each type of clay, TABLE 1 Data on.aeta potentials, adsorption, and jlocculation I FLOCCULATYPE OF COLLOIDAL CLAY
Id1 GRATION YELOCITY* ,hLL NEGATIVE)
‘TONVALU F O R XCI
TEMPERATJIRl
(IN TIRMS O F 8 CONCENTRATIONS)
C.
-Li-clay.. ............... Na-clay. . . . . . . . . . . . . . . . K-clay. . . . . . . . . . . . . . . . . . NH4-clay . . . . . . . . . . . . . . . Rb-clay . . . . . . . . . . . . . . . . . Cs-clay . . . . . . . . . . . . . . . . . . H-clay . . . . . . . . . . . . . . . . . . Mg-clay . . . . . . . . . . . . . . . . . Ca-clay . . . . . . . . . . . . . . . . . Sr-clay . . . . . . . . . . . . . . . . . . Ba-clay. . . . . . . . . . . . . . . . . La-clay.. . . . . . . . . . . . . . . . .
3.45 3.31
30.3 28.9
3.48 3.25 3.02 2.84 3.18 3.27 3.06 3.01 2.74
33.2 30.3 30.3 30.3 30.3 32.8 30.3 30.3 21.2
Th-clay ...........
3.11
32.2
2.57
33.7
M.B.-clay..
........
* Microns
58.8 57.6 56.4156.0 54.9 51.2 48.4 53.9 52.6 51.8 50.8 45.5
68.0 66.5 48.7 50 37.4 31.2 14.5 31.32 28.80 25.76 26.75 13.96
21.6 11.2 7.8 5.4 5.6 1.5 2.9 3.0 2.6 2.3 0.86
FLOCCULATION VALUE FOR COMMON CATION A 0 CHLORIDE
[S-CONCENTRATION
26 14.8 7.8 4.9
4.2 0.36 0.63 0.55
0.GO 40.5
1
0.0
per second per volt per centimeter.
t Interpolated from a corresponding monovalent series (Li, Na, K) of Putnam clay.
but the average value of the zeta potentials calculated according to the equation { =
E ‘ (300)2
where 5 = zeta potential, q = viscosity of the solution (HzO) in poises, p = migration velocity of particle in em. see.-’, measured at a depth of 0.2113 times the thickness of the cell.
595
STABILITY OF COLLOIDAL SYSTEMS
D
= dielectric constant (80),
H = potential gradient in volts cm.-l, and 300 = ratio of electrostatic unit to practical electromagnetic unit of potential. Although equation 1 is subject to criticism, we prefer to report the results as zeta potentials rather than migration velocities. The migration velocity was found to be directly proportional to the potential difference between the electrodes, and appeared to be independent of the size of the particles. As the concentration of the sol increased the velocity decreased, as shown in figure 2. All data reported were measured at a sol concentration of 0.001 per cent on the basis of the weight of H-clay. The sols were kept in stock solutions of 0.5 per cent and 1.0 per cent, and it was observed that after dilution to 0.001 per cent the zeta potentials varied with the age of the diluted sol (figure 1). Final measurements were 60c 55
La-clay 0.01%
3
b
9 I2 IS Age rflrr dilution drys
-
18
21 Cancrntiaiian-
%
FIQ.2 FIG.1. EFFECT OF DILUTION AQE ON THE POTENTIAL OF CLAYPARTICLES FIG 2. EFFECT OF CLAYCONCENTRATION ON THE ZETA POTENTIAL OF CLAY PARTICLES FIa. 1
taken four to six days after dilution. All clays were negative, even the Th- and La-clays, in spite of the fact that addition of thorium chloride or lanthanum nitrate to a K-clay yields positive particles. Evidently during the process of removal of the excess electrolyte by leaching with distilled water the stabilizing chloride ions were replaced by OH- and HC03- and a negative clay resulted. Somewhat similar observations have been reported by Lottermoser ( 6 ) . The zeta potential of the Th-clay appears to be too high, as will be demonstrated later. Flocculation values
It is difficult to reproduce satisfactorily flocculation values for clay sols because of the great influence of such factors as method of shaking, age of sol, mode of diluting, etc. The following procedure was adhered to rigidly. Into a 75-cc. test tube were placed 5 cc. of 2.66 per cent clay sol (on the basis of H-clay), x cc. of redistilled water, and y cc. of electrolyte solution.
596
HANS JENNY AND R. F. REITEMEIER
+ +
The final volume (5 x y) was always 35 cc. For fifteen minutes the tube was gently shaken with a mechanical device and care was taken that no air bubbles formed. Ten cc. of the mixture was then transferred into another 75-cc. tube which contained 25 cc. of water, and both tubes were shaken fifty times. This procedure permitted the determination of flocculation values at two different clay concentrations. In this paper only the values for the second (dilute) system are given. The tubes were put into a thermostat (30°C.), and after six hours the presence or absence of sedimentation was recorded. The flocculation values are expressed in terms of symmetry concentrations, which notation furnishes more instructive figures than does the customary designation as millimoles per liter.
40
M.8:
-
FIG.3. EFFECT OF CHARQE AND SIZEOF ADSORBED CATIONSUPON THE POTENTIAL OF
COLLOIDAL CLAYPARTICLES (Li-CLAY,
Ca-CLAY, ETC.)
ZETA POTENTIALS A S INFLUENCED BY CHARGE AND SIZE O F THE
ADSORBED CATIONS
In view of the fact that all clays prepared carry the same number of adsorbed cations, namely 60 milliequivalents per 100 g. of clay, the zeta potentials can be directly related to the nature of the ion in the outer swarm of the electric double layer.. According to the data in table 1 and the curves in figure 3, certain regular trends are evident, and in particular an ionic charge and size effect come into prominence. For rare gas type ions of equal size the zeta, potentials of the clays tend to be lower as the electric charge of the adsorbed i o n i s higher. The restriction of the comparison to ions of equal size is essential because certain mono-clays (e.g., Cs-clay) have lower potentials than some of the di-clays (Mg-clay, etc.). Evidently the statement often found in literature that clays with adsorbed divalent cations have lower potentials than mono-clays is too general, and applies only to ions with equal crystal lattice radii (Goldschmidt's values). The r81e of ionic size can be formulated as follows: For rare gas type ions of equal valency the zeta potential i s the higher the smaller the adsorbed cation. This observation is in agreement with Wiegner's idea (lo), according to
STABILITY OF COLLOIDAL SYSTEMS
597
which the most hydrated ion (small ions are very strongly hydrated) brings about the highest zeta potential. On the basis of Helmholtz’s classical picture of the electric double layer, the potential is higher as the number of electric charges (e) and the average distance (6) of the double layer are greater. For ionic crystals such as clay particles, dissociation can be considered as complete (heteropolar linkage) and consequently the magnitude of the zeta potential becomes solely a function of 6. On account of the water shell, the centers of hydrated cations cannot approach the negative wall of the particle as closely as those of the smaller non-hydrated ions. Consequently ions with small crystal lattice radii but great effective sizes (hydrodynamic radii) widen the distance of the double layer and cause high potentials as demonstrated in figure 3. ZETA POTENTIAL AND IONIC EXCHANGE
The mechanism of ionic exchange can be visualized as follows : Colloidal clay particles are plate-shaped crystals which hold on their surface adsorbed cations. Owing to heat motion and Brownian movement the adsorbed ions are not at rest but oscillate, and at times are at a considerable distance from the wall. If it so happens that on account of Brownian movement a cation of an added electrolyte slips between the negative wall and the positive oscillating ion, the electrolyte cation will become “adsorbed” while the surface ion remains in the solution as an exchanged ion. The more loosely an ion is held the greater is the average distance of oscillation and the greater is the possibility of replacement or, vice versa, the more tenaciously an ion sticks to the surface the less readily it will be released by the cations of an electrolyte added to the colloidal system. The average distance of oscillation corresponds directly to the average thickness of the electric double layer, and on the basis of the picture outlined one would conclude at once that clays with high zeta potentials contain easily exchangeable ions. To test the validity of this conclusion it becomes necessary to learn the ease of replacement of the ions adsorbed on the clays listed in table 1. For this purpose data previously published were used which refer to the very same clay systems used in the present study. The release of the polyvalent cations was accomplished by addition of potassium chloride, while that of the monovalent cations was calculated from the exchange values with ”1-clay. Since potassium ions and ammonium ions are equally well adsorbed and released, the data permit quantitative comparison. According to figure 4 it is evident that ions which are easily released are also responsible for high zeta potentials. Lithium and sodium ions are loosely held, and about 70 per cent of them are exchanged by addition of an equal number of ammonium ions (symmetry concentration), while the
598
HANS JENNY AND R. F. REITEMEIER
clays with low potentials (La-clay, Th-clay) have exchange values below 20 per cent. The divalent cations occupy an intermediate position. Neglecting the wave-like shape of the curves, the general trend of the exchange-zeta potential relation is of parabolic nature as seen in figure 5. The relationship can be satisfactorily described by the equation
E = 3.55.10-1056.42
(2)
where E represents the percentage release of the adsorbed ion at 1 S concentration, and { the zeta potential in millivolts. The curve rises in a very pronounced fashion, approximately as the sixth power of the potential.
FIG. 4
FIG. 5
FIQ.4. RELEASEOF ADSORBEDCATIONS IN RELATION TO ZETA POTENTIAL For example, release of lithium from Li-clay or barium from Ba-clay. Exchange expressed as symmetry values. FIQ.5 . T H E INTENSITY OF IONIC EXCHANGE INCREASES APPROXIMATELY AS
THE
SIXTHPOWEROF THE ZETA POTENTIAL Note the indication of a maximum potential. Black points refer to release by calcium; the white points to exchange with potassium and ammonium.
According to the strong adsorption of thorium the value of the zeta potential of the Th-clay appears to be questionable. The curves in figure 5 further indicate that the ionic exchange is zero if the zeta potential is zero, in other words, the adsorbed ions are so tightly held that they become an integral part of the particle with a corresponding annihilation of the double layer. Inspection of the upper part of the curve is equally instructive. Since ionic exchange cannot exceed 100 per cent, the trend of the curve strongly suggests that the clay systems investigated possess a definite maximum potential which is slightly above 60 millivolts. These data can be taken as a confirmation of Hevesy's concept (2) and also of Freundlich's statement (1) that the potential of colloidal particles t,ends to stay below 100 millivolts.
599
STABILITY O F COLLOIDAL SYSTEMS ZETA POTENTIALS IN RELATION TO
FLOCCULATION^
VALUES
The contention of colloid chemists that the zeta potential bears a close relationship to the stability of a colloidal system is well illustrated by figure 6. A series of clays carrying equal amounts but different kinds of cations were flocculated with potassium chloride according to the method mentioned on pages 595-6. It takes much more electrolyte to flocculate a clay with a high potential than one with a low potential. The relation between the flocculation value and the zeta potential also is of parabolic form, showing that the stability increases many-fold with a slight rise in the zeta potential. To flocculate a clay with maximum potential, extrapolation would give a value of about 50 S, which corresponds to an electro-
!” FIG.6 FIG.7 FIQ. 6. FLOCCULATION OF MONO-AND POLY-CLAYS BY POTASSIUM CHLORIDE The higher the initial potential of the colloidal particle the greater the flocculation value. The critical potential has a value of about 42 millivolts. FIG.7. FLOCCULATION VALUESOF VARIOUSCATIONSIN RELATIONTO THEIR CRYSTAL LATTICERADII
lyte concentration of 0.0326 N potassium chloride for the systems in question. The curve in figure 6 tends to cut the X-axis at a potential which is far above zero, and therefore is in accord with the concept of the critical potential, advanced by Powis (7). This value appears to correspond to about 42 millivolts for the type of experimental procedure applied. In confirmation of this magnitude, the methylene blue clay with a potential of 40.5 millivolts sediments completely without the addition of electrolyte. 1
“Flocculation” and “coagulation” are used synonymously in this paper.
600
HANS JENNY AND R. F. REITEMEIER FLOCCULATION VALUES AS AFFECTED BY CHARGE AND SIZE O F THE FLOCCULATING IONS
Charge of ion The well-known flocculation rule of Schulze-Hardy, as formulated by Wetham (9), states that the reciprocal flocculation values expressed in moles follow the proportion z a 11:XI1 :XI11 :XIV (3) where the Roman subscript denotes the valency of the flocculating ion. This rule is grossly violated in the case of clay systems (table 2). For NHrclay and Ca-clay the average flocculation values for the mono- and poly-valent cations yield quotients of the form 11:x11:2.03x111:3.21~1~ (NHeclay) 11:x11: 1.76x111:3.14X1v (Ca-clay)
(4) (5)
In other words, the polyvalent cations are much less effective coagulators than is indicated by the valency rule. For the two clay systems given, the order of magnitude of effectiveness is much better expressed by a proportion of the type 11:XII :2x111:3 x 1 ~
(6)
where the exponents of Wetham’s equation have become mere factors.
Size of ion In figure 7 the flocculation values have been plotted against the crystal lattice radii of the corresponding ions, and it is clearly seen that for a given type of clay the flocculation value is higher as the size of the ion is smaller. The more highly hydrated a cation, the smaller is its efficiency as a coagulat or. FLOCCULATION VALUES AND EXCHANGE ADSORPTION
In the flocculation of ultramicrons, two important cases must be separated. First, the cation of the electrolyte and the colloidal particles possess common ions only (e.g., K-clay + potassium chloride); secondly, the coagulation agent contains only foreign ions with respect to the particle. In the former case flocculation results from repression of the double layer (diminishing of 6). In the second case it constitutes a combination of ionic exchange and the repression phenomenon. In regard to the common ion effect the data in table 1vary considerably for different clay systems. Since the osmotic pressures of dilute strong electrolytes of equal normalities vary but little, the great difference in the flocculation values must be attributed to the magnitude of the original zeta
601
STABILITY O F COLLOIDAL SSSTEMS
potential rather than to the repression efficiency of the various salts. Indeed, the figures for the common ion effect are of the same order of magnitude as the potassium chloride-set values (table 1). Whenever the flocculating electrolyte contains a foreign cation, exchange adsorption is bound to occur. A comparison between the flocculation TABLE 2 . Relation between Jlocculation values and exchange adsorption NH~CLLY ELECTROLYTE
Flocculation value (S)
dsorption
8.0 8.0 5.4 4.9 4.7 2.8 0.98 1.22 1.27 1.16 0.90 0.75
32.0 33.5 51.33 50 62.56 68.78 84.89 65.44 63.56 71.67
LiCl.. . . . . . . . . . . . . . . . NaCl . . . . . . . . . . . . . . . . KCl. . . . . . . . . . . . . . . . . . NHrCl.. . . . . . . . . . . . . . RbCl. . . . . . . . . . . . . . . . . CSCl. ................ HCl. ................ MgC12. ............... CaCla. ............... BaClr. . . . . . . . . . . . . . . . La(NO3)a. . . . . . . . . . . . ThCla.. ..............
(Sj
80.89
CWCLAY
H-CLAY
'locculation value ( S )
dsorption (8)
'locculatior value (S)
4.8 4.5 3.0 2.5 1.87 1.17 0.55 0.59 0.55 0.55 0.47 0.36
13.08 12.74 28.80 29.35 43.85 50.83 77.80 47.53 50 52.96
2.9 2.7 1.5 1.3 1.17 0.73 0.36 0.47 0.47 0.35 0.18 0.16
80.24
idsorption
(8)
6.6 6.2 14.5
28.20 39.73 50 15.78 26.89 23.78
'I
FIG.8. RELATION BETWEEN ADSORBABILITY OF A MONOVALENT IONAND COAGULATING POWER
ITS
values and the ionic exchange data (table 2) points toward an intimate association of the two phenomena. For any specific clay system (NH,clay, Ca-clay, etc.) those ions which are poorly adsorbed yield high flocculation values, while cations easily taken in by the colloidal particle coagulate the system at relatively low electrolyte concentrations. The two
602
HANS JENNY AND R. F. REITEMEIER
sets of values suggest a direct correlation which is of negative exponential type, and the experimental points satisfy the empirical equations
F, = 7.16e-0.0332E(for Ca-clay) F,
= 3.52e-0.0431 (for H-clay)
(7) (8)
where F , represents flocculation value and E the intensity of electrolyte adsorption expressed in terms of symmetry values. The reciprocal of the flocculation value can be taken as an index of the flocculation power of a cation, and the relation of the latter to the adsorbability of the ion is graphically represented in figure 8. The functions are of the form
F, F,
= 0.140e0~0332~ (for Ca-clay) = 0.284e0.M31E (for H-clay)
(9) (10)
where F , represents the flocculating power and E the adsorbability (exchange). For the sols tested, the coagulating effect of'an ion increases exponentially with its adsorbability. The position of the various curves stresses the fact that for a specific ion its flocculative effect depends largely on the nature of the ion initially adsorbed. FLOCCULATION BY X-RAYS
\
The sols listed in table 1were subjected to x-ray treatment. Th-, La-, and H-clay flocculated after a dosage of 10,503 r units, while the di-clays required about double the amount. None of the mono-clays (except Hclay) showed any signs of sedimentation, even after a dosage of 20,503 r units. Zeta potentials of treated and untreated samples of Th-, Mg-, H-, and K-clays were measured, and it was observed that in every case the zeta potential was reduced in magnitudes varying from 3.2 to 7.8 per cent. Conductivity measurements failed to reveal the presence of significant amounts of coagulating electrolytes. It must be concluded' that the efficiency of x-rays in destroying the stability of the clay systems dependsamong other things-on the nature of the adsorbed cation. It appears that the x-rays directly modify the composition of the electric double layer. MECHANISM OF FLOCCULATION WITH SPECIAL REFERENCE TO THE R ~ L E
PLAYED BY IONIC EXCHANGE
The data secured in this investigation enables one to draw a rather detailed picture of the significance of ionic exchange in the problemof colloid stability. Hitherto, too much emphasis has been attributed to the behavior of the flocculating electrolyte alone, while the results reported in this paper indicate that the nature of the cations initially adsorbed on the colloid is equally important.
STABILITY OF COLLOIDAL SYSTEMS
603
Every electrolyte coagulation is associated with an ionic exchange reaction. Two specific cases of flocculation will be discussed. First, that of a clay system which contains adsorbed monovalent cations of a highly hydrated nature, and secondly, the behavior of particles carrying either non-hydrated monovalent cations or polyvalent ions. In the first case, which deals with clays containing highly hydrated monovalent ions (e.g., Na-clay), the zeta potential is high because the ions are but loosely held and oscillate over considerable distances. The effective width of the double layer is great (or, the degree of dissociation is high). Although the addition of sufficient amounts of any electrolyte will cause the system to settle out, the magnitude of the flocculation value is a function of the properties of the coagulating cation. If it is also monovalent and strongly hydrated (e.g., Li+) the extent of ionic exchange will be moderate, in the neighborhood of 50 per cent. The zeta potential becomes but little altered and flocculation has to be brought about by the repression of the double layer (or, decrease in dissociation), which requires large amounts of electrolytes of the order of 15 to 20 S. An entirely different mechanism prevails if the cation of the added electrolyte is monovalent but not hydrated, as CS+,or polyvalent as La+++. Under such conditions, ionic exchange assumes a dominating influence. At symmetry concentration, from 75 to 95 per cent of the cations change places. The colloidal particle acquires an entirely new coat consisting of ions which are strongly attracted by the inner layer, and as a result, the zeta potential drops sharply. Very little electrolyte is necessary to repress the new outer swarm of ions to the critical distance which is . characteristic of the critical potential. The flocculation values are small, and of the order of 1 S. In regard to the second case, which embraces clays carrying non-hydrated monovalent ions (Csf-clay) or polyvalent cations (La+++-clay), the zeta potentials are low to begin with, because the oxygen and hydroxyl ions of the crystal lattice act as powerful adsorbents, and the average oscillation distance of the adsorbed ion is relatively small. Addition of electrolytes possessing hydrated monovalent cations (lithium chloride) will produce a modest exchange only (less than 25 per cent) which, nevertheless, tends to raise the average distance of the double layer and consequently the zeta potential. The stability of the system is definitely augmented, but the repression effect of the electrolyte finally counterbalances the peptization due to ionic exchange and precipitation takes place. The flocculation values are of medium magnitude (2 to 10 S ) , but higher than the common ion values. If the coagulating cation is of similar nature to the one initially adsorbed (non-hydrated, or polyvalent), ionic exchange assumes again greater dimensions (about 50 per cent) without, however, greatly modifying the
604
HANS JENNY AND R. F. REITEMEIER
zeta potential. Flocculation is accomplished mainly through repression (or decrease of ionization), which is effective at very low electrolyte concentration, often less than 1S. SUMMARY
1. Zeta potentials of Putnam clay particles depend on the charge and size of the adsorbed ion. For rare gas type ions of equal size, the potential tends to be lower as the valency of the ion is higher. If the charge of the ion is kept constant, the potential is higher as the adsorbed cation (crystal lattice radii) is smaller. 2. Colloidal particles with high potentials carry adsorbed ions which are easily exchangeable. The ease of replacement increases as the sixth power of the zeta potential. Data indicate the existence of a maximum potential which amounts to about 62 millivolts. 3. The flocculation value of a given electrolyte increases potentially with the zeta potential. The critical potential was found to be 42 millivolts. 4. The valency rule of Schulze-Hardy is grossly violated in the case of clay systems. For monovalent cations the flocculation value is the higher the smaller the size of the ion. 5 . The flocculating power of a cation is a positive exponential function of its ionic exchange ability. 6. A detailed picture of the mechanism of flocculation in relation to ionic exchange is presented. REFERENCES FREUNDLICH, H. : Kapillarchemie, p. 349. Leipzig (1923). HEVESY,G. VON: Kolloid-Z. 21, 129 (1917). JENNY, H.: J. Phys. Chem. 36, 2217-58 (1932). JENNY, H., AND GIESEKINQ, J. E.: In print. KRUYT,H. R.: Colloids, p. 286. ,John Wiley and Sons, New York (1930). LOTTERMOSER, A., AND RIEDEL,W.: Kolloid-Z. 61, 30-9 (1930). POWIS,F.: Z. physik. Chem. 89, 186-212 (1915). (8) TUORILA, P.: Kol1oid.-Z. 44, 11-22 (1928). (9) WHETHAM, W. C. D.: Phil. Mag. [51 48, 474-7 (1899). (10) WIEGNER,G. : Kolloid-Z. 36 (Erganzungsband), 341-69 (1925). (1) (2) (3) (4) (5) (6) (7)
