CATAPHORESIS AND T H E ELECTRICAL KEUTRALIZATION O F COLLOIDAL MATERIAL BY SANTE MATTSON
1. Introduction I n a paper on the relation between the electrokinetic behavior and the exchange capacity of colloidal soil materials the author has established a quantitative relationship between the charge of the particles and certain properties of the soil material. This work emphasizes the importance of cataphoresis as applied to the study of electrical neutralization and to a determination of the isoelectric point. While the electrical neutralization and mutual flocculation of oppositely charged colloidally dispersed material has been studied and much discussed in the literature, no attempts appear to have been made to study this behavior systematically in its application to problems of practical importance. Many such problems present themselves. The removal of colloidally suspended impurities is a matter of serious concern in many industries. The purification of water by the use of alum is an application of the above principle; but, here again, the different factors, qualitative and quantitative, which govern the electrical behavior of the suspended particles have not been investigated systematically. As a result of this the waterworks engineer often finds himself confronted with perplexing difficulties. The use of alum and other electrolytes in the reclamation of alkaline soils suggests great possibilities, and research work in this direction should be accompanied by a study of the electrokinetic behavior of the different electrolyte-soil systems. The removal of colloidal material from raw sugar and molasses is another problem of economic importance which deserves a thorough investigation. In many processes, such as the preparation of emulsions, sprays and precipitates to be used in pigments, etc., a high degree of dispersion is desirable and a study of the electrokinetic behavior of the dispersed material under different conditions would undoubtedly be of fundamental importance. Many other applications of cataphoresis could be suggested and it is safe to predict an important future for this method of dealing with colloidal behavior. It is the object of this paper to give a detailed description of the cataphoresis cell used by the author and to give a few additional applications. 2. Cataphoresis The electrical migration of suspended particles has in most cases been measured by observing the movement of the boundary between the suspension and the clear dispersing liquid when placed in a U-tube either of the type used by E. F. Burton,' or A. Coehn.* Phil. Mag., (6)11, 44 (1906). 15, 653 (1909).
* Z.Elektrochemie,
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
1533
This method is not accurate, however. In the first place the conductivity of the suspension is greater than that of the dispersing medium even if the latter consists of an ultrafiltrate of the former. The potential gradient is therefore not uniform. The movement of the particles at the boundary is proportional to the drop of potential a t that point and this is constantly changing due to ionic migration. Then, since the current must flow for a considerable time to produce a measureable displacement, the effects of heating, electrolysis and polarization may be considerable; changes in concentration of the different ions must affect the charge of the particles. This probably accounts for the different rates with which the ascending and descending boundaries often move.
0
FIG.I
The U-tube method is further limited to dispersed material and cannot be used after flocculation takes place, nor can the isoelectric point be determined. Some of these difficulties have been overcome by the micro and ultramicroscopic methods of Cotton and MoutonI1 The Svedberg,* R. Ellis,$ F. PowisI4and others. The most elaborate and accurate ultramicroscopic measurements have been made by The Svedberg and Hugo Andersons who have determined and pointed out the several errors connected with such measurements. They worked with a closed parallelopiped chamber and photographed the path of the particles in different depths of the chamber. From the velocities observed they calculated the electrosmotic movement of the liquid. Their results were in good agreement with theory. They obtained the best values when an alternating current was used. This method would probably be found too elaborate for most laboratory practice. I n a previous paper6 the author briefly described a simple cataphoresis cell with which about twenty measurements can be made in one hour with a fair degree of accuracy. Another but somewhat improved cell has later been constructed. Since this cell has never been fully described and in response to several requests, a detailed description is given below. The cell is shown in Fig. I and consists of a thick-walled tube of 2 . 3 5 mm. inside diameter and 2 2 . 3 cm. long, terminating in two larger tubes as shown. “Les ultramicroscopes et les objets ultramicroscopiques” (1916) S o v a Acta SOC.Sc. Upsaliensis, (4) 2, 149 (1907). Z. physik. Chem., 78,321 (1911). Z. phvsik. Chem., 89,91 (1914). KollGd-Z., 24. f56 (1919). 6 Mattson: Kolloldchem. Beihefte, 14, 227 (1922). 1
2
I534
S A N T E .MATTSON
At 0 the tube is ground down to within 0.2 mm. of the inner wall. The plane surface is then polished or a piece of thin cover glass is pasted on the rough surface by means of Canada balsam. At right angles to 0, on the side of the tube where the light is to enter, another plane surface is similarly made. This need not be made wider than the bore of the tube. Two large platinum electrodes are placed in the larger tubes A and B as near the entrance to the small tube as possible. The apparatus is sealed on a piece of wood and this is then firmly screwed onto the microscope platform. The cell is filled through tube A, which must be a little taller than B, the air escaping through stop-cock SI. It is emptied through stop-cock Sp. Washing is accomplished rapidly in the same way. The particles are illuminated on the principle of
FIG.2
the ultramicroscope, the source of light being an electric arc or a high-power incandescent lamp. The optical arrangement, which is similar to that of the ultramicroscope of Siedentopf and Zsigmondy, is shown in Fig. 2 , where A is the arc lamp provided with a collecting lens, Q is a trough containing a saturated solution of alum to adsorb the heat, S is an adjustable slit, L is a lens with a focal length of 12 cm., 0 is a low-power objective (16 mm.) so placed that the image of the slit is focused inside the tube T directly under the microscope. T shows the tube in two positions. The electrodes are connected to a 2 2 0 volt direct current circuit. A so-watt lamp is placed in series. By means of the commutator C the current is reversed*. The microscope used in this work was a Bausch and Lomb with an 8 mm. objective and eyepiece 7.5 giving a magnification of 150 diameters. The working distance of this objective (NA 0.50) is 1.6 mm. The eyepiece was provided with a scale which covered 0.4 mm. on the objective micrometer. The P.D. at the electrodes deviated slightly from 2 0 5 volts. The potential gradient was therefore 205/22.3 = 9.2 volts per cm. and the observed velocity V1 of a particle expressed in p/sec. in a gradient of one volt per cm. = 400/9.2 S where S is the time in seconds a particle requires to cover the scale. The cataphoretic movement is measured in both directions, each observation covering on the average about I O to 20 seconds, a stop-watch being used. When the movement of the particles of an electronegative suspension is observed in a tube of the above description, it will be found that, observing successive layers from the top to the bottom of the tube, particles moving next to the wall move toward the cathode as if they carried a positive charge. This cathodic movement slows up further down, until a layer is reached where A cataphoresis cell of the above description is now, supplied by Eirner and Amend, New York, and by the Arthur H. Thomas Company, Phladelphia.
CATAPHORESIS AND ELECTRICAL NEUTRALIZATIO?;
I535
the particles are apparently a t rest. Below this layer the apparent movement is reversed, being now anodic as if the particles carried a negative charge, their speed increasing down to the center of the tube. The same changes in apparent movement repeat themselves from the center to the bottom of the tube. The explanation is as follows: At the glass-water boundary is a double electrical layer, the water being positive and the glass surface negatively charged. The water will therefore be electrosmotically transported along the walls of the tube toward the cathode. Since now the tube is filled and the stop-cock closed, as much water as flows along the walls in one direction must return through the center in the opposite direction. In an annular layer somewhere between the axis and the wall the liquid must be a t rest. The moving water carries the particles along and the observed velocity of the particles is VI = v st V where u is the velocity of the particles relative to the liquid and V the velocity of the liquid. I n the annular layer where v = 0, v1 = v. On the basis of some theoretical deductions of M. v. Smoluchowskil the following expression for the velocity of a liquid in the different parts of a closed capillary tube such as here described has been developed.
where r a c z
distance from axis of tube, radius, a constant determined by the P.D. of the double layer, a very small constant which, for so large tubes as here considered, can be put = 0. From this expression it Rill be found that V = o when r = o.go7a. The apparent speed of migration of the particles in a suspension of powdered quartz was measured in different parts of the tube. The second column in Table I gives the observed Yl in p/sec. I volt/cm. of the particles for different r values. The latter are expressed in terms of scale divisions on the microscope micrometer screw. The diameter of the tube was 524 divisions (a = 2 6 2 ) . * I n a layer 185 ( = 0.707 X 262) scale divisions from the axis the particles moved with a speed of - 2.9 p per sec. toward the anode. This is according to the formula the true speed of the particles since the liquid is at rest in this layer. Nearer the wall, a t 2 1 7 scale divisions from the axis, the particles showed no apparent movement. Here the movement of the liquid must be equal and opposite to that of the particles or + 2 . 9 plsec. By adding this value to the observed velocities of the particles VI we get V; the velocity of the liquid at different depths of the tube as shown in the third column. = = = =
Graetz: “Handbuch Elektr. Magn.”, 7,383 (1921).
* In measuring the diameter of the tube in microscope scale divisions it is necessary
to use a low-power objective with a working distance equal a t least to the depth of the tube. By allowing a sediment to settle on the bottom and a few gas bubbles to collect a t the top of the tube these points are easily found.
I536
SANTE MATTSON
TABLE I Observed velocities of particles and calculated velocities of liquid in different depths of the tube r values
Velocity of particles V1 observ.
262
+s
237
+2.2
217
10.0
185 162 I37
62
-2.9 -4.3 -6.2 -7.9 -8.7 -9.7
37
-10.2
I12
87
0
.4
-10.2
p/sec. ’I
l’ ’I
)’ lJ
Velocity of liquid
V = VI i 2.9
+ 8 . 3 p/sec.
V
Velocity of liquid = c (r*-a2/2)
+ 7 . 8 plsec.
ss.1 ’)
+5.0
”
+2.9
+2.9
If
fO.0
fO.0
-1.4 -3.3
-1.8 -3.5 -4.9 -6.0 -6.8 -7.5 -7.8
-5.0
l1
-5.8
’l
-6.8
”
-7.3 -7.3
”
I’ It
” ” ’I
’’
We can also find V from the formula V = c(rz - a2/2),if we know the value of c. Putting the value of V = +2.9 p a t r = 2 1 7 we get c = 0.000228. Using this value for c we get the V values shown in column 4 which check fairly well with the V values of the third column. These two series of V values representing the movement of the liquid in the different annular layers in the closed tube are expressed by the curve in Fig. 3. FIG. 3 The arrows represent velocity vectors. Electrosmotic movement of water By keeping the microscope focused on in a closed capillary tube. a laver 0 . 7 of the radius from the axis of the tube the true velocity of the particies is obtained directly. At this point the liquid must be a t rest, independentIy of any change in the rate of electrosmose, provided of course that the stop-cocks are kept well greased to prevent leaking. The cell must however always be kept absolutely clean. Certain electropositive materials such as acid proteins and basic dyes adhere tenaciously to the electronegative surface of the glass. The latter might thereby receive a different charge in different parts of the tube, causing an irregular electrosmotic flow of the liquid. The cell should be given a frequent cleaning with acid and alkaline solutions and should be left filled with a sulphuric acid bichromate mixture when not in use. With the apparatus above described it is possible to measure the electrical migration not only of the individual particles but also that of the flocs after a suspension is flocculated and to study the effect of different electrolytes upon the charge and, what perhaps is the most important, the isoelectric
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
IS37
point of any system can be determined rapidly. The accuracy may be increased by taking the average of several measurements in each direction. The particles under observation are 11 cm. away from either electrode and electrolysis cannot affect the charge of those particles since the current flows only a short time. The great distance between the electrodes allows the use of a high potential. This reduces the polarization potential to less than one percent of the electrode potential. I n the following tables the migration velocities of the particles are expressed in N/sec. a t apotential gradient of I volt/cm. From themigrationvelocity the electrokinetic potential 5 is calculated from the Helmholtz-Perrin formula: v = -l-HD 4*t)
where v = velocity of the particles, H = potential gradient in volt/cm. D = dielectric constant of the liquid, q = viscosity of liquid, hence the P. D. of the double layer:
The viscosity of water at the temperature of the laboratory is taken from the Smithsonian tables and the dielectric constant D is put equal to that of pure water a t the same temperature. When v = 2.9, p = 2 . 9 X 104 cm.
l-=
4 .rr X
.OI
X
.00029
I
x
X 300 X 300
-
.041 volt
80
300 X 300 is placed in the numerator to convert H and { into absolute units. 3. The Electrical Neutralization of Colloidal Clay by Aluminum Salts I n the following experiments the electrodialyzedl supercentrifuge fraction of the Sharkey clay soil was used. 2 5 cc. of the clay suspension containing I O mgms. of clay were rapidly mixed with 2 5 cc. of the salt solution and the mixture placed in test tubes provided with rubber stoppers. After 24 hours the degree of flocculation, which in the tables is represented with the letter z, was observed. One z signifies slight and four 2’s complete flocculation. The tubes were then shaken and the cataphoresis measurements made. Table I1 shows the gradual neutralization of the negative charge of the particles. It will be seen that the lowest concentrations of AICla cause a proportionally much greater reduction of the charge than the higher concentrations, which only cause a very gradual further reduction of the negative charge until the mixture is isoelectric a t a concentration of about 1 . j milliequivalents per liter. This is due to the fact that it is the products of hydrolysis Mattson: J. .4gr. Res., 33, 553 (1926).
1538
SANTE MAWSON
TABLE I1 The electrical neutralization of electrodialyzed Sharkey clay by AlC13, .zoo gm. clay in one liter AlCl? Flocculation P. D.
milliequv./liter 0
after 24 hours 0
P/Bec.
millivolts
-3.20
-45.0
0 .I
XXX
-1.60
-22.5
0 . 2
-0.80
-11.3
-0.59
-
1.5
xxxx xxxx xxxx xxxx xxxx
i
2 . 0
xxxx
+O.
4.0 8.0
xxxx xxxx
+0.49 +0.65
0.4 0.7 I
.o
-0.45 -0. I 7
o I4
8.4 6.3 - 2.4 i 0.0
+
2.0
+ 7.0 + 9.1
of the aluminum salt rather than the trivalent A1 ion which reduce the charge most powerfully. Since the hydrolysis is proportionally greater in the more dilute solutions, the electrical neutralizing power is likewise proportionally greater. That i t is primarily the products of hydrolysis of the salts of aluminum and iron which are responsible for the electrical neutralization and flocculation of electronegative colloids has already been pointed out1 but because of the importance of this fact for establishments where these substances are used for purification and clarification purposes the following experiment will be given here. In a series of tubes the clay suspension was acidified and alkalinized with increasing quantities of HC1 and NaOH respectively. was then added at the rate of 1.5 milliequivalents per liter which, according to the previous experiment, was just sufficient to render the untreated suspension isoelectric. Table I11 gives the quantities of acid and alkali added, the rate of flocculation after mixing as well as the condition after 24 hours together with the charge on the particles or flocs in the different mixtures. As will be noted the addition of the acid weakens the electrical neutralizing power of the A1C13 evidently as already pointed out by reversing the hydrolysis, while a partial alkalinization causes activation. In the proportion of 1.5 milliequiv. A1CI3 to 1.25 NaOH the particles attained their maximum electropositive charge moving a t the rate of 3.1 I.( per second toward the cathode. The pH a t this point of highest activation was found in a number of cases to be about 5 . 2 . h greater proportion of alkali reduces the positive charge, passes the isoelectric point and, with an excess of alkali, the system becomes strongly electronegative due to the adsorption of the OH ions. It should be noted that the clay was electrodialyzed and therefore adsorptively unsaturated and strongly acid. Some of the ,base was therefore adsorbed by the clay. This accounts for the fact that the system was still Mattson: Kolloidchem. Beihefte, 14, 227
(1922).
‘539
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
TABLE I11 The effect of HCI and NaOH upon the electrical neutralizing power of A1C13 in clay suspension, 1.5 milliequiv. AlCI, per liter was added to the acidified and alkalinized suspensions Milliequiv. Flocculation after p/sec. P.D. acid or base
mixing
24 hrs.
5.00 HC1
slow
XXXX
-0.48
I,
xxxx
-0.40
1)
1.50
”
.SO
”
-
I
volt cm.
xxxx
-0.21
xxxx xxxx xxxx xxxx
*O
f O
+0.37 $0.98 +I.72 +2.15
.SO
”
.75 I .oo
”
0
XXXX
”
0
xxx
f2.60
1.25
”
0
xx
+3
.40
”
0
xxxx
i-2.20
1.50
”
.oo 4.00
”
0
0
”
0
0
.30
I
2
”
rapid
- 6.7 5.6
-
rapid very rapid most rapid slow
. I O NaOH
millivolts
XXXX
I10
+0.43 -3.20 -3.50
+
2.9 5.2
f13.8 +24.3 +30.3 +36.3 +43.7 +31.0 6.0 -45.0 -49.4
+
electropositive in the presence of equal proportions of A1C1, and NaOH. The proportion of A1C13 and NaOH which in the above case was found most efficient is of course of no importance since this proportion will vary with the reaction of the electronegative suspension. I n his earlier work referred to above the author found that CuSOa behaves in the same way as the salts of iron and aluminum. Without alkali the CuSOl caused only the usual reduction of the charge on quartz particles produced by other divalent cations. If the sulphate was partly alkalinized it charged the particles positively, while an excess of alkali gave rise to a second reversal in the sign of charge. The above results are represented graphically in Fig. 4. The sharp bend in the lower part of the curve shows a very narrow limit of maximum activity. This limit corresponds to the formation of the maximum quantity of the electropositive colloidal oxychloride. On the more acid side the normal AlC& predominates, and while the A1 ions suppress the negative charge considerably they do not entirely neutralize it. On the alkaline side the normal Al(OH)3 is formed and this material is, in the slightest excess of alkali, itself electronegative. Alumina is an electrical ampholyte and its maximum activity as a positive sol appears to coincide with a pH of about 5.2. I n order to determine the electrical neutralizing power of alkalinized AlCl3 in the proportion of 1.2 j milliequivalents of NaOH to 1.5 of illC13 experiments where carried out with the same clay suspension by adding increasing quantities of the two electrolytes in this proportion; it was found that slightly more than 0.5 milliequivalents of the alkalinized AICls was sufficient t o render 0.2 gm. of the clay material isoelectric as compared to 1.5
1540
SANTE MATTSON
milliequivalents of normal A1C13. This is equivalent to .0085 gm. A1203in the former and .oz55 gm. A1203 in the latter case. The isoelectric ratio of A1203 to clay is therefore under the most favorable condition .0085/.z = .043 which means that 43 mgms. of A1203are sufficient for the electrical neutralization of one gram of the clay here used. It should be added that different clays differ very much in this respect. The red and yellow varieties which have a high sesquioxide content are only weakly electronegative and are easily neutralized, while clays with a high silica content are strongly electronegative and show a high isoelectric ratio.'
-40 -20 2
f:
0
f *EO
s +40 4
UCI
2
0
2
MILLIEOU~YAL€NTS NaOH
4
FIG.4 Displacement of the electrokinetic potential by HCl and NaOH in an isoelectric mixture of 1.5milliequivalents AlCls and 0 . 2 gram clay per liter (comp. Table 111.).
From the above experiments it follows that, where an aluminum salt is to be used as a flocculating agent of electronegative material, the highest efficiency can only be attained by adjusting the pH to a point a t which the products of hydrolysis of the aluminum salt constitute an electropositive sol of maximum activity. For the chloride of aluminum this p H is about 5.2. Whether acid or alkali is to be added to the suspension depends of course upon the reaction of the latter. It is evident that the addition of a salt of aluminum to an alkaline water would a t first result in the formation of electronegative Al(0H) which would have no effect upon the stability of the electronegative material in suspension. A great saving of alumina would be effected by the addition of a certain quantity of acid and by a determination of the isoelectric ratio. I n view of the proposed treatment of alkaline soils with salts of aluminum the importance of investigations along the above lines wilr be realized. If insufficient amounts of the salt were applied, the normal AI(OH), would be formed which, in excess of alkali, is itself electronegative and would therefore be without effect. An excess of salt might be injurious to plants and would not improve the soil structure as much as the optimum amount, which could be determined by the above method of electrical neutralization. It should be added that the floc produced by the partly alkalinized AlCh was found to be much more stable than that formed by AlC13 alone or with HCl. A few decantations caused dispersion of the floc produced by the last
* Mattson: J. Am.
SOC. Agron., 18, 458 (1926).
1541
CATAPHORESIS A S D ELECTRICAL NEUTRALLZATIOS
two methods while the first-mentioned floc remained stable after a great number of washings. A significant fact is that the greatest activity of the aluminum compound as an electrical neutralizing agent lies in a region of great insolubility. Yery little aluminum is therefore left in the solution if flocculation takes place at the isoelectric point. Experiments with alum showed that while the flocculating power of this salt is equal to that of the chloride,' its neutralizing effect on the negative charge of the clay particles is weaker. X concentration of alum of 1.5 milliequivalents per liter with respect to the A1 ion reduced the electrokinetic potential of the clay particles from -45.0 t o -2.4 millivolts (comp. Table 111). Even a concentration of 8.0 milliequivalents per liter left the particles
z
0 2 4 FIG.j Displacement of the electrokinetic potential by HC1 and NaOH in a mixture of 4
(
equivalents alum K z " ' ~ ' ' ' )
and
0.2
I .j
milli-
gm. clay per liter.
slightly electronegative. A partial alkalinization activated the alum but to a lesser degree than in the case of AlC13. Hydrochloric acid and sodium hydroxide were added to the clay suspension in a series of tubes as in the experiment with AlCl, and for comparison 1.5 milliequivalents per liter of recrystalized alum were added. Fig. s shows the displacement of the electrokinetic potential caused by the acid and alkali. The difference in behavior of the two salts of aluminum is due to the sulphate ions which are present in excess in the alum solution. It will be shown later that the anions, especially the divalent and polyvalent, cause a displacement of the charge of colloidal particles in the electronegative direction. The addition of KzS04 to the AlCl,-clay mixture showed a similar displacement. Influence of Anions and Cations on the Electrokinetic Potential and the Electrical Neutralizing Power of Clay Particles In a previous paperz the author has shown that the quantities of methylene blue required to render a given quantity of clay isoelectric is directly proportional to the base exchange capacity of the clay material a t the same pH. This was explained on the assumption that the methylene blue cation displaces the common cations forming a nondissociated adsorption compound with the clay complex. In the presence of other electrolytes the quantity 4.
Flocculation takes place over a, wide range on either side of the isoelectric point. J. Am. Sac. Agron., 18, 458 (1926).
* Mattson:
1542
SANTE MATTSON
of the dye required to neutralize the negative charge on the clay particles may be greatly affected, the effect being in general such that the more active anions increase the isoelectric ,ratio of methylene blue to clay while the active cations decrease this ratio. This is illustrated in Fig. 6, where the abscissas represent the quantities of methylene blue added to the suspensions and the ordinates the corresponding velocities of electrical migration in p/sec. for I volt/cm.
FIG.6 Influence of anions and cations on the electrokinetic potential and the electrical neutralizing power of clay particles.
The results were obtained by mixing 2 5 cc. of the suspension containing milligrams of clay to which the electrolytes had been added with 2 5 cc. of the methylene blue solution and after 2 4 hours measuring the cataphoretic movement. The concentration of the electrolytes was in each case 1.5 milliequivalents per liter. It will be seen that anions, especially those with a high valence together with the OH ions, cause a displacement of the isoelectric point in the direction of greater methylene blue concentrations, while the di- and trivalent cations cause an opposite displacement. While the more active anions effect a greater steepness of the curves in the electronegative region and a flattening of the curves in the electropositive region, the active cations produce an opposite effect. Where both of the ions are active as in Ca(OH)2 the curve becomes flatter in both regions. It will also be noted that the electrical neutralizing power of the clay is independent of the initial charge of the particles. The addition of Ca(OH)2 reduced the migration velocity from 3 . 2 to 1.0 p/sec. while the quantity of methylene blue required to neutralize the negative charge of the particles increased from about 1.6 to 5.0 milligrams. The hydroxide, phosphate, and ferrocyanide of sodium all increase the migration velocity to about 4 p/sec. but the electrical neutralizing power of the clay has become proportionally much greater. IO
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
I543
These facts lead to the recognition of two factors of electrokinetic energy similar to all other forms of energy: an intensity factor and a quantity factor. The former is represented by the electrokinetic potential and the latter expresses itself in the form of the electrical neutralizing power and represents the total quantity of adsorbed ions. The electrokinetic potential is due to the existence of an electrical double layer resulting from an unequal adsorption of ions of opposite sign of charge. But it appears improbable that all of the adsorbed ions exist in the form of a double layer. Calculations made by the author1 indicate that only a small fraction of the ions adsorbed by soil materials exist in this condition. If this were not so it would be difficult to explain the above observations. The adsorption of one ion in excess of the other of a pair of ions must be limited by electrostatic attraction. The value of this limit will depend upon the nature of the ions, size and hydration probably being the most important factors as already pointed out by Wiegner.* From the above experiment it is evident that the association tendency of the Ca ions is greater than that of the more highly hydrated Na ions or, what amounts to the same thing, the dissociation tendency of the Na ions is the greater. The fact that much more methylene blue was required to neutralize the negative charge of the clay after Ca(OH)* had been added, although this addition resulted in a reduction of the initial charge, is to be explained as follows : The methylene blue cations which are strongly adsorbed neutralize the free charges of adsorbed anions. The Ca ions in the outer layer are thereby released and diffuse into the solution where they are electrically balanced by the anions of the dye. As the electrokinetic potential is reduced the Ca ions in the inner layer are set free and this displacement continues until all the negative charges due to adsorbed anions have been neutralized by the methylene blue cations, when the material is isoelectric. We find a perfect analogy in the pH and titration value of an acid solution. Just as an acid may have a low hydrogen ion concentration but a high titration value, or vice-versa, colloidal particles may have a weak charge but a high electrical neutralizing power or vice versa. The two processes differ however inasmuch as the titration value of an acid is a fixed quantity while the electrical neutralization of colloidal materials, which depends upon ionic equilibria, varies with the nature and concentration of the ions. In flocculating a suspension of denatured albumen with Na-acetateacetic acid mixtures L. Michaelis and P. Rona3found that, for flocculation the most active hydrogen ion concentration was displaced toward the acid side by the active anions while cations caused a displacement toward the alkaline side. This displacement of the p H a t which flocculation took place is in reality a displacement of the isoelectric point. The acid side in their experiment corresponds to the positive side in the above experiments. The methylMattson: J. Agr. Res., 33, 553 (1926). Kolloid-Z., 36 (Zsigmondy Festschrift) 341 (1925). Biochem. Z., 94,225 (1919).
I544
SANTE MATTSON
ene blue in the above case plays the same part as the H ions in the case of albumen which is an ampholyte. We see therefore the same principle disclosed in both cases.
5. The Electrical Behavior of the Colloidal Material in Milk, Raw Sugar and Molasses The proteins in milk consist chiefly of casein and albumen. These are true ampholytes and like gelatin are isoelectric at a hydrogen ion concentration of 2 x IO-^ or a pH of 4.7 as determined by L. Michaelis.’ The p H of fresh milk is usually 6.6 and the electrical migration of the proteins is anodic. The proteins exist in the form of proteinates and their ions as anions, hence the electronegative behavior. On the acid side of the isoelectric point the proteins combine with acids and become cathodic. When milk sours, or when an acid is added, the caseinates and albuminates are decomposed and the anodic movements become slower until a t a pH of 4.7 there is no migration. A further increase in the hydrogen ion concentration results in a reversal in the direction of migration. The proteins have become electropositive. Table IV shows the effect of HCl upon the electrical behavior of the milk proteins.
TABLE IV Effect of HCl upon the electrical migration of milk proteins. 0.2 cc. fresh pasteurized milk diluted to 2 0 cc. HC1 cc. 0.01 N in 20 cc.
Flocculation
0
0
0.5
0
1.0
0
1.2
XXXX
1.4 1.6 1.8
0
2.0
0
’
p/sec. within 1 / 2 hour
P. D. millivolts
-2.92 -2.24 -1.41 fo +1.26
-41.2 -31.6 -19.8 *O
+17.7
PH
6.7
5.7 4.9 4.7 4.6
0 0
The initial migration velocity was - 2 . 9 2 plsec. The calculated P.D. was - 4 1 . 2 which agrees well with the values observed by J. Loeb2 in a solution of Na-gelatinate a t the same pH. At a concentration of 1.2 cc. N/IOOHC1 in 2 0 cc. of the diluted milk the proteins were isoelectric and coagulated completely. The action of the HC1 is, according to the now generally accepted view, purely chemical in nature, the proteins being true ampholytes. The action of a basic dye like methylene blue upon the milk proteins is different, and probably best accounted for by assuming an adsorption and electrical neu1
2
“Die Wasserstoffionenkonzentrationen”, 54 ( I 914). “Proteins and the Theory of Colloidal Behavior”, 138 (1922).
I545
CATAPHORESIS AND ELECTRICAL NEUTRALIZATIOS
tralization of oppositely charged bodies, the negative proteins and the positive ions of the dye. Table V shows the effect of methylene blue on the electrical behavior of milk proteins. It required about 4.1 mgm. of the dye to produce electrical neutralization of 0 . 2 cc. fresh milk when diluted to 2 0 cc. with freshly distilled water. Coagulation, it will be seen, extends slightly on either side of the isoelectric point.
2
6
4
FIG.7
FIG.8
Electrical neutralization of milk proteins at var ing pH values by methyLne blue.
Electrical neutralization of colloidal material in raw sugar and molasses by methylene blue.
TABLE V Electrical neutralization of milk proteins by methylene blue. 0 . 2 cc. fresh pasteurized milk diluted to 20 cc. Methylene blue mgms.
Flocculation
0
0
r/sec. within hour
P. D.
-2.92
-41.2
1.0
0
-2.30
-32.4
2 .O
0
-2.00
3.0
X
-1.05
-28.2 -14.8 - 3.4
4.0 4.2 4.5
XXXX
-0.24
xxxx xxx
+0.33 +o. 78
+ 4.6 +II.O
5.0
X
+I.O8
i-15.2
6.0
0
+2.
f29.8
I1
Since souring renders the milk less electronegative, the same sample of milk was placed in the cooler and at 48 hour intervals the experiment with methylene blue was repeated. After two days the pH of the diluted milk was 6.2, after four days it was 5.1, and after six days it was 4.7, and the
1546
SANTE MATTSON
coagulum was isoelectric. The results of these and the above experiment with methylene blue are shown in Fig. 7 in which the abscissas give the concentration of the dye and the ordinates the calculated P. D.’s. It will be seen that, as the acidity of the milk increases, less of the methylene blue is required to reach the isoelectric point. It thus appears that the freshness of milk may be determined by its electrical neutralizing power. Fig. 8 shows the electrical behavior of the colloidal material in raw cane sugar and in molasses from beets. Each tube contained 100 mgms. of the sugar or molasses in 50 cc. water. The abscissas show the number of mgms. methylene blue added to each tube. It will be seen that the colloidal material in the molasses required about three times as much of the dye for electrical neutralization as did the raw sugar. The molasses which was very dark in color contained of course much more colloidal material than the raw sugar. It was not determined whether the content of colloidal material varied in the same proportion as the amount of methylene blue required. Since the presence of different electrolytes and a difference in reaction modifies the electrical neutralizing power of colloidal material, as has been shown above, an exact proportion between the two quantities should not be expected.’ 6. Electrokinetics of Barium Sulphate A study of the electrokinetic behavior of precipitates will undoubtedly be of great value where a control of certain factors such as the size of crystals, dispersion or aggregation are desired. As this subject will receive a more
detailed discussion later only one example will be given here.
TABLE VI The electrical charge of BaSOc when formed with an excess of KzS04 and of BaClz and the pH of the supernatant solution C.C. 0.1 N
KlSO,
BaCll
PlSeC. I
P. D.
volt em.
millivolts
-3.8 -3.0
-53.5 -42.3
-1.1
+ 5.6
-15.5
6.9 6.6 6.4
+22.6
5.2
+39.5 +60.5
4.8
+0.4 +1.6 +2.8
+4.3
PH 7.1
5.0
I After the completion of the above work, the methods developed by the author have, a t his suggestion been applied by M. S. Badollet and H. S. Paine, U. S. Bureau of Chemistry, to a uandtative study of the colloidal materials in sugar-house liquors. These investigatorsyound the basic d e, night blue, more suitable for their work than methylene blue. The value of their work is lowever doubtful for although the make free use of a manuscript, which the author handed over t o Mr. Badollet regarJng dascription of methods and apparatus they appear to have failed to grasp the brinciples of electrosrnose and cataphoresis. I n explaining the adjustment of the apparatus they refer t o a point “where the colloidal particles do not show progressive movement toward either electrode” when no current iS flowing. What kind of movement they were observing when no current was flowing and what significance such a point could have is difficlut to say. M. S. Badollet and H. 9. Paine: Int. Sugar J., 28, 23-28,97-103,137-140(1926).
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
I547
Different proportions of accurately standardized N/IO BaClz and K 8 0 4 solutions were rapidly mixed in a total volume of 50 cc. The p H of the distilled water and the two solutions was 6.4. Table VI gives the sign of charge and the electrical migration of the BaS04 crystals. The last column gives the p H of the supernatant solution colorimetrically determined. It will be seen that with an excess of the sulphate ions the precipitate is electronegative but with an excess of Ba ions it is electropositive. This must be due to an adsorption of the respective ions. But the association tendency or, to put i t otherwise, the solution tension of the Ba and the SO4ions must be different because, in equivalent proportions of the reagents, the precipitate is electropositive indicating a slight excess of Ba in the surface layer. Bas04 is therefore normally, in the absence of surface active anions, electropositive and might be written thus: (BaS04)Ba., where z is very small. That the change of the sign of charge on the crystals actually is due to an association of one ion in excess of the other is evident from a corresponding displacement in the pH of the solution. The displacement to the acid side with an excess of BaCh is more pronounced than the displacement to the alkaline side with an excess of K2S04. Why the adsorption of Ba ions should increase the H ion concentration in the solution and why the adsorption of the SO4 ions should have the opposite effect is easily explained. The adsorbed Ba ions attract an equivalent number of anions which surround the crystal forming the outer ionic layer which, in this case, is electronegative. If the attracted anions consisted exclusively of the C1 ions of the BaClz the pH of the outside solution would be unaffected. But the C1 ions are displaced by the more active OH ions of the water resulting in the formation of a trace of HC1 in the solution. Similarly, the adsorption of the SO4 ion attracts an equivalent of cations forming, in this case, an electropositive outer ionic layer. Now it is well known from studies of base exchange that the H ion has a strong displacing power. The H ions of the water displace partially therefore the K ions of the KnS04with the result that a trace of KOH is formed in the outside solution. This leads to the conclusion that electronegative materials or acidoids must have a higher H ion concentration in a layer near the surface than that of the outside solution. Conversely electropositive materials or basoids must show a lower H ion concentration a t the surface. That acid soils and peat show a greater acidity t o litmus paper than their water extracts has often been observed.' By removing the liberated base by continued leaching of BaS04 with a KzSOa solution and by a subsequent treatment with a strong chloride solution, an exchange acidity similar to that of soils should be developed. The above property of BaS04 may be assumed to be general to a greater or lesser degree for all precipitates. It is improbable that the solution tension of the anions and cations in the crystal lattice is ever exactly the same but varies with the hydration, the diffusibility and other properties of the ions. The fact that precipitates of the same material vary greatly in their Mattson: Kolloidchern. Beihefte, 14, 227
(1922)
1548
SANTE MATTSON
behavior is undoubtedly due to slight differences in composition. Such differences might be expressed by the general formulas (BA), B, or (BA), A. depending upon the nature of the material and the conditions under which it is formed. The quantity z may be chemically insignificant but very important as a factor governing the physical character of the material.
7. The Electrokinetics of Certain Soluble Electrolytes In the case of certain soluble electrolytes the effect of the unequal solution tension of the two ions in the crystal lattice shows itself in a striking manner. Salts of highly hydrated cations and complex polyvalent anions such as the phosphate and ferrocyanide of sodium form, when dissolved, a transient turbidity. A crystal of the ferrocyanide when placed in water and viewed through the microscope fairly appears to explode. Little chips of crystals break away as if repelled by a considerable force and pass only very gradually in solution. These crystal chips carry a very high electronegative charge as determined cataphoretically. By shaking a few crystals of the ferrocyanide with water a transient suspension was formed and, by working rapidly, the cataphoretic movement of the particles could be determined before solution was completed. The particles migrated toward the anode with the extraordinary velocity of 8.4 microns per second corresponding to an electrokinetic potential of I I 7 millivolts. The fact that the ferrocyanide crystals break up in water into small fragments before dissolving must be due to this very high electronegative charge which is developed on their surface. This charge is the result of the diverging tendencies of the two ions; the Na+ ion to diffuse into the water and the Fe(CN)ri---- ion to remain attached t o the crystal lattice forming a micelle 4 Na.+. whichmight be represented by the formula:((Naa F e c y ~(Fecys),----] ) The surprising slowness with which the crystal fragments dissolved, considering the high solubility of the salt, is probably due to the layer of the ferrocyanide ions impeding the outward diffusion. If the breaking up of the ferrocyanide crystals in water into small fragments is due to a greater solution tension of the Na ions, then this breaking up ought to be prevented by increasing the osmotic pressure of the Na ions by adding a sodium salt of a weakly adsorbed anion. By adding NaCl to the water before the addition of the ferrocyanide it was found that the latter dissolved “normally” without the formation of any turbidity, thus verifying the above conclusions. The presence of an excess of Na ions reduced the diffusion of this ion from the surface of the ferrocyanide and, the C1 ions not being appreciably adsorbed, the P.D. of the double layer did not bssume the above high magnitude but remained within the limits of stability of the crystals. This behavior of a soluble electrolyte to undergo a crystal dispersion before solution takes place will be met with only where the solution tension of the two ions differs greatly as in the case of compounds in which one of the ions has a tendency to become heavily hydrated, i.e. to be strongly adsorbed by the water, while the other ion is strongly attracted and ad-
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
I549
sorbed by the solid surface, Foremost among the former ions are the alkali cations of low atomic weight aqd volume such as Na and Li which, because of their small size and the resulting strong electric field, attract and adsorb the water molecules most strongly as pointed out by von Hevesyl and by Wiegner and Jenny.2 Among the strongly adsorbed ions are the complex and polyvalent ions and notably the OH ions. The transient turbidity formed when NaOH is dissolved is probably due to this tendency of the OH ion and is to be explained as was done in the case of the ferrocyanide. The ions which cause a dispersion of their own crystals before the latter dissolve are therefore the same ions which, when added to a suspension of an insoluble material, cause the highest degree of dispersion provided of course that the suspended material has the same sign of charge as that of the most strongly adsorbed ion. Thus the electronegative clay is most readily dispersed by the hydroxide, phosphate and ferrocyanide of sodium while the electropositive ferric hydroxide is most efficiently flocculated by these electrolytes. 8. Conclusion
I n conclusion to the above work a brief definition of the conditions which lead to a potential difference at the interface of in-water-suspended materials will be given. Two forms of electrokinetic potential may be recognized as follows: ( I ) dissociation potential and ( 2 ) adsorption potential. The former results from an unequal diffusibility or an unequal solution tension of the ions in the material itself while the latter is formed through an unequal adsorption of ions foreign to the material but present in the solution. A dissociation potential difference is formed by dispersed materials when one of the ions is diffusible and when the other ion, because of its complexity is nondiffusible or when, because of its low solution tension it remains associated with the molecules of the solid and becomes by virtue of this association a nondiffusible ion. The proteins in combination with acids and bases: [R.COO-]Na+ and [R.XH+&l- and the oxychloride of iron: [(Fe203)=FezO2++]z C1- may be mentioned as examples of this type of materials. A dissociation potential is also formed by electrolytes in which both ions are diffusible but possess a different solution tension. Examples of this type are [(BaSO&. Ba++]S04-- and [(iVarFeCy& FeCye----]4Na+. The sign of charge will be that of the ion with the lowest solution tension. If the difference in tension of the two ions is small as in BaS04the charge may be reversed by adding an excess of the ion having the highest tension. While the dissociation potential may be of either sign of charge, the adsorption potential appears to be always electronegative in aqueous suspensions. The fact that the great number of insoluble and chemically inert substances charge themselves electronegative in water and in solutions of ordinary electrolytes might be explained by considering the chemical nature of the water itself. Since the electrokinetic potential depends upon the most minute Kolloid-Z., 21, 129 (1917). Proc. First Int. Congress Soil Science, Second Commission, 40 (1927).
* .\bstracts
I550
SANTE MATTSON
concentrations of ions, water must, for the subject here dealt with, be looked upon as an electrolyte. In the second place water is an ampholyte of the most ideal type since, by dissociation, an equal number of H and OH ions are formed. One of these ions, the OH, has been shown to be one of the most strongly adsorbed ions.' Now it is generally assumed that the water molecules, or rather groups of associated water molecules, arrange- themselves in an oriented position a t interfaces due, probably, to an inequality in the strength of the electric fields at phase boundaries. This means that adsorbed water is structurally different from ordinary water in which the molecules have a random distribution. The question arises: is the adsorbed water also chemically different from ordinary free water? If groups of associated water molecules are electrical bipoles and if the polar orientation is such that the OH ions (and anions in general) are attracted toward the interface then it follows that adsorbed water must be more acidic than water in the free condition. The fixation of the OH ions a t the interface leads to the liberation of a proportionally greater number of H ions, which, because of electrostatic forces, are held in the form of an outer layer surrounding the adsorbent. This may be represented by the following scheme:
[/adEorbentI
- H H H
a
.0.0.0.0.]H+ H H H H
This would account for the sign of charge of the great body of very different materials (acidoids) which in water charge themselves electronegative. It would also account for the often observed fact that these materials show a higher H ion concentration a t their surface than that of the surrounding solution. Further, since the H ions may be displaced by other cations, the source of the acid liberated by the neutral salt treatment is also accounted for. The adsorption of water weakens its basic and strengthens its acidic properties and the adsorbed water exhibits therefore the character of an acid. The adsorption potential must be looked upon as being superimposed upon the dissociation potential. The sign of charge is determined by the algebraic sum of both. This conceptionof themechanism of the two potentials explains several observations which would otherwise be difficult to account for: All materials which by dissociation charge themselves electropositive may be charged electronegative by increasing the OH ion concentration. As examples of such materials may be mentioned the proteins, the colloidal basic dyes, the metallic oxides and barium sulphate. All of these are therefore ampholytoids. Materials which remain electropositive independent of the p H (basoids) are unknown. A great number of materials are always electronegative (acidoids) retaining this charge independent of the pH. Not even the trivalent cations appear to be able to change the sign of charge of these materials. The change in sign of charge of clay brought about by the addition of AlCla has been 1
Mattson: Kolloidchern. Beihefte, 141,
227 (1922).
CATAPHORESIS AND ELECTRICAL NEUTRALIZATION
1551
shown to be due to the products of hydrolysis, i.e. to the oxychloride, and the resulting positive charge is evidently due to a dissociation of this complex. The adsorption potential appears therefore to be strictly electronegative as a consequence of the attraction of the anions (OH and others) to the interfacial side of the adsorbed water. The negative charge of the noble metals which, because of their low solution tension, would be expected to be electropositive, as pointed out by Freundlich,’ is explained by this adsorption. The observation made by Freundlich that the electrokinetic potential does not always parallel the Nernst potential, has been explained by him, and is further made more comprehensible by the above deductions. The above work was performed in the Bureau of Soils of the United States Department of Agriculture. summary
An ultramicroscopic cell which permits a rapid determination of the cataphoretic movement of colloidal particles has been described. Several applications of cataphoresis have been given as follows: ( I ) The electrical neutralization of clay suspensions by salts of aluminum. The experiment showed that it is the products of hydrolysis of these salts rather than the trivalent cations which are active and that an adjustment of the pH to a value of about 5.2 is essential for the highest degree of efficiency. ( 2 ) The electrical neutralization of clay by the basic dye methylene blue in the presence of different anions and cations showed that the isoelectric blue ratio methyll;; was increased by the former and decreased by the
(
)
latter ions, the effect increasing with the valence. (3) The quantities of methylene blue required to neutralize the electronegative proteins in milk decreased with the acidity (i.e. increased with the freshness) of the latter. The electrical neutralization of the colloidal materials in raw sugar and in molasses indicated a proportionality between the neutralizing power and the quantities of colloidal material present. (4) A study of the electrokinetic behavior of BaS04 showed that this material is, by itself and in the presence of an excess of Ba, electropositive while in the presence of an excess of SO4 it is electronegative. The electropositive condition causes a decrease and the electronegative condition causes an increase in the pH of the solution. The former phenpmenon was explained on the basis of a difference in solution tension of two ions and the latter by assuming an adsorption of the OH and H ions of the water respectively. ( 5 ) The behavior of certain soluble electrolytes, such as the ferrocyanide of sodium, of forming a turbid suspension before dissolving was cataphoretically investigated. The crystal chips which in water break away from the larger crystals were found to carry an extraordinary high electronegative “Kapillarchemie”, 339 (1922).
I552
SANTE MATTSON
charge. The great solution tension of the highly hydrated Na ions and the low tension of the tetravalent anion (which is a highly adsorbed ion) was held responsible for this behavior. In conclusion it was pointed out that all electrolytes may give rise in water to a dissociation P.D. at the phase boundaries, the sign of charge being determined by the solution tension and osmotic pressure of the respective ions. But the cause of the electronegative charge of the great number of inert materials is to be sought in the adsorbed layer of oriented water molecules which apparently attract the anions to the interfacial side of this layer giving rise to an adsorption potential. Adsorbed water is held to be more acidic than free water. Soil Research Laboratory, AgricuUura.2Experiment Station, New Bruszdek, N . J. 19Z8.
