272
MAXBENDERAND HENRY MOUQUIN
Vol. 56
BROWNIAN MOVEMENT AND ELECTRICAL EFFECTS’ BY MAX BENDER^
AND
HENRYMOUQUIN
Department of Chemistry, New Yo& University, New York, N . Y . Received February 6 , 1961
1. Brownian activity stops in the Taggart shallow cell when the electrolyte changes from alkali t o acid, because the particles become attracted to the glass walls as a result of the change8 in electrostatic forces. 2. In this connection a means of estimating the sign and intensity of electrical charge on a glass surface in the presence of different solutions of electrolytes is resented. 3. Even though a suspension is at the isoelectric point, flocculation may not hap en. For instance, with s falerite, flocculation did not occur in acid medium (even 0.1 M H~SOI)or in concentrated Ea&, the SUB ensions being cfose to the isoelectric point. With aluminum chloride, flocculation occurred onlv in alkaline solution, not in $stilled water or acid. 4. The Brownian movement of sphalerite particles was found to be diherent in one electrolyte medium com ared with another. It was lowest in the alkaline aluminum chloride flocculating medium, the activity being that pregcted by Einstein’s theory. In alkaline medium only, i t was somewhat over 1/2 that of Einstein, and in acid medium only, almost 2 / ~ . 5. The displacement due to Brownian movement of sphalerite particles in 0.001 M Na2COsmedium is about greater when measured in a shallow cell than when in a dee cell. 6. This enhanced displacement of sphalerite in the shallow cell along with the observations of Henri and Lucas ingcates that the shallow cell serves to accentuate the effect of electrical charge on Brownian movement, for the walls of the cell are close to the particles. Thus the variations in Henri’s results might be explained. 7. I n samples of suspension of low electrolyte content and near theisoelectric point, particles (in the deep cell) were observed to have markedly different Brownian motion, as well as different cataphoretic velocities. Those particles with higher Brownian activities generally had a higher cataphoretic velocity (or zeta potential). 8. Likewise, qualitative observations in the shallow cell oint to electrical environment affecting pedesis. Attraction forces or repulsion forces or a combination of both are seen to Be involved with the Brownian activity, the effects acting over arelatively “long range.” 9. A reasonable agreement was found between the time-displacement data observed for B halerite and the Einstein expression which theoretically relates time and displacement. Comparison of the data with that ottained by Henri for latex shows still closer correlation even though the systems and conditions under consideration are marked1 different. 10. This research indicates that electrical forces influence the Brownian movement. Nevertheless, molecurar collisions (density fluctuations) must be considered, and the Einstein relationships between the displacement and the time, the viscosity, the temperature and the radius hold a t least roughly. But allowance should be made for the ionic content of the medium, and the zeta potential of the particles and nearby walls.
Introduction Reliable data on the quantitative aspects of Brownian movement are rare. The early experimenters largely concentrated their efforts in demonstrating the validity of the Einstein Occasionally, however, there have appeared results which seemed to indicate that factors other than those envisaged in the classical equation might influence the amplitude of the Brownian movement, notably the pomible effect of electrolytes on aqueous suspensions. Unfortunately, none of the previous workers in this field have published data giving simultaneous measurements of size, charge and amplitude of Brownian movement on given particles under known conditions. The experimental part of this paper is an attempt to remedy this lacuna. Admittedly, much more information is necessary before arriving at any final explanation. Nevertheless, a very brief analysis of the various theoretical possibilities has been appended herein. Henri45 in 1908, claimed that the pedesis of rubber latex globules varied in intensity depending on the presence and nature of electrolyte dissolved in the suspending medium. His particles were spherical, having a relatively uniform radius of about one micron. The greatest displacement was in distilled water. This activity was practically four times that calculated with the Einstein formula. When (J) The data cited in this paper arq from M. Bender, “Brownian Movement-Electrical Charge-Flocculation,” Ph.D. Dissertation, New York University, 1949. See also this thesis for a comprehensive survey on Brownian movement literature relative to this work. (2) Calco Chemical Division, American Cyanamid Company, Bound Brook. New Jersey. (3) A. EinRtein, Ann. P h v s . , 17, 649 (1906); 19, 371 (1906): transltrtions by A. L). Cowper. in “hventiaations on the Theory of the Brownian Movement,” Methuen Co., Ltd., London, 1926. (4) V. Henri, C o n p f . tend.. 146, 1024 (1908) (6) V. Henri, {bid., 147, 62 (1908).
the medium was alkaline, Henri observed a displacement one-half that which he found in distilled water and when acidic, it was one-ninth. Henri used a shallow ceL6 Henri’s experimental technique, involving cinematography, appears to have been above any real important criticism. Actually, there was in his work an approximate check of the classical displacement-time relationship and the acknowledgment of this by Perrin,’ Chaudesaigues,8and Svedbergg indicates approval of Henri’s methods. Burton,1° refers to Henri’s “very reliable results of the motion in pure solvents and also in these solvents after impurities were added. ” Despite the great amount of work by Perrin and his collaborators and by Svedberg,l’ which tended to conform with the classical theory, Henri’s work was not actually disproven and we have Duclauxl* as late as 1938 in his extensive review on Brownian movement pointing out in no uncertain terms the contradictions in the literature regarding the possible effect of electrical charge on pedesis. A recent disagreement with the classical ideas of Brownian movement was voiced by Taggart and (6) Distinction should be made between shallow cell (generally prepared by placing a drop of the suspension being examined between microscope glass slide and cover-glass) and deep cell. From observations, the shallow cell usually has an inside height no greater than 35r. Yery often this height is considerably less depending on the investigator’s technique. The deep cell, on the other hand, has an inside height of over 10011. (7) J. Perrin, Ann. Chim. P h y s . . 18, 1 (1909). ( 8 ) Chaudeaaigues, Compt. rend., 147, 1044 (190s). (9) T. Svedberg, “Colloid Chemistry,” 2nd Ed., The Chemical Catalog Co., Inc., New York, 1928. (10) E. F.Burton, “The Physical Properties of Colloidal Solutions,” 3rd Ed., Longmans, Green, and Co., London, 1938, p. 68. (11) T. Svedberg. Nova Acta Regiae Societatis Scientiarum Upsaliensis Ser. I V , 3, N I (1907), or “Studien sur Lehre yon den Kolloiden L&ungen,” Dissertation, 1907; translation in Ion, 1, 373 (1900). (12) J. Duclaux, “Trait6 de Chimie Physique,” Tome 11, Hermann & Cie, Paris, Chapitre V (1937) and Chapitre VI (1938).
Feb., 1952
BROWNIAN MOVEMENT AND ELECTRICAL EFFECTS
his collaborators1+16in connection with studies pertaining to ore flotation. The claims were that the Brownian activity (of mineral particles like quartz, galena, sphalerite and pyrite, etc.) could be stopped or revivified at will by the addition of given electrolytes to the aqueous suspension, and that there wm a region of solubility of the surface compound on the particle (ie., between 1 and 40 mg. per liter) only within which, Brownian movement occurred. In this work Taggart took care to keep the suspension sufficiently dilute so that there would not be a variation in actual particle size. His observation cell was shallow. In 1945, Kellogg” criticized Taggart’s viewpoints. This was based on qualitative experiments including comparative observations in both deep (500 p ) and shallow cells, the correlations he found between slime-coating and lack of Brownian movement in a shallow cell or at the bottom of a deep cell, and his observations of Brownian activity where the “surface compounds” were outside the limits of Taggart’s Brownian movement-solubility curve. Finally, one more disagreement with classical theory can be given. LucaslS in 1938 mentioned retarding the Brownian motion of Hevea rubber globules by diluting the latex with 7% sodium chloride solution. He made use of this effect in his ultraviolet photographic work. Since the particles were caused to be in subdued movement, he was able to obtain clear photographic images. The observation cell was shallow, it being prepared by placing a quartz cover-glass over a drop of the BUSpension on a quartz slide, and blotting out excess liquid. Spence,l9 in 1908, had made similar observations of reduced Brownian movement activity of Hevea latex in saline solution. Experimental and Discussion Although most of this literature is consistent in the sense of pointing out the inadequacies of the classical theory of Brownian movement, nevertheless, i t is difficult to deduce interconnecting theory (at least on a general basis) between the different authors. Much of it, as outlined above, is qualitative, and more quantitative displacement, and electrical charge data are now attempted to the solution of this problem. The experiments in this work are grouped in several sections. First, a resume of qualitative observations and tests made along the lines of previous workers is given. Second, the electrical charges were meamred on particles and on cell walls under a variety of conditions. From these it also waa possible to establish the necessary conditions of electric neutrality on the particles. Third, the amplitude of Brownian movement and the particle sire were measured simultaneouslp. Finall , qualitative experiments were carried out in order to he& more fully develop the theoretical implications. One series of these observations shows connections between the zeta potential of particles and their Brownian activity. Another series indicates how attraction forces and repulsion forces become interrelated with the Brownian movement
.
(la) T. C. Fitt, “The Brownian Movement and the Reactions at the Surface of Zinc Sulfide particles in Solutions of Sodium Arsenate,” Doctoral Thesis, Columbia University (194-). (14) T. C. Fitt. A. W. Thomas and A. F. Taggart. A.I.M.E. Tech. Pub. No. 1575 (1943). (15) A. F. Taggart, J . Phrs. Chem., 86, 130 (1932). (16) A. F. Taggart, T. C. Taylor and A. F. Knoll, Transactions, A.I.M.E., 117, 217 (1930): or A.I.M.E. Tech. Pub. No. 312 (1930). (17) H. H. Kellogg, A.I.M.E. Tech. Pub. No. 1841 (1945). (18) F. F. Lucas, Ind. Enu. Chsm., 8 0 , 146 (1938). (19) D. Spence, India Rubber Journal, 36, 233 (1908).
273
I. Preliminary Observations and Tests.-These were carried out in verification and extension of the work both of Taggart and of Kellogg. Preliminary tests showed that the finely divided particles of sphalerite conformed well to Taggart’s predictions; accordingly, this material was used in the later and quantitative parts of this research. Numerous attempts showed that Taggart ’s results could not be duplicated in deep cells. For example the Brownian movement of sphalerite particles could not be st.opped under deep cell conditions by addition of dilute sulfuric acid-quit,e different t,owhat is observed in shallow cells. The solubility criterion advanced in the Taggart hypothesis fell down when ap lied t80such simple suspensions a8 silver iodide or calcium sutate, both having a solubility range outside the critical Furthermore, the typical adhesion to thc glaas bottom and top) of a cell could be demonstrated both by a regloll. simple ow technique and even by micromanipulation. Finally, it was observed that chemical treatment with diethyl germanium oxidelo of t,he glass cell surface delayed stopping of Brownian movement in the Taggart cell thus proving that the cell surface was involved in the reaction. 11. Electrokinetics. a. Particle Charge and Wall Charge.-Although the above observations point quite conclusively that adhorence to cell walls (glass) is the cause for Tag art’s particles having ceased their Brownian movement, t i e phenomena involved can be better understood as well as confirmed if the electrical charge dat,a are known for the particles and surface(s) under considerat,ion. It was decided to use the Northrup-Kunitz rectangular cell in this connection because not only could it be employed to measure particle electric charge, but also hydrodynamic theory (see Abramson)ll predicts that data pcrtaining to glass wall charge can be obt>ainedby an analysis of the electro-osmotic flow occurring in such a cell. The direction and amount (or velocity) of the elect;ro-osmot.ic flow naturally depends on the cell wall. By algebraically adding the true particle cataphoretic velocity ( L e . , that observed at the usual Smoluchowski levels in the cell, which levels are at or near l / b and ‘ / b the cell height) and the velocity of the particles when they arc close to the glass cell surfaces, it becomes possible to evaluate the direction and velocity of the liquid (electro-osmotic flow) a t the walls, and accordingly obtain a relative idea of the sign and amount of charge on the glass. The direct values of these liquid velocities were used and no attempt was made to translate them to zeta potentials by means of any of the standard equations such aa t,he Helmholtz equation. After preliminary experiments showed that t.hc usual hydrodynamics of a rectangular cataphorcsis cell were being obeyed and t,hat the amount of bow in the curve of a cell height v?. particle velocity was affected by the acidity of the suspension, it was considered valid to calculate velocity data of liquid at the g l a s ~surface due to electroendosmosis, and use this liquid velocity to show relative charge of the cell walls. The results of such calculations are givcn in Table I for six suspensionszzof sphalcrite in alkaline (pH 10.1, 8.9), distilled water (pH 6.6), and acid (pH 4.3, 2.9, 2.6) media. True average cataphoretic velocity (C.V.) is givcn for these six suspensions and also for t.wa addit.iona1 ones of greater acidity, namely, 0.01 and 0.1 M HzSOd. Zcta potential data are also contained therein. Figure 1 represents a graph of this C.V. and liquid vclocity data as a function of pH. Note how markedly the
A
(20) This material wa8 contributed by Dr. 11. 11. Anderson of the Chemistry Department, N.Y.U., 1949. (21) H. A. Abramson. “Electrokinetic Phenomcna, and Their Application to Biology and Medicine,” The Chemical Catalog Coinpany, Inc., New York, N. Y., 1934. (22) All the suspensions employed in the research, contained 0.0125% by weight of sphalerite in given electrolyte solution of given concentration. This gave an optimum number of particles in the field of the microscope for observing cataphoresis or Brownian movement displacement. Also, this concentration of particles was sufficient to clearly display flocculation phenomena. In preparation, the 0.0125% suspension was formed by dilution of 0.025% suspension in distilled water volumetrically in half with the requisite electrolyte solution. The distilled water suspension was obtained by grinding that weighed out quantity of powdered sphalerite necessary for 1 liter, in the “Diarnonite” mortar and pestle with a few drops of water. More water was gradually added and grinding continued. Then the slurry was transferred to a volumetric flask and the balance of the water gradually added with shaking.
274
MAX BENDER AND HENRY MOUQUIN
Vol. 56
TABLEI AVERAGEC.V.," ZBTAPOTENTIAL AND AVERAGELIQUID VELOCITY' AT DIFFERENT pH 10.1
(0.001 M ) (NarCOa)
t (mv.) Velocity H20"atglass 0 p/sec./volt/cm. at'25'. that way. ~ ' A R T I C L M C.V.
% HrSOi
a
6.6
4.3 (0.00006
(Dist. H20)
M)
(HrSOr)
1 0 2 20.0 3 33.3 4 50.0 5 66.7 6 100*.0 p/sec./volt/cm.
2.11 - 2.09 - 1.51 - 2.10 -19.6 -27.3 -27.5 -27.2 - 2.95 - 3.54 - 2.71 - 2.48 The pH value given is probably approximate.
2.9 (0.001 M )
2.6 (0.002 M )
-
-0.08 -0.44 (0) -1.0 -5.8 (0) +1.28 means that the glass is charged
(HrSOd)
-
C.V. av. (true)
CHANGES IN
8.9
(0.OOOl M ) (NarCOs)
e
1.12 -14.6 - 0.38 or sign
+ -
TABLE I1 (TRUE)AND LIQCIDVELOCITY AT THE GLASSSURFACE AS 0.002 M H&Ol ITE IN 0.001 M Na2C01
Molarity HzS04
Molarity NanCOa
0 0 0 0.000500
0.001000
Molarity NazSO4
0 -2.11 0.000400 -2.31 0 ,000667 -1.86 0 .000500 -0.76 .001000 0 .000333 -1.49 .002000 0 0 -0.08 Positive or negative sign means that the glass is charged that way. I
I
l
IS .~DDF,D
SPHALER-
-2.95 -3.98 -1.12 + O . 83 +1.09 +1.28
- 19.4 - 1.0 I
TO
Li uid"-b vePocity
-27.5 -30.0 -24.2 - 9.9
l
(1.45)b (0.1 M ) (HzSOd
(HaSOr)
I (mv.)
C.V. true"
.000400
charges of both sphalerite particles and glass cell wall decrease in negativity with increasing acidity below pH 4 . Cell wall charge decreases faster than particle charge .and even becomes posit,ive while the particles still remain somewhat negative. This immediately suggests electrostatic attraction forces a t a t least certain regions of HzSOa acidity which forces are capable of causing particles to adhere and stop their Brownian movement.
2.2 (0.01 .W
(HtSOr)
l
l
1
1
- 4.0
-3.0
-2.0
. -1.0
,x
8
0
d
+1.0
.e
-3.0
+2.0
-2.0
+3.0
I r a
.t: -1.0
B
g
t4.0
o
.
C\
.tntc.,,~rpuritl~,r
+2.0 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0
PH. Fig. 1.-C. V. (true) of particles and velocity of liquid at glass surface us. pH. Yet another series of C.V. and cell surface liquid velocity determinations was carried out. This time the electrolyte content of each sphalerite suspension considered was representative of given proportions of 0.002 M HzSO4 to 0.001 M Na&Oa, thus duplicating conditions at different times in the original Taggart cell where the Brownian movement of particles in alkaline medium was stopped by addition of acid. The data are given in Table I1 and are plotted in Fig. 2. Note how the wall charge becomes positive, the particle charge still remaining negative, when the suspension has 50% or more of 0.002 M HzSO~,the balance being 0.001 M N ~ z C O ~There . was no appreciable flocculation for any of the six suspensions and Brownian motion waa vigorous for all. Thus, Taggart's results can be explained by the fact that in acid medium the negative particles become attached to the positive glass upon coming close to i t as a result of their normal settling. Revivification occurs when alkali solution is added to the cell because the glass surface becomes negative again and repcls the particles, thc smaller oncs bccoiiiiiig resuspcndcd (in the vicinity of the glaw surface).
t 0
x
cv.itrue)ol'particles vglority or liquid at glass surface isgn corrrsponds to charge ofe;hss)
10
20 30 40 50 60 70 80 90 100 HzSO4, %. Fig. 2.--Particle C.V. (true) and liquid velocity at the surface for different mixtures of 0.001 M NazCOs and 0.002 M HzS0,.
b. Isoelectric Point.-With evidence a t hand that the Brownian movement is not stopped by given electrolytes, the next step is to prove whether pedesis at least varies with the electrical environment. In making the comparison, it should be of advantage to consider at least one suspension where the particles are not charged, Le., they are at the isoelectric point (LP.). .Furthermore this system should be one where there is a tendency toward f l o c ~ u l a t i o nso~ ~that the differences in the suspensions being compared will represent extremes (more than is the case if a non-flocculated isoelectric were used). Also, the presence of floccules would be a check on the isoelectric point having been determined correctly by the cataphoretic measurements technique used in this work. Kone of the sulfuric acid suspensions considered so far (Tables I and 11)showed flocculation. This was t8rueeven (23) Flocculation (coagulation) is defined in all this work as the grouping together of the majority of the "individual" particles of a suspension, which are no greater than 1 micron in diameter and mostly submicroscopic in size, to become macroscopic in sise. Actually the "individual" particles of all the suspensions were mostly aggregates judging from high resolution dark field microscope observations. In the work, care was taken to consider those particles only whose dark field haloes (at the lower microscope resolution where cataphoresis and Brownian rlisidaceincnt werc measured) showed thcin to be fairly regular in shape.
VeL., 1952
“
BROWNIAN MOVNMENT A N D ELECTRICAL EFFECTS
--:
when the particle charge was prac-3*0 tically zero as for 0.002 M and 0.1 M H&304. Likewise, NaCl used by Lucasl* (see Introduction) to decrease the Brownian activity of -2.0 Hevea rubber latex, did not flocculate the sphalerite particles even at the high concentration of 1 N where the suspension was practitally at the isoelectric point. Aluminum chloride then was studied as a possible flocculent for $ 0 the 0.0125% suhalerite. Various u aluminum co&ehtrations were tried in basic medium (0.001 NazCOa), +l.o in distilled water and in acid medium(O.OO1 M H2S04),the truecataphoretic velocity being measured each time. No flocculation was +2.0 evident in distilled water or in acid solution at any of the aluminum concentrations tried including those which brought the suspensions to
I
I
I
I
I
I
275 I
I
1 -
-
\
~rwlectric Point o.cowooNAIC1,
-
I
I
from the equation iepresent the hest avaihble approximil111 tions. Second, they have been checked esperinientallg by C.V. (TRUE)us. KORJIALITY AICI, IN ALKALINE‘SOLUTION Perrin and his collab~rators,~J Henri‘ and others. A i d third, the range in temperature (viscosity) and part,icle size C.V. over which the adjustments were made was not too great. Normality (dsec./ AlCli PH volt/cm.) .t (inv.1 Most of t.he displacement measurcnients were made in tho deep Northrup-Kunitz cell where the part’icles are well re-2.79 -36.3 0 * oO0100 9.5 moved from the walls. This eliminates wall effects (electri-13.6 .000300 8.5 -1.04 cal), which, it has already been shown, may complicate -16.8 .OW360 7.9 -1.29 Brownian nioveinent studies. Also, with the particle:: being too siiiall for their size to be determined by direct nii- 3.9 .000375 7.9 -0.30 croscopy, it was at least possible in the deep cell to follow - 1.5 .000399 7.8 -0.12 the settling of each with the microscope fine adjustment’and .000400 7.7 t-0.02 0.3 determiiic its rate of fall. The rate of fall could then be - 0.8 .oO0425 7.6 -0.06 converted to particle size by employing Stokes’ law. .000480 6.9 +30.0 +2.31 Pai,ticle size was chosen small enough to obtain measurable I
