THE JOURNAL OF
PHYSICAL CHEMISTRY (Registered in
U. 8. P a t e n t Office) (Copyright, 1854, by the American Chemical Society)
VOLUME58
OCTOBER 18, 1054
NUMBER10
T H E SIZE OF SODIUM MONTMORILLONITE PARTICLES 13 SUSPENSION FROM ELECTRO-OPTICAL BIREFRINGENCE STUDIES BY ALLANKAHNAND DONALD R. LEWIS Publ.ication No. 44, Exploration and Production Research Division, Shell Development Co., Houston, Texas Received March 6 , 1964
The size of sodium montmorillonite particles in aqueous suspension was determined from electro-optical birefringence “decay curves.” Five suspensions, each of a narrow particle-size range, were prepared from a batch of sodium montmorillonite by means of a Spinco Model L ultracentrifuge. The electro-optical birefringence behavior of these suspensions was determined a t a weight concentration of 0.05% sodium montmorillonite. On the assumption that the particles are &late spheroids in which the majQr axis is much larger than the minor axis, the semimajor axes of the particles were found to range from 2500 t o 12,300 A.
Introduction I n the study of aqueous clay suspensions, the determination of the size and shape of the clay particles is of great importance. Of the numerous techniques available for the measurement of the size and shape of colloidal particles, the electro-optical birefringence technique has been chosen for several reasons. First, external forces are applied to the suspension for only a fraction of a second during measurement. Second, in addition to determining the rotational diffusion constant of the particles, information about the electrical properties of the particles may be obtained. Benoit’ and O’Konski and Zimm2 have described methods for orienting colloidal particles in a suspension by applying a rectangular voltage pulse and observing the rate of orientation. O’Konski and Zimm used a repeated rectangular voltage pulse; Benoit used a single rectangular voltage pulse, which is also the type used in the present study. At the end of a pulse the applied voltage decreases t o zero and the particles return to random orientation. The rate of relaxation for this return to random orientation may be used to determine the major dimension of the particle. Benoit has applied electro-optical birefringence methods to measure the size of particles of tobacco mosaic virus, thymonucleic acid and vanadium pentoxide. In this paper, the application to sodium montmorillonite is discussed. H . Benoit, Ann. phys., [I21 6, 561 (1951). (2) C. O’Konski and B. Zimm. Science, 11, 113 (1950).
(1)
Theoretical A suspension of sodium montmorillonite is normally optically isotropic. When an electric field is applied the suspension behaves optically like a uniaxial crystal and becomes birefringent. The birefringence is attributed to the orientation of the particles under the influence of the electric field.3 Upon removal of the field, the particles return to random orientation and the birefringence decays. The rate of decay is given to a good approximation‘ by where An, is the initial value of the birefringence and An is the value a t time 2. D is the rotational diffusion constant. On the basis of a great deal of qualitative evidence, the montmorillonite particle is considered to be disc-shaped (an oblate spheroid) .4,b Then according to Perrin’s formula for oblate spheroids in which the major axis is much larger than the minor axis6 where a is the semimajor axis of the particle and q is the viscosity. (3) C. E. Marshall, Trans. Faraday Soc., Z6, 173 (1930). (4) C. E.Marshall, “ T h e Colloidal Chemistry of the Silicate Minerals,” Academic Press, Inc., New York, N. Y., 1949, p. 69. ( 5 ) 0. J. Kelley a n d B. T. Shaw, Proc. Soil Sci. Am., 7 , 58 (1942). (G) A. E. Alexander a n d P . Johnson, “Colloid Science,” Vol. 1, Oxford University Presu, New York, N . Y., 1949, p. 386.
801
ALLANKAHNAND DONALD R. LEWIS
802
Vol. 58
PM
----------ON OPTICAL BENCH
Fig. I.-Schematic
arrangement for single-pulse operation,
The quantitative determination of birefringence is based on well-known optical principles.' When the suspension is in an electric field it acts as a uniaxial crystal with its optic axis parallel to the field so that a beam of plane-polarized light entering the suspension perpendicular to the field becomes elliptically polarized. For the greatest convenience in the interpretation of results, the light entering the suspension is plane-polarized a t 45" to the electric field. The light emerging from the suspension is passed through a quarter-wave plate, the principal axes of which are also a t 45" to the electric field. The light emerging from the quarter-wave plate is found to be plane-polarized but at an angle 0 from the original plane of polarization. e
= T xd(~n)
(3)
where X is the wave length of light used, d is the thickness of the birefringent layer and An is the birefringence, which equals the refractive index parallel to the electric field minus the refractive index perpendicular to the electric field. Experimental Optical Equipment .-The general arrangement of the optical equipment in our investigations was similar t o that usually used in the investigation of the Kerr effect. As shown schematically in Fig. 1, a beam of light was collimated into a narrow pencil, passed through a filter F, which transmitted a narrow wave length region, and then into a Nicol prism N!, which plane-polarized the beam of light. After traversing the cell which contained the suspension made birefringent by an electric field the beam was no longer plane-polarized but was ellipitically polarized. The quarterwave plate Q, with its axes also a t 45" to the direction of the electric field, again rendered the light beam plane-polarized. The light beam finally emerged from the second Nicol prism Nz and was detected by the photomultiplier tube PM. All the equipment shown in Fig. 1 above the dashed line was mounted on an optical bench. The light source L was a 12-ampere 6-volt projection lamp which was powered by a storage battery. While the lamp was being o erated the battery was simultaneously charged from a Eattery charger so that the principal function of the battery was that of a voltage regulator for the lamp. The lens on the lamp housing focused the image of the filament a t infinity so that a beam of parallel light emerged from the lamp. The light beam was collimated into a narrow pencil by means of irises I1 and 1 2 . The iris 11 was a Rapax No. I camera shutter unit, without any lens, which contained an iris diaphragm so that it would be used to interrupt the beam of light. Is was a simple multiple-leaf iris diaphragm. The filter F was a oBaird interference filter with its transmission peak a t 5466 A. (7) S. Prooopiu, Ann. phys., [lo] 1, 213 (1924).
The polarizer prism N1 had a 9-mm. aperture. The cell was made of optical glass with accurately flat parallel faces which had been annealed to be as strain-free as possible. The two electrodes EI and E%,which were immersed in the clay suspension, were made from sheets of platinum inch thick and 10 mm. wide in the direction parallel t o the propagation of the light. The quarter-wave plate Q was designed for use at 5461 A. The analyzer prism, Nz was of exactly the same ty e as the polarizer prism. A 9 3 1 4 hotomultiplier tube I$M was used to detect any light wkch passed through the analyzer. The signal from the photomult,iplier tube went to one channel of a dual-channel oscilloscope. The two polarizing prisms NI and NZand the quarter-wave plate Q were mounted in worm-gear-driven mounts which permitted complete rotation of their optical axes. The position of any of these optical components was determined by means of a mechanical counter which was coupled to the driving shaft of the worm gear. One unit on the mechanical rotation counter corresponded to 0.2" of arc. Electronic Equipment.-The basic components of the electronic equipment are also shown in Fig. 1. The pulse generator produced a single rectangular voltage pulse by using a circuit which operated in the manner described by Reich.* The maximum voltage of the peak of the pulse was approximately 100 volts. The voltage gradient in the cell was controlled by varying the distance between the electrodes. Five pulse widths from 2 to 200 milliseconds were available. The pulses which were generated were all of one olarity with respect to ground. A succession of pulses of tge same polarity, if applied t o the cell, would cause a net transfer of montmorillonite from the suspension t o the anode. The equivalent of reversing the polarity of the pulse was accomplished by means of a switching arrangement which automatically reversed the role of the two electrodes E1 and E*. A time delay of 10 seconds was provided from the time of initiation of the pulse until the switching of the rela units, which reversed the polarity of the electrodes. T l i s permitted the recording of the pulse and response on the face of the oscillograph to be completed without any possibility of transient voltages being injected into the pulse or response by the switching of the electrodes. A z-axis intensity modulation permitted timing be imposed on the trace across the oscilloscope. provided by a signal from an audiooscillator which passed through a wave shaper and was then amplified before being applied to the z-axis of the oscilloscope. Materials.-The montmorillonite was obtained from a sample of bentonite from Clay Spur, Wyoming, specially selected to be free of accessory mineral^.^ The montmorillonite was converted to the sodium form in an ion-exchange column, similar t o the one described by Lewis.lO This material contained a wide variety of particle sizes and was separated into a series of fractions, each of a narrow particlesize range, by a Spinco Model L ultracentrifuge. Five
'$i%'k:
(8) H. 6. Reich, "Theory and Application of Electron Tubea," MoGraw-Hill Book Co., Inc., New York, N . Y . , 1937, p. 360. (9) This material was provided through the courtesy of Baroid Sales Division, National Lead Company, Houston. (10) D. R . Lewis, I n d . Eno. Chern., 46, 1782 (1953).
c
SIZEOF SODIUM MONTMORILLONITE PARTICLES
Oct., 1954
particle-size fractions were obtained. These were labeled, in order of decreasing size, RI, Rz, Rs, R,, Rb. Experimental Method. Calibration.-The polarizer and quarter-wave plate were set up a t 45' to the plane of the electrodes. The analyzer could then be set a t any position and the trace on the dual-channel oscilloscope from the phototube (response trace) was positioned to coincide with the signal trace corresponding to zero voltage between the electrodes. The apparatus was calibrated by rotating the analyzer and determining the vertical displacement of the response trace as a function of analyzer position. If the analyzer was initially crossed with the polarizer, the displacement of the trace on rotation of the analyzer was found to be accurately proportional to sin2 0 . This indicated that the displacement on the oscilloscopewas directly proportional to the intensity of light striking the phototube. Birefringence Curves.-With the suspension in the cell, the voltage pulse was applied to the electrodes. The rise in voltage of the pulse simultaneously triggered the sweep of the oscilloscope which was common for both channels. A permanent record of the traces on the oscilloscope was made by means of a camera attachment.
Results All the samples were run at a weight concentration of 0.05% sodium montmorillonite. Preliminary experiments showed that this was below the concentration range where the calculated value of
803
D,the rotational diffusion constant, is a function of the concentration. The marked effect of particle size on the birefringence decay curve is shown in Figs. 2 and 3. I n Fig. 2, the decay curve GH of the largest size fraction R1 is shown. At H, 274 milliseconds after the field has been removed, the sample stir1 has 62% of the birefringence a t G. In Fig. 3, taken with the smallest size fraction, Rb in the cell, 62% of the initial birefringence has been reached after the field has been off for only 4 milliseconds. I n Fig. 4, the time scale for an Rg fraction has. been expanded so that the decay curve may be obtained more precisely. The discontinuities in the oscilloscope traces of Fig. 4 are caused by the timing signals. The time interval between two successive discontinuities is 2 milliseconds, The value of D may be obtained from a curve such as that in Fig. 4 and the calibration curve. From equation 1 log An = log Ano
-Dt 2.303
From equation 3, for a given wave length of light and thickness of birefringent material
e
0
A
.-.log e = log eo
C F
D
H
G
I
3
= -Dt
2.303
Therefore, a plot of e versus t on semilogarithmic paper should yield a straight line of slope 2.61 D. From equation 2, the value of a, the semimajor axis of the particle, may now be obtained. In Fig. 5, 0 is plotted against t on semilogarithmic paper for section TU of Fig. 4. The deviations from linearity are undoubtedly due to the fact that the sample actually contained a range of particle sizes. The calculated value of D is based on the straight line which is drawn through the first four points.
t
"\ Nl
FJ-----
= KAn where K is a constant
o R
2
t ( MILLISECONDS),
Fig. 5.-Semilogarithmic plot of birefringence decay curve T U of Fig. 4. Fig. 2.-Birefringence curve of sample R, : voltage pulse, ABCD; birefringence curve, EFGH. Distance AB corresponds to 192.5 milliseconds. Fig. 3.-Birefringence curve of sample Rs: voltage pulse, IJKL; birefringence curve, MNOP. Distance IJ corresponds to 192.5 milliseconds. Fig. 4.-Birefringence curve of sample Rg. Distance QR corresponds to 10.3 milliseconds.
The results on the various size fractions of sodium montmorillonite are presented in Table I. It can be seen from Table I that the semimajor axis of the particles in each of the fractions is very large in comparison to the equivalent spherical radius. However, the values of the equivalent
ALLANKAHNAND DONALD R. LEWIS
804 TABLE I
PARTICLE-SIZE DATAON FRACTIONS OF SODIUM MONTMORILLONITE Values of D obtained from suspensions containing 0.05y0 by weight of sodium montmorillonite. Equiv.a splierical radius,
Fraction
A.
RI
>1380
*
% by wt. of total sodium montniorillonite
D, sec. - 1
a, A.
27.3 0.23 12,300 Rz 810-1 380 15.4 0.40 10,300 RO 400-810 17.0 7.9 3,800 R4 230-400 17.9 17 3,000 R!, 70-230 22.4 29 2,500 a Calculated from speed and duration of centrifugation, from data given in Technical Manual for Ultracentrifuge Model L, Specialized Instruments Corporation, Belmont, California.
spherical radius were obtained a t an initial sodium ,montmorillonite concentration of about 1% and not a t 0.05y0. On the basis of published data on asymmetric large molecules” and some unpublished data from this Laboratory, it is still unlikely that the equivalent spherical radius of the particles would be more than twice as large as those given. There is less than a factor of five difference between the semimajor axis values of the Rl and Rg fractions. This is confirmed qualitatively also by inspection of electron micrographs of these sam(11) Reference 6, p. 276.
Vol. 58
ples. l2 The electron micrographs show, however, that the particles of the R1 fraction are considerably thicker than the particles of the R6fraction. In addition to the size data obtainable from the (‘decay curves,” the electro-optical birefringence “rise curves,” such as the portion ST of Fig. 4, might be expected to yield information about the electrical properties of the sodium montmorillonite parti’cles. Benoit’ has derived equations for ‘(rise curves” on the assumption that the orientation of the particles in the electric field was due solely to some combination of permanent and induced dipoles in the particles. It was not found possible to fit our data to any of these equations.
DISCUSSION HERBERT L. DAvrs.-Have you considered or measured the effect of p H , of cations and anions, and of substances known to produce marked effects on the clay slips used commercially? This excellent technique might provide useful information on the magnitude and influence of charge and hydration, as these influence viscosities, plasticities and other properties of clay dispersions. Comparable systems with sodium hydroxide, silicate or phosphate would be relevant. Additions of cationic matcrials such as barium chloride, aluminum chloride or quaternary compounds might produce notable effects of academic and practical interest. ALLANKAHN.-LJP to the present we have only used pure materials in our measurements. We hope however to be able to use this birefringence technique to study the interaction of clays with materials such as you mentioned. (12) Electron micrograplis were prepared by Dr. Thomaa F. Bates Dept. of Mineralogy, Pennsylvania State University, State College, Pennsylvania.
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