Electrooptical characteristics of chrysotile asbestos sols

Electrooptical Characteristics of Chrysotile Asbestos Sols ROLAND ISHERWOOD AND BARRY R. JENNINGS Optics Group, 3. 3". Thomson Physical Laboratory, University of Reading, Whiteknights, Reading, RG2 2AF, Berks, United Kingdom Received January 21, 1985; accepted April 15, 1985 Results are presented of an optical study on aqueous suspensions of the serpentine asbestos mineral chrysotile. Using a specially designed apparatus, photon correlation light scattering in the presence and absence of applied electric pulses have enabled the size and electrophoretic mobility to be measured. Supplementary electric birefringence measurements on the same system have additionally yielded size distribution data, permanent dipole moments, and electrical polarizabilities. A comparative analysis of these electrical and optical parameters gives information on the nature of the surface electrical characteristics of the particles which is discussed in terms of the properties of chrysotile. © 1985AcademicPress,Inc.

INTRODUCTION

Measurements of the spectral linewidth of the light scattered from colloidal suspensions have become increasingly popular as a means of determining the translatory (Dr) and to some degree the rotary (DO diffusion coefficients of the particles. The chaotic Brownian motions of the particles in the medium result in fluctuations in the scattered intensity which broaden the spectral characteristics. From the diffusion coefficients, particle size parameters can be evaluated. To date, the majority of measurements have been undertaken on unperturbed media. Valuable additional information can be obtained by subjecting a medium to an electric field. This was initially demonstrated by Ware and Flygare (1) who showed that the accompanying electrophoretic motion of the particles influenced the spectral characteristics in a discrete manner. The radiation scattered from such systems undergoes a frequency shift which is dependent upon the translatory velocity of the particles in the field. Superposition of this inelastically scattered radiation with that elastically scattered from the medium results in the generation of a measurable beat frequency signal on the photocathode of a de-

tecting photomultiplier. Analysis of this signal is readily undertaken using an autocorrelator. The combination of the unperturbed and the electrically induced phenomena result in the rapid measurement of Dt and the electrophoretic mobility (u). In addition, a colloidal medium responds to a voltage electric pulse by becoming birefringent as long as the particles are geometrically and optically anisotropic. Such birefringence can be detected by transmission measurements through a pair of crossed polarizers. The birefringence is transient in nature with the maximum equilibrium value being a direct response of the interaction of any permanent dipole moment (~) and the anisotropy (Aa) of the electrical polarizability (a) with the effective electric field (E). It is this electrical coupling which causes particle orientation. The rates of response to the pulse, however, are direct indicators of Dr. Hence, by performing electric birefringence measurements, these aforementioned parameters are also readily obtainable. Recently, we have constructed a single apparatus which is capable of measuring all of these various optical phenomena in a single set of experiments. Technical details of the system and its use can be found elsewhere (2).

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Journalof ColloidandInterfaceScience,Vol. 108,No. 2, December 1985

ELECTROOPTICS OF CHRYSOTILE

463

Chrysotile is one of the serpentine asbestos In this paper we report experiments on minerals which is of needle-like morphology. chrysotile sols which both illustrate the value The fibrous silicates have historically had ex- of the optical apparatus and evaluate the electensive commercial use, but are currently un- trical parameters of the mineral in aqueous der suspicion because of the suspected detri- dispersion. mental medical hazard they produce (3). Like THEORETICAL BACKGROUND all serpentine minerals, chrysotile consists of an extended layer of SiO4 silica tetrahedra, in (i) Scattering characteristics in the absence which hydroxyl ions interspace the apical ox- of applied fields. In dilute suspension the minygens of the tetrahedra to form a regular close- eral particles undergo chaotic, random mopacked array. Above this is situated a layer tions which produce local concentration flucof Mg ions with their associated hydroxyls: tuations. These, in turn, are the origin of light the complete chemical formula being scattering phenomena. In the present appaMg3Si2Os(OH)4. With chrysotile, there is a re- ratus, photon counts of the scattered intensity stricted repeat distance of the silicate layer with are recorded in a suitable photomultiplier over respect to that of the Mg layer so that the various intervals in time. These counts are structure bends with the Si layer on the inside statistically analyzed via an autocorrelator of the curve (4). This forms tubes or scrolls, which produces an intensity autocorrelation which adopt a spiral cross section as indicated function C(r). This is a measure of the timein Fig. 1. Although there are many fibrous and dependent correlation between the scattered lamellar silicate structures, it is not at all ap- intensity fluctuations. The parameter r is the parent why chrysotile should give rise to the correlation sample time. The autocorrelation medical hazards of which it is suspected. From function has a temporal behavior given by the the physicochemical view point, it is desirable equation (5) to know the surface charge and electrical C(r) = A exp(-2FK27 -) + 1, [1] properties of the particles which must ultimately be of significance in understanding the where K = (47rn/X)sin 0/2, with A a constant binding behavior of these particles to biological of the apparatus, n the medium refractive index for radiation of wavelength X in the mecells and genetic material.

"~

~

x

x

x

x

,y

•

OH •

Mg

O, OH Si

FIG. 1. Schematicrepresentation of a fiber bundle of chrysotile,showingthe scroll-likemicrostructure and the related curved lattice structure (Yada, Ref. (4)). The inset electron mierographshowspolystyrene calibration spheres of 0.26 #m. Journal of Colloid and Interface Science, Vol. 108, No. 2, December 1985

464

ISHERWOOD

dium, and 0 the angle of observation to the forward direction of the incident beam. In Eq. [1], the parameter I' is an experimental diffusion coefficient which for rigid spherical molecules, is a single exponential function. By plotting the variation in I' with K 2 and extrapolating to zero angle of observation (K 2 ~ 0) and zero r, Pecora (6) has shown that the parameter/" can be equated with the z average Dr. At higher angles, Dr increasingly influences C0-) in a complicated manner. Although Dr can be obtained in principle from such angular dependence, this parameter is more readily accessible via the electric birefringence procedure discussed below.

(ii) Scattered intensity changes in electric fields. The majority of mineral particles are

AND

JENNINGS

centration c, the observed birefringence can be expressed as (9) An =

27rc • Ag. q~, n

[4]

where Ag expresses the anisotropy in g. Strictly, this is a function of the particle shape. The form used here is applicable to rigid particles with cylindrical symmetry in which the geometrical, optical, and electrical axes for the particles are coincident. The parameter 4~ is an orientation function which depends upon the electrical characteristics of the particles and the nature of the applied field. When sinusoidal fields of rms amplitude E are applied, ~b and hence the observed An contain a dc equivalent or "steady" component supplemented with an "alternating" component of twice the frequency w of the applied field. This alternating component can be readily filtered out from any detected response. The steady component then has the form (9)

charged in aqueous media (7). The imposition of low-voltage electric fields on such media result in electrophoretic migration of the particles. The frequency shift in the scattering that accompanies such motion can also be described in terms of a change in the fluctuations of the scattered intensity. Using a heterodyne (two-beam optical mixing) method, the intensity correlation function experiences a cosine damping factor in addition to the exponential property of Eq. [1], hence C(z) has the form with k and T the Boltzmann constant and the absolute temperature, respectively. The (8) resultant permanent dipole moment t~ C(r) = B exp(-DtK2r)cos{(K • v)~-}, [2] = f~u~ - / ~ and the polarizability anisotropy where B is simply a modified form of the con- Aa = (al - a2), with subscripts 1 and 2 instant A, and v is the particle velocity. From dicating values associated with the unique this equation, it can be seen that the period symmetry axes and the quadrature axes, reT -1 of the oscillations in the correlation func- spectively, can then be isolated from the different frequency dependence. In practice, this tion can be expressed by is achieved by analyzing the initial slopes of X1 1 curves of An vs E 2. With dc-applied fields, both T . . . . . [3] u E sin0 the Ac~ and ~t2 factors contribute to the obas u = v/E. Hence u can be readily obtained. served effect with ~o = 0. A similar slope for (iii) Electric birefringence measurements. data in response to a high-frequency field inThe alignment of particles which exhibit an- eluded only the As contribution. Hence t~ and isotropy in their optical polarizabilities (g) per As can be isolated. For highly anisometric particles, the fieldunit volume, results in a birefringent medium. free decay of An, following the termination of The amplitude of such birefringence (An) dethe electric pulse, obeys the single exponential pends directly upon the degree of alignment equation (10) and orientation of the particles. For light of wavelength X, in a medium of volume conJournal of Colloid and Interface Science, Vol. 108, No. 2, December 1985

An = Anoexp{--6Drt}.

[6]

465

ELECTROOPTICS OF CHRYSOTILE

This, however, is only true for a monodisperse suspension. The value An0 is that value of An at which the decay commences and for which t .= O. With a monodisperse medium, a simple semilogarithmic plot reveals Dr directly. With a polydisperse sample, all components contribute to the decay process and an average must be obtained or a deconvoluting procedure adopted to isolate the contributions of the various components. MATERIALS A N D M E T H O D S

The chrysotile sample was extracted from the Cassiar mine in the USSR and was kindly supplied by Messrs. Cape Fibres Ltd. Suspensions were difficult to prepare. Initial attempts to grind the material in an agate mortar and pestle were unsuccessful as the fibers tended to mat together. Both wet grinding and placing the material in a high-torque blender, produced gel structures which were unacceptable. Furthermore, air bubbles became incorporated in this gel, and resulted in flotation of the material. An alternative approach was therefore adopted. First, the mineral was cleaved into approximately 0.5- to 1-mm lengths with a steel blade and an agate mortar. The p o w d e r was then ground and dispersed in freshly deionized, doubly distilled water, subjected to the blender for 2 min and ultrasonification for 3 min. This was followed by centrifugation at 2000g for 15 rain. The supernatant was then decanted off and used as the stock suspension. Previous studies by Hodgson (11) indicated that when placed in 4 M hydrochloric acid, 60% weight loss was observed in a matter of minutes, owing to the leaching of the MgOH layer. With our own sols, it was found that, even under an atmosphere of nitrogen, the pH rose from 7.5 to 10 over 24 h and to pH 8 within 7 h. As the changes were negligible during 2 h, all electrical measurements were confined to this period.

mium laser provided stable incident radiation o f 15 m W power at a wavelength of 441 nm. A variety of optical components then defined a state of polarization and intensity of the beam prior to its being incident on the sample. This was held in one of two specially designed cells. For field-free scattering measurements, it was ofcyfindrical form. For the field-induced measurements, it was a fiat, rectangular cell which facilitated the application of pulsed electric fields by a suitably placed pair of vertical electrodes. A detection optical arm was mounted coaxially with the cell center and was radially positioned about the scattering volume over the angular scattering range of 10 ° < 0 < 135 °. The optical arm held various apertures and polarizers which defined the light falling on a photon counting photomultiplier. The output was discriminated, amplified, and directed to a 48-channel correlator whose output was displayed on an oscilloscope as the temporal autocorrelation function C(z). The facility also existed for analyzing this output signal directly in terms of the characteristics of C(r). For the chrysotile sample, a typical display is shown in Fig. 2 from which F was determined at various low angles. The data so measured are given in Fig. 3. From the limiting value at 0 ~ 0, Dt = 7.5 (+1.2) × 10 -13 m z s -1 was obtained. A translatory diffusion coefficient can be analyzed in terms of the length L of a rod of diameter d according to the Perrin formula (12)

C(TI .°° °'°°°° °.•

° • ,°° °'°°°°°°°°°°°°°°°° ......

;o

2'0

3'o

°°

~o

CHANNEL NUMBER

(i) Scattering measurements. Full details of the method o f use of the apparatus have been given elsewhere (2). Briefly, a helium-cad-

FIG. 2. Typical scattering correlation function, obtained from a chrysotile sol. The scattered light was detected at 0 = 10 ° over a sample time of 0.8 ms. Journal of Colloid and Interface Science, Vol. 108, No. 2, December 1985

466

ISHERWOOD AND JENNINGS the front of the sample cell. In this manner, oscillating autocorrelation functions were obtained. From Eq. [3], it is seen that the period T-I varies as E, and hence the applied voltage V. Figure 4 shows a plot of these two variables from which some continuous curvature is observed save at the very lowest applied fields. From the limiting initial slope, a value of u = 1.3 (+0.8) X 10 -8 m 2 s -~ V -b, was obtained.

% x r.._

(iii) Electric birefringence measurements. For these data an eye-response photodiode was used as the detector to record any light transFIG. 3. Variationof the experimentalphoton correlation diffusion coefficient(r) with sin2(0/2). The extrapolated mitted through an optical array of a polarizer, sample cell, quarter-wave plate, and analyzer. data to 0 = 0 gave the value of Dr. These components were arranged with the quarter-wave plate and polarizer in parallel optical azimuth, with E applied vectorially at O, = ~ In L" [71 45 ° to the polarizer azimuth and with the analyzer offset by some 10 ° from the "crossed" Using this, a value of Lz = 2.7 #m was ob- position. Such an optical geometry affords a tained. The more complicated but exact means of calculating An directly from the reBroersma (13) theory gave a value within 10% corded optical transmitted light intensity. For of this and hence within the experimental un- the sample in question, fields of up to 5 X 105 certainty. V . m -1 and 30 ms duration were applied. Oi) Electrophoretic measurements. For These were of either step-like dc pulses or these measurements a train of electric pulses bursts of sinusoidal fields with f = 5 kHz. The was applied to the cell. Individual pulses were variation of the birefringence with E 2 for each of reversing polarity (to reduce electrode po- type of field is shown in Fig. 5. It is seen that larization and continuous migration). Pulse whereas the high-field limit is similar, the lowdurations of 50 to 10 ms and amplitude 5 to field behavior is different. This immediately 15 V were applied. A dead time of 1 to 3 s indicates the presence of both ~t and Aa as was introduced between the pulses and the electrical contributions to the orientation correlator was delayed after each pulse so that mechanism. From the initial slopes of this figthe initial 2 to 10 ms did not contribute to the ure under the two conditions, and using Eq. correlation count. This was to avoid scattering phenomena accompanying the orientation of the particles prior to their translation. Hence, T-1 by carefully designing the electronics, heating [s-q 20 and electroosmosis effects were reduced to a m i n i m u m by suitable consideration of the electric field strength, pulse duration, repetition rate, and sampling time. In order to achieve the heterodyne correlation function, a small amount of the incident o 5 1'6 l's light had to be optically mixed with that scatVoltage [V] tered from the sample during the electrophoFIG. 4. Inverse of the correlation oscillation period retic motion. This was achieved by collecting against the appliedvoltageobtained from electrophoretic the light scattered from an internal scratch on light scatteringdata on the chrysotilesuspension. Sin 2 ( e / 2 ) x l O z

/7. j.JJ

_/I"

Journal of Colloid and Interface Science, Vol. 108, No. 2, December 1985

467

ELECTROOPTICS OF CHRYSOTILE

__! ...... I

sets of data of Fig. 6 one can calculate directly the median m and the breadth a ofa lognormal distribution of particle sizes. Adopting such a procedure yielded m = 0.87 # m and tr = 0.70. From these parameters, the related averages in particle length L, are obtained directly as

i i

2 .

<11

Ln = m exp(o-2/2) i

1'0

2'0

3'0

~

100

i

~ 300

Lz = m exp(5a2/2).

[8]

E 2 x 1 0 -9 [ V m - I ] 2

FIG. 5. Induced birefringence and amplitude as a function of E z for dc (+) and 5 kHz ac ( . ) applied fields. In the latter case, only the "steady" component was recorded.

[5], values o f # = 2.7 (+0.7) X 10 -2~ Cm and Aa = 3.4 (+0.5) × 10 -z9 F m 2 were obtained. The birefringence decay rate was found to be very field dependent. This is expressed in Fig. 6 by the two semilogarithmic plots of the birefringence decay as functions of time. The different time scales are apparent as is the curvature in each plot. Such curvature is indicative of polydispersity. Larger particles generally have a larger electric dipole and thus require a smaller field to orient them. They do, however, require a longer time for their rotation; hence the low field data has an enhanced contribution from the larger particles. This has been exploited in this research group as a means of evaluating particle size distributions from birefringence decay data. Details can be found elsewhere (14). From the two

,/ ./

0-6

0'/* "E ~" 0.2

0-1

Adopting this procedure the averages given in Table I were obtained. (iv) Electron microscopy. Electron microscopic data were also recorded on the same sample which indicated their fibrous nature. Measurements were made of some 750 chrysotile particles to obtain the length data given in Table I. Simultaneously, attempts were made to measure the diameter of some 150 such particles. This was a difficult procedure but yielded an average diameter of 44 nm, which was used when required in Eq. [7]. DISCUSSION

(i) Particle geometry. Table I presents a comparison of the particle lengths obtained by the various methods. It is seen that both the optical methods, which have a tendency to measure the higher averages in the distribution, are in concord. For the lower particle sizes, the micrograph and birefringence data are also in agreement. From the micrograph of Fig. 1, one notices that the needles associate with extensive lateral overlapping, leading to overestimation of lengths with the larger particles. It is the authors' belief that the optical measurements provide a more complete and reliable description of the particles within the suspension.

/

¢ 0 t [ms]

t+O

TABLE I 0'./+

08

tIms]

FIG. 6. Analysis of the decay Of the induced birefringence for chrysotile. Frame A is for a low field of 2.6 × 104 V . m -1 and frame B for a high field of 5.5 × 105 V . m -l. From the initial slopes, Dro = 4.4 s-l and Dr0o = 81 s -l were obtained, respectively.

Geometrical Parameters for Aqueous Chrysotile Rod lengths (#rn)

L.

L,

Photon correlation scattering Transient electric birefringence Electron microscopy

-0.87 0.87

2.70 2.29 4.49

Journal of Colloid and Interface Science, Vol. 108, No. 2, December 1985

468

ISHERWOOD

Electrical P a r a m e t e r s for A q u e o u s C h r y s o t i l e

, fKaKl(~a)) Q = 3~r~(L + z a ) ~ ~u,

Value

u

1.3 X 10 -8 m 2 s -1 V - t

tt aa Q

2.7 x 10 -25 C m 3.4 X 10 -29 F m 2 7 X 10 -17 C

K-1 ~" q

70 nrn +22 mV 1.4 x 10 -17 C

(ii) Electrical properties. According to the predictions of Eq. [3], linearity might have been expected in Fig. 4. Although the origin of the curvature awaits further study, we note that from electrical resistance data, it has been reported that chrysotile rods have a high surface conductivity (15). This might be the source of the nonlinearity at the higher applied voltages. From the experimental value of the polarizability anisotropy (Table II), it is apparent that the polarizability itself is significant. If this parameter has its origins predominantly in the particle interface (see below), then one might expect the particles to have a relatively "thick" electric double layer. Under such conditions, with the product parameter Ka small, where K-1 is the double-layer width parameter and a the particle surface radius of curvature, then the following three equations apply (16). First, u = 6~r~7 '

[9]

where 7, e, and ~"are the solvent viscosity, solvent relative permittivity, and the interfacial zeta-potential, respectively. Furthermore,

QDt u = kT

JENNINGS

geometry of the particles, Q may be expressed as (17)

T A B L E II

Parameter

AND

[10]

with Q as the net surface charge. With the electric double-layer model of the interfacial properties, this parameter may be more realistically associated with the charge at the shear plane of the layer for which the zeta-potential is applicable. Finally, in terms of the rod-like Journal of Colloid and Interface Science, VoL 108, No. 2, December 1985

[11]

where Ko and K1 are tabulated, modified Bessel functions. Using the experimentally determined values in the interrelated forms of these equations, leads to values ofQ = 7 × 10-17 c, ~-1 = 70 nm, and ~ = +22 mV. All three values are realistic. The zeta-potential is the most common parameter used to characterize the double-layer properties and hence the charged nature of the particle/medium interface. The most popular means of evaluating this parameter is via the electrophoretic mobility u, obtained from direct microscopic observations. Measurements on large particles is thus implied. A thorough study by Chowdhury and Kitchener (18) has indicated the variability of u and hence ~"on the origin and purity of chrysotile. The major impurity is brucite-Mg(OH)2. Auxiliary potentiometric titration analyses on the present Cassiar chrysotile by the providers of the material indicated a low brucite content, under which conditions a value of 34 mV has been reported (18) from microelectrophoresis experiments. This supports the result of the present study. It also indicates the value of the present method which is extremely fast and is not restricted to large particles. Submicroscopic samples can be measured as here with ease and relative speed. When one considers that particle size data are also obtained via measurements of Dr, the value of the electrophoretic, photon scattering method over conventional electrophoretic apparatus is apparent. Transient birefringence (and intensity scattering, Ref. (19)) are even faster. They yield the parameters #, Aa, L, and some measure of the distribution of L. Little attention has been given to these two electrical parameters in colloid science, even though they have a major role in structure determinations and the electrical descriptions of molecules, biopolymers, and polymers. We have been unable to

ELECTROOPTICS

find values in the literature for # and Aa for chrysotile. We do note a similarity between the values reported here and literature values for other rod-like clay minerals. The value of ~t = 2.7 × 10-25 Cm can be compared with 8.3 for crocidolite asbestos (20), with 10 for polygorskite (21), and 6.5 for halloysite (22), all in units of 10-25 Cm. Similarly, the result of Aa = 3.4 × 1 0 -29 Fm 2 compares well with 2 for crocidolite (2), 2.6 for polygorskite (21), and 14 for halloysite (22). The close similarity of these values for minerals of similiar shape in the same aqueous medium, yet of quite different crystallographic structure, suggests that the lattice structure, is not the prime origin for the observed electrical parameters. Whereas classical theories (23) based upon this concept allow for the effect of particle shape, they are valid only for uncharged particles in strictly nonpolar solvents. An alternative approach is to consider the possible origins of the surface charge on isolated mineral particles. These are known to be three, namely isomorphosis replacement within the mineral lattice, broken bonds at the particle surface and exposed hydrogen atoms from hydroxyl groups at the surface. Collectively, these properties account for the ionexchange capacity of the direct particles (7). Here again, we have been unable to find a value in the literature for this parameter for chrysotile asbestos. We have, however, estimated this parameter in the analysis of electrical surface data for the rod-like asbestos crocidolite (24). In that study, the particles were similar to those of the present study in size and electrical characteristics. Yet it was shown that an unrealistically small value of the cationic exchange capacity would be invoked to account for the observed electric polarizability. Similarly, the chrysotile studied here would have an exchange capacity as small as 2 N 10-3 meq per 100 g of dry clay. Such neutrality is unlikely. Of greater significance is the increasing realization that the electrical properties of the interface and of the surface charged double layer are the significant origins of the electrical

469

OF CHRYSOTILE

polarizability. This is not unreasonable as it has been recognized as the source of apparent surface charge for electrophoretic phenomena for some while. Considerations of this influence on dielectric and polarization phenomena have been increasing during the recent decade (25-30). Should this be the mechanism for a then three important academic principles follow. First, it should be possible to relate the polarizability to the double-layer charge properties. Second, under defined conditions, a could be related to u. Third, measurement of a could replace or supplement u as a means of estimating ~. Theoretical expressions have been developed by several authors to estimate the polarizability of rigid polyelectrolytes (28, 29, 31, 32). That due to Mandel (32) is the most accessible to our experimental parameters. For a rod with N charged surface sites, and a total number n of counterions per particle, each with valency Z for an electronic charge of e, then ,yZ2NL 2

a - - 12kT

[121

for a rod of length L. The parameter 3' = Zn/ N is thus indicative of the counterion charge associated with each site. The total counterion charge associated with the Mandel rod surface is then q = (neZ). Hence Eq. [12] may be expressed in the form

q

12kTa ZeL2

[ 13]

for which all parameters are experimentally determined. Using the birefringence data with Z = 1, then q = 1.4 × 10-17 C. This should be compared with Q = 7 × 10-17 C from the electrophoretic data. Both q and Q are of the same order of magnitude. Whereas Mandel developed his equation for a hypothetical charged rod surface, a more realistic experimental description relates to the charge properties at the double-layer shear plane to which Q has been assumed to apply. Such concord in values should encourage refinement of the theories of both u and a. Even in terms of the Journal of Colloid and Interface Science, VoL 108, No. 2, December 1985

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ISHERWOOD AND JENNINGS

foregoing a n e s t i m a t e can b e m a d e o f ~"f r o m A a w h i c h i s o f t h e s a m e m a g n i t u d e as t h a t f r o m u. W h e n o n e considers the speed with w h i c h Ac~ can b e d e t e r m i n e d , together with the fact t h a t b o t h size a n d size d i s t r i b u t i o n d a t a can be d e t e r m i n e d s i m u l t a n e o u s l y , the value o f i n d u c e d birefringence e x p e r i m e n t s for n o n s p h e r i c a l m i n e r a l s a n d colloids is apparent. Finally, it s h o u l d b e m e n t i o n e d t h a t a series o f e x p e r i m e n t s are in progress in which the c o u n t e r i o n c l o u d s u r r o u n d i n g r o d a n d disks h a p e d m i n e r a l s is m o d i f i e d b y either p H charges or the a d d i t i o n o f surfactants. T h e c o m b i n a t i o n o f u a n d A a d a t a are especially v a l u a b l e b o t h in i n d i c a t i n g t h e surface charge characteristics a n d in studying the zero p o i n t o f charge. U n d e r this c o n d i t i o n A~ is n o t zero (as is u) owing to the ever p r e s e n t " b u l k " or "classical" c o n t r i b u t i o n to the polarizability. This c o n d i t i o n offers a m e a n s o f e s t i m a t i n g this c o n t r i b u t i o n to the polarizability. ACKNOWLEDGMENTS The authors thank the Science & Engineering Research Council and Messrs. Cape Fibres Ltd. for a CASE studentship to Roland Isherwood, at which time this study was initiated. Messrs. British Petroleum Ltd. are thanked for an award which has enabled the studies to be continued. REFERENCES 1. Ware, B., and Flygare, W., Chem. Phys. Lett. 12, 81 (1971). 2. Isherwood, R., and Jennings, B. R., Soc. Photo-Opt. Instrum. Eng. 492, 116 (1984). 3. Buchanan, W. D., in "Asbestos: Properties, Applications, and Hazards, Vol. 1" (L. Michaels and S. G. Chissick, Eds.), Chap. 12. Wiley, Chichester, 1979. 4. Yada, K., Acta Crystallogr. 23, 704 (1967). 5. Chu, B., "Laser Light Scattering." Academic Press, New York, 1974. 6. Pecora, R., J. Chem. Phys. 49, 3 (1968).

Journalof Colloidand InterfaceScience.Vol.108,No. 2, December1985

7. Van Olphen, K., "An Introduction to Clay Colloid Chemistry." Wiley, London, 1963. 8. Crosignani, B., di Porto, P., and Bertolotti, M., "Statistical Properties of Scattered Light." Academic Press, New York, 1975. 9. Yoshioka, K., and Watanabe, H., "Physical Principles and Techniques of Protein Chemistry" (S. J. Leach, Ed.), Part A. Academic Press, New York, 1969. I0. Benoit, H.,Ann. Phys. (Paris) 6, 561 (1951). 11. Hodgson, A. A., in "Asbestos: Properties, Applications, and Hazards, Vol. 1" (L. Michaels and S. G. Chissick, Eds.), Chap. 3. Wiley, Chichester, 1979. 12. Perrin, F., J. Phys. Radium 7, 1 (1936). 13. Broersma, S., J. Chem. Phys. 32, 1632 (1960). 14. Oakley, D. M., and Jennings, B. R., Clay Miner. 17, 313 (1982). 15. Chowdhury, S., Ph.D. thesis. University of London, England, 1973. 16. Shaw, D. J., "Electrophoresis." Academic Press, New York/London, 1969. 17. Overbeek, J. Th. G., Adv. ColloidSci. 3, 97 (1950). 18. Chowdhury, S., and Kitchener, J. A., Int. J. Miner. Process 2, 277 (1975). 19. Jennings, B. R., in "Molecular Electro-Optics" (S. Krause Ed.), p. 181. Plenum, New York, 1981. 20. Isherwood, R., and Jennings, B. R., Clay Miner. 18, 313 (1983). 21. Stoylov, S. P., in "Proceedings, 4th International Congress on Surface Activity, Brussels." p. 171. 1964. 22. Bhanot, M., and Jennings, B. R., J. Colloid Interface Sci. 56, 92 (1976). 23. Langevin, P., Radium (Paris) 7, 249 (1910). 24. Jennings, B. R., and Bhanot, M., Clay Miner. 12, 217 (1977). 25. Eigen, M., and Schwarz, G., J. Colloid Interface Sci. 12, 181 (1957). 26. O'Konski, C. T., and Haltner, A. J., J. Amer. Chem. Soc, 79, 5634 (1957). 27. Stoylov, S. P., in "Chemistry, Physics and Applications of Surface Active Substances" (J. Overbeek, Ed.), p. 171. Gordon & Breach, New York, 1967. 28. Takashima, S., Adv. Chem. Sci. 63, 232 (1967). 29. Oosawa, F., "Polyelectrolytes." Dekker, New York, 1971. 30. Morris, V. J., and Jennings, B. R., Biochem. Biophys. Acta 392, 328 (1975). 31. Schwarz, G., J. Phys. Chem. 66, 2636 (1962). 32. Mandel, M., Mol. Phys. 4, 489 (1961).