2498
J. Phys. Chem. 1981, 85, 2438-2439
NSF grant DMR 77-27428 to M.N. and Department of Energy Office of Basic Energy Sciences Contract DEAS03-76SF0034 to M.A.E.; equipment used in this work was acquired with funds provided by NSF grants
MPS75-06135, GP32304, and CHE79-10965. The technical support of the laser system by Spectra-Physics, Inc. is gratefully acknowledged. Samples were prepared in the laboratory of Professor Paul Boyer at UCLA.
Microelectrophoresis of Asbestos Fibers. Comparison of Theory with Experimental Data Joseph E. Schiller' and Sequoyah L. Payne U.S. Depertment of the Interlor, Bureau of Mines, Twin Cnles Research Center, Mlnneapolis, Minnesota 554 17 (Receivd: May 21, I M l )
The relationship has been determined between the electrophoretic mobility of asbestos fibers, electrolyte concentration, and fiber orientation in an electric field. The observed behavior is compared with Henry's model for charged colloidal cylinders, and we have found no previous reports that experiments have been performed to test Henry's theory. For electrolyte concentrations between lob and M, theory predicts that the mobility of cylinders perpendicular to the applied field should be 0.80 to 0.98 times as fast as those parallel to the electric field. The observed relative mobilities are 0.63 to 0.78. The lower mobility of perpendicular fibers in the real case is suggested to be due to the fibers not being perfect cylinders as the model assumes.
Introduction Zeta potentials have been determined for amphibole asbestos fibers and amphibole cleavage fragments from a number of sources.1*2 Microelectrophoresis was the method used in those studies, and measurements were generally restricted to elongated particles aligned parallel to the applied field. Since the particles are observed directly in microelectrophoresis, the interesting effects of particle shape and orientation on electrophoreticmobility were also studied. The electrophoretic mobility of short asbestos fiber segments was found to be much less than could be accounted for by shape; it was thus revealed that the ends of amphibole asbestos fibers have a positive surface charge while the lateral surfaces are negative., In this paper, the observed relationship of asbestos fiber orientation, electrolyte concentration, and electrophoretic mobility is compared with the expected interdependence of these quantities predicted by electrokinetic theory. The classical treatment of the behavior of charged colloidal cylinders in an electric field was performed by Henr9 and more recent publications by Stigter have extended the earlier work.68 The resulting equation for the mobility of cylinders4 is
where U is the velocity of the particle, X is the electric field strength, E and 1are the dielectric constant and viscosity of water, respectively, 5 is the potential at the plane of shear of the particle, and F ( K ~is )a "geometric factor" that (1) Light, W. G.; Wei, E. T. Environ. Res. 1977, 13, 135. (2) Schiller, J. E.; Payne, S. L.; Khalafalla, S. E. Science Submitted
for publication. (3)Schiller, J. E.; Payne, S. L. Rep. Znuest. U.S., Bur. Mines 1980, RZ 848s.
(4)Henry, D.C. Proc. R. SOC.London, Sec. A 1931,133, 106. (5)Stigter, D.J. Phys. Chern. 1978,82, 1417. (6) Stigter, D.J. Phys. Chern. 1978,82, 1424.
TABLE I : Values of KNO, Concentration,Double-Layer Thickness ( K - I ) , and the Ratio of Particle Radius to Double-Layer Thickness
KNO, concn, M 4.2 x 10-5 3.3 x 1 0 - 4 1.4 x 10-3 6.3 x 10-3
(K
b) K-',
4.7 x
cm
lo+
1.7 X 8.1 x 10-7 3.8 x 10-7
Kb 11
29 62 130
depends on the ratio of the radius of curvature of the surface of the particle to the thickness of the double layer. For cylinders oriented parallel to the electric field, F ( d ) = 4, and for perpendicular cylinders, F ( K ~ranges ) from 4 to 8, depending on the ratio of particle radius to doublelayer thickness. Numerical values of F ( K ~for) cylinders with different values of Kb were tabulated by Abramson.' Amphibole asbestos fibers are generally not perfect cylinders.8 Although they appear to have a somewhat irregular cross section, a cylindrical model was chosen as a first approximation for explaining the electrophoretic behavior of amphibole asbestos fibers. Experimental Procedure and Results The material studied was anthophyllite asbestos from the International Union Against Cancer (UICC)." Suspensions in distilled water (0.01% wt/vol) were prepared by magnetic stirring and ultrasonic agitation. The pH was adjusted to 7 by using 0.01 M HNO, and KOH, and the mixtures were sealed and allowed to stand for 15-20 h. If needed, the pH was readjusted to 7 just prior to microelectrophoresismeasurements. Ionic strengths ranging from 4.1 X to 6.4 X M were obtained by adding KNOS to some of the suspensions. Conductance was used as a measure of KNO, concentrations, since unmeasured quantities of "Os and KOH were used to adjust the pH. (7) Abramson, H. A.; Gorin, M. H.; Moyer, L. S. Chern.Reu. 1939,24, 345. (8) Franco, M. A.; Hutchinson, J. L.; Jefferson, D. A.; Thomas, J. M. Nature (London) 1977,266, 520. (9) Timbrell, V.; Rendall, R. E. G. Powder Technol. 1971-72,5,279.
Thls artlcle not subject to U S . Copyrlght. Published 1981 by the American Chemical Society
The Journal of Physical Chemlstry, Vol. 85, No. 17, 1981 2490
Letters
The determination of particle orientation and electrophoretic mobility was done by using a rectangular cell electrophoresis system and petrographic microscope equipped with a camera. The rectangular cavity in the cell had a width of 12 mm, a depth of 0.95 mm, and a length of 3 cm. The polished platinum electrodes were separated by 7.4 cm, and the microscope had an overall magnification of 375X. The cell was filled with the suspension of asbestos fibers and positioned so that the microscope was focused exactly 0.20 mm above the bottom of the cell. Times were recorded only for highly elongated fibers (length to width ratios of at least 10) that were exactly parallel or perpendicular to the electric field as they moved. A total of about 20 parallel and 20 perpendicular fibers were timed in each suspension. The standard deviation of individual transit times was about &lo%,and a reverse in electrode polarity did not affect the transit time. The distance traveled was 220 pm, and the applied voltage was 175 V. The transit times for the 20 fibers were averaged, and the mobility for fibers of each orientation was determined by using the average transit time. Photographs of the particles observed showed them to be 0.5-2 pm wide and 10-25 pm long, and the average fiber diameter was 1
m.
Discussion of Results The DebyeHuckel approximation was used to estimate the double-layer thickness of particles in KNOBby the expression, 6' (cm) = 3.04 X lo4 M-'j2. Calculated values for double-layer thickness for colloidal particles in solutions used in this study are given in Table I. A radius of 0.5 pm was used to calculate the values of Kb that are also presented in Table I. The comparison of actual behavior with that predicted by theory, using the equations for cylindrical particles, is best done by comparing the expected and observed ratios of mobility for fibers oriented parallel and perpendicular to the field in each solution studied.1° For fibers parallel to the field, eq 1 becomes
and for fibers in the same solution that are perpendicular to the field, the mobility is (3)
(10) Stigter, D., personal communication.
TABLE 11: Estimated Values of F ( K ~ Predicted ), Values of Ul/Ull, and t h e Experimental Values of Ul/UII
est F(Kb)
4.97 4.43 4.22 4.09
VlKJll predicted from eq 4
0.80 0.90 0.95 0.98
exptl
0.63 0.64 0.76 0.78
The constants are eliminated by dividing eq 3 by eq 2 to give the ratio of mobilities of perpendicular and parallel fibers for a given solution. This ratio is
UI = - 4 -
(4) UIl F(Kb) The predicted and observed ratios of UL/UIIare given in Table 11. The values of K b in Table I would normally be considered so large that there should be no observable difference in mobility for fibers oriented parallel and perpendicular to the external field. The actual ratios of mobility for perpendicular and parallel orientation are considerably less than unity (Table 11). Even for K b of over 100, perpendicularly oriented fibers moved only about 0.8 times as fast as those parallel to the field. The predicted ratios of mobility for perpendicular and parallel orientations cannot be calculated exactly for the large values of ~b in Table I. For K b greater than 3, values of F(Kb)in Table I1 were ~ ) 1 / ~ by b using the data estimated from a plot of F ( K vs. presented by Abramson." These estimated values of F ( K ~ ) were used in eq 4 to calculate the predicted ratios of U,/ Ull. Although this extrapolation of Abramson's data gave only approximate predicted values of F(Kb), the relative mobility of perpendicularly oriented fibers is clearly much less than expected. The difference between the actual behavior of asbestos fibers and that predicted very likely results from the fibers not having smooth cylindrical surfaces as assumed by our model. Amphibole asbestos fibers may have somewhat irregular cross-sectional shapes. Indentations in the surface along the fiber axis may cause the actual plane of shear to be some distance outside the Stern layer for a portion of the surface of fibers that are perpendicular to the applied field. The observed potential will therefore be less than the potential at the Stern plane. Fibers parallel to the field would not have water trapped in these elongated cavities since the flow is along the fiber axis. Parallel fibers would have their plane of shear near the Stern plane, and they would not experience a decrease in mobility as a result of having the plane of shear relatively far from the particle surface.
