Environ. Sci. Technol. 1996, 30, 1176-1179
Electrokinetic Movement of Settled Spherical Particles in Fine Capillaries CHENG-CHUEH KUO AND KYRIAKOS D. PAPADOPOULOS* Department of Chemical Engineering, Tulane University, New Orleans, Louisiana 70118
In certain soil remediation schemes using electrical fields, through the soil’s porous network there may be transport of colloidal and larger-than-colloidal particles, which can affect remediation in several ways. In the first visual investigation of the electrokinetic motion of particles in fine capillaries, an experimental methodology is described for studying the effects of capillary diameter, ionic strength and pH. The technique is applied to the case of negatively charged spherical particles having a diameter of 6.76 µm, after they have been allowed to settle inside cylindrical capillaries with diameters of 10, 110, and 700 µm, respectively. Electrokinetic velocities are reported at several values of applied electric field. In most cases, the direction of the particles’ movement is toward the cathode, implying that electroosmotic flow is dominant. At pH 4 and in the 10-µm capillary, the direction of electrokinetic movement depends on the applied field. At low electric fields, the particles move toward the cathode, whereas above some critical value for the electric field, movement takes place toward the anode.
Introduction Recent interest in electrically driven separations (1) stems in general from the many industrial applications to which they lend themselves, and in particular to the electrokinetic remediation of soil (2-6) in environmental cleanup operations. Whereas the main interest in such processes lies in an attempt to concentrate and confine heavy metals close to an electrode, in noncompact soils concurrent colloidal transport may be significant in altering the microstructure of the porous medium and in facilitating the transport of adsorbed metal ions and organics. As this paper shows, the direction and extent of such transport may be dictated by two opposing driving forces on the particle, of an electrophoretic and an electroosmotic nature, respectively. Electrophoresis and electroosmosis are two phenomena controlling the transport of a dispersed phase through a continuous one, under the influence of an applied electric * To whom correspondence should be addressed; telephone: (504)865-5826; fax: (504)865-6744; e-mail address: pops@ che.che.tulane.edu.
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field (7-9). The dispersed phase can be constituted by charged liquid droplets or solid particles while the dispersion medium may consist of an aqueous or a non-aqueous solution. More specifically, electrophoresis is the movement of a charged particle in a liquid, caused by an electric field, while electroosmosis refers to the movement of a liquid close to a charged surface due to its inclusion of the surface’s counterions. While electrophoretic movement of particles is in the direction of the electrode with opposite charge to the particles’ surface, electroosmotic flow of a liquid takes place toward the electrode that has the sign of a solid surface’s charge. The net charges on particle or wall surfaces, either positive or negative, are due to the dissociation of ionizable groups at surfaces or the adsorption of potential-determining ions from solution. Several theoretical studies have addressed corrections and extensions to the classical electrophoretic equations of Smoluchowski, Hu ¨ ckel, and Henry, a number of which have specifically studied the influence of solid boundaries in the proximity of the moving particles (10-13). No experimental studies that include the objectives of this work have previously been reported in the literature. In conventional microelectrophoretic measurements, the electrophoretic mobility is determined by tracking the particles’ movement along a stationary layer, where there is no effect of electroosmotic flow, and therefore negatively charged particles move toward the anode while positively charged particles migrate toward the cathode. In this paper, the particles’ movement is tracked after they have settled, so that they are clearly inside the domain of electroosmotic flow. In measuring the particles’ electrokinetic velocity, the parameters that were varied are pH, ionic strength, pore radius, and applied field.
Materials, Experimental Setup, and Methods Three different pHs (4, 6, and 11) were used in this study. pH solutions were all prepared using water, which was deionized and purified by a Barnstead E-pure system. The pH was adjusted with NaOH and HCl solutions freshly prepared from 99.99% pure stocks of these chemicals supplied by Aldrich. An Orion 720A pH-meter was used to measure pH. Fifteen milliliters of surfactant-free suspension of sulfated polystyrene latex particles was purchased from Interfacial Dynamics Corporation (IDC), and it was used as received, without further purification. The mean diameter of these microspheres is 6.76 + 0.845 µm, with detailed IDC specifications listed in Table 1. To avoid the influence of surrounding particles on the electrokinetic velocity of an individual particle, ultradilute latex suspensions were necessary. Therefore an extra-fine injection micropipet, described below, was used to withdraw and inject a trace amount (about 10-3 µL) of 4.2% wt suspension into 20 mL of the various pH solutions described above. One of the key components in the experimental apparatus is the fabrication of microcapillaries of various sizes, in which the electrokinetic movement of particles was observed directly. Drummond precision-bore glass micropipets, having a length of 78 mm and an inside diameter of 700 µm, were purchased from Curtin Matheson Scientific, Inc. These micropipets were centrally pulled with a Narishige PB-7 Micropipette Puller to make microcapillaries
0013-936X/96/0930-1176$12.00/0
1996 American Chemical Society
TABLE 1
Specification of Polystyrene Particles Used in This Studya mean diameter % solids density of polystyrene at 20 °C refractive index of polystyrene surface charge density area/charge group mequiv/g specific surface area particle no./mL
6.76 µm ( 12.5% 4.2 ( 0.1 wt % 1.055 g/cm3 1.591 at 590 nm 3.80 µC/cm2 473 Å/charge group 0.00033 8413 cm2/g 2.49 × 108
a Note: This table is a reproduction of a supplier’s (Interfacial Dynamic Co.) analysis data.
FIGURE 2. Snapshots of particle electrokinetic movement in a 10 (a) and a 110-µm (b) capillary, respectively.
chosen to fasten the working capillary without crushing it during the experiments.
FIGURE 1. Schematic representation of the microelectrokinetic cell.
with inside diameters of 10 and 110 µm. By appropriate combinations of pulling distance, pulling weight, pulling speed, and heating power, any desirable inside diameter between 2 and 200 µm can thus be achieved. The 110-µm capillary was made by center-pulling a 700-µm capillary once by 10 mm in length at a heater setting of 650, while the 10 µm micropipettes were prepared by center-pulling a 700-µm capillary by 3 mm and then again pulling the narrowest section by another 10 mm. To the ends of a working capillary, two cylindrical reservoirs were attached, 10 mm in inside diameter and 11 mm in height. Silicone tubing was used to seal the two ends of the capillary to the 2 mm diameter holes drilled on the sides of the reservoirs as shown in Figure 1. A given microcapillary was flushed and filled with a certain pH solution via injection micropipets, which were also used to introduce the latex particles close to the narrow section of the microcapillary. The technique for preparing injection micropipets is similar to the one described above except that the pulling length was extended to 50 mm and the narrowest part was cleft with a glass tubing knife. The capillary holder was designed to hold capillaries of different sizes, to have a versatile and secure attachment to the microscope’s stage, and to provide a sturdy support for the narrowest microcapillaries that broke easily. A large circular hole in the center of the capillary holder accommodates the need of closure between the working capillary and the high magnification objective of the inverted microscope, while taking advantage of more working space on the stage for one reservoir cup and one electrode at each end of the capillary. The groove across the length of the holder is used to keep the glass capillary in place. The two sets of screws and washers made from nylon were
The custom-made microelectrokinetic cell, as previously described, was placed on the stage of an Olympus inverted microscope IMT2-001A. After the capillary was micropipet-flushed and filled with aqueous medium of a desired pH, a minute amount of suspension with only a few (1-5) spherical polystyrene particles was injected into the capillary via a micropipet by using a pressure-controlled microinjector IM-200 obtained from Narishige Co. The threedimensional movement (up and down, left and right, forward and backward) of this second micropipet is micromanipulated by the MN-2/MO-303 system, which was also purchased from Narishige. Silver electrodes of 0.25 mm diameter were supplied by Alfa Products, the electric field applied across the capillary length was controlled by the power unit of a commercial electrophoresis system (Zeta-Meter ZM-80), and voltage was measured with a digital voltmeter (FLUKE 73). A COHU high-performance 4910 RS-170 CCD camera captured the electric field-induced motion of particles through the capillary, which was monitored on a Sony Trinitron PVM-1943MD super-finepitch color video monitor. The acquired images were processed by the Bioscan Optimas image analysis system (Meyer Instruments), and the motion sequence was recorded by a SONY SVO-9500MD professional videocassette recorder for further analysis of particle velocity and pore dimensions. Snapshots of particle motion (e.g., Figure 2) were obtained from a Mitsubishi CP210U color video copy processor. Particle velocities were calculated by timing the particle movement over the full length of the monitor’s screen or over a fixed distance measured between two reference spots marked previously on the capillary surface. A similar experimental setup was recently used in our laboratory to study bacterial motility in fine capillaries (14) and the coalescence of oil droplets suspended in aqueous medium (15, 16).
ζ-Potential and Particle Size Measurements The ζ-potential on the surface of polystyrene particles in different pH solutions was measured using the Doppler
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electrophoretic light scattering analyzer (DELSA 440) from Coulter Electronics. A drop of 4.2% wt latex solution was suspended in 20 mL of deionized water at various pH values without addition of any salt. The suspension was gently hand-shaken and then sonicated for two 15-s intervals in a 67-kHz frequency and a 85-W power ultrasonic bath (Model ME6.4) from Mettler Electronics. This sonication time was necessary to well-disperse latex particles without fracturing them at the same time. The same procedure was used on glass powder from crushed capillaries in order to obtain an estimate of the ζ-potential of the capillary wall at various pHs. The crushed glass suspension was made by dispersing ∼0.005 g of fine glass powder in 20 mL of water. Reported ζ-potential values of both latex particles and crushed glass powder represent the average of multiple runs, which in turn were the average of ζ-potentials measured at two stationary layers of the electrophoresis cell. It should be noted that the ζ-potential of crushed glass represents the ζ-potential of the capillary only approximately. The size distribution of the crushed glass particles was measured with a Coulter submicron particle analyzer model N4MD (data not shown), and the mean effective diameter was 2 µm. Using the same instrument, the size of the latex particles was confirmed.
Electrokinetic Velocity of Settled Particles Electrokinetic velocities were measured for three different capillary diameters, three pHs, and two ionic strengths. In all experiments the electric field was varied from 0 to 30 V/cm, and electrokinetic velocity measurements were made on the same particle for a given pH. Before a certain field was applied across the capillary, the latter was allowed to reach equilibrium, and it was microscopically ensured that there were no remnant convection currents from the experiment of the previous voltage. Also to this effect, particle velocities were measured using both polarities. It should be noted that the electric field experienced by the particle is very different than the one applied across the capillary. This is due to changes in electrical conductivity in the vicinity of solid surfaces as well as the squeezing of the electric field lines inside the capillary, and especially (11) in the intervening space between the particle and the glass wall. The same three capillaries were used for all experimental data shown in Figure 3 (a-c). In these experiments, the pH values of 4, 6, and 11 were reached by adding minute amounts of HCl and NaOH in 0.01 KCl solutions, thus keeping a nearly constant ionic strength. At high ionic strength the Debye length, κ-1, is ∼3 nm for all three pHs, whereas for low ionic strength, it has the values of 30.4, 238.3, and 9.6 nm for pHs 4, 6, and 11, respectively. It is seen that at high ionic strength the particle electrokinetic velocity increases near-linearly with the applied voltage in solutions of pH 6 and pH 11 for all three different capillary sizes of 10, 110, and 700 µm. For these two pHs, similar trends are exhibited in the case of low ionic strength, the results being shown in Figure 4 (b and c). In all cases of high ionic strength, the particle velocity increases with decreasing capillary diameter, and the direction of particle motion is toward the cathode. Since both the wall and the particles have a negative charge at all pHs studied (Figure 5), the particles are transported chiefly by electroosmotic flow. Figure 6 illustrates the right direction of electroosmotic flow and a particle whose electrophoretic direction would be toward the left. A
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FIGURE 3. Electrokinetic velocity of a 6.76-µm spherical latex particle in three cylindrical capillaries and three pHs, at high ionic strength.
FIGURE 4. Electrokinetic velocity of a 6.76-µm spherical latex particle in three cylindrical capillaries and three pHs, at low ionic strength.
surprising observation pertaining to the capillaries of different diameters is that at pH 11 and for both ionic concentrations (Figures 3c and 4c), the electrokinetic velocity is greatest in the capillary of intermediate diameter, with the following order: 110 > 10 > 700 µm. At pH 6, the respective order is 10 > 110 > 700 µm, which is expected
FIGURE 5. ζ-potential of polystyrene particles and a crushed glass suspension as function of pH.
FIGURE 6. Schematic depiction of a negatively charged particle in a microcapillary and the electroosmotic velocity profile of the fluid.
since the electroosmotic effect on the particles’ movement is more important in capillaries of higher curvature or smaller diameter. At pH 4, particle movement reversal is seen, which is not shown in the other pHs. At low ionic strengths and above some critical applied electric field, the particles were observed to move toward the anode for the two smaller capillary diameters, as shown in Figure 4a, with the highest speeds being exhibited in the smallest of the two (10 µm). In the case of 0.01 KCl aqueous medium, direction reversal was seen only in the 10-µm capillary, and even then the phenomenon was slightly pronounced (Figure 3a). A first explanation for movement reversal may be found by the lower absolute value of ζ-potential that the wall has at pH 4, as opposed to pHs 6 and 11. A lower ζ-potential implies a weaker electroosmotic flow and therefore a smaller electroosmotic effect on the particles’ movement. It should not go unobserved, however, that also the absolute potential on the latex spheres drops even faster than that of glass with decreasing pH. This behavior is in apparent disagreement with Keh and Anderson’s work (10), which would predict that the particles should move toward the anode for all pH > 7.8, a range where the absolute potential on the wall is lower than that on the particles. However, their paper considers a particle moving along the axis of a capillary tube, and thus cannot provide a basis for comparison as cannot the work of Keh and Chen (11). A theoretical explanation therefore may not be provided at this stage.
For a given capillary diameter, it is instructive to notice the effects of pH and ionic strength. In the case of the 110 and 700-µm capillaries, and for 0.01 KCl solution, there is little variation in electrokinetic velocity with varying pH, whereas drastically different trends are seen in the 10-µm capillary. In the case of low ionic strength, in all capillaries the electrokinetic velocity varies substantially with pH. Since at a lower ionic strength the electrical double layer is more extended, the added sensitivity of electrokinetic velocity on pH is expected. At the present time, there are no theoretical models to predict or explain the behavior we have observed in the systems we studied. Therefore, it is not possible to quantitatively separate the effects of electrophoresis from those of electroosmosis. There are several effects that will have to be addressed in future fluid mechanical studies, one being the viscous drag or lift exerted on the particle by the fluid that exists in the intervening space between the particle and the glass wall. The squeezing of the fluid’s streamlines in that space is causing a local acceleration of the fluid, which could act on the particle in several ways, for example, by causing it to rotate. Future experiments at the microscopic level should target the determination and the importance of the particle-tocapillary wall distance for single-capillary systems, while macroscopic experiments should address the electrokinetic movement of suspended particles through packed beds.
Acknowledgments Support for this work was provided by NSF EPSCoR Grant EHR-9108765 and the Louisiana Board of Regents. We thank Wei Chen for her assistance in the experiments and also Ashoke SenGupta and Zewen Liu for their input at the beginning of this work.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
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Received for review June 14, 1995. Revised manuscript received November 6, 1995. Accepted November 13, 1995.X ES950413D X
Abstract published in Advance ACS Abstracts, February 1, 1996.
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