Langmuir 1995,11, 798-800
798
Electrophoretic Mobility and Dielectric Response Measurements on Electrokinetically Ideal Polystyrene Latex Particles M. R. Gittings and D. A. Saville” Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544 Received August 31, 1994. I n Final Form: December 30, 1994@ We measured t h e dielectric response a n d electrophoretic mobility of a polystyrene latex to test t h e classical theory of electrokinetics. Measurements of the low frequency dielectric response of the latex at various (low) volume fractions were made using the four-electrode signal-processingspectrometer of Myers a n d Saville (J.Colloid Interface Sci. 1989, 31, 448). Electrophoretic mobilities over a range of NaCl concentrations were determined with a Coulter DELSA440. Good agreement between the e-potentials derived from the two complementary measurements was found using t h e standard model of electrokinetics a n d the (-potential from the upper branch of the mobility-(-potential relation. To our knowledge this is the first time that classical electrokinetic behavior has been found with “off-the-shelf” latex particles.
Introduction F o r some time it has been recognized that most colloidal particles are “nonideal” in the sense that complementary electrokinetic measurements o n the same particles yield ambiguous results. Thus, 5-potentials derived from electrophoretic mobility measurements differ f r o m those inferred from electrical conductivity, streamingpotentials, or dielectric response measurement^.'-^ Part of the problem is ascribed t o the a s s u m p t i o n s of the standard model, viz.,a uniformly charged, smooth, surface whose characteristics are unaffected by dynamic processes. This disagreement leads, in turn, t o the development of m o r e sophisticated models. Often these models take account While of conduction processes behind the shear s u c h models are sometimes capable of providing an internally consistent interpretation, this comes at the expense of increased complexity. Moreover, there is always the nagging possibility that the flaw lies elsewhere. T o provide a f i r m basis for more elaborate treatments, it would be extremely helpful t o have model particles which conform t o the tenets of the classical theory. Heat treatment does provide s u c h particles o n o c c a ~ i o nbut ,~~~ it is essential t o expand the list of possibilities. Classical particles can also serve as a substrate for adsorbed polymer and measurements o n s u c h particles provide unambiguous insight i n t o the role of“hairy surfaces”. As part of a wider study on the role of adsorbed polymer, we measured the response of electrokinetically “ideal” particles. Insofar as we are aware, this is the first i n s t a n c e in which s u c h behavior has been observed with o t h e r than “heat-treated” particles.
Experimental Methods
Monodisperse polystyrene latices with a diameter of 156 nm (measured by transmission electron microscopy and having a
* To whom correspondence
a Abstract
should be addressed. published in Advance ACS Abstracts, February 15,
1995. (1)OBrien, R. W.; Perrins, W. T. J . Colloid Interface Sci. 1984,99, 20. (2) Zukoski, C.F.; Saville, D. A. J . Colloid Interface Sci. 1986,107, 322. (3)Rosen, L. A,; Saville, D. A. Langmuir 1991,7,36. (4)Zukoski, C. F.; Saville, D. A. J . Colloid Interface Sci. 1986,114, 32. (5) Mangelsdorf, C.S.; White, L. R. J . Chem. SOC.,Faraday Trans. 2 1990,86,2859. (6) Rosen, L. A.; Baygents, J. C.; Saville, D. A. J . Chem. Phys. 1993, 98,4183. (7)Kijlstra, F.;van Leeuwen, H. P.; Lyklema, J. J. Chem. SOC., Faraday Trans. 1992,88,3441. (8)Rosen, L. A.; Saville,D. A. J . Colloid Interface Sci. 1990,140,82. (9)Rosen, L. A.;Saville, D. A. J . Colloid Interface Sci. 1992,149, 542.
4.2% coefficient of variation) were obtained from Interfacial Dynamics Corporation (IDC). The latices are stabilized by sulfate charges having a pK. < 2 and a titratable surface charge density of -1.08 pClcm2, according to IDC. Salt solutions were made using filtered, deionized, doubly distilled water from a KMnOd KOH first stage and recrystallized NaCl from Fluka Chemical Corp. The latices were washed 12 times by repeated centrifugation using a Beckman L5-65 ultracentrifuge. After each centrifugation the supernatant was discarded and the latices resuspended in the doubly-distilled water. A final centrifugation/ decantation was used to concentrate the latices for experimentation. The concentrated suspension showed no bubble stability and had no styrene odor. A hydrodynamic size of 160 nm was measured using photon correlation spectroscopy. Extra caution was taken in order to prevent contamination of the suspensions from the negatively charged oligomeric compounds in the oil on human skin by wearing gloves. Studies in our laboratory have shown, for example, that positively charged amidine latices can exhibit negative mobilities with even the slightest trace of contamination. Negatively charged latices may also be affected; however, the electrokinetic effects of the contaminant would be hidden since the contaminant is also negatively charged. Electrophoretic mobility measurements were made at 25 “C using electrophoretic light scattering with a Coulter DELSA440. Each mobility measurement was performed at volume fractions of the order of to minimize particle interactions. Mobilities were measured over a span of electrolyte concentrations ranging M HzC03 (from dissolved COz) to M NaC1. from 1.5 x Joule heating was minimized by requiring the temperature difference between the cell electrodes to be 50.3 “C. Likewise, the temperature rise at either electrode throughout measurement was restricted to 0.1 “C, to minimize temperature’s effect on the mobility. Mobility data were obtained across the width of the cell and fitted to a parabola from which the electrophoretic mobility a t the stationary layers was determined. This procedure accounts for the influence of electroosmotic flow at the cell walls and Poiseuille backflow. For all of the data “fits” the R2 value (a measure of the “goodness-of-fit”)was greater than 0.986. The low frequency dielectric response of the latices was determined using the four-electrode signal-processing spectrometer of Myers and Saville.’O The four-electrode design accounts for polarization effects by requiring two end electrodes to pass current and two midstream, probelike, electrodes to measure voltage. Small drifts in temperature (-0.0001 “C) are nulled electronically by monitoring changes in the suspension resistance at a reference frequency throughout the experiment. Further information on the details of the technique can be found elsewhere.8J0 Measurements were taken at 25 “C in 1mM NaCl a t particle volume concentrations between 2% and 6% over a frequency range of 0.5-100 kHz. Within this frequency range, only relaxation of induced electrical dipoles will be perceptible (10)Myers, D.F.;Saville, D. A. J . Colloid Interface Sci. 1989,131, 448.
0743-746319512411-0798$09.00/00 1995 American Chemical Society
Langmuir, Vol. 11, No. 3, 1995 799
Electrokinetically Ideal Latex Particles
5.0
lo
I
4-
4.8 -
4.4 4.6
3-
0 0
4.0
2.
0
4.2 -
0
-
0
0 1.
3.8 1
3, theory shows that the relation between mobility and (-potential has a maximum, representing a competition between increasing electrokinetic charge on the particle and polarization effects arising from increasing electroosmotic drag. In 1mM NaCl ( K a = 8.1), the mobility of our particles is -4.24pmcmN.s, so the numerical solution yields two possibilities for the 5-potential (-100 and -140 mV) (see Figure 2). The dielectric response data for mM NaCl were compared with the theoretically predicted response a t both [-potentials using the standard theory as described by deLacey and White (13). Theoretical parameters, such a s the dielectric and conductivity increments, are based on infinite dilution of the suspension and, therefore, all of the experimental data were extrapolated to zero volume fraction for comparison purposes. Because the dielectric response measurements were performed a t low volume fractions, the dielectric constant (E)and the conductivity (u)can be expanded in the volume fraction (#I a s follows:
+ Ao(f)@ + ;A20(f)4' +
1
100
150
200
250
300
-
