A New Instrument for the Measurement of Very Small Electrophoretic

Chapter 23

A New Instrument for the Measurement of Very Small Electrophoretic Mobilities Using Phase Analysis Light Scattering

Downloaded by EMORY UNIV on February 17, 2016 | http://pubs.acs.org Publication Date: June 10, 1998 | doi: 10.1021/bk-1998-0693.ch023

Walther W. Tscharnuter, Fraser McNeil-Watson, and David Fairhurst Brookhaven Instrument Corporation, Brookhaven Corporate Park, 750 Blue Point Road, Holtsville, NY 11742

A new instrument, the Brookhaven ZetaPALS, based on the principles of phase analysis light scattering (PALS) applied to the measurement of electrophoretic mobilities, has been produced. Such measurements are particularly useful in the study of dispersions in non-polar media, since for a given zeta potential, the mobility is proportional to the dielectric constant. It is also potentially useful where mobilities are low and the use of high electric fields (a traditional remedy) is inappropriate. This is the case when the ionic concentration and hence conductivity of the medium is high. The present PALS configuration has been shown to be able to measure mobilities up to three orders of magnitudes lower than conventional L D E . The device is based on developments reported elsewhere but has a number of new features. In particular all the signal processing is digital and the optical system a reference beam configuration. In this paper we present data showing that on suitable samples both the PALS technique and conventional L D E can be performed on the same instrument, and that the techniques have good agreement.

An understanding of many aqueous dispersions is based on their electrophoretic mobilities, which is of importance in determining, for example, suspension stability, rheological properties and coating behaviour. This arises because the mobility, defined as the velocity the particle attains per unit electric field, can be related to the so-called zeta potential. This potential is defined to be the potential at the surface of shear where the particle with a shell of electrostatically attracted counter-ions moves

©1998 American Chemical Society

In Particle Size Distribution III; Provder, Theodore; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

327

328 through the bulk solution. The value of the zeta potential is not the same as the surface potential, due to the presence of the counter ions: however it is the relevant potential for calculating the interaction energy of the dispersion. The electrophoretic mobility, u., can be shown to be related to the zeta potential, by


n where r\ is the viscosity, e is the dielectric constant of the dispersing medium, f(ica) is a function of the particle size (radius a) and 1/K the thickness of the double layer of counter ions and ions surrounding an individual particle. In many aqueous based dispersions with a moderate ion concentration and not too low particle size, where Ka is 100 or more, f(ica) = 1.5, and the equation (1) becomes the so-called Smoluchowski relationship. For particles in non-polar media, where Ka2 is known, the reversal of sign between polar ethanol and non-polar toluene or non-polar xylene is expected on electron donor-acceptor arguments. (19,20). The reduction in the relatively high value of 0.255xl0' m W to 0.155xl(r n^vY on drying is also explicable when it is recognized that traces of water will stay on the surface of the particle due to the hydrophobicity of the toluene and largely determine the surface charge. It is well recognized that trace amounts of water can dramatically effect the surface charge of particles in non-aqueous media (21,22). The final result was obtained on another proprietary commercial sample sent for electrophoretic mobility analysis: casein dispersed in a P E G having an average M W of 200. The measurement was made at 37° C. Even at this temperature the viscosity is still 27 cP, so a small value of mobility is to be expected. This measured value of -0.025xl0" m V* s' is to be contrasted with the typical literature value for casein in water of about -6.0xl0" mW (23).


8

8

8

2

1

1

1

8

1

Results II : Measurements on quartz spheroids over a range of ionic molarities To investigate the performance of the ZetaPALS over a range of salt conditions, rather wider than so far demonstrated, a set of measurements on the material BCR66 were performed. This material is a certified reference material made of crushed quartz spheroids of size range 0.35 microns to 3.5 microns (24). It is used to check the accuracy and precision of particle sizing instrumentation and also as a calibrant. The samples were prepared by simple dispersion of the BCR66 into D.I. water containing different concentrations of KC1 using ultrasonics; no additional dispersing agent was employed. The results obtained are tabulated below in Table 3 and plotted in Figure 7. As expected for quartz in aqueous suspension, all the measured mobility values are negative. In low electrolyte concentration (< 0.01M) the zeta potential is calculated to be about -65mV, typical of literature values for quartz (16). The measurements above 0.1M could not be performed by conventional L D E . Data was obtained even in saturated salt (>4M) but the reproducibility was not felt to be acceptable enough, at this time, to justify publication, though the mobility obtained, interestingly, was still negative. The general trend of the effect of increasing electrolyte concentration, shown in Figure 7, including the extremum at 0.001M (probably due to slight specific adsorption of chloride ions) is also in agreement with expectation.


339

Table 3 B C R 6 6 in K C I

Mobility

Molarity


0.0001

-4.30+/-0.10

0.001

-4.56+/-0.05

0.01

-3.50+/-0.12

0.1

-2.10+/0.10

0.5

-1.75+/-0.20

1.0

-1.33

2.0

-0.73

BCR66 in KCI

molarity

Figure 7 B C R 66 in KCI

Conclusions The Brookhaven ZetaPALS instrument has been used to measure the electrophoretic mobility of a range of samples some of which are also able to be measured by conventional LDE. The agreement between the two techniques, where appropriate, is shown to be excellent. Examples of measurements on samples beyond the reach of conventional L D E due to low dielectric constants, high viscosities, and high ionic strengths, have also been clearly demonstrated; routine measurements of such systems are now possible. With a current sensitivity of up to three orders of magnitude better than conventional LDE, the PALS methodology represents a quantum leap in the technological development of commercially available instrumentation to measure the electrophoretic mobility of colloidal dispersions.


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References 1. O'Brien, R.W.; White, L.R. J.Chem.Soc., Farad.II, 1978, 74, 1607 2. Bangham, A. et al. Nature, 1958, 182, 642 3. Morrison,I.D.;Tarnawskyj, C.J. Langmuir, 1991, 7, 2358 4. Kornbrekke,R.E.et al. Langmuir, 1992, 8, 1211 5. Seaman, G.V.F. IntheRed Blood Cell; Surgenor, D., Ed.; Academic Press: 1975 6. Beretta, D.A.; Pollack S.R. J.Ortho.Res., 1986, 4, 337 7. Coffey, M.D.; Lauzon, R.V. Int.Symp.Oilfield Chem., 1975, SPE No.5302, 93 8. Yousef, A.A.; Bibawy, T.I. Tenside Deterg., 1976, 13, 316 9. Novotney, V. J.Appl. Phys., 1979, 50, 324 10. Pohl, H.A. In Dielectrophoresis; Cambridge University Press: 1978 11. Miller, J.F. Ph.D. Thesis; University of Bristol 1990 12. Miller, J.F.; Schätzel, K., Vincent, B. J.Coll.Int.Sci., 1991, 143, 532 13. Uzgiris,E.E.Prog.Surf.Sci., 1981, 10 53 14. McFadyen, P.; Fairhurst, D. Proc.Brit.Ceram.Soc., 1993, 51 15. Hackley, V.A. et al. Coll. and Surf., 1995, 98, 209 16. Hunter, R.J. In Zeta Potential in Colloid Science; Academic Press: 1981 17. Miller, J.F. et al., Coll. and Surf., 1992, 66, 197 18. Bradbrook, S. PhD.Thesis, University of Bristol 1996 19. Labib, M.E.; Williams, R, J.Coll. andInt.Sci.,1984, 97, 356 20. Fowkes, F. et al. In Colloids and Surfaces in Reprographic Technology; Hair, M and Croucher, M.D. Ed.; ACS Symposium Series: 1982, 200 21. McGown, D.N. et al. J.Coll.andInt.Sci.,1965, 20, 650 22. Kosmulski, M.; Matijevic, M. Langmuir, 1991, 7, 2066 23. Moore, D.H. In Electrophoresis: Theory, Methods and Applications; Bier, M. Ed.; Academic Press: 1959 24. European Community Bureau of Reference, 1980, EUR 6825


A New Instrument for the Measurement of Very Small Electrophoretic

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