Bruce B. Weiner, Walther W. Tscharnuter, David Fairhurst Brookhaven Instruments Corporation Holtsville, NY US

Transcription

1 Zeta Potential: A New Approach by Bruce B. Weiner, Walther W. Tscharnuter, David Fairhurst Brookhaven Instruments Corporation Holtsville, NY US A paper presented at the Canadian Mineral Analysts Meeting, September 8-12, 1993 held in Winnipeg, Manitoba, Canada. Bruce B. Weiner, Walther W. Tscharnuter, David Fairhurst Brookhaven Instruments Corporation, Holtsville, NY The zeta potential (ZP) is a function of the surface charge which develops when any material is placed in a liquid. It is a very good index of the magnitude of the electrostatic repulsive interaction between particles. The ZP is commonly used to predict and control dispersion stability. The characteristics of the solid-liquid interface may also influence, inter alia, adhesion, flotation and, in more concentrated systems, rheological behavior. Reliable ZP determinations are thus of practical concern to may industries including ceramics preparation, processing and application. A group of phenomena, known collectively as Electrokinetic Effects, can be exploited as the basis for determining the zeta potential (ZP). The most widely studied, and of most practical relevance to colloidal-sized paniculate suspensions, is particle electrophoresis, or microelectrophoresis, the movement of charged particles suspended in a liquid under the influence of an applied electric field. This offers the possibility of measuring the complete mobility spectrum. Over the past decade, the technique of Electrophoretic Light Scattering (ELS) has been used extensively to characterize the ZP of colloidal sized particles. ELS measurements are many times faster than conventional manual techniques. Although the basic ELS illuminating optics and signal processing appear deceptively simple, in practice, careful optimization is needed to obtain usable signals. Aside from the optical problems in either the cross beam or the reference beam configurations, an inherent limitation to obtain precise and repeatable results is the electro-osmotic effect, which up to now could only be accounted for by careful and time consuming scans across the sample cell. The transformation from electrophoretic mobility to ZP is not a trivial matter; the assumption of a simple, constant, relationship (commonly the Smoluchowski limit of the Henry equation) is not generally applicable. Of prime importance is the need to know the particle size, which can be determined rapidly by Dynamic Light Scattering (DLS) using a newly developed single board real time correlator with a dynamic range exceeding 10 decades. The present paper will address these questions and discuss recent advances in instrument design to reduce or even eliminate (in the important case of electro-osmosis) the various instrument limitations. The importance of the ZP to ceramic processes, and the factors which effect the magnitude of the ZP, are illustrated using examples from measurements of various oxides and non-oxides commonly used in ceramics formulations.

2 Attraction of colloidal particles is fundamental. It arises from a variety of dispersion forces, and it is a relatively long range force. The energy of attraction varies as 1/r 2, where r is the distance between particles. All colloidal dispersions will eventually aggregate unless there are sufficient forces to prevent adherence of particles. For particles in water or other polar liquids, the most common force of repulsion arises from charges on the particles. As a result of the charge at the surface of the particle in the liquid an electrical double layer is formed. One part of the layer consists of the charges on the particle's surface. The other is the increased concentration of counterions near the surface and the corresponding decrease of coions. These changes of ion concentrations near the surface decrease further from the surface due to the thermally driven motion of ions

3 ELECTROKINETIC PHENOMENA ARISES WHEN TWO PHASES MOVE WITH RESPECT TO EACH OTHER WITH AN ELECTRIC DOUBLE LAYER AT THE INTERFACE FOUR RELATED PHENOMENA: ALL INVOLVE RELATIVE MOVEMENT BETWEEN RIGID AND MOBILE PARTS OF THE DOUBLE LAYER AND SOLVENT CHARGE SURFACE. IONS ELECTROPHORESiS MOVEMENT OF CHARGED SURFACE STATIONARY LIQUID APPLIED ELECTRIC FIELD 2. ELECTRO-OSMOSIS MOVEMENT OF LIQUID STATIONARY CHARGED SURFACE APPLIED ELECTRIC FIELD (i.e., the complement of electrophoresis) 3. STREAMING POTENTIAL MEASURED ELECTRICAL FIELD MOVEMENT OF LIQUID - MECHANICAL STATIONARY CHARGED SURFACE (i.e., the opposite of electro-osmosis) 4. SEDIMENTATION POTENTIAL MEASURED ELECTRICAL FIELD MOVEMENT OF CHARGED PARTICLES STATIONARY LIQUID (i.e., the opposite of electrophoresis) When charged electrodes are placed in a suspension of charged particles, the particles move toward the electrode of opposite charge. The simplest arrangement for observing the movement is a Van Gils cell.

4 The electrophoretic velocity is proportional to the electric field, with the proportionality constant called the electrophoretic mobility. ELECTROPHORETIC MOBILITY Zeta potential is proportional to the electrophoretic mobility, at least in the simplest case. Zeta potential is the parameter which determines the strength of the repulsion between particles. As such, it is this quantity, not mobility, that is important. ZETA POTENTIAL In the Van Gils' cell, and all other commercially available cells, except one, the electric field lines cut across the charged walls of the cell. Electroosmotic movement of the liquid occurs. In a closed cell this forces a flow in the cell that results in two, equal counterflow patterns. The measured velocity of particles anywhere in the cell is the sum of the electroosmotic and electrophoretic velocities. The electroosmotic velocity is zero at two special planes, called stationary planes, in the cell. If the velocity is measured at these special positions, it is equal to the desired electrophoretic velocity

5 The stationary planes are very close to the cell walls. Unfortunately, electroosmotic velocity changes most rapidly around the stationary planes. The depth of field of most optical systems is large enough that averaging occurs. Thus, it is difficult to ignore the effect on the measured velocity of the electroosmotic part For accurate measurements it is necessary to verify, each time, the positions of the stationary planes (also called stationary levels). Prof. Bob Pelton at the Univeristy of Toronto has shown that large errors ensue if the levels are not verified. Due to the rapid change around the stationary planes of the electroosmotic velocity, it is particularly difficult to measure an accurate width of the electrophoretic mobility distribution. The distribution width is an indication of the distribution of zeta potential. It is an important parameter for determining the longterm stability of the dispersion.

6 Theory shows that the observed velocity distribution across the cell is parabolic, except in the case of very low mobility. In this special case, near the walls, the velocity may change direction. In any case, where electroosmosis exists, it is essential, for accurate measurements, to measure at several positions across the cell. The data are then fit to a parabola. If the fit is good, then the average electrophoretic velocity, at the stationary planes can be determined by interpolation. In recent years the technique of Electrophoretic Light Scattering (ELS) has been applied to the measurement of the velocity of charged particles in a liquid. This has the advantage of measuring a large number of particles simultaneously without the subjective and time-consuming character imposed by classical, visual observation. Unfortunately, all commercially available instruments, except the ZetaPlus, a new instrument, also suffer from the problem of electroosmosis. In an ELS instrument a focused beam of laser light passes through the cell. A part of the laser beam is diverted before it reaches the cell. This beam is combined with the scattered light. This beam is variously called the reference beam or local oscillator. Its purpose, as shown later, is to determine the sign of the charge on the particle, and, hence, the sign of the zeta potential.

7 In the ZetaPlus the electrodes are entirely within the cell. The electric field does not cut across the cell walls. As a result no electroosmosis occurs, and the velocity measured between the electrodes is only due to electrophoresis. There is no electroosmotic effect, nor any stationary planes to verify. A large number of advantages arise as a result of using this configuration. Since there are no standards to which any particle electrophoresis measurements can be compared, it is only possible to compare different techniques. Here the electrophoretic mobility of TiO, is compared as a function of ph using a classical Van Gils' cell ( Rank Mk II instrument) with visual measurements and the ZetaPlus cell with ELS measurements. The agreement is excellent, indicating the essential accuracy of the new design. It is worthwhile pointing out that each point plotted using the Van Gils' cell is the result of interpolation at the stationary plane after measurements across the cell were made. Such measurements take a long time. In contrast the ELS measurements are much faster, and no interpolation is required.

8 Measurement of the Zeta Potential of Degussa P25 Titaniom Oxide in aqueous suspension: Comparison of the BI-ZetaPlus and the Rank Mk II When particles move perpendicular to the laser beam the frequency of the scattered light is shifted. This Doppler shifted frequency contains the information on the electrophoretic mobility. Since frequency is inherently a positive quantity, it is necesssary to add a local oscillator (reference beam) in order to separate positive and negative shifts. Doppler shifts greater than the frequency of the local oscillator arise from positively charged particles. Doppler shifts less than the frequency of the local oscillator arise from negatively charged particles. The local oscillator frequency in the ZetaPlus is 250 Hz. Doppler shifts are generally less than 100 Hz The measured Doppler shift frequency is proportional to the electrophoretic velocity, the proportionality constant is a well-known function of laser wavelength, liquid refractive index, and scattering angle.

9 In addition to motion of the particle due to electrophoresis, Brownian motion is also important. The resulting diffusion of the particles broadens the measured mobility distribution. The diffusional broadening is inversely proportional to particle size and is a strong function of scattering angle: the smaller the angle, the less the diffusional broadening. In the ZetaPlus the scattering angle is only 15.

10 Zeta potential is a parameter that characterizes the stability of an electrostatically stabilized dispersion. Although zeta potential is not a direct measure of surface charge density most surfaces are too complicated for such a direct proportionality to exist it changes when the surface charge changes. Any added species which changes the surface charge will cause some change in the zeta potential. A common example occurs when the ph is changed in dispersions of oxides or other surfaces that are either acidic or basic. The ph at which the zeta potential is zero is called the isoelectric point (I.E.P.). At this ph the repulsive forces are zero, and aggregation will occur.

11 Such a list, while instructive, can be misleading. The zeta potential is determined by the surface chemistry. Even a small percent of a component, if preferentially adsorbed at the surface of the particle, will largely determine the surface charge density, the resulting zeta potential, and the stability, or lack thereof, of the dispersion. In the following examples, a commercially available TiO 2 sample, one sold as pure titanium dioxide, acts like either alumina or silica depending on which of these components is at the surface. The lesson is this. It is not the bulk properties of the material which determine stability; it is the surface properties.

12 Since small changes in surface chemistry have a large effect on dispersion stability, it is also a mistake to assume that batch-to-batch and lot-to-lot variations in will not affect dispersion. In the example shown here the I.E.P. varies by nearly two ph units, and this was the same bulk material from the same supplier tested over a five year period. It is not possible to measure zeta potential once, or rely on average properties found in the literature. It is necessary to make measurements on a continuing basis