Introduction

The flow of particles in suspension is affected to a great extent by their electrostatic surface charges.  Like-charged particles repel.  Unlike-charged particles attract.  It is well known that suspensions can be flocculated or deflocculated by controlling the electrostatic surface charges of the suspended powders.  High surface charges (either positive or negative values) cause deflocculation.  Low surface charges (zero or near-zero positive or negative values) will cause flocculation.

The initial way to control surface charges on particles is to control suspension pH.  A knowledge of the isoelectric point of each species of suspended powder plus knowledge (or control) of suspension pH allows this to be accomplished.  Each particle will initially have a surface charge that is positive, neutral, or negative, simply because the particle is suspended in water and the suspension has a pH that is below, at, or above (respectively) the isoelectric point of that particle. 

In this article, we will discuss IsoElectric Point (IEP), zeta potential, and a way to measure IEP without the need for a zeta potential analyzer.

Iso-Electric Point

Each different particle species has a unique isoelectric point, IEP, which is defined as the pH at which the suspended particle has zero electrostatic surface charge.  For example, silica's IEP is near pH 2.  Alumina's, by contrast, is in the vicinity of pH 9-9.5.  Each powder has an IEP that varies with its composition -- and particularly, as a function of the powder's dominant cation(s).

Although each powder has a unique IEP, all particles behave similarly at pH values above and below their own IEPs.  At pHs less than a particle's IEP, the particle will carry a positive electrostatic charge.  At pHs greater than a particle's IEP, the particle will carry a negative electrostatic charge.  As one adjusts pH from a powder's IEP toward more acidic values, the excess concentration of H⁺ ions produces positive electrostatic surface charges.  As one adjusts pH from a powder's IEP toward more basic values, the excess concentration of OH^- ions produces negative electrostatic surface charges.

Practical Effects of IEP

What practical effect does this have on suspensions?  Consider a suspension containing a mixture of silica and alumina powders.  If the pH of the suspension is approximately neutral (pH ~ 7), all silica particles will have net negative electrostatic surface charges.  All alumina particles, however, will have net positive electrostatic surface charges.  The reason for this is pH 7 is more basic than the IEP of silica (pH ~ 2), but more acidic than the IEP of alumina (pH ~ 9).  As a result, at pH ~7 the silica and alumina particles will have opposite charges and will be attracted to each other, but they will repel other particles of their particular type.  That is, silica particles will repel silica particles; alumina particles will repel alumina particles; but silica particles and alumina particles will attract.  The nature and strength of the attractive and repulsive forces will be functions of the particle sizes, the surface areas, the composition of the mixture, and the magnitudes of the electrostatic surface charges of both the silica and alumina particles. 

If one of the two constituents is colloidally sized while the other consists mostly of larger particles, at pH ~7 the colloidal particles will coat the larger particles.  If both types of particles are relatively large, the attractive forces between the particles will pull them together with strengths that will vary with particle size, surface areas, and magnitude of electrostatic surface charge.

Another example of a practical affect involves the dispersion of soluble inorganic cations in a powder suspension.  If a cation is to be dispersed in a suspension, it would be helpful to do so at a pH below the particles' IEP where the electrostatic surface charges are positive.  Since particles will be positively charged below their IEP, they will naturally repel the approach of cations to their surfaces.  This will make it relatively easy to achieve an excellent homogeneous dispersion of the cation throughout the suspension.  When the desired level of homogeneity is achieved, the pH of the whole suspension can be titrated to a higher value (i.e., above the IEP) where the surface charges will change to negative values.  Once this occurs, the cations will be attracted to and adsorbed by the negative surfaces -- preserving the level of homogeneity already achieved in the mixture.  It is easier to titrate and change the pH of a suspension while preserving the quality of mixture, than to attempt to disperse cations throughout a suspension populated by negatively charged particles.

Chemical Additives

Electrostatic surface charges of powders can be altered by a wide variety of suspension additives.  As we demonstrated in the PPC textbook (Funk and Dinger, Predictive Process Control, ...), very low concentrations of anionic polyelectrolytes changed the net surface charges of alumina particles from positive to negative.  The organic additives coated the surfaces of the alumina particles, masked the positive alumina surface charges, and caused the particles to behave as if they were charged particles of additive. 

The concept of IEP, however, refers to clean particle surfaces -- clean of additive chemicals of any type.  To measure IEP values, therefore, one must have pure, clean powder surfaces that are NOT contaminated by any additive chemicals.  Inorganic salts as well as organic additives all hinder measurements of IEP.

Zeta Potential

The standard way to measure electrostatic surface charge is to measure a particle's zeta potential, ζ.  Zeta potential is defined as the electrostatic potential at the shear plane of a particle.  The shear plane is a small distance from the particle surface.  It is the distance at which the attached fluid layer can be sheared away as the particle moves through bulk fluid.  The layer of fluid inside the shear plane always travels with the particle.  All fluid outside the shear plane, however, shears away during particle movement and reforms when movement of the particle ceases.  The zeta potential is the value of the electrostatic potential at this distance from the particle surface. 

Zeta potential has typical values from +100mV through 0mV to -100mV.  Highly deflocculated values are associated with zeta potentials with absolute values greater than ~60mV.  That is, zeta potentials from +60mV to +100mV and from -60mV to -100mV are indicative of highly deflocculated suspensions.  Flocculation usually occurs when zeta potentials are zero or near zero -- that is, between +10mV and -10mV.

Typically, to measure zeta potential one actually measures electrophoretic mobility (EPM).  The EPM of a particle is essentially proportional to zeta potential.  So by measuring EPM, one can determine zeta potential.  To measure EPM, one subjects particles to a constant electric field and then measures the velocity of the particles traveling in that field.  The velocity of the particles per strength of electric field defines the EPM.  Zeta potential is then calculated from EPM.

Zeta potential analyzers tend to be expensive instruments which are not required for 24/7 operations of typical ceramic production plants.  Every once in a while, a ceramic engineer may want to know (or will need to know) the effective surface charge of suspended particles or the IEPs of raw material ingredients.  Apart from buying a zeta potential analyzer, or sending samples out to be analyzed during those times when the IEP would be helpful, how can one measure IEP?   That is covered in the next section.

IEP Measurements

This simple method to measure isoelectric points (IEPs) uses test tubes and a test tube rack.  Mix a sample of fine powder with distilled water in a relatively large beaker.  Split this into eleven small sample beakers that can each hold enough suspension to fill a test tube.  Using a pH meter or pH paper, measure the pH of the original solution.  Then, using nitric acid, titrate the first small beaker to the next lower integer pH value below the initial pH.  When the desired pH is achieved, stir the suspension well, and fill one of the test tubes with it.  Label this test tube and place it in the rack.  Follow this same procedure to produce samples with all other integer pH values down to pH 2.  When completed, the left end of the rack should contain samples that are pH 2, 3, 4, 5, 6, and 7, for example.  Repeat this same process with the other beakers by titrating with potassium hydroxide to raise the pH to achieve all integer pH values up to pH 12.  When completed, the rack will then contain eleven samples at all pH values (left to right) from pH 2 to pH 12.  The pH of the initial sample (powder plus distilled water) will vary with each powder, so the number of samples adjusted with acid and the number adjusted with base will vary in each case. 

Allow the test tubes to sit quiescent over night or over the weekend.  Then, examine the results.  The powder at the IEP will be flocculated and settled, and the fluid above the sediment will be perfectly clear.  This sample should have a distinct sediment layer with a perfectly clear supernatant liquid.  Samples on either side of this sample (moving out towards the pH 2 and pH 12 samples) will be more and more well dispersed and cloudy with no distinct lines between sediment and clear supernatant fluid.  The key to identifying the pH at the IEP is that the suspension will be flocculated.  At both higher and lower pH values away from the IEP, the suspension will be more and more deflocculated, more and more cloudy, more and more well dispersed, and less and less settled. 

If necessary, after this first test, a second test can also be performed in which the pH increment between test tubes is set to 0.2.  The center test tube in the rack should contain suspension at the pH identified as the IEP from the first test.  Test tubes to either side of this first one should be set to increments of 0.2 pH from it.  This will allow identification of the IEP to the nearest 0.2 pH.

Summary

Isoelectric point is a fundamental characteristic of each ceramic powder ingredient.  Knowing the IEP and the pH of the body slip during processing, one can predict the behavior of each powder.

The IEP measurement method described in this article is an easy, inexpensive way to identify the isoelectric point of powder samples without the necessity for an expensive zeta potential analyzer.  All that is needed is some time, a rack full of test tubes, acid and base solutions, and a pH meter or pH paper.

Miscellany

Suggested topics for future issues of this E-zine .... Please continue to send your ideas or questions for future topics.  Thanks.  Until next time ...

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Copyright © 2006  Dennis R Dinger

103 Augusta Rd, Clemson, SC 29631   (864) 654-5731

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