the isoelectric point
For an oxide powder in water, the surface charge is not fixed — it depends on the acidity of the water around it. Make the water more acidic and the surface picks up protons and goes positive; make it more basic and it gives up protons and goes negative. Somewhere in between there is one particular pH where the positive and negative just cancel and the particle carries no net charge at all. That crossover pH is the isoelectric point (IEP) — the single worst pH to try to disperse a slurry, because with no charge there is no electrostatic repulsion and the particles flocculate.
It is a fingerprint of the material. Silica has a very low isoelectric point, around pH 2 to 3, so it is negatively charged and disperses across almost the whole practical pH range. Alumina sits high, around pH 8 to 9; titania and zirconia land in the middle, near pH 6. At the isoelectric point the zeta potential reads zero, the DLVO energy barrier drops away, viscosity and yield stress shoot up as a particle network forms, and a slurry that poured freely a moment ago turns into a lump. So the golden rule of formulating a water-based slip is: find the isoelectric point, then work well away from it (at least a pH unit or two) where the charge — and the repulsion — is strong.
The isoelectric point is also a tool, not just a hazard. Sometimes you deliberately push a slurry toward its isoelectric point to make it flocculate on purpose — for instance to build a yield stress that holds shape in gel casting, or to consolidate a body quickly. A dispersant often works by shifting the effective isoelectric point: adsorb a negatively charged polymer onto alumina and its zero-charge pH slides far to the acid side, so the powder now stays negatively charged and dispersed across a much wider range. One caution: the pure-oxide isoelectric point can move once dispersants, impurities, or dissolved ions are present, so always measure it for your actual system rather than trusting a textbook value.
You want to slip-cast pure alumina in water. Its isoelectric point is near pH 9, so you deliberately acidify the slip to about pH 4 — well below the IEP — where every particle carries a strong positive charge, the zeta potential is high, and the slurry is fluid and stable. Cast at pH 9 by mistake and the batch would flocculate into an uncastable lump.
The isoelectric point is the pH of zero net charge — the pH to avoid when you want a dispersed, castable slip.
The textbook isoelectric point is for the clean oxide in water; adsorbing a dispersant, or dissolved impurity ions, can shift it by several pH units. Measure the isoelectric point of your real slurry — do not assume the handbook number applies.