A suspension with a zeta potential of -20 mV would be classified as stable or unstable?

• A. Stable
• B. Unstable
• C. Moderately stable
• D. Not enough data

Answer: (B)

Zeta potential reflects how strongly charged particles in a suspension repel each other. Strong repulsion (large magnitude of zeta potential) tends to keep particles apart and the dispersion stable. In practice, a high magnitude, often cited as above about 30 mV (positive or negative), indicates good electrostatic stabilization. When the magnitude is smaller, the electrostatic repulsion is weak and van der Waals attractions can dominate, leading to particle aggregation and an unstable suspension.

With a zeta potential of −20 mV, the magnitude is only 20 mV, which is below the typical threshold for robust electrostatic stabilization. The negative sign just shows the charge direction on the particles; stability depends on how large that magnitude is. Therefore, this value indicates insufficient repulsion to prevent aggregation, making the suspension unstable.