Zeta Potential of Fumed Silica: Surface Charge, pH Dependence & Formulation Stability
Fumed silica surface charge swings from positive to strongly negative across the pH scale, making zeta potential the single most critical parameter for predicting dispersion stability in waterborne systems.
Fumed silica surfaces are populated with silanol groups (Si–OH) at a density of 2–3 OH/nm². In aqueous media these silanols undergo pH-dependent ionization: below pH 3 a small fraction accepts protons (Si–OH₂⁺), producing a weak positive charge, while above pH 3 progressive deprotonation (Si–O⁻) drives the surface strongly negative. The resulting zeta potential—measured by electrophoretic mobility—quantifies this net charge and directly predicts whether particles will repel each other or flocculate.
For hydrophilic grades such as SEMISIL 200 (BET 200 ± 25 m²/g), silanol density is at its maximum, giving the steepest charge-vs-pH curve and the strongest electrostatic stabilization above pH 5.
The isoelectric point (IEP) of unmodified fumed silica falls near pH 2–3, one of the lowest among common oxide particles. Below the IEP, zeta potential is slightly positive (+10 to +20 mV); above it, charge increases roughly linearly until leveling off around −45 to −55 mV at pH 9–10. This means most industrial waterborne systems (pH 7–9) operate well into the electrostatic stabilization regime.
Hydrophobic grades shift the IEP upward to pH 3–4 because surface methyl or dimethylsilyl groups replace ionizable silanols, reducing charge density. Formulators switching between hydrophilic and hydrophobic grades must re-evaluate pH targets accordingly.
Colloidal stability requires zeta potential magnitudes above ±30 mV as a practical minimum. At 5 wt% loading in deionized water, hydrophilic fumed silica at pH 7 typically measures −35 to −40 mV—adequate for shelf-stable thixotropic networks. Adding electrolytes (\>0.05 M NaCl) compresses the double layer and can drop the effective potential below 20 mV, causing rapid gel collapse or hard sedimentation.
For anti-settling packages in waterborne coatings (pH 8–9), maintaining ionic strength below 0.1 M and selecting high-BET grades (≥200 m²/g) maximizes electrostatic repulsion and thixotropic recovery.
Adjust system pH to 7–9 before dispersing fumed silica to exploit the strong negative charge regime and accelerate wetting. Pre-dispersion at low pH (near the IEP) causes irreversible hard agglomerates that high-shear mixing cannot fully break. Use deionized or low-conductivity water (
The table below summarizes typical zeta potential values for standard hydrophilic fumed silica (BET 200 m²/g) at 1 wt%…
The table below summarizes typical zeta potential values for standard hydrophilic fumed silica (BET 200 m²/g) at 1 wt% in deionized water, measured by laser Doppler electrophoresis at 25 °C. Values shift ±5 mV with BET surface area changes of ±50 m²/g.
| pH | Zeta Potential (mV) | Stability Rating |
|---|---|---|
| 2.0 | +15 to +20 | Unstable (near IEP) |
| 3.0 | 0 to −5 | Unstable (IEP zone) |
| 4.0 | −15 to −20 | Marginally stable |
| 5.0 | −25 to −30 | Threshold stable |
| 7.0 | −35 to −40 | Stable |
| 9.0 | −45 to −50 | Highly stable |
| 10.0 | −48 to −55 | Highly stable |
For maximum dispersion stability in waterborne systems, formulate at pH 7–9 with low ionic strength (
The isoelectric point (IEP) of unmodified hydrophilic fumed silica is near pH 2–3, meaning the net surface charge is zero at this pH. This is among the lowest IEPs of any commercial oxide filler. Above pH 3, the surface becomes progressively more negative due to silanol deprotonation.
A zeta potential magnitude above ±30 mV is the practical threshold for electrostatic stability. At pH 7 in deionized water, hydrophilic fumed silica typically reaches −35 to −40 mV, which provides adequate repulsion for shelf-stable thixotropic networks in coatings and adhesives.
Zeta potential increases in magnitude (becomes more negative) as pH rises above the IEP of ~pH 2–3. At pH 5 it reaches approximately −25 mV; at pH 7, −35 to −40 mV; and at pH 9–10, −45 to −55 mV. This is driven by progressive ionization of surface silanol groups.
Yes. Surface treatment with dimethyldichlorosilane or hexamethyldisilazane replaces ionizable silanols with non-polar groups, shifting the IEP upward to pH 3–4 and reducing the absolute zeta potential by 10–15 mV at any given pH compared to hydrophilic grades of equal BET area.
Dissolved electrolytes compress the electrical double layer surrounding each particle, reducing the effective zeta potential. Monovalent salts (NaCl) destabilize above ~0.05–0.1 M, while divalent cations (Ca²⁺) cause flocculation at concentrations as low as 0.005–0.01 M due to more efficient charge screening.
Higher BET surface area means more silanol groups per gram and a steeper charge-vs-pH curve. Grades at 200–300 m²/g (such as SEMISIL 200) provide the strongest electrostatic stabilization. Below 130 m²/g, the reduced silanol density may require steric co-stabilizers in demanding formulations.
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