2026-04-29
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Fumed Silica Surface Chemistry: Hydrophilic vs Hydrophobic Grades Explained The surface chemistry of fumed silica determines everything from dispersibility in water-based systems to compatibility with non-polar resins. This guide walks through silanol group chemistry, common modification agents, and how to match the right grade to your application.
Fumed silica is produced by flame hydrolysis of silicon tetrachloride at over 1000 °C. The result is a…
Fumed silica is produced by flame hydrolysis of silicon tetrachloride at over 1000 °C. The result is a three-dimensional network of amorphous SiO₂ with an extremely high surface area (90–400 m²/g). The key to its versatility lies not in bulk composition — all grades are pure SiO₂ — but in the chemical nature of the surface.
Two families of grades dominate industrial use:
Native silanol (Si–OH) surface. Disperses readily in water and polar solvents. Absorbs moisture. Compatible with water-based coatings, polar adhesives, and aqueous systems.
Silanols replaced with organosilane or silicone groups. Repels water. Compatible with non-polar resins, oils, and hydrophobic polymer systems. Lower moisture pickup.
Key insight: Choosing the wrong surface type causes dispersion failure. A hydrophilic grade in a silicone oil system will form hard agglomerates; a hydrophobic grade in a water-based coating will float or cluster rather than dispersing uniformly.
Fresh fumed silica carries approximately 2–3 silanol groups per nm² of surface. These Si–OH groups are responsible for…
Fresh fumed silica carries approximately 2–3 silanol groups per nm² of surface. These Si–OH groups are responsible for the strong thickening and thixotropic behavior of hydrophilic grades through hydrogen bonding — both between silica particles and between silica and polar solvent molecules.
Single Si–OH not adjacent to another. Most reactive with modification agents. High hydrogen-bonding potential.
Two Si–OH groups on neighboring Si atoms, bridged by hydrogen bonds. Common on lower surface area grades.
Two Si–OH groups on the same Si atom. Less common; reduced reactivity toward silane coupling agents.
The density and distribution of silanols varies with surface area — higher BET grades (e.g., Aerosil 380 at 380 m²/g) carry more silanols per gram than lower BET grades (e.g., Aerosil 90 at 90 m²/g), resulting in stronger inter-particle networking and higher viscosity build at equivalent loading.
Post-synthesis surface treatment reacts the reactive silanol groups with organosilane or silicone reagents, replacing…
Post-synthesis surface treatment reacts the reactive silanol groups with organosilane or silicone reagents, replacing polar Si–OH with non-polar organic groups. The degree of substitution — typically 40–80% of available silanols — determines the balance between hydrophobicity and residual polar character.
| Agent | Group Introduced | Resulting Grade Example | Best For |
|---|---|---|---|
| HMDS (hexamethyldisilazane) | –Si(CH₃)₃ (trimethylsilyl) | Aerosil R972 | Silicone systems, non-polar coatings, powder flow |
| DDS (dimethyldichlorosilane) | –Si(CH₃)₂ (dimethylsilyl) | Aerosil R974 | PVC plastisols, non-polar resins, epoxy |
| PDMS (polydimethylsiloxane) | Silicone polymer chains | Aerosil R202 | Silicone rubber, silicone sealants, HTV compounds |
| Octylsilane | –C₈H₁₇ (octyl) | Aerosil R805 | Low-polarity solvent coatings, alkyd systems |
| Aminosilane | –NH₂ (amino) | Various grades | Epoxy adhesives, reactive coupling |
Note on partial treatment: Most hydrophobic grades retain 20–60% of original silanols unreacted. This gives them an amphoteric character — they are water-repellent but not completely non-polar. Always verify the carbon content specification (wt% C) to assess the degree of surface coverage.
The following table compares representative grades across the hydrophilic–hydrophobic spectrum. Aerosil (Evonik) grades are used as reference; equivalent grades from Cabot (CAB-O-SIL), Wacker (HDK), and OCI/Tokuyama follow the same chemistry.
| Grade | BET (m²/g) | Surface Type | pH (4% disp.) | Carbon Content | Typical Use |
|---|---|---|---|---|---|
| Aerosil 90 | 90 ± 15 | Hydrophilic | 3.7–4.7 | — | Powder flow, mild thickening |
| Aerosil 200 | 200 ± 25 | Hydrophilic | 3.7–4.7 | — | General thickening, coatings, adhesives |
| Aerosil 380 | 380 ± 30 | Hydrophilic | 3.7–4.7 | — | High-viscosity systems, waterborne coatings |
| Aerosil R972 | 110 ± 20 | Hydrophobic (HMDS) | — | 0.6–1.2 wt% | Silicone sealants, non-polar coatings, flow |
| Aerosil R974 | 170 ± 20 | Hydrophobic (DDS) | — | 0.7–1.3 wt% | Epoxy, PVC plastisol, non-polar resins |
| Aerosil R202 | 100 ± 20 | Hydrophobic (PDMS) | — | 4.0–8.0 wt% | HTV silicone rubber, silicone compounds |
| Aerosil R805 | 150 ± 25 | Hydrophobic (octyl) | — | 3.5–5.5 wt% | Alkyd coatings, solvent-borne systems |
Highlighted rows are most widely used in general industrial formulation.
Use this quick-reference to select between hydrophilic and hydrophobic based on system polarity: - Water-based coatings…
Use this quick-reference to select between hydrophilic and hydrophobic based on system polarity:
Water-based coatings & paints: Hydrophilic (Aerosil 200 / 380) — high thickening efficiency, stable in aqueous dispersion.
Solvent-borne coatings (polar solvents: MEK, acetone): Hydrophilic (Aerosil 200) or mildly hydrophobic (R974) — test for compatibility at 10% solids.
Solvent-borne coatings (non-polar: xylene, mineral spirits): Hydrophobic (R805, R972) — hydrophilic grades will not wet and will agglomerate.
Epoxy adhesives & sealants: Hydrophobic (R974, R972) — prevents moisture absorption that degrades epoxy cure.
Silicone rubber (HTV/RTV): Hydrophobic PDMS-treated (R202, HDK H20) — PDMS chains entangle with silicone polymer for reinforcement.
Polyurethane sealants: Hydrophobic (R972, R974) — avoids moisture reaction with isocyanate groups.
Pharmaceutical powders & free-flow: Hydrophilic (Aerosil 200) — food-grade/pharma-grade available; improves powder flow.
Cosmetics / personal care: Both — hydrophilic for aqueous gels, hydrophobic (R972) for anhydrous products and color cosmetics.
Rule of thumb: If your system contains more than 10% water or polar solvent (ethanol, IPA, acetone), start with a hydrophilic grade. For non-polar systems or when moisture sensitivity is a concern, use a hydrophobic grade.
Tell us your base resin, solvent system, and target viscosity — we’ll identify the right fumed silica grade for your formulation.
Generally no. Hydrophilic fumed silica has a polar, silanol-rich surface that requires a polar environment to disperse properly. In non-polar solvents (mineral spirits, xylene, silicone oil), the particles will not wet and will form hard agglomerates. A hydrophobic grade with an appropriate surface treatment (octyl, HMDS, PDMS) is required for non-polar systems.
Carbon content (wt% C) measures the amount of organic surface treatment on hydrophobic grades. Higher carbon content means more complete silanol substitution and greater hydrophobicity. For example, Aerosil R202 (PDMS-treated) has 4–8 wt% C, while R972 (HMDS-treated) has only 0.6–1.2 wt% C — R202 is significantly more hydrophobic.
Yes, but the mechanism differs. Hydrophilic grades form hydrogen-bond networks. Hydrophobic grades build structure through weaker van der Waals interactions, resulting in softer, more shear-reversible thixotropy. Dosage requirements are typically 20–40% higher for hydrophobic grades to achieve equivalent viscosity to a hydrophilic grade in compatible systems.
The simplest test is the methanol wettability test (MWN): mix the silica with methanol/water solutions of increasing methanol concentration. The MWN value (% methanol at which the silica begins to wet) is a standard QC parameter — hydrophilic grades wet at 0% methanol; fully hydrophobic grades may require 50–70% methanol to wet. Most supplier data sheets report this value.
Yes. Blending the two can tune surface polarity for intermediate systems. A common approach is 70% hydrophilic + 30% hydrophobic to get improved moisture resistance in a waterborne coating while retaining high thickening efficiency. Pre-disperse each grade separately then combine for best results.
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