Particle Size and Aggregate Structure of Fumed Silica
Primary particles of 7-40 nm fuse into fractal aggregates of 100-300 nm that determine every performance property — reporting a single particle size is technically misleading.
Primary Particle Formation in the Flame
Primary particles form when SiCl₄ hydrolyzes in a hydrogen-oxygen flame above 1800 °C. Residence time and flame temperature control the final diameter: shorter residence yields smaller particles (7-10 nm, ~380 m²/g BET) while longer exposure allows growth to 20-40 nm (~50-150 m²/g). These amorphous spheres are the fundamental building block — but they never exist individually in any commercial product.
- SEMISIL 380 grade — 7 nm primary particles, 380 m²/g BET, maximum thickening efficiency per unit weight
- SEMISIL 200 grade — 12 nm primary particles, 200 m²/g BET, balanced viscosity build with easier dispersion
- Low-surface-area grades — 20-40 nm primaries, 50-150 m²/g, used where transparency or low thickening is required
Aggregates vs Agglomerates: The Critical Distinction
Fumed silica has a three-level hierarchy that formulators must understand. Primary particles (7-40 nm) sinter together in the flame to form aggregates of 100-300 nm — these are permanently fused and cannot be broken by shear. Aggregates then loosely cluster via hydrogen bonding into agglomerates of 1-40 µm. Agglomerates are what you receive in the bag; aggregates are what you measure after proper dispersion. Laser diffraction on a dry powder reports agglomerate size — not the functional unit.
- Aggregates (100-300 nm) — Permanently sintered, fractal branching, the true functional unit in liquid systems
- Agglomerates (1-40 µm) — Loosely H-bonded clusters, broken down during dispersion, size depends on shear input
- Why it matters — Thixotropy comes from aggregate networking, not agglomerate size — under-dispersed product wastes material
How Aggregate Structure Drives Application Performance
The fractal, chain-like shape of fumed silica aggregates creates a percolation network in liquid media at loading levels as low as 1-3 wt%. Higher BET grades (smaller primaries) produce more branched aggregates with greater chain length, building viscosity faster. In reinforcement applications, aggregate morphology determines tensile strength gain in silicone rubber — branched aggregates interlock with polymer chains more effectively than compact clusters.
- Thixotropy and anti-settling — Branched aggregates form hydrogen-bonded networks at rest; shear breaks bonds reversibly, giving controlled flow
- Reinforcement in elastomers — High-surface-area aggregates (300-400 m²/g) increase tensile strength 3-5× in silicone compounds
- Surface treatment effect — Hydrophobic treatment (DDS, HMDS) caps silanols, reducing H-bonding but improving dispersion in non-polar systems
Measuring Particle Size Correctly
No single technique captures all three structural levels. BET nitrogen adsorption gives primary particle surface area — the most reliable proxy for primary size. Dynamic light scattering (DLS) on dilute dispersions reports aggregate size (100-300 nm), but only after proper sonication. Laser diffraction of dry powder measures agglomerates (1-40 µm), which is useful for incoming QC but tells you nothing about in-use performance. Always request BET data alongside any particle size claim.
- BET adsorption — Gold standard for primary particle characterization; directly correlates to thickening efficiency
- DLS / photon correlation — Reports hydrodynamic aggregate diameter after dispersion; sample prep is critical
- Laser diffraction — Measures agglomerate size; useful for powder flowability QC, misleading for performance prediction
Grade Selection by Particle Size and Surface Area
Selecting a fumed silica grade starts with matching BET surface area to your performance target. The table below maps common grades to their structural parameters and primary applications.
| Grade | Primary Particle (nm) | BET (m²/g) | Aggregate Size (nm) | Primary Applications |
|---|---|---|---|---|
| SEMISIL 380 | 7 | 380 ± 30 | 100-150 | Maximum thixotropy, anti-settling in low-polarity systems |
| SEMISIL 200 | 12 | 200 ± 25 | 150-200 | General-purpose thickening, adhesives, coatings |
| 150 m²/g type | 14-16 | 150 ± 20 | 180-220 | Silicone reinforcement, moderate viscosity build |
| 50 m²/g type | 30-40 | 50 ± 15 | 250-300 | Low thickening, improved transparency, free-flow aid |
FAQ
What is the primary particle size of fumed silica?
Primary particles range from 7 to 40 nm depending on manufacturing conditions. Flame temperature and residence time determine the diameter — higher surface area grades like 380 m²/g have ~7 nm primaries, while 50 m²/g grades have 30-40 nm primaries. These particles are amorphous silica spheres formed by hydrolysis of SiCl₄.
Why is reporting a single particle size misleading for fumed silica?
Fumed silica exists as a three-level hierarchy: primary particles (7-40 nm), aggregates (100-300 nm), and agglomerates (1-40 µm). Each level affects different properties. A single number fails to capture whether the measurement refers to the fused aggregate or the loose agglomerate, leading to incorrect grade comparisons.
What is the difference between fumed silica aggregates and agglomerates?
Aggregates are permanently sintered chains of primary particles, typically 100-300 nm, that cannot be broken by mechanical shear. Agglomerates are loose clusters of aggregates held together by hydrogen bonds, ranging 1-40 µm, which break apart during dispersion. Only aggregates determine in-use performance.
How does BET surface area relate to fumed silica particle size?
BET surface area is inversely proportional to primary particle diameter. A 380 m²/g grade has ~7 nm primaries, while a 200 m²/g grade has ~12 nm primaries. BET is the most reliable and reproducible measurement for comparing fumed silica grades because it directly reflects primary particle geometry.
What particle size measurement should I request from suppliers?
Request BET surface area as the primary specification — it is standardized (ISO 9277) and directly correlates to thickening performance. If aggregate size data is available via DLS, request the dispersion protocol used. Avoid relying on laser diffraction numbers alone, as these measure agglomerates and vary with powder handling.
How does aggregate structure affect thixotropy in coatings?
Branched, high-surface-area aggregates form hydrogen-bonded networks throughout the liquid at 1-3 wt% loading. At rest, these networks resist flow and prevent settling. Under shear, hydrogen bonds break reversibly, allowing controlled application. Higher BET grades build thixotropy more efficiently per unit weight.