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…
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 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.
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.
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.
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.
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 |
When evaluating fumed silica, always specify BET surface area and dispersion protocol — a single ‘particle size’ number without structural context leads to mismatched grade selection and wasted reformulation cycles.
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.
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.
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.
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.
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.
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.
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