Structure-Property Relationship of Fumed Silica Nanostructures
The nanostructure of fumed silica is the core factor determining its physicochemical properties.  Its particle size, specific surface area, aggregation morphology, and surface chemical state collectively constitute the "structure-function" mapping relationship. The following is a systematic analysis:

1. Typical Characteristics and Characterization Parameters of Nanostructures
Structural Parameters | Fumed Silica (Gas Phase Method) | Fumed Silica (Precipitation Method) | Characterization Method
Primary Particle Size | 8–19 nm | 11–100 nm | Transmission Electron Microscopy (TEM)
Specific Surface Area | 150–400 m²/g | 60–180 m²/g | BET Nitrogen Adsorption Method
Aggregate Morphology | Chain/Branched Three-Dimensional Network | Spherical/Agglomerates | Scanning Electron Microscopy (SEM)
Surface Hydroxyl Density | 2–3 Si–OH/nm² | 5–6 Si–OH/nm² | Fourier Transform Infrared Spectroscopy (FTIR), TGA-DSC
Crystallinity | Completely Amorphous | Partially Microcrystalline (containing hydrates) | X-ray Diffraction (XRD)
Note: Fumed silica produced by the gas phase method, due to high-temperature flame hydrolysis synthesis, has smaller particles, a narrower distribution, no crystalline water, and a more uniform structure; the precipitation method, due to the chemical precipitation and drying process, easily forms a hydrated gel structure, leading to large fluctuations in specific surface area.

2. Regulation Mechanism of Nanostructure on Key Properties
① Reinforcement Performance: Interfacial Crosslinking Dominated by Specific Surface Area
Mechanism: High specific surface area (>200 m²/g) allows fumed silica to form a large number of hydrogen bonds and physical entanglements with rubber molecular chains, constructing a "nanofiller-polymer" three-dimensional network.
Supporting Data:
Fumed silica (specific surface area 300 m²/g) can increase the tensile strength of silicone rubber from 0.4 MPa to over 16 MPa, an increase of more than 40 times. Precipitated silica exhibits optimal overall rubber properties when its specific surface area is between 161–190 m²/g (Type B); while a specific surface area higher than 190 m²/g (Type A) provides stronger reinforcement, it easily adsorbs accelerators, leading to delayed vulcanization.
② Rheological properties and processability: Aggregate morphology determines dispersion behavior
Chain-like aggregates (fumed silica): Form a spatial network, significantly increasing system viscosity and thixotropy, suitable for high-viscosity rubber compounds, coatings, and sealants.
Spherical aggregates (precipitated silica): Good fluidity, but prone to sedimentation, requiring the addition of dispersants.
Dispersion bottleneck: Nanoparticles easily aggregate due to van der Waals forces, leading to uneven performance. Silane modification can reduce surface energy and improve compatibility in organic matrices.
③ Thermal and electrical properties: Structural density determines insulation and heat insulation capabilities
Thermal conductivity: The porous chain-like structure hinders phonon transport, making silica an excellent thermal insulation filler. Composite materials can have a thermal conductivity as low as <0.05 W/(m·K) at 800°C.
Electrical insulation: High purity (>99.8%) and low hydroxyl density (fumed silica) allow it to maintain high volume resistivity even at high temperatures, making it suitable for electronic packaging and cable insulation.
④ Oil absorption value and adsorption capacity: Synergistic effect of pore structure and surface activity
Oil absorption value (DBP) is positively correlated with specific surface area, reflecting its adsorption capacity for organic substances.
High oil absorption value (>200 mL/100g) is suitable for thickening inks and toothpaste, but excessive adsorption will consume additives in the formulation, requiring a balance in dosage. 3. Precise Control of Nanostructures through Preparation Processes
Process Type | Reaction Conditions | Structural Control Results | Performance Impact
Gas Phase Method | SiCl₄ + 2H₂ + O₂ → SiO₂ (1800°C flame) | Nanoscale, anhydrous, high purity, chain-like aggregates | High reinforcement, high insulation, high thermal stability
Precipitation Method | Na₂SiO₃ + H₂SO₄ → SiO₂·nH₂O (aqueous phase precipitation) | Micron-scale, hydrated, high impurity content, spherical aggregates | Low cost, good processability, but weak reinforcement, poor heat resistance
The gas phase method allows for particle size control through flame temperature and reaction time; the precipitation method controls the crystal nucleation rate through pH, temperature, and aging time, achieving specific surface area grading (Classes A–F).

4. Cutting-Edge Structural Design: From Passive Fillers to Functionalized Nanoplatforms
Core-Shell Structure: SiO₂@Al₂O₃ core-shell composites reduce volume shrinkage from 10.49% to 3.47% at 1000°C, achieving structural stabilization^[2006.12758v3]^.
Hollow Porous Structure: Mesoporous silica is constructed through template methods, increasing the specific surface area to over 800 m²/g, used for catalytic carriers and drug sustained release.
Surface Functionalization: Silane coupling agents (KH-550, KH-570) react with Si–OH to form an organosilicon layer, achieving:
Hydrophobization: Contact angle increases from <10° to >120°;
Interfacial covalent bonding: Forming Si–O–C bonds with rubber molecules, improving dynamic fatigue life.