Precipitated silica is a white, powdery, X-ray amorphous silicic acid and silicate product, primarily composed of silicon dioxide (SiO₂). Its unique physical properties and microstructure give it broad application value in fields such as rubber, cosmetics, electronics, and environmental protection. I. Basic Physical Properties
1. Appearance and Density
Appearance: White amorphous powder or flocculent, light and loose
Density: True density is approximately 2.0 g/mL, apparent density is approximately 0.2 g/mL, indicating a porous structure.
Melting point: Up to 1610-1750°C, showing excellent thermal stability.
2. Solubility and Stability
Solubility: Insoluble in water, solvents, and acids (except hydrofluoric acid), but soluble in caustic alkalis and hydrofluoric acid.
Stability: High temperature resistance, non-flammable, odorless, and tasteless, with excellent chemical stability and electrical insulation.
pH value: Usually between 5.0-8.0, weakly acidic.
3. Specific Surface Area and Porosity
Specific surface area: Varies depending on the application, generally 145-300 m²/g, much higher than traditional carbon black.
Pore structure: Possesses abundant microporous and mesoporous structures with a large pore volume, which gives precipitated silica excellent adsorption capacity.
DBP absorption value: 2.00-3.50 cm³/g, indicating good oil absorption performance.

II. Microstructural Characteristics
1. Molecular Composition and Structure
Chemical formula: Usually expressed as SiO₂·nH₂O, where nH₂O exists in the form of surface hydroxyl groups.
Hydroxyl group types: Three main hydroxyl groups exist on the surface—isolated hydroxyl groups, adjacent hydroxyl groups, and siloxane groups, among which adjacent hydroxyl groups are crucial for the adsorption of polar substances.
Crystal structure: Amorphous structure, more chemically active compared to crystalline silicon dioxide.
2. Particle Morphology and Size
Primary particle size: Usually between 10-40 nanometers, belonging to nanoscale materials.
Particle shape: Mainly spherical, but also rod-shaped and flake-shaped; the specific morphology can be controlled by the preparation process.
Aggregation state: Primary particles are in face-to-face contact, forming a chain-like connection structure (secondary structure), forming loose aggregates.
3. Surface Characteristics
Surface activity: Due to the large number of hydroxyl groups on its surface, precipitated silica has high surface activity and hydrophilicity.
Surface modification: Its surface properties can be altered through chemical treatment to produce hydrophobic or hydrophilic precipitated silica, adapting to different application needs.
Surface charge: It carries a charge in solution, giving it good dispersibility and stability.

III. Preparation Methods and Structural Relationships
1. Gas Phase Method
Process characteristics: Produced by the vaporization of silicon tetrachloride at high temperatures and reaction with hydrogen and oxygen.
Structural characteristics: The product is an amorphous, flocculent, translucent solid colloidal nanoparticle with a small particle size (10-20 nm) and a large specific surface area (100-400 m²/g).
Purity: Can reach 99%, but the preparation process is complex and expensive.
2. Precipitation Method
Process characteristics: Produced by the reaction of water glass with acid to form silicic acid, followed by decomposition.
Structural characteristics: The average particle size of the original particles is 17-28 nm, and the specific surface area is 50-250 m²/g.
Applications: Mainly used as a reinforcing agent for natural and synthetic rubber, and as an abrasive in toothpaste.

IV. Physical Properties and Application Relevance
1. High Specific Surface Area and Adsorption Performance
The high specific surface area and abundant pore structure of precipitated silica give it excellent adsorption capacity, enabling it to effectively adsorb gases, solutions, and organic substances. This characteristic makes it useful as an adsorbent in cosmetics to remove skin impurities, in environmental protection for wastewater treatment, and in the pharmaceutical field for drug storage and release. 2. Optical Properties and Applications
Light Scattering Performance: Silica effectively scatters light, making it widely used in white and translucent materials.
Refractive Index: n = 1.544, giving it important applications in the field of optical materials.
Applications: Used as a whitening agent and optical filler in coatings, plastics, and cosmetics.
3. Electrical and Thermal Properties
Electrical Insulation: Possesses excellent electrical insulation properties, widely used as insulation material in electronic devices.
Thermal Conductivity: Has low thermal conductivity and high thermal insulation performance, widely used in thermal insulation materials.
Thermal Stability: Does not decompose at high temperatures, allowing it to maintain stable performance in high-temperature environments.

V. Relationship between Structure and Performance
The microstructure of silica directly determines its macroscopic properties. Its nanoscale particle size and high specific surface area endow it with excellent adsorption and dispersion properties; the presence of surface hydroxyl groups gives it good hydrophilicity and chemical activity; and the porous structure provides a huge internal space, enhancing its adsorption and storage capacity. These characteristics enable silica to significantly improve the strength, wear resistance, and weather resistance of products in the rubber industry; to act as an adsorbent and thickener in cosmetics; to serve as an insulating material and conductive additive in the electronics industry; and to be used as an adsorbent in environmental protection for wastewater treatment and air purification.
Research on the physical properties and microstructure of silica is constantly deepening. By adjusting its preparation process and surface modification, its performance can be further optimized and its application fields expanded, providing more possibilities for materials science and industrial applications.