Silica (precipitated silica) is an important inorganic chemical product. Its unique physical and chemical properties are determined by its chemical composition and crystal structure. The following is a detailed analysis of its molecular structure, crystal structure, surface hydroxyl characteristics, and their impact on performance.

I. Molecular Structure of Silica
The molecular structure of silica (SiO₂) has the following characteristics:
Basic Unit: The silicon atom is located at the center of a regular tetrahedron, and four oxygen atoms are located at the four corners of the tetrahedron, forming the basic structural unit [SiO₄].
Bonding Method: Silicon atoms and oxygen atoms are bonded via covalent bonds, with each oxygen atom linked to two silicon atoms, forming a three-dimensional network structure.
Chemical Formula: SiO₂ is the simplest formula, representing the ratio of silicon and oxygen atoms in the crystal, not a single molecular formula.
Bonding Characteristics: Covalent bonds are dominant at room temperature, with the proportion of ionic bonds increasing with increasing temperature.

II. Crystal Structure Characteristics of Silica
Silica is primarily amorphous silica. Its crystal structure characteristics include:
Structural Type: Amorphous material in which silicon and oxygen atoms are arranged in either short-range order or long-range disorder.

Difference from crystalline state:
The angle of the Si-O-Si bond bridge in amorphous SiO₂ is not fixed (1/2). 10-180°), peak at 144°
Low density (2.15-2.25 g/cm³), no fixed melting point
Loose and uneven network structure, with irregular pores

Oxygen atom states:
Bridging oxygen: connects two silicon atoms
Non-bridging oxygen: connects only one silicon atom

III. Surface Hydroxyl Properties and Formation Mechanisms
Silica has a large number of silanol groups (-SiOH) on its surface, which exhibit the following characteristics:
Forming mechanism: Unpaired oxygen atoms in the amorphous structure combine with hydrogen to form hydroxyl groups

Chemical activity:
Can form hydrogen bonds or react with other groups
Easily oxidizes at room temperature, forming a protective oxide layer

Influence of hydroxyl content:
High hydroxyl content increases the material's hydrophilicity but reduces its heat resistance.
In the rubber industry, hydroxyl groups react with epoxy groups, affecting processing properties.

IV. Mechanisms of Structural Properties Affecting Properties
Silica's structural properties influence its application properties in many ways:

1. Physical Properties
Porous Structure: Provides high surface area and porosity, enhancing adsorption capacity
Particle Size Distribution: Nanoparticles (10-100 nm) provide excellent reinforcement

2. Chemical Properties
Surface Hydroxyl Groups: Promote chemical bonding with polymers, increasing composite strength
Modification with silane coupling agents improves dispersibility in rubber
Chemical Stability: Resistant to acids and alkalis (except HF), suitable for a variety of environments

3. Application Performance
Rubber Industry:
Improving tire wear resistance, fracture resistance, and aging resistance
Optimizing interfacial bonding with rubber through surface modification

Other Applications:
Used as a rheology control agent in coatings
Used as an abrasive and thickener in toothpaste
Utilizing its high insulating properties in electronic materials

Summary
The excellent performance of silica (amorphous silica) stems from its unique chemical composition and structural properties:
The three-dimensional molecular network provides a basic framework
The amorphous crystal structure creates porosity
Surface hydroxyl groups impart reactivity
These structural properties together determine its key application value in multiple industrial sectors.
Through surface modification and process control, the structural properties of silica can be further optimized to meet the performance requirements of different application scenarios.