Precipitated silica (chemical formula: SiO₂·nH₂O) is an amorphous silicon dioxide material with characteristics such as high specific surface area, porous structure, and abundant surface hydroxyl groups. Its chemical properties are mainly determined by the surface hydroxyl functional groups. It can undergo physical adsorption and chemical reactions with various substances and is widely used in rubber, catalysts, coatings, and other fields.
I. Chemical Characteristics of Precipitated Silica
1. Basic Composition and Structure
Chemical composition: Precipitated silica is a porous substance, and its composition can be represented as SiO₂·nH₂O, where nH₂O exists in the form of surface hydroxyl groups.
Microstructure: Its macroscopic structure is similar to carbon black. The particles are spherical, and the individual particles are in face-to-face contact, forming a chain-like connection structure (secondary structure). The chain branches interact through hydrogen bonds, forming aggregates.
Types of surface hydroxyl groups: Three main types of hydroxyl groups exist on the surface of precipitated silica particles:
Isolated hydroxyl groups: Contain positively charged H atoms, which easily form hydrogen bonds with negatively charged atoms.
Adjacent hydroxyl groups: Very important for the adsorption of polar substances.
Siloxane groups: Exist on the surface of dehydrated precipitated silica and are not easily removed at elevated temperatures.
2. Physicochemical Properties
Solubility: Soluble in caustic alkalis and hydrofluoric acid, insoluble in water, solvents, and acids (except hydrofluoric acid).
Electrical properties: Has excellent electrical insulation and high-temperature resistance (melting point approximately 1610℃).
Specific surface area: Different applications have different ranges, generally 145-165 m²/g (HT2), 165-185 m²/g (HT1), and 200-300 m²/g (HT3).
Acidity/Alkalinity: Fumed silica is acidic (pH value approximately 6), while precipitated silica is alkaline.
Water absorption: The surface hydroxyl groups give it strong hydrophilicity. The original particles are extremely fine, and after absorbing moisture from the air, they form aggregated fine particles. 3. Classification of Silica
By production method:
Fumed silica: Under normal conditions, it is a white amorphous flocculent translucent solid colloidal nanoparticle (particle size less than 100 nm), with a purity of up to 99%, a particle size of 10-20 nm, and a specific surface area of 100-400 m²/g. It is expensive.
Precipitated silica: Includes traditional precipitation method (using sulfuric acid, hydrochloric acid, CO₂, and water glass as raw materials) and special precipitation methods (using ultra-gravity technology, sol-gel method, etc.).
By surface properties:
Hydrophilic silica: The surface contains a large number of hydroxyl groups, which easily adsorb water molecules.
Hydrophobic silica: After surface modification treatment, the hydroxyl groups are replaced, making it difficult to absorb water.

II. Reaction Mechanism of Silica
1. Physical Adsorption Mechanism
Van der Waals force: The large specific surface area and porous structure of silica allow organic substances to interact with the surface through Van der Waals forces and be physically adsorbed.
Hydrogen bonding: Surface hydroxyl groups form hydrogen bonds with polar substances, enhancing adsorption capacity, especially the adsorption of polar substances by adjacent hydroxyl groups is very important.
Reversibility: Physical adsorption is usually reversible and does not cause chemical changes in organic substances, making it suitable for environmental purification to remove organic pollutants in water and harmful gases in the air.
2. Chemical Reaction Mechanism
Esterification reaction: The hydroxyl groups on the surface of silica can undergo esterification reactions with organic anhydrides to form ester compounds.
Nucleophilic addition reaction: The gaseous amino groups on the surface can undergo nucleophilic addition reactions with organic acyl chlorides to form amides and other products.
Reaction with alkali: Silica can react with caustic alkali to produce silicate, the reaction equation is: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O.
Reaction with hydrofluoric acid: The reaction equation is: SiO₂ + 4HF → SiF₄ + H₂O.
3. Cross-linking Reaction Mechanism
Chemical cross-linking: Adjacent functional groups exist between silica and other substances, and cross-linking reactions are achieved through the formation of covalent bonds. The functional groups in the crosslinking agent react with the hydroxyl and amino groups on the surface of precipitated silica, forming C-O, C-N, and other bonds.
Physical crosslinking: Crosslinking structures are formed between precipitated silica molecules through non-covalent bonds such as hydrogen bonds, van der Waals forces, and hydrophobic interactions, without destroying the material matrix structure.
Anionic crosslinking: Crosslinking structures are formed between precipitated silica molecules through anionic reactions, mainly relying on acid-base reactions or anionic substitution reactions in the reactants.
4. Catalytic Reaction Mechanism
Surface active centers: As a catalyst carrier, the active functional groups on the surface of precipitated silica can combine with organic substances, producing a synergistic effect and effectively promoting catalytic reactions.
Adsorption-activation mechanism: In catalytic reactions, precipitated silica adsorbs reactant molecules, bringing them closer to the active sites and causing them to react, converting the reactants into products.
Application examples: In organic synthesis, precipitated silica catalysts can accelerate oxidation, reduction, esterification, and other reactions; in dye degradation, by calculating the adsorption characteristics of reactants on the surface of precipitated silica, suitable materials for catalytic degradation can be selected.

III. Modification Reaction Mechanism of Precipitated Silica
1. Surface Modification Methods
Silane coupling agent modification:  Part of the functional groups on the coupling agent chemically react with the hydroxyl groups on the surface of precipitated silica, changing the surface group structure and distribution, and improving compatibility with the matrix.
Ionic liquid modification: Using ionic liquid modifiers instead of traditional organic phase modifiers offers advantages such as being liquid at room temperature, high conductivity, and high stability.
Macromolecular interface modification: Using macromolecular polymers containing polar groups as modifiers improves the compatibility between precipitated silica particles and the matrix.
2. Si69 Modification Mechanism
Chemical reaction: The amino and thiol groups in the Si-69 molecule undergo nucleophilic substitution reactions with the active hydrogen on the surface of precipitated silica, tightly binding Si-69 to the precipitated silica. Interface Bonding: The thiol groups in the Si-69 molecule undergo an addition reaction with the double bonds in the rubber molecules, forming thioether bonds; the siloxane bonds undergo a substitution reaction with the active groups on the surface of the precipitated silica, forming strong chemical bonds.
Performance Improvement: The surface activity of the modified precipitated silica increases, strengthening the interfacial bonding force with the rubber matrix and significantly improving the mechanical properties and aging resistance of the rubber.
3. Modification of Precipitated Silica
Organic Wet Modification: During the preparation process, polyethylene glycol (PEG-6000) and n-butanol are used to modify the precipitated silica, greatly improving the agglomeration phenomenon, with primary particle sizes around 20 nm.
Modification Effect: After modification, silymethyl (Si-CH) groups are formed on the surface of the precipitated silica, the content of silanol (Si-OH) groups decreases, hydrophobicity increases, and the oil absorption value (DBP value) increases from 2.1 mL·g⁻¹ to 3.1 mL·g⁻¹.

IV. Application Basis of Precipitated Silica
The chemical characteristics and reaction mechanisms of precipitated silica determine its applications in multiple fields:
Rubber Industry: As a reinforcing agent, improving the strength, hardness, and wear resistance of rubber materials.
Environmental Purification: Utilizing its high specific surface area and adsorption capacity to remove organic pollutants from water and harmful gases from the air.
Catalyst: As a catalyst carrier, promoting organic synthesis reactions.
Food and Cosmetics: As an anti-caking agent, thickener, and whitening agent, utilizing its high specific surface area and adsorption capacity.
Electronic Materials: As an insulating material, utilizing its electrical insulation and high-temperature resistance.
The study of the chemical characteristics and reaction mechanisms of precipitated silica is of great significance for developing new functional materials and improving the performance of existing materials. Through surface modification technology, the surface chemical properties of precipitated silica can be controlled, expanding its application fields and improving its application effects in rubber, coatings, catalysts, and other fields.