Silica sol is a colloidal solution, odorless and non-toxic. Its molecular formula can be expressed as mSiO2.H2O, and its appearance is milky white or light blue translucent liquid. Silica sol is a sol with colloidal characteristics, particles that are approximately spherical, and negatively charged. The three factors of zeta potential, Brownian motion, and sufficient solvent barrier give it aggregation stability and kinetic stability. However, the colloidal particles are metastable phases and always have a tendency to spontaneously aggregate. As long as one of the three major stabilizing factors is weakened, it will automatically aggregate to produce gel or precipitate. When silica sol condenses into gel, it is impossible to make it sol again by heating or adding solvent, so it is an irreversible colloid.
Generally speaking, the larger the specific surface area, the greater the surface energy. The silica sol system is an unstable system with a large surface energy. It has a tendency to automatically reduce surface energy. It is easy for small particles to spontaneously aggregate into large particles and even form gel. There are many factors that affect the stability of silica sol, such as electrolyte, temperature, concentration, pH and particle size. The pH value has a great relationship with the stability of silica sol. Silica sol is relatively stable in the alkaline range (pH9.5-10.5) and part of the acidic range (pH2~4).
Finished silica sol is generally divided into alkaline silica sol and acidic silica sol, among which alkaline silica sol is more stable and is used more. The mass fraction of SiO2 is generally 10%~40%, its particle size is 5~100nm, and the specific surface area is 50~400m2/g.
The wide application of silica sol in scientific research and various industrial fields is closely related to its performance characteristics. Silica sol has many excellent properties such as large specific surface, high adsorption, high dispersion (several nanometers to tens of nanometers), high fire resistance and thermal insulation, etc. From its properties, the following characteristics can be found:
① The silica sol particles attached to the solid surface can increase the friction coefficient;
② No curing agent is required, it can firmly adhere to the solid surface and form a solid film, and the film forming temperature is very low;
③ It can form a solid gel by drying or sintering, so it has a certain durability;
④ It can form a gel with a large surface area and uniform pores, and can evenly disperse the powder to increase the stability of the suspension;
⑤ Through Si-OH groups and adsorbed water, it can improve wettability and anti-static properties;
⑥ It can be immersed and filled into porous materials to make the surface smooth;
⑦ By evenly mixing the particles, the mechanical, optical and electrical properties of organic resins can be modified and enhanced;
⑧ Sol is a liquid system that can undergo homogeneous reactions. Using silica sol instead of silica as a raw material for the reaction can increase the reaction rate.
Silica sol has many excellent properties and characteristics, so as a fine chemical product, it is widely used in chemical, material, textile/papermaking, electronics and other industries.