The principle of hydrophobicity of fumed silica is mainly to make the hydroxyl groups on the surface of silica react chemically with organic compounds, cover a layer of organic molecules on the surface of silica, thereby changing from hydrophilic to hydrophobic, and improving its wettability with organic molecules. Different types of fumed or precipitated silica can be used as raw materials, and the silane groups react with suitable compounds, and the surface is hydrophobic due to the non-hydrolyzed methyl groups connected to the surface.

Currently, the common organic compounds used to react with the hydroxyl groups on the surface of silica to achieve its hydrophobic properties include n-octyltrimethoxysilane, hydroxy silicone oil, dimethyldichlorosilane, etc. Silane groups are also often involved in such reactions, by chemically reacting with the hydroxyl groups on the surface of silica, covering the surface with organic molecules, thereby changing the hydrophilicity of silica from hydrophilic to hydrophobic, and improving its wettability with organic molecules. In addition, silane coupling agents also play an important role in this process, which can improve the dispersibility and surface hydrophobicity of nano-silica.

The surface covering organic molecular structure used to make fumed silica hydrophobic has the following significant characteristics: First, it can change the hydrophilicity of silica to hydrophobicity, improve its wettability, dispersibility and interfacial bonding strength with organic molecules, thereby improving the comprehensive performance of the material. Secondly, the hydrophobic silica can be well dispersed in the organic phase through the hydrophobic groups, and can form a strong interaction with the organic phase through the silanol groups, so that the incompatible inorganic phase silica and the organic phase can establish a solid connection. Furthermore, the hydrophobic modified fumed silica is added to the organic phase, and due to the effect of its surface silanol and other structures, its performance in the organic phase can be further optimized.

The principle of hydrophobicity of fumed silica is mainly to make the hydroxyl groups on the surface of silica react chemically with organic compounds, cover a layer of organic molecules on the surface of silica, thereby changing from hydrophilicity to hydrophobicity and improving its wettability with organic molecules. Different types of fumed or precipitated silica can be used as raw materials, and the silane groups react with suitable compounds to make it hydrophobic due to the non-hydrolyzed methyl groups connected to the surface. Hydrophilic fumed silica is treated with a modifier to obtain hydrophobic fumed silica, whose surface hydroxyl groups are reacted or shielded and hydrophobic. Hydrophobic fumed silica is usually made by using different types of fumed or precipitated silica as raw materials and reacting with suitable compounds through silane groups. For example, Degussa AEROSIL R972 is obtained by reacting silane groups with dimethyldichlorosilane.

The principle of hydrophobic fumed silica is to make the surface hydroxyl groups react with organic compounds to cover organic molecules, thereby changing from hydrophilic to hydrophobic, improving the wettability with organic molecules. Its preparation process includes pyrolysis and wet method, presenting white flocculent powder, non-toxic, tasteless, odorless, environmentally friendly and pollution-free. The surface of the particles retains hydroxyl groups and is hydrophilic. The primary particles and aggregates have specific particle sizes. It can also obtain particles with fine particle sizes and high specific surface areas by controlling gas phase reactions. It has applications in defoaming agents and other aspects.

During the hydrophobic process of fumed silica, the number of methyl groups connected to the surface has a specific quantitative effect on the hydrophobic property. It is known that the principle of hydrophobicity of fumed silica is mainly to make the hydroxyl groups on the surface of silica react chemically with organic compounds, and cover a layer of organic molecules on the surface of silica, thereby changing from hydrophilic to hydrophobic, and improving its wettability with organic molecules. In addition, the prepared hydrophobic silica contains hydrophobic groups methyl, and the tensile mechanical properties and hydrophobicity of the silicone rubber sample with hydrophobic silica added are significantly improved, but there is no exact description of the specific quantitative relationship between the number of methyl groups and hydrophobic properties.

In addition to methyl groups, there are many other groups that can make fumed silica hydrophobic. For example, hydrophobic fumed silica can be obtained after silicone oil treatment. During the treatment process, the surface hydroxyl groups are reacted or shielded, thereby achieving hydrophobicity. In addition, through chemical reagent treatment, the surface hydroxyl groups are replaced by corresponding hydrophobic groups (such as some specific organic groups), which can also make the fumed silica hydrophobic. At the same time, different types of gas phase or precipitation silica are selected as raw materials, and the hydrophobic effect may also be achieved through the reaction of silicon hydrocarbon groups with suitable compounds. However, the specific types of groups will vary depending on the treatment method and reaction conditions.