Hydrophobic silica

Hydrophobic silica is a form of silicon dioxide (commonly known as silica) that has hydrophobic groups chemically bonded to the surface. The hydrophobic groups are normally alkyl or polydimethylsiloxane chains. Hydrophobic silica can be processed in different ways; such as fumed silica, precipitated silica, and aerosol assisted self assembly, all existing in the form of nanoparticles.

Hydrophobic silica has an orthorhombic crystal structure (its space group name is Pmna under the bipyramidal point group). Orthorhombic structures are the product of stretching a cubic lattice along two of its orthogonal pairs, resulting in a rectangular prism shaped crystal structure.

Hydrophobic silica displays water resistant properties because of its nanostructure and chemical properties. When applied to a surface of a material, the nanoparticles adhere to the host material and prevent liquids from permeating the rough texture. The water only comes into contact with the tips of the nanoparticles coating the outside of the material. Due to lack of attraction, the water is repelled from the hydrophobic silica.

Initially, silica is hydrophilic due to the presence of the silanol (Si-OH) groups on the surface of the particle. These silanol groups can chemically react with various reagents to render the silica hydrophobic. There are many different methods of processing silica to become hydrophobic, mainly by adding hydrocarbon groups.

Fumed silica can react with chlorosilanes in a fluidized bed reactor at 400 °C

...
Wikipedia