All forms of SAS may be surface-modified to produce silica that is more hydrophobic. The difference between the amorphous and crystalline silica forms arises from the connectivity of the tetrahedral units. Amorphous silica consists of a non-repeating network of tetrahedra, where all the oxygen corners connect two neighbouring tetrahedra. Although there is no long range periodicity in the network there remains significant ordering at length scales well beyond the SiO bond length. The amorphous structure is very “open”, i.e., channels exist through which small positive ions such as Na+ and K+ can readily migrate. Pyrogenic amorphous silica is produced in closed reactors

by the hydrolysis of (alkyl)chlorosilanes (e.g. SiCl4, HSiCl3. CH3SiCl3) in an oxygen/hydrogen flame at temperatures between 1200 and 1600 °C. Nucleation, condensation and coagulation of SiO2 Cyclopamine molecules generate proto-particles of SiO2 which combine to primary particles. Under the conditions of the reaction

zone, primary particles form SiO2 aggregates; aggregates then form agglomerates of SiO2. It is important to note that primary Selleck MK-2206 particles do not exist outside the reaction zone. The relatively high temperature yields a product that has low water content ( Fig. 2). Precipitated silica and silica gel consist of randomly linked spherical polymerized primary particles. OSBPL9 The properties are a result of the size and state of aggregation of the primary particles and their

surface chemistry. Precipitated silica and silica gels can be produced from various raw materials. The most relevant process in industry is from sodium silicate solutions by acidification with sulphuric acid to produce a gelatinous precipitate. The precipitate is filtered, washed, dehydrated and milled to produce precipitated silica with typically broad meso/macroporous pore structures reflected in the pore size distribution, or silica gels with generally more narrow microporous or mesoporous structure with average pore diameters between 2 and 50 nm. By controlling the washing, ageing, and drying conditions, the important physical parameters such as porosity, pore size, and surface area can be adjusted to produce a range of different silica gel types with well-defined particle size distributions. Amorphous mesoporous silica with uniformed pores in the size range between 1.5 and 50 nm can be synthesised by reacting tetraethylorthosilicate (TEOS) with a template of surfactant molecules, typically amphiphilic polymers, under either alkaline or acidic conditions. The surfactants are later evacuated from the mesopores by a calcination step or by washing with a solvent. Form and diameter of the mesopores are determined by the type of surfactants used in the synthesis (Mou and Lin, 2000 and Napierska et al., 2010).