If the pH is reduced below 7 or if salt is added, then the units fuse together in chains to result in silica gel. If, however, the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and gradually grow to colloidal silica

(silica sols). The maximum concentration at which this step can be carried out is in the range of 10–15%. Higher concentrations will also result GSK-3 activity in gelation. The resulting colloidal suspension is stabilised by the addition of KOH, NaOH, NH3 or HCl in amounts of up to 10% by weight. An alternative method for stabilisation is based on electrostatic repulsion of the particles. Substitution of some of the Si atoms by Al is known to increase the negative colloidal charge, especially at pH ranges below the neutral point leading to higher repulsive forces between the sol particles. The resulting suspension can then be concentrated, usually by evaporation of the liquid phase. Maximum silica concentrations in the end product depend on particle size and range between approximately 30 wt% for 10 nm particles and about 50% for 50 nm particles. Higher concentrated suspensions are not stable. Hydrogen ions from the surface of colloidal silica tend to dissociate in aqueous solution, resulting in a negative charge. Spherical colloidal silica particles in suspension

can selleck chemicals llc also be obtained by the Stöber method (Stöber et al., 1968), by which controlled growth of particles of near uniform size and porosity is achieved by hydrolysis of alkylsilicates and subsequent condensation of silicic acid in an ethanolic solution with catalytic amounts of ammonia. For further details on the manufacture of pyrogenic silica, precipitated silica and silica gel; the reader is referred to the Best Available Techniques (BAT) Reference Documents ( BREF, 2007). SAS are a distinct, manufactured form of silicon dioxide; they typically contain Histone demethylase less than 1% of

impurities. Silicon dioxide is described as a white fluffy powder or granules; and is hygroscopic (EFSA, 2009). The tendency to be solvated by water depends on the SAS type, with saturation concentrations usually increasing with increasing surface area. Generally, SAS have a tendency to supersaturate and surface-treated hydrophobic SAS have lower solubility as compared to the hydrophilic forms. For the analysed SAS, the saturation concentration was reached within a few hours (Alexander et al., 1954, Borm et al., 2006a, ECETOC, 2006 and Vogelsberger, 1999). Particle size distribution curves and the accuracy of measurements depend on the particular method used, on sample preparation and whether the measurement was performed in solid or liquid phase (for details see ECETOC, 2006 and ISO, 2008).