Particles, hydrolysis product, surface aggregation

Radioactive tracers adsorb not only on solid container surfaces and precipitates but on any kind of solid material suspended or in contact with the solution. Dust, cellulose fibers, glass fragments, organic materials, etc., are examples of substances that readily adsorb radioactive tracers from solution. If the solution contains large molecules as, for example, polymeric metal hydrolysis products, these also tend to adsorb trace elem ts. In addition to sorption, the presence of such material in the solution can lead to the phenomenon of radiocolloid formation, which is the attachment of radionuclides to semicolloidal aggregates in solution. If the solution is kept at sufficiently low pH and extremely free from foreign particles, sorption and radiocolloid formation are usually avoided as major problems. 

The synthesis of uncoated and surface-modified silica particles via the hydrolysis of tetraethoxysilane (TEOS) in a homogeneous alcoholic solution of water and ammonia is well documented in the literature [1-7]. TEOS and other metal aUtoxides (including those of titanium and iron) can be hydrolyzed in reverse microemulsion systems for the production of gels or metal oxide particles [8-16]. Systems incorporating surfactant aggregates have also been exploited in the synthesis of a number of other ultrafine particulate materials [13,15,17-20]. 

At low pH, acid catalysis promotes hydrolysis but hinders both condensation and dissolution reactions, " leading to small and homogeneous particles. Base catalysis of sol-gel hydrolysis and condensation reactions, in contrast, promotes fast condensation and dissolution. This leads to the production of an inhomogeneous system due to rapid condensation of the hydrolyzed precursor monomers and to dense silica particles formed by the ripening of aggregates during the collision of droplets. As a result, the microparticles show essentially no porosity, with the particles being stabilized by a water/surfactant layer on the particle surface that prevents particle precipitation. ... 

---