is completed at neutral pH and ammonia has a role in stabilizing the individual carbon particles by preventing self-adhesion and aggregation, as well as acting as a catalyst. Advantages and applications One major advantage of the Stöber process is that it can produce silica particles that are nearly monodisperse, and thus provides an ideal model for use in studying colloidal phenomena. It was a pioneering discovery when first published, allowing synthesis of spherical monodisperse silica particles of controlled sizes, and in 2015 remains the most widely used wet chemistry approach to silica nanoparticles. The process provides a convenient approach to preparing silica nanoparticles for applications including intracellular drug delivery and biosensing. The mesoporous silica nanoparticles prepared by modified Stöber processes have applications in the field of catalysis and liquid chromatography. In addition to monodispersity, these materials have very large surface areas as well as uniform, tunable, and highly ordered pore structures, which makes mesoporous silica uniquely attractive for these applications. Aerogels Aerogels are highly porous ultralight materials in which the liquid component of a gel has been replaced with a gas, and are noteworthy for being solids that are extremely effective thermal insulators with very low density. Aerogels can be prepared in a variety of ways, and though most have been based on silica, materials based on zirconia, titania, cellulose, polyurethane, and resorcinol—formaldehyde systems, amongst others, have been reported and explored. The prime disadvantage of a silica-based aerogel is its fragility, though NASA has used them for insulation on Mars rovers, the Mars Pathfinder and they have been used commercially for insulating blankets and between glass panes for translucent day-lighting panels. Particulate gels prepared by the Stöber process can be dehydrated rapidly to produce highly effective silica aerogels, as well as xerogels. They key step is the use of supercritical fluid extraction to remove water from the gel while maintaining the gel structure, which is typically done with supercritical carbon dioxide, as NASA does. The resulting aerogels are very effective thermal insulators because of their high porosity with very small pores (in the nanometre range). Conduction of heat through the gas phase is poor, and as the structure greatly inhibits movement of air molecules through the structure, heat transfer through the material is poor, as can be seen in the image at right where heat from a Bunsen burner transfers so poorly that crayons resting on the aerogel do not