Colloidal silicas are suspensions of fine
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid) is a solid that lacks the long-range order that is a characteristic of a crystal. The terms "glass" and "glassy solid" are sometimes used synonymousl ...
, nonporous, and typically spherical
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
particles in a liquid phase. It may be produced by
Stöber process from
Tetraethyl orthosilicate (TEOS).
Properties
Usually they are suspended in an aqueous phase that is stabilized electrostatically. Colloidal silicas exhibit particle densities in the range of 2.1 to 2.3 g/cm
3.
Most colloidal silicas are prepared as
monodisperse suspensions with particle sizes ranging from approximately 30 to 100 nm in diameter. Polydisperse suspensions can also be synthesized and have roughly the same limits in particle size. Smaller particles are difficult to stabilize while particles much greater than 150 nanometers are subject to sedimentation.
Manufacture
Colloidal silicas are most often prepared in a multi-step process where an alkali-silicate solution is partially neutralized, leading to the formation of silica
nuclei. The subunits of colloidal silica particles are typically in the range of 1 to 5 nm. Whether or not these subunits are joined depends on the conditions of polymerization. Initial acidification of a water-glass (
sodium silicate) solution yields Si(OH)
4.
If the pH is reduced below 7 or if salt is added, then the units tend to fuse together in chains. These products are often called
silica gel
Silica gel is an amorphous and porosity, porous form of silicon dioxide (silica), consisting of an irregular three-dimensional framework of alternating silicon and oxygen atoms with nanometer-scale voids and pores. The voids may contain wate ...
s. If the pH is kept slightly on the alkaline side of neutral, then the subunits stay separated, and they gradually grow. These products are often called
precipitated silica or silica sols.
Hydrogen ion
A hydrogen ion is created when a hydrogen atom loses or gains an electron. A positively charged hydrogen ion (or proton) can readily combine with other particles and therefore is only seen isolated when it is in a gaseous state or a nearly particl ...
s from the surface of colloidal silica tend to dissociate in aqueous solution, yielding a high negative charge. Substitution of some of the
Si atoms by
Al is known increase the negative colloidal charge, especially when it is evaluated at pH below the neutral point. Because of the very small size, the surface area of colloidal silica is very high.
The
colloidal suspension is stabilized by pH adjustment and then concentrated, usually by evaporation. The maximum concentration obtainable depends on the on particle size. For example, 50 nm particles can be concentrated to greater than 50 wt% solids while 10 nm particles can only be concentrated to approximately 30 wt% solids before the suspension becomes too unstable.
Applications
*In
papermaking
Papermaking is the manufacture of paper and cardboard, which are used widely for printing, writing, and packaging, among many other purposes. Today almost all paper is Pulp and paper industry, made using industrial machinery, while handmade pape ...
colloidal silica is used as a drainage aid. It increases the amount of cationic
starch
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diet ...
that can be retained in the paper. Cationic starch is added as
sizing
Sizing or size is a substance that is applied to, or incorporated into, other materials—especially papers and textiles—to act as a protective filler or glaze. Sizing is used in papermaking and textile manufacturing to change the absorption ...
agent to increase the dry strength of the paper.
*High temperature
binders
*
Investment casting - used as the inorganic binder in
moulds
*An
abrasive - for polishing
silicon wafers
*
Carbonless paper
*As binder or silica source in
catalysts
Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
*
Desiccant
*As lubricant in the manufacture of pharmaceutical capsules and tablets.
*Stabilizing and rigidizing refractory ceramic fiber blankets
*
Abrasion resistant coatings
*Increasing
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...
- used to coat waxed floors,
textile fibers,
cardboards and
railway track
Railway track ( and UIC terminology) or railroad track (), also known as permanent way () or "P way" ( and Indian English), is the structure on a railway or railroad consisting of the rails, fasteners, sleepers ( railroad ties in American ...
s to promote traction
*Antisoiling – fills
micropores to prevent take up of dirt and other particles into textiles
*Surfactant – used for
flocculating,
coagulating,
dispersing,
stabilising etc.
**Liquid silicon dioxide (colloidal silica) is used as a wine and juice
fining agent.
*Absorbent
*Colloidal silica is used in
concrete densifiers and
polished concrete.
*In manufacturing
Quantum dots, small semi-conductors used in various scientific research settings.
*Reduced porosity and increased protection from alkali-silica reaction (ASR).
See also
*
Silica dioxide
References
{{Reflist
Silicon compounds