Protective colloid

Protective colloids (also protective colloids ) are almost exclusively high-molecular compounds that are involved in precipitation reactions, i.e. H. in reactions in which a solid phase is separated from a homogeneous liquid phase, prevent clumping ( agglomeration , aggregation , flocculation, coagulation ) of the primary particles. A distinction must be made between the protective colloids and the dispersing additives , which are subsequently added to a suspension.

Mechanism of action

The protective colloids attach themselves to the primary particles with their hydrophobic part and turn their polar, i.e. H. hydrophilic part of the molecule of the aqueous phase. As a result of this accumulation at the interface, they reduce the interfacial tension and prevent agglomeration of the primary particles. In addition, they increase the viscosity at the interface and, since all protective colloids are able to bind large amounts of water, the viscosity of the dispersion in question increases at the same time .

In order to compare the protective effect of different protective colloids, Richard Zsigmondy defined the so-called gold number in 1901 . It is the amount of protective colloid that prevents a colloidal gold solution from changing color . Instead of the colloidal gold solution, Wolfgang Ostwald used a colloidal congo rubin solution and defined the congo rubin number in 1919 . V. Bermann defined an iron number. In these definitions, the hydrogen concentration of the hydrosols and the protective colloids is not taken into account.

Application examples

Light-sensitive layers in analog photography

In analog photography , the light-sensitive layers are made of silver bromide with the help of a protective colloid. The classic protective colloid is gelatine, which prevents the crystals from clumping together during the reaction between silver nitrate and bromine salt and thus ensures even distribution.

Suspension polymerizations

A suspension polymerization prevents the monomer droplets and product beads from clumping together during the reaction.

Types of protective colloids

They are mostly water-soluble polymers such as partially saponified polyvinyl acetate , polyvinyl alcohol , polyvinylpyrrolidone ; Cellulose ethers (Tylose) such as methyl cellulose , hydroxyethyl cellulose , hydroxypropylmethyl cellulose ; Polyacrylates , starch , proteins , alginates , pectins and gelatins .

Sources and individual references

↑ Raph. Ed. Liesegang: Colloid-chemical technology: A handbook of colloid-chemical approach in the chemical industry and technology . Springer, 1932, ISBN 3-642-49919-8 ( limited preview in Google book search).

[1] Raph. Ed. Liesegang: Colloid-chemical technology: A handbook of colloid-chemical approach in the chemical industry and technology . Springer, 1932, ISBN 3-642-49919-8 ( limited preview in Google book search).