Question

Explain the method by which colloidal solutions of ink and graphite are prepared.

Hint:

Remember: - Aquadag = Graphite + Water (Aqua) - Oildag = Graphite + Oil These are used extensively as lubricants in machinery where high temperatures are involved.

Answer 1

Concept: Colloidal solutions of substances like ink and graphite are typically prepared using mechanical dispersion methods. Since these substances consist of large particles that are insoluble in the dispersion medium, they must be physically broken down into particles of colloidal size (1 nm to 1000 nm).
The most common method for preparing these sols is using a Colloid Mill. • The solid substance (graphite or ink pigment) is first ground into coarse particles. • It is then mixed with the dispersion medium (like water) to form a suspension. • This suspension is passed through a colloid mill, which consists of two metal discs rotating at high speed (7000 rpm) in opposite directions with a very narrow gap between them. The intense shearing force between the discs grinds the coarse particles down to colloidal dimensions.
Once the particles are reduced to colloidal size, they have a tendency to aggregate back into larger particles. To prevent this, a stabilizing agent is added: • For Ink: Protective colloids or specific resins are added to keep the pigment particles dispersed. • For Graphite: Tannin is often used as a stabilizing agent to produce a stable colloidal solution of graphite in water, known commercially as Aquadag. If dispersed in oil, it is called Oildag.