Types of Colloids

The colloids can be classified in different ways, which are as follow;

• Based on the interaction between dispersed and dispersion phases
• Based on the type of dispersed phase or colloidal particles' size
• Based on the charge on the colloidal particles

1) Based on the interaction between the dispersed phase and dispersion medium

They can be of two types based on interaction between its two phases; lyophilic colloids and lyophobic colloids.

i) Lyophilic Colloids (Sols)

What is a lyophilic colloid?

The term 'lyophilic' means liquid loving. The colloids that show a strong affinity between the dispersed and dispersion phase are called lyophilic colloids. Common examples of lyophilic colloids include glue, rubber, proteins, gelatin, starch, and more.

Characteristics of lyophilic colloids sols

• They have a strong affinity or attraction between dispersed phase (colloidal particles) and the dispersion medium.
• The colloidal particles are solvated or coated by one or more layers of dispersed phase or medium.
• They are stable sols as a strong affinity between particles and dispersion medium does not let the solutes settle down at the bottom.
• They are easy to prepare due to the strong affinity between their two phases. They form naturally without requiring mechanical agitation. For example, they can be prepared by shaking, mixing, or heating.
• Their particles are usually small in size, but, they can be large molecules and polymers like proteins and starch.
• They are reversible sols as when the dispersion medium is added to the coagulated colloidal particles it again becomes a colloidal sol.
• The surface tension of the sol is less than the dispersion medium.
• The viscosity of the lyophilic sol is higher than its dispersion medium due to solvation.
• They are more hydrated and do not tend to coagulate.
• The charge on particles is affected by the pH of the medium.
• Their coagulation is possible by using a large amount of electrolyte.
• Their particles are not visible with the naked eye or under a microscope and show weak a Tyndall effect due to their small size.
• The colloidal particles may form a network by cross-linking and may entrap dispersion medium in this cross-linked network.
• Their particles have little or no charge.
• All particles of the dispersed phase have the same charge so they tend to repel each other, which makes the sol stable.
• Due to less or no charge, their particle may not move towards anode or cathode when an electric field is applied.

ii) Lyophobic Colloids (Sols)

What is a lyophobic colloid?

The 'term' lyophobic means water-hating. They are those colloids or sols in which there is no affinity or attraction between the dispersed phase and dispersion medium as the dispersed phase does not have any attraction for the dispersion medium. They are solvent hating colloids. For example, metal in water, ferric hydroxide, aluminium hydroxide in water, etc.

Characteristics of lyophobic sols:

• They lack attraction or affinity between the dispersed phase and the dispersion medium.
• They cannot be formed easily as they lack affinity between their two phases; special methods like condensation is used to form a lyophobic sol.
• They are less stable.
• Their dispersed phase particles are aggregates of a large number of smaller particles.
• They are irreversible. Their particles left behind after evaporation or coagulation cannot be converted back into sol by adding the dispersion medium.
• The surface tension of sol is the same as that of the dispersion medium, their viscosity is also almost the same.
• They are easy to coagulate as they are unstable, only a small amount of electrolyte can coagulate them.
• Their particles can be seen by using a microscope.
• Their particles scatter the light passed through them and thus show a strong Tyndall effect.
• Their particles can have a positive or negative charge. The same charge on the particles keeps them suspended without coagulation. It makes them stable, however, they are less stable than lyophilic sols.
• Their charged particles tend to move towards electrodes when an electric field is applied.

Points to remember:

• When the dispersion medium is water, the lyophilic sol is known as hydrophilic sol and lyophobic sol is known as a hydrophobic sol.
• When the dispersion medium is alcohol, it is called alcosol and when the dispersion medium is benzene it is known as benzosol.
• When the dispersion medium is water, it is known as aquasol or hydrosol.

Difference between lyophilic sols and lyophobic sols

2) Based on their particles' size

The colloids are further classified into three types based on their particles' size, which are as follows;

i) Multimolecular Colloidal sols: Their particles are aggregates of atoms or molecules whose size is less than 1 nm. So, after aggregating their size fall in the colloidal range. The particles of aggregates are held together by Van Der Wall's forces and are lipophilic in nature.

For example, gold sol in which gold atoms aggregate to form colloidal particles. Other similar examples include sulphur sol and silver sol.

ii) Macromolecular colloids: It refers to colloids in which a single large macromolecule behaves as a dispersed phase particle. It is lyophilic in nature. For example, starch in water, proteins in water, enzymes in water, etc. They are generally lypophobic in nature.

iii) Associated colloids: It refers to colloids that act as normal electrolytes at low concentrations, but behave as colloids when their concentration increases and they become aggregate particles. These aggregate particles are called micelles.

For example, when the concentration of soap increases in water, it forms micelles with the size in the colloidal range and thus forms an associated colloidal sol. If we dilute these sols, they will return to their original state or become individual ions.

Furthermore, the micelles are formed above a specific temperature (Kraft temperature), and above a specific concentration (critical micelle concentration).

Difference between Multimolecular colloids and Macromolecular colloids

3) Based on charge on their particles

The colloids can be of two types based on the charge on dispersed phase (colloidal particles). The colloidal particles can have a positive or negative charge, accordingly a colloidal sol can be a positively charged sol or negatively charged sol as described below;

i) Positively charged sols: They have a positive charge on colloidal particles. For example, haemoglobin present in the blood is a positively charged sol, whereas blood is a negatively charged sol. Some other examples are hydrated oxides such as Al₂O₃.xH₂O and oxides like TiO₂ and dyes like Methylene blue.

ii) Negatively Charge Sol: They contain negatively charged colloidal particles. Common examples include metal sol such as Cu, Ag, Au sols and metal sulphides such as AsS₃ and Acid dyes like Eosin, etc.