Colloid

Etymology:

The word colloid is derived from a Greek word kolla “glue” and an English word oid “like”. Hence kolla +oid (colloid) = glue-like. This word was first used in the mid-19 ^th century for substances that have been in a gelatinous or gluey state, like starch in water.

In surface chemistry, the word colloid refers to acolloidal solution which is an intermediary between true solution and suspension.

A true solution is a mixture in which a solute is completely dissolved in a solvent (like salt in water) to make a homogeneous mixture. Since homogeneous mixtures consist of a single phase so you cannot see the two substances mixed up to form a solution. You can only see the solvent.

Figure 1: Salt solution in water

A suspension is that mixture in which solute does not dissolve in a solvent, rather it either settles down as precipitates (e.g. sand stirred in water) or comes at the top (e.g. oil in water). Thus it forms a heterogeneous mixture. A heterogeneous mixture consists of two or more than two phases. And all the phases are visible through the naked eye.

Figure 2a: Sand stirred in water Figure 2b: Oil in water

Since a colloidal solution is an intermediary between true solution and suspension, it exhibits the properties of both. That is, the solute neither completely dissolves in the solvent nor does it settle down or come at the top. Rather the solute particles stay dispersed in the solvent and that’s why colloidal solutions are called dispersions. A colloidal solution (or dispersion) may seem homogeneous but it is heterogeneous in nature. This means you cannot see the two phases but they are there.

Figure 3: Glass of milk (colloid)

Examples of colloids

Some of the colloids are given below:

Ruby Glass (solid in solid colloid) Paint (solid in liquid colloid)

Smoke (solid in gas colloid) Pumice stone (gas in solid colloid)

Dispersed phase and dispersion medium

It is important to note that for colloidal solutions, we use the words dispersed phase and dispersion medium as constituents of the solution. Here dispersed phase and dispersion medium are analogous to solute and solvent respectively for a true solution.

Particle size

The colloidal solutions are also characterized by their particle size, which refers to the size of particles of the dispersed phase. The colloid particles are larger than particles of a true solution (0.1nm-1nm) but smaller than that of suspension (>10³nm).

Hence,

“A substance would be in a colloidal state if the particle size of its dispersed phase is in the range of 1nm to 10³ nm”

Also because

1nm = 10Aº

We can also say that in a colloidal state, the particle size (of dispersed phase) ranges from 10Aº-10⁴Aº

Types of colloids

Only eight types of colloidal systems exist in nature or can be formed. These colloidal systems along with the examples are given below

Dispersed Phase	Dispersion Medium	Name of colloidal system	Examples
Solid	Solid	Solid sol	Ruby Glass
Solid	Liquid	Sol	Paints
Solid	Gas	Aerosol	Smoke
Liquid	Solid	Gel	Jellies
Liquid	Liquid	Emulsion	Milk
Liquid	Gas	Aerosol	Fog
Gas	Solid	Solid foam	Pumice stone
Gas	Liquid	Foam	Froth, Whipped cream

Note that the colloid of gas with gas is not possible. This is because gases upon mixing, diffuse into each other and hence a colloid cannot be formed.

Colloids are also given specific names depending upon the nature of the dispersion medium present in them. Some examples of such colloidal systems are stated below

Dispersion Medium	Name of colloid
Water	Aquasol or Hydrosol
Gas	Aerosol
Alcohol	Alcosol
Benzene	Benzosol

Properties of colloids

• They are heterogeneous in nature
• Their particle size is in the range of 1nm to 1000nm
• Their dispersed phases neither dissolve in dispersion medium nor do they settle at bottom or on the surface
• The particles of their dispersed phase are either single macromolecules or aggregates of molecules or atoms
• They pass slowly through a semi-permeable membrane (or parchment). This is because their particles are relatively larger than particles of a true solution