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Understanding the Classification of Colloids

29th Oct 2020 @ 10 min read

Surface Chemistry

Colloids can be classified in two ways:

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

2) Based on the type of the particles of dispersed phase

Below is an illustration of how colloids are classified:

Colloid Classification

Let’s take a look on them one by one

1) Classification based on the nature of interaction between dispersed phase and dispersion medium

In this classification the affinity of dispersed phase towards dispersion medium or alternatively the strength of intermolecular forces present between sol particles will be the decisive factor

Based on this interaction colloids can be classified as:

Lyo means liquid and the terms philic and phobic refer to love and fear respectively

Hence

Lyo + philic = liquid loving

Lyo + phobic = liquid fearing/hating

This leads us to their basic definitions, i.e.

Lyophilic sols are those which have strong affinity between dispersed phase and dispersion medium whereas lyophobic sols are those which have low affinity between dispersed phase and dispersion medium

Here affinity means force of attraction between dispersed phase and dispersion medium

Let’s discuss the properties of these sols

Lyophilic sols:

· There is a strong affinity between dispersed phase and dispersion medium

· They can be prepared directly by shaking of the dispersed phase with dispersion medium. Also their colloidal state is recovered by merely shaking of the dispersion medium

Reason: There is very strong force of attraction between dispersed phase and dispersion medium

· Because of their affinity towards dispersion medium they are also called intrinsic

· When an electric field is applied, the particles of the sols may or may not migrate towards either electrode

· The surface tension of the sol/dispersion is lower than that of the dispersion medium (i.e. when the colloid is formed, the surface tension is lowered)

Reason: Presence of a very strong intermolecular force results in stronger bonding of dispersed phase with dispersion medium. Because of this all other forces acting on a sol particle balance each other. Hence surface tension is lowered

Lyophobic sols:

· There is a low affinity between dispersed phase and dispersion medium

· They cannot be prepared directly (i.e. shaking with the dispersion medium). Rather indirect methods are used for preparation of these colloids

Reason: There is a very low or no force of attraction present between dispersed phase and dispersion medium

· Because of their reluctance towards dispersion medium they are called extrinsic colloids

· The particles of sol migrate towards cathode or anode when subjected to an electric field

· The surface tension of the sol/dispersion is same as that of the dispersion medium (i.e. surface tension doesn’t change even after the formation of colloid)

The differences between the two colloids can be summarized as:
 

 

LYOPHILIC SOLS

LYOPHOBIC SOLS

Affinity b/w sol particles

(Intermolecular Forces)

Strong affinity

Low affinity

Method of Preparation

Direct Method

(Shaking with dispersion medium)

Indirect Methods

Ease of preparation

Easy to prepare

Difficult to prepare

Type of colloid

Intrinsic colloids

Extrinsic colloids

Migration of sol particles

(Under electric field)

Particles of the sol may or may not migrate towards electrodes

The particles of sol migrate towards electrodes

Surface tension

(of the sol/dispersion)

Lower than that of the dispersion medium

Higher than that of the dispersion medium

Examples

Gum, Starch, Gelatin etc.

Metal Sulphide sols, Metal sols

2) Classification based on the type of particles of dispersed phase

In this classification the factor that differentiates different colloids is, the size or the concentration of the particles of dispersed phase forming a colloidal solution

This way colloids can be classified into three groups

Now we will discuss these colloids one by one

Multi-molecular colloids

In order to form a colloidal solution, the size of dispersed phase particles should be in the range 1nm-1000nm. But there exist many kinds of molecules which are smaller than 1nm, hence are incapable of forming colloids in standalone state.

But sometimes, many such molecules when combined can fall into the range (1nm-1000nm) where the colloidal solution is possible.

“A molecule which is formed by aggregation (combination) of many small molecules is called multi-molecule”

And

The colloidal solution formed by treatment of these multi-molecules with dispersion medium is called multi-molecular colloid

Examples: Starch in water, Sols of gold/Sulphur etc.

The atoms or molecules in a multi-molecule are held by weak Vander Waal’s Forces

Macromolecular colloids

In such colloids a single molecule is big enough that it falls in the range (1nm-1000nm). Such a molecule is called macromolecule.

When a single molecule is big enough to form colloidal solution on direct treatment with dispersion medium, it is called macromolecule and the colloid thus formed is called macromolecular colloid

Examples: Polymers like rubber, nylon, polythene, starch, cellulose, proteins, enzymes etc.

In macromolecular colloids containing polymers, some polymeric species themselves form ions and thus can migrate in an electric field. Such macromolecules are called “colloidal electrolytes”

Associated colloids

Certain substances when dissolved in a medium at low concentration behave as normal molecules and do not make colloids

But at higher concentration these substances exhibit colloidal properties due to the formation of aggregated particles

When the particles of certain substance at higher concentrations aggregate (combine) to form a colloidal solution, the colloids formed are called associated colloids

Example: Micelle formation (in soaps and detergents) is an example of associated colloid

How a Micelle is formed?

When stearic acid reacts with sodium hydroxide (NaOH), sodium stearate is formed

Micelle Formed

Sodium stearate C17H35COO-Na+ consists of two ions, C17H35COO- and Na+. For the first ion C17H35COO- the first part C17H35 is hydrocarbon while the 2nd part COO- is a polar one. Thus for a polar substance like water, the hydrocarbon part of the 1 st ion is non-dissolvable (hydrophobic) while the polar part is dissolvable (hydrophilic)

We can also denote C17H35COO- by the following bone line diagram

Bone Line Diagram

Here the tail of the bone line diagram is hydrocarbon which is hydrophobic, while the head of the diagram ( COO- is polar and hence is hydrophilic

When these bone lines (or C17H35COO- ions) are lower in concentration they maintain their individual existence in a polar substance like water.

Micelle not formed diagram

But when their concentration is increased up to a certain level, the hydrophilic parts ( COO- ) of C17H35COO- will make formation of a group and this group will make hydrogen bond with water molecules. Meanwhile the hydrophobic parts ( C17H35 of each ion) will hide themselves behind hydrophilic parts, in order to avoid water molecules.

The molecule thus formed is called Micelle molecule, which is shown below

Micelle Formed at higher concentration

Micelle Formation

But a Micelle molecule is formed only above a certain temperature and concentration. They are called Kraft temperature and Critical Micelle Concentration (CMC).

Kraft temperature: The temperature above which a Micelle is formed is called Kraft temperature. Micelle cannot form below Kraft temperature

Critical Micelle Concentration (CMC): The minimum concentration required for formation of a Micelle molecule is called critical Micelle concentration. Below this concentration, a Micelle cannot form.

Hence whenever the above two conditions (i.e. Kraft temperature and Critical Micelle Concentration) are fulfilled for a hydrocarbon ion ( C17H35COO- in our case) its hydrophilic parts will form a group and make hydrogen bond with water molecules while the hydrophobic parts will hide themselves inside. The Micelle thus formed will exhibit the properties of a colloidal solution. And this type of colloid will be called associated colloid.

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Colloids Classification Surface Chemistry

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