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Introduction
A mixture in which a substance with microscopically spread insoluble particles (this is known as the dispersed phase) is suspended throughout another substance (known as the dispersion medium) is termed as a colloid in chemistry.
Colloids are broadly classified into lyophilic and lyophobic colloids based on their affinity with each other.
As the name suggests, lyophilic colloids are lovers of liquid, and lyophobic colloids are haters of liquid. The difference between lyophilic colloids and lyophobic colloids is based on their affinity for the dispersion medium, preparation, stability, reversibility, solvation, viscosity, surface tension, visibility, migration, hydration, and coagulation.
What are Colloids?
Colloids are typically heterogeneous mixtures where one substance is suspended in another substance. The substance that is suspended has microscopically spaced insoluble particles. The substance that holds the suspended substance is called a dispersion medium.
Classification of substances in a colloidal suspension
Dispersion medium
The substance into which the dispersed phase is dispersed, suspended, or distributed is known as the dispersion medium. It is also known as the continuous medium.
Dispersed phase
The substance that is dispersed or suspended or distributed is known as the dispersed phase. The diameter of the dispersed phase particles is approximately 1 nanometer to 1 micrometre.
For example, in milk, fat globules are the dispersed phase suspended into the dispersion medium of water.
According to some definitions, it is required that the particles must be dispersed in a liquid. However, other definitions include substances like aerosols and gels.
Some colloidal suspensions are translucent because of the Tyndall effect. The Tyndall effect results from light scattering by suspended particles in an excellent colloidal suspension. Other colloidal suspensions may be opaque or have a slight colour.
Colloids are broadly classified based on the state and interaction between the suspended element and the element that suspends the said element.
Classification based on the state of the dispersed phase and the dispersion medium
The three main types of colloids based on the state of phase and the medium are:
Sol
In this type of colloid, solid is dispersed in a liquid
Ex- blood, pigmented ink, cell fluids, paint, antacids and mud.
Gel
In this type of colloid, the liquid is dispersed in solid
Ex- curd, cheese, butter.
Emulsion
In this type of colloid, the liquid is dispersed in a liquid
Ex- milk is an oil in water emulsion
Classification based on the interactive affinity of the dispersed phase and the dispersion medium
Colloids are further classified into lyophilic colloids and lyophobic based on their interactive affinity with each other.
Lyophilic colloids / Lyophilic sols
These are lovers of solvents. When the colloidal suspension has water as a dispersion medium, they are termed hydrophilic.
Lyophobic colloids / Lyophobic sols
These are haters of solvent. When the colloidal suspension has water as a dispersion medium, they are termed hydrophobic.
The difference between Colloids and solutions
- A colloidal suspension consists of two media that do not mix. Specifically, it is a heterogeneous mixture of discrete elements of one medium termed as a dispersed phase which is dispersed in a continuous second medium termed as the dispersion medium. The two substances can be very different. More specifically, they can be a gas, a liquid, or a solid.
- A colloid constitutes a dispersed phase and a continuous phase. Colloidal particles are more significant. In a colloidal suspension such as milk, the colloidal particles are fat globules and not individual fat molecules.
- On the contrary, in a solution, the solute and solvent constitute only one phase.
- A colloid has two phases – one that is dispersed and another that is continuous. Colloidal particles are also a more prominent example of milk. The fat globules are colloidal particles.
- The solute and solvent in a solution are in the same phase. The solute dissolves in a solvent, and each of them has its ions or molecules. In a salt solution, for example, the sodium chloride (NaCl) crystals dissolve, resulting in the Na+ and Cl− ions being surrounded by water molecules.
Lyophilic colloids or lyophilic sols
In a lyophilic colloidal suspension, the dispersed phase has a high affinity for the dispersion medium. Owing to their ability to reconstitute the addition of the dispersion medium after separation, they are termed reversible sols. Lyophilic colloids are known for their stability. They do not gel quickly. Lyophilic sols are highly solvated. A layer of the dispersion medium covers the dispersed phase particles in lyophilic sols. They can guard lyophobic colloids against electrolytes, forming a protective layer. Lyophilic colloids need the addition of electrolytes or suitable solvents for coagulation. Gum, gelatin, protein, starch, rubber, etc., are the best examples of lyophilic colloids.
Lyophobic colloids or lyophobic sols
In a lyophobic colloidal suspension, the dispersed phase has no affinity for the dispersion medium. Hence, the formation of the lyophobic colloid is complex, or it is not readily formed. They are also termed irreversible sols. Lyophobic colloids cannot be reconstituted simply by adding a dispersion medium after precipitation. They are unstable, and the addition of electrolytes results in coagulation. Coagulation happens even on shaking or heating of the colloidal suspension. Lyophobic colloids need a stabilising agent and specific methods to be prepared. Some common examples of Lyophobic colloids include sulphide compounds of metals.
Difference between lyophilic colloids and lyophobic colloids
Lyophilic colloids | Lyophobic colloids |
dispersed phase’s affinity for the dispersion medium | |
There is a high affinity for the dispersion medium from the dispersed phase. | The dispersed phase has zero affinity for the dispersion medium. |
Preparation of the colloidal suspension | |
Preparation is easy. It requires the dispersed phase to be mixed, shaken or heated with the dispersion medium. | Preparation is tedious and requires unique methods and the intervention of electrolytes for stabilisation of the procedure. |
The stability of the colloidal suspension | |
Lyophilic colloids is highly stable. | Lyophobic colloids are unstable. |
Reversibility | |
The addition of the dispersion medium results in the reconstitution of the suspension, and hence they are reversible. | Colloidal suspensions are irreversible, and once they are precipitated, they cannot be reconstituted by adding a dispersion medium. |
Solvation of the dispersed phase | |
in lyophilic colloids are highly solvated and is covered by a layer of the dispersion medium. | Dispersed phases in lyophobic colloids are not solvated. |
The surface tension of the dispersed phase | |
is lower than the dispersion medium. | The surface tension of the dispersed phase is the same as the dispersion medium. |
The viscosity of the dispersed phase | |
is much higher than the dispersion medium. | The viscosity of the dispersed phase is the same as the dispersion medium. |
Visibility of particles | |
There is zero visibility of particles even under an ultramicroscope. | Particles are detectable under an ultramicroscope. |
Migration of particles | |
may occur in either direction, or migration would not occur in an electric field as they are not carriers of any form of charge. | Migration of particles occurs towards the cathode or anode in an electric field as they are carriers of a charge. |
Reaction with electrolytes | |
The addition of electrolytes in small quantities has no effect. | Coagulation takes place as a result of the addition of electrolytes. |
Hydration | |
Results in extensive hydration | Does not result in hydration |
Examples | |
Gum, starch, etc. | Metals like Silver (Ag) and Gold (Au), hydroxides like aluminium hydroxide Al(OH3), ferric oxyhydroxide Fe(OH)3 metal sulphides like arsenic trisulfide As2S3 , etc. |
Conclusion
Colloids are broadly classified into lyophilic and lyophobic colloids based on their affinity with each other.
Lyophilic colloids are lovers of liquid, and lyophobic colloids are haters of liquid, as the name suggests. The dispersed phase has a high affinity for the dispersion medium in lyophilic colloids, whereas there is zero in lyophobic colloids.
Lyophilic colloids differ from lyophobic colloids based on their affinity for the dispersion medium, preparation, stability, reversibility, solvation, viscosity, surface tension, visibility, migration, hydration, and coagulation.
