Colloidal Solutions

What are Colloidal Solutions?

A colloidal solution is a mixture in which the solute particles remain suspended throughout the solution and their size is between the particles' size of true solutions and suspensions. We can say that in colloidal solutions, the particles' size is more than the particles' size of a true solution but less than the particles' size of suspensions. For example, in true solutions particles' size is less than 1 nm, in colloids between 1 to 1000 nm and in suspensions more than 1000 nm. The other characteristic feature of the colloidal solution that separates it from suspension is that its particles do not settle at the bottom when left undisturbed.

A colloidal solution is also called a colloid. It is a heterogeneous solution, however, it may look like a true solution due to the small size of particles. So, to distinguish a colloid from a true solution, the term 'sol' is used instead of the term 'solution' such as colloidal sol. Furthermore, their particles are known as colloidal particles.

A colloid is made of two components, which include the dispersed phase and the dispersion medium. These components are described as follows;

Dispersed phase: It is made of colloidal particles that are distributed or dispersed throughout the colloidal sol. It is also called the discontinuous phase and can be associated with the solute phase of a true solution.

Dispersion phase: The component of a colloid in which colloidal particles or dispersed phase remains suspended is called the dispersion phase or dispersion medium. It is also called the continuous phase and can be associated with the solvent phase of a true solution.

Furthermore, the colloids are heterogeneous mixtures as they lack the properties of true solutions.

Types of colloidal solutions or colloids

The dispersed phase and dispersion medium of a colloid sol can be present in any physical state such as solid, liquid or gaseous state of matter. So, there could be nine types of colloidal solutions, however, only eight types are possible as the gaseous dispersed phase in a gaseous dispersion medium forms a homogeneous mixture so it becomes a true solution and cannot be considered a colloid.

So, colloids can be of the following types based on the physical state of their components;

Properties of Colloids

• They are heterogeneous solutions as they have a non-uniform composition with two different phases; dispersed phase (discontinuous phase) and the dispersion medium (continuous phase).
• The size of colloidal particles that make the dispersed phase ranges from 1 to 1000 nm.
• The colloidal particles can pass through ordinary filter paper. However, they cannot pass through a semi-permeable membrane.
• They are stable sols as their particles do not settle at the bottom if the sol is left undisturbed. However, by applying strong force sedimentation of colloidal particles can be caused.
• Their particles cannot be seen with the naked eye.
• Their particles show continuous zig-zag movement that is called Brownian movement. A similar charge on the colloidal particles causes this movement as they tend to repel each other due to the similar charge.
• The charge on colloidal particles is the same, however, it can be positive or negative. The dispersion phase also has a charge opposite to that of the dispersed phase's charge. If the charge of colloidal particles is positive, the charge on the dispersion phase or medium will be negative.
• The colloidal sols show the Tyndall effect as the colloidal particles scatter the beam of light that is passed through the sol.
• Colloidal particles do not coagulate, however, they can be coagulated by using electrolytes.
• Their particles are uniformly distributed throughout the colloid. They diffuse or tend to move from a region of higher concentration to a region of lower concentration in order to get distributed uniformly in the dispersion medium.
• The viscosity of a colloid is given by the Einstein equation. E = E₀ (1+2.5 V). E₀ = viscosity of the dispersion medium. V = vol. in parts of a cubic centimetre occupied by the dispersed colloidal particles.
• Colloidal sols show electrical properties such as electrophoresis in which the colloidal particles tend to move to oppositely charged electrodes when an electric current is passed.

Applications of colloids

Colloids offer a wide range of application in our day to day life. Some of which are described below;

Electrical precipitation of smoke: The harmful particles present in the smoke of the factories are removed before the smoke is released into the air. The smoke is passed through a chamber that is fitted with plates with a charge opposite to that of smoke particles. The oppositely charged smoke particles are attracted by the charged plates and the particles lose their charge and settle down at the bottom of the chamber in the form of a precipitate. For example, the harmful charged colloidal particles like carbon get discharged and settle down at the bottom and clean hot air is discharged into the atmosphere.

Purifying water: There are lots of impurities present in water as dispersed phase or colloidal particles. These impurities are removed by using alum. When alum is added into the water, the impurities tend to clump together and water is left undisturbed to allow sedimentation of clumped impurities. Afterwards, the clean water is separated by filtration or decantation.

Sewage Disposal: The dirt and mud particles in sewage disposal also act as colloidal particles as they carry an electric charge. When they are passed through the plates carrying an opposite charge, they are coagulated as electrophoresis occurs. The unwanted particles settle down at the bottom as precipitate which is removed.

Medicines: Most of the medicines that we take are colloids. Actually, they are prepared in the form of colloids as it increases their surface area and thus their absorption by the body. Some examples are as follows;

• Argyrol, which is a silver sol, is an eye lotion.
• Colloidal antimony is used in kala-azar
• Milk of magnesia is used for various stomach disorders
• A copper colloid is used as an anticancer drug
• Cough syrups and antibiotics like penicillin, streptomycin are also given in the form of colloids to improve their absorption by the body.

Tanning of leather: The animal skin has positively charged particles, which gives it colloidal properties. The animal skin is soaked in tannin solution that contains negatively charged colloidal particles, so due to the opposite charges coagulation of colloidal particle of skin takes place and they settle down in the skin. It makes the leather hard and durable.

Rubber Industry: The latex, which is obtained from the rubber tree and used to produce rubber, is also a colloid. The rubber particles in latex have a negative charge. This charge is neutralized and thus rubber particles are separated from latex through coagulation.

Industrial Products: There are also many industrial products that are produced in the form of colloids such as paints, cement, lubricants, synthetics plastics, and more. Their colloidal nature gives them the required consistency and thickness.

Cleaning action of soaps and detergents: The cleaning action of soaps and detergents is also dependent on colloid formation. The soap or detergent particles form micelles when water is added. These micelles act as dispersed phase or colloidal particles that help remove the dirt or stains from the clothes.

Food industry: There are various food items in the food industry that are produced in the form of colloids such as jam, cheese, jelly, cake, ice cream, butter, whipped cream, etc.

Cosmetic industry: There are various cosmetics products that are also produced in the form of colloids such as face cream, moisturizers, body lotions, etc.

Medical treatment: They are used in the plastic surgery of different body parts and also used in dentistry and wound dressing material.

Artificial rain: The colloidal sols are also used to produce artificial rain. The clouds contain the colloidal water particles that carry a charge. When an electrolyte such as a silver iodide is sprayed from an aeroplane over the clouds coagulation of water particles occurs and they form water drops and start falling down in the form of rain. Electrified sand is also used to cause the coagulation of water particles.