Properties of Colloidal Solutions

Colloidal solutions, also known as colloidal suspensions, are a mixture in which the compounds are suspended in a fluid in a regular pattern. A colloid is a very small particle that is consistently distributed throughout another substance.

Colloidal systems can exist in all three phases of matter: gas, liquid, and solid. A colloidal solution, on the other hand, usually refers to a liquid mixture. The size of the constituent elements is the major distinguishing feature between a real solution and a colloidal solution.

A colloidal suspension is another name for a colloidal solution. It’s characterised as a mixture of particles from a variety of different substances. Colloidal solutions are made up of particles of a size intermediate to that of a real solution, ranging from 1 nm to 500 nm. In many situations, it is classified as a two-phase, heterogeneous system or a homogeneous system.

Properties of Colloidal Solutions

1. Colligative properties: Because colloidal particles are larger aggregates, the number of particles in a colloidal solution is lower than the number of particles in a true solution. As a result, when compared to true solution values at the same concentrations, the values of colligative properties are of low order.
2. Tyndall effect: When a homogeneous solution placed in darkness is observed in the direction of light, it appears clear; when observed from a direction at right angles to the direction of the light beam, it appears perfectly dark. Colloidal solutions may appear reasonably clear or translucent when viewed at right angles to the passage of light, but they exhibit a mild to strong opalescence when viewed at right angles to the passage of light. This is known as the Tyndall effect. Tyndall cone is the name given to the bright cone of light. It is because colloidal particles scatter light in all directions in space. In the colloidal dispersion, this light scattering illuminates the path of the beam.
3. Colour: The wavelength of light scattered by the dispersed particles determines the colour of the colloidal solution. The wavelength of light is also affected by particle size and nature. The colour of the colloidal solution changes depending on how the light is received by the observer. A mixture of milk and water, for example, appears blue when viewed through reflected light and red when viewed through transmitted light. The finest gold sol is red in colour; as the particle size increases, it becomes purple, blue, and finally golden.
4. Brownian movement: When colloidal solutions are viewed through a powerful ultramicroscope, the colloidal particles appear to move in a continuous zig-zag pattern across the entire field of view. This is known as the Brownian movement, and it is independent of the nature of the colloid but is affected by particle size and solution viscosity. The faster the motion, the smaller the size and the lower the viscosity. The Brownian movement has been explained as a result of the particles being bombarded unbalanced by the molecules of the dispersion medium. The Brownian movement has a stirring effect that prevents particles from settling and is thus responsible for the stability of sols.
5. Charge on colloidal particles: Colloidal particles are always electrically charged. This charge is the same on all particles in a given colloidal solution and can be positive or negative.
6. Electrophoresis: Electrophoresis experiments confirm the presence of charge on colloidal particles. When a potential difference is applied across two platinum electrodes immersed in a colloidal solution, the colloidal particles gravitate towards one of the electrodes. Electrophoresis is the movement of colloidal particles under an applied electric potential. Positively charged particles are drawn to the cathode, while negatively charged particles are drawn to the anode. When electrophoresis, or particle movement, is prevented, the dispersion medium begins to move in an electric field. This is known as electroosmosis.
7. Coagulation or precipitation: The presence of charge on colloidal particles accounts for the lyophobic sols’ stability. If the charge is removed in some way, the particles will come closer together to form aggregates (or coagulate) and settle down under the force of gravity.

Coagulation (or Sol precipitation)

The process of settling colloidal particles is known as coagulation or sol precipitation.

Coagulation of lyophobic sols can be accomplished in the following ways:

1. By electrophoresis: Colloidal particles are discharged and precipitated as they move towards oppositely charged electrodes.
2. By mixing two oppositely charged sols: When oppositely charged sols are combined in nearly equal parts, their charges are neutralised and partially or completely precipitated. When hydrated ferric oxide and arsenic sulphide are mixed, they precipitate. Mutual coagulation is the name given to this type of coagulation.
3. By boiling: When a sol is boiled, the adsorbed layer is disturbed due to increased collisions with dispersion medium molecules. This reduces the charge on the particles, causing them to settle in the form of a precipitate.
4. By persistent dialysis: Traces of the electrolyte present in the sol are almost completely removed during prolonged dialysis, causing the colloids to become unstable and eventually coagulate.
5. By the addition of electrolytes: Colloidal particles precipitate when an electrolyte is added in excess. The reason for this is that colloids interact with ions that have a charge opposite to that of the colloids. This causes neutralisation, which leads to coagulation. The coagulating ion is the ion responsible for neutralizing the charge on the particles. A negatively charged ion causes positively charged sol to precipitate, and vice versa.

Emulsions

Emulsions are colloidal liquid-liquid systems that consist of finely divided droplets dispersed in another liquid. When two immiscible or partially miscible liquids are shaken together, a coarse dispersion of one liquid in the other is obtained, which is known as an emulsion. 

In most cases, one of the two liquids is water. Emulsions are classified into two types.

• Oil dispersed in water (O/W type)
• Water dispersed in oil (W/O type)

Water serves as the dispersion medium in the first system. Milk and vanishing cream are examples of this type of emulsion. The liquid fat is dispersed in water in milk. Oil serves as a dispersion medium in the second system. Butter and cream are two common examples of this type. Oil-in-water emulsions are unstable and sometimes separate into two layers when left standing. A third component known as an emulsifying agent is usually added to an emulsion to stabilize it. Between the suspended particles and the medium, the emulsifying agent forms an interfacial film.

Any amount of the dispersion medium can be used to dilute emulsions. When the dispersed liquid is mixed, it forms a separate layer. Emulsion droplets are frequently negatively charged and can be precipitated by electrolytes. They also demonstrate the Brownian movement and the Tyndall effect. Emulsions can be separated into constituent liquids by heating, freezing, centrifuging, and other methods.

Colloids around us

The majority of the substances we encounter in our daily lives are colloids. Colloids make up a large portion of the food we eat, the clothes we wear, the wooden furniture we use, the houses we live in, and the newspapers we read.

Examples of colloids

• The blue colour of the sky: Dust particles and water suspended in the air scatter blue light, which reaches our eyes and causes the sky to appear blue to us.
• Fog, mist and rain: Moisture condenses on the surfaces of dust particles when a large mass of air containing dust particles is cooled below its dewpoint. These colloidal droplets continue to float in the air as mist or fog because they are colloidal in nature. Clouds are aerosols composed of small droplets of water suspended in the atmosphere. Colloidal droplets of water grow larger and larger in size as a result of condensation in the upper atmosphere, eventually falling like rain. When two opposingly charged clouds collide, rain can fall.
• Blood: It’s a colloidal solution of an albuminoid compound. The styptic action of alum and ferric chloride solution is caused by blood coagulation, which forms a clot that prevents further bleeding.
• Soils: Humus acts as a protective colloid infertile soil, which is colloidal in nature. Soils absorb moisture and nutrients due to their colloidal nature.
• Formation of delta: River water is a colloidal clay solution. A variety of electrolytes can be found in seawater. When river water meets seawater, the electrolytes in the seawater coagulate the colloidal solution of clay, causing it to deposit and deltas to form.

Sample Questions

Question 1: What are the types of colloidal solutions?

Answer:

Colloids are classified into sol, emulsion, foam, and aerosol. Sol is a colloidal suspension of solid particles suspended in liquid. Emulsion is a mixture of two liquids. When a large number of gas particles are trapped in a liquid or solid, foam is formed. Aerosol is made up of small liquid or solid particles dispersed in a gas.

Question 2: What is the Brownian movement?

Answer:

When colloidal solutions are viewed through a powerful ultramicroscope, the colloidal particles appear to move in a continuous zig-zag pattern across the entire field of view. This is called Brownian movement.

Question 3: Write the dispersed phase and dispersion medium of paints.

Answer:

The dispersed phase is solid, and the dispersion medium is liquid.

Question 4: Write the dispersed phase and dispersion medium of butter.

Answer:

In solidified butter, liquid is dispersed in solid. As a result, liquid is the dispersion medium and solid is the dispersed phase.

Question 5: Give reasons why physisorption decreases with an increase in temperature.

Answer:

Physisorption decreases as temperature rises due to weak van der Waals forces of attraction that weaken as temperature rises.

Question 6: Explain electrophoresis.

Answer:

Electrophoresis is the movement of colloidal particles in an electric field towards a positive or negative electrode. It happens when colloids have a positive or negative charge.