Properties of Colloids for IIT JEE

A colloid is a mixture in which one substance is distributed in another substance, and not dissolved. One is called a dispersed phase and other is called the dispersing phase. Unlike solutions the dispersed phase is not dissolved but is microscopically visible. 

The term colloid in Greek means “glue like structure”. It was first used in 1860s to denote mixtures like starch and Gelatin. Colloids can be proteins, polymers, alloys, clouds, etc. They can be present in any physical state. They can be an aggregate of hundreds to thousands of molecules, or can be a single molecule with large atomic mass also. The dispersed phase is particles and can be also called colloidal particles (ranging from 1 to 1000mm diameter). The dispersed phase is distributed in such a manner that even after filtering or centrifugation, they are inseparable.

Colloids and suspension are often put together or examples of both are confusing. But there is distinct characteristic of both, that can easily differentiate the two. Properties such as size of dispersed particle, filterability, settling property, visibility, separation and diffusion are responsible for differentiating suspension from colloids.

Overview:

Different type of dispersed phase and dispersion medium forms different types of colloids:

There are two of the either methods to prepare the colloidal system, either create small dispersed particle by shaking, shearing or by condensation of dispersed particles. Thus:

1. On the basis of interaction between phases:

These classifications can be further divided into two groups: a) lyophilic colloids b) lyophobic colloids.

a) Lyophilic Colloids: The type of colloids in which the dispersed phase is having great affinity for the dispersion medium. Once both the phases come into contact, they pass into the colloidal state; also they are reversible in nature. These colloids are the most stable. Example: Gum, Starch, Gelatine.

b) Lyophobic Colloids: The type of colloids in which the dispersed phase has very less affinity for the dispersion medium. They do not form colloids so easily; special methods have to be implemented for their formation. They are irreversible, are unstable and require stabilization agent. Example: alloys, paints, etc.

Stabilization of Colloids:

When the two phases of colloids remain dispersed or indistinct and appear as one, they are said to be stable. Once they get distinct into two different phases and can be defined as two separated phases, such a colloid is termed as unstable. Generally, stability is hindered by one of the processes of sedimentation and aggregation, which are enforced by colloid’s tendency to reduce the surface energy. In order to achieve stabilization, interfacial tension between the colloid articles needs to be reduced.

Aggregation occurs as a result of attractive forces. (Vander Waals force between particles is more dominant over the repulsive force Electrostatic forces). In order to achieve stability against aggregation, mechanism named electrostatic stabilization and stearic stabilization comes into action. 

Electrostatic stabilization – It refers to the repulsion occurring due to like static charges. Both the phases have different charged affinities. The dispersing phase has electrical double layer at interface, and in the dispersed phase the particles are too small or have very less mass and kinetic energy to overcome the electrical charge of dispersing phase.

Steric stabilization – It refers to the repulsion that occurs as a result of coating of dispersed phase with polymer. Due to the coating, attractive forces reduce between particles and the range required for aggregation gets reduced.


Properties of Colloids:

1. Brownian Movement:

It is also known as Brownian motion, after the scientist who discovered it in 1827, named Robert Brown (a botanist). It is defined as a continuous random movement of microscopic particles dispersed in liquid or gas, particularly in a zig-zag manner. The theory of Brownian motion based on kinetic theory of matter, was elaborated by Einstein. The resulting equation correlates average square distance (movement) at identical time intervals.

The movement of dispersed particle result due to the impact of the surrounding of dispersion medium. The forces which impact the movement of dispersed particle are unequal in different direction, thus resulting into the zigzag movement. This type of movement is generally independent of the nature of dispersed (colloidal) particle. It is observed that this movement is extra rapid when colloidal particle size is less and the solution is less viscous. So, it is inferred that less viscous the dispersion phase, more extreme will be the Brownian movement. The cause can be explained as the unbalanced bombardment of particles by molecules of dispersion medium (example - water).

Importance of Brownian motion:

a) It provides the proof for kinetic theory of gases. The rapid movement and kinetic motion of molecules in liquid is well confirmed by the Brownian motion itself.

b) Avogadro’s number is easily determined. With the help of ultra-microscope, the number of colloidal particles can be counted in the given mass of colloidal mixture, which ultimately gives the number of molecule in one mole of substance.

c) Stabilize the colloidal mixture: As we know that more the Brownian motion, the less is the viscosity of the colloidal mixture. In similar manner, more vigorous the Brownian motion of colloidal particles, the more is the stability of the colloidal mixture. As the zigzag movement prevents the particle from settling, the colloid remains stable without the use of any stabilization agent.

d) Size of colloidal particle: Due to continuous rapid movement of colloidal article, they tend to colloid to each other and result into shear and stress which results in the reduction of their size. Also, it has been proved that under the influence of Brownian motion and gravitational force, colloidal particles get arranged or distributed in vertical columns depending upon the density of colloidal particle in dispersion medium.

2. Heterogeneity:

Unlike solutions, colloids are not a homogenous mixture. They are heterogeneous in nature, such that particles of dispersed medium and dispersion medium remain within their own surface boundaries and the two phases are distinct at a microscopic level.

3. Electrical properties:

Each and every particle present in any type of mixture, by default contains a charge on its surface. The particles of dispersed phase may contain throughout positive or negative charge, and the same opposite charge exists on particles of dispersion medium. It should be noted that throughout the phase the charge present on each and every colloidal particle will be the same. Following are some of the electrical properties of colloids:

a) Electrophoresis: As mentioned above, all particles in the colloidal mixture are charged by either of the charge. Hence, passing an electric current through colloidal mixture creates an external electric field in it. As a result of it, the charged particles tend to migrate towards oppositely charged electrodes and later on get discharged to give precipitate. Thus, this method can be used to separate the two phases of colloidal mixture, as ultimately all the particles lose its charge and result into coarse, agglutinated mass.

b) Electro-Osmosis: It is the phenomenon when the dispersion medium itself starts moving in the electrical field. This happens only when the movement of dispersed colloidal particle is prevented under certain situation. Electro-osmotic flow of dispersion medium particle is caused by the Coulomb force induced due to the electrical field on net mobile electric charge of the whole colloidal system.

c) Coagulation: The colloidal system remains stable for a longer time, the only reason being the repulsion between similarly charged large numbers of dispersed colloidal particles. If an electrolyte or oppositely charged solution is added, it causes neutralisation and charge loss resulting into precipitation. Such a process is called flocculation or coagulation.

4. Tyndall effect:

It is also known as Tyndall scattering. It was discovered in 19^th century and named after a physicist, John Tyndall. Basic layman definition describes it as the reflection of light by dispersed colloidal particle present in a transparent medium. The physicist’s observation was that when a strong beam of light is passed through a colloid, the path of light becomes visible and when the same is observed at the right angle of light beam, the dispersed particle appeare to be reflecting the light focused on them. Thus, the phenomenon was known as the Tyndall effect and illuminated or reflected path of light was called The Tyndall core.

The colloidal particles absorb the light energy and become self-luminous which is then further scattered from their surfaces. The path of light is visible from side only because the maximum intensity of scattered light is in the right-angled plane of original light source. The intensity of scattered light depends on the refractive indices of dispersed colloidal particle and dispersion medium. More the lyophobic the system, more is the Tyndall effect.
This phenomenon is responsible for comet tails, blue sea water, tinge of smoke, blue sky during the day, etc