Colloids

Double layer

A double layer is a structure that appears on the surface of an object when it is exposed to a fluid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object. The first layer, the surface charge (either positive or negative), comprises ions adsorbed onto the object due to chemical interactions. The second layer is composed of ions attracted to the surface charge via the coulomb force, electrically screening the first layer. This second layer is loosely associated with the object. It is made of free ions that move in the fluid under the influence of electric attraction and thermal motion rather than being firmly anchored. It is thus called the "diffuse layer".There are two types of double layers.They are as follows:

i)Helmoltz double layer

ii0Diffused layer model

Helmoltz double layer

When a solid say glass rod comes in contact with the solution,the glass surface adsorb hydroxyl ion from the solution and its surface gets negatively charged.The negatively charged glaa surface adsorb counter ions from the solution.Because of this,two layers of develop across the interface.Out of these two layers,one layer of charge is fixed at the interface called fixed art of double layer and the another layer that remains in solution side is known as movable part of double layer.These layers of charge is known ass Helmoltz double layer or parallel plate capacitor model because layers of charge are arranged in a parallel fashion.

Helmoltz Double Layer

Diffused layer model

According to this model,when a solud comes in contact with a solution of layer of charge developed on its interface,this surface consists either positive or negative charge.This layer of charge adsorb counter ion from the solution but due to the disorder in thermal energy in a solution,a diffused layer will be formed.The diffused layer consist of charge of both signs but net charge is equal and opposite to each other.It means electrified interface is neutral.Such type of layer of charge developed at the interface is known as diffused layer model.

Zeta Potential

When a solid comes in contact with the solution,there is a formation of the double layer at the interface of solid.Among these layers,one is the fixed part and another is the movable part.The potential originated at the interface of double layer is known as zeta potential.

The zeta potential is caused by the net electrical charge contained within the region bounded by the slipping plane, and also depends on the location of that plane. Thus it is widely used for quantification of the magnitude of the charge. However, zeta potential is not equal to the electric surface potential in the double layer because of the formation of the double layer at the interface at different locations. Such assumptions of equality should be applied with caution. Nevertheless, zeta potential is often the only available path for characterization of double-layer properties.

The zeta potential is a key indicator of the stability of colloidal dispersions. The magnitude of the zeta potential indicates the degree of electrostatic repulsion between adjacent, similarly charged particles in a dispersion. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation. When the potential is small, attractive forces may exceed this repulsion and the dispersion may break and flocculate.

Electrophoresis or Cataphoresis

The migration of disperesed phase(solute particles) under the influence of electric field in colloidal solution is known as electrophoresis or cataphoresis.

Applications

i)Electrophoresis is used to separate different sol particles from the colloidal solution.

ii)Electroplating of rubber from latex on metal part can be done by using electrophoresis.

Electroosmosis

IIf migration of disperesed phase is prevented by using semipermeable membrane then dispersion medium should migrate under the influence of electric field.This migration of dispersion medium under the influence of eectric field is known as electroosmosis.Electroosmosis can be used for the purification of the colloidal solution.

Question:Why lyophilic sol are more stable than lyophobic sol?

Both lyophilic and lyophobic sol particles are identically charged.Because of this identical charge<an alectrostatic repulsive force will be created between them so thet the particles cannot come closer and hence they do not combine.But in the case of lyophilic sol,there is a better solvent particle attracting affinity so thst lyophilic sol particles are sheated with solvent molecules.Due to this sheat formation,lyophilic sol are more stable than lyophobic sol.

Protective Colloids

Lyophilic sol are more stable than lyophobic sol so,to increase the stability of the lyophobic sol,a small amount of lyophilic sol are added.These addedlyophilic sol surround the lyophobic sol particles so that the lyophobic nature of sol particles changes to lyoplilic nature and increases stability.

These colloids that are added to lyophilic sol to increase the stability of lyophobic sol are called as protective colloids.Gelatin,albumen,starch,etc are some of the examples of protective collloids.The protective power of such colloidscan be measured by the hepl of gold number.Gold number is defined as the number of milligrams of protective colloids which just prevent yhe coagulation of 10 ml of gold soll when 1 ml of 10% solution of sodium chloride is added to it.Lesser is the value of gold number,higher will be its protective power.

References

Paula, P.Atkins & J.de. Elements of Physical Chemistry. New Delhi: Oxford University Press, 2009.