Detergency of Soap

What is Detergency?

The compounds which can remove dirt/oil from a material & keep it in solution by the means of suspension are called detergents. This property is called detergency.
It’s a property of a compound which is closely related with surface tension.
A satisfactory detergent must possess the following properties:

(1) Good wetting characteristics in order that the detergent may come into intimate contact with the surface to be cleaned.
(2)Ability to remove or to help remove dirt into the bulk of the liquid.
(3)Ability to solubilize or to disperse removed dirt and to prevent it from being re-deposited on to the cleaned surface.

Read this post to learn about Soap & Micelle

As we learned earlier soap molecules have a hydrophilic head (water seeking) & hydrophobic tail (water-repelling), it won’t be distributed in the water uniformly rather the hydrophobic tail would orient itself to the surfaces.

Hyphobic Head & Hydrophilic Tail

(The long alkaline chain is the hydrophobic and carboxylate ion is the hydrophilic part)

At the interfaces (ex. fabric water, oil water),the long alkyl chain tends to move away from the water phase as a result surface tension decreases because the force acts opposite to the inward pull of water molecules . So, we can say that soap is a surface active compound which reduces surface tension of water. And this property imparts detergency.

Let’s consider an oil drop on a fabric –

Different forces acting on an oil drop

Different Forces Acting on an Oil Drop

The shape of the oil drop will depend on the following 3 forces:
TL =The surface tension between the oil drop and surrounding aqueous phase
TS = The surface tension between the fabric & surrounding aqueous phase
TLS = The surface tension between the fabric and oil drop

We can see that if TLS + TL increases in relation to TS the area of contact between oil and fibre will decrease. This will lead the shape of the oil to be like a sphere with the decreasing of contact angle (θ)

When the forces are in equilibrium, their relationship is expressed by Young’s equation neglecting gravity effect:

Relation between contact angle and different forces

Here, 3 cases are considerable-

Contact angle & wetting properties (detergency)

Contact Angle & Wetting Properties (Detergency)

Hydrophobic Impurities

Here are two images to understand the concept clearly –

removal of hydrophobic dirt (detergency)

Removal of Hydrophobic Dirt (Detergency)

roll up mechanism (detergency)

Roll-Up Mechanism (Detergency)

  • The Hydrophobic tails saturates the dirt/oil & fabric.
  • Fibre water & oil water surface tension decreases.
  • The surface tension between fibre and oil is unchanged so from the equation (1) we will get a negative value of cos that means the contact angle is now more than 90
  • This unchanged surface tension between fibre and oil & repulsion between the negative charges of fibre water and oil water interface will cause the the oil to reduce it’s surface area by rolling up
  • The oil begins to lift from the fibre & is removed into the solution where it’s suspended by electrostatic repulsion.
  • The oil/dirt won’t be re-deposited because of the negative charged repulsion between oil/dirt water & fibre water interfaces

Hydrophilic (Polar) Impurities

  • The hydrophilic head orients itself to the polar dirt
  • A second layer is formed where the hydrophobic tail orients itself to the first layer of tails
  • The rest of the mechanism is similar to the previous system

REFERENCES

Books:
Dyeing and Chemical Technology of Textile Fibres by E.R. Trotman

Textile Preaparation & Dyeing by A K Roy Choudhury