Detergency of Soap: Potential Uses in Textile Wet Processing

Detergency of Soap
Anik Yusuf
Department of Textile Engineering
Ahsanullah University of Science & Technology, Dhaka
Email: anik.yusuf@gmail.com





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.
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.

(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.

Let’s consider an oil drop on a fabric
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 fabric will decrease. This will lead the shape of the oil to be like a sphere with the decreasing of contact angle (Ө)

When the forces are in equilibrium, their relationship is expressed by Young’s equation neglecting gravity effect:
The value of cosӨ varies from +1 (Ө=0 degree) through 0 (Ө=90 degree) to -1(Ө=180 degree)

Here, 3 cases are considerable-

As we can see there are 2 liquid phases (water and oil) now which liquid phase will displace the other one can be known by the contact angle. If the angle is more than 90 degree than the value of cosӨ is negative which means TLS > TS so obviously the water phase will start to displace the oil phase which is in contact with the fabric.

***To remove oil/dirt from a fabric, the soap solution must increase the contact angle which must be more than 90 degree

For Hydrophobic impurities:
Here are two images to understand the concept clearly -

Roll up:
  • 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 cosӨ that means the contact angle is now more than 90 degree
  • This unchanged surface tension between fibre and oil & repulsion between the negative charges of fibre water and oil water interface will cause 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
For 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 
 

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