Detergency

It is a process by which soil is removed from a surface and undergoes solubilization or dispersion.

Result of several physicochemical phenomenons taking place at the interface of three phases : surface/soil/detergent.

The phenomenons are :

– Wetting of surface

– Removal of soil from surface

 – Avoiding re-deposition of soil on surface.

Basic Principal:

1.Wetting of surface: The detergent must come into contact with the surface so that adherence forces of detergent with subtract of surface should be higher than  adherence forces of soil with subtract of surface

          F_{detergent/surface} > F_{soil /surface}   ( Here F = adherence force )

It lower the superficial tension of the detergent solution and the interfacial tensions between aqueous bath, soil and surface

2.Removal of soil:The detergent solution wets the surface, is absorbed by it and lowers the surface’s attraction to allow the soil to separate itself from the surface.

3.Avoiding re-deposition

Chemical reactions

 – lipids undergo saponification

 – mineral soil undergoes solubilization

 – soil undergoes emulsification

 Liquid soil = hydrophobic ; detergent solution = hydrophilic.

Preparation of detergent

During the preparation of detergents, long-chain hydrocarbon obtained from petroleum fractions is converted into an organic aid through a series of steps.  The organic acid is then neutralized with sodium hydroxide, NaOH solution to produce a detergent. Examples are of detergents are: Sodium alkyle sulphate and sodium alkylbenzene sulphonate.

(Above pictures taken from https://image.slidesharecdn.com/soapanddetegents-150725134721-lva1-app6891/95/soap-and-detegents-8-638.jpg?cb=1437832077)

Dispersing Agents (Solid in water):

These are organic compound. A solid material dispersed in a liquid requires an additive to make the dispersion process easier and more stable – this is the role of the dispersing agent, or dispersant. Dispersing Agents function in a manner similar to emulsifying agents. The difference is that solid particulate matter, rather than insoluble oils, is dispersed. The nature of the lyophilic part of the surfactant molecule must be such that it adsorbs onto the particle's surface. These assist the process of dye particles size reduction and help to dissolve dye particles in water. Pigment colour paste one of the most known example in which dispersing agent is used to disperse pigment in water. Disperse dye also contains dispersing agents.

How they work?

When we shake solid (like sand) in water, it seems that solid is dissolved. However after some time solid particles are separated out from water. These solid particles attract each other. That is why they come together and separate out from water. To separate these particles from each other, there is a need of energy. Also, solid particles must be stabilised after they have been separated from each other. The particles will move to each other and glue together again when particle-particle repulsion is insufficient. The spontaneous process of gluing together of solid particles in a liquid is called flocculation. The functionality of a dispersant is to prevent flocculation. Dispersants do their job because the molecules adsorb on the solid-liquid interface and assure repulsion between the particles.

What is the difference between Soaps and Detergent:

Soaps

Soap is made from fat and alkali by specification method. There are not suitable for delicate clothes.  They work well in hot water. They don't produce lather with hard water and are not able to clean the cloth. Lot of water is needed to remove soap from the cloth.

Detergents

Detergents are carbonic compounds which are not alkaline. They are suitable for delicate clothes. They clean both in hot and cold water. They clean the cloth even in hard water. They are removed very easily so less quantity of water is needed.

     3. Emulsifying Agents (Oil in water)

When water and oil are mixed together and vigorously shaken, a dispersion of oil droplets in water - and vice versa - is formed. When shaking stops, the phases start to separate. 

However, when an emulsifier is added to the system, the droplets remain dispersed, and a stable emulsion is obtained.

An emulsifier consists of a water-loving hydrophilic head and an oil-loving hydrophobic tail. The hydrophilic head is directed to the aqueous phase and the hydrophobic tail to the oil phase. 

2. Detergents

Detergents are surfactants that help remove soils from solid surfaces. Over and above reducing water's surface tension, detergents must adsorb onto the soil's surface to aid in spontaneous release. Detergents must also keeps the soil suspended to prevent re-deposition. 

Ø  Detergents are surfactants used  for removal of dirt.

Ø  Detergency involves:

•      Initial wetting of the dirt and the surface to be cleaned.

•      Deflocculation and suspension, emulsification or solubilization of the dirt particles
•      Finally washing away the 

dirt.http://d25smtqkk0nuqw.cloudfront.net/wp-content/uploads/2015/11/biorf11.gif

Following can be understand from the above figure:
1. Dirty Substrate (Any textile material)  come in contact with detergent or surfactant in the presence of water
2. Detergent molecules start coming toward dirt or soil. As the dirt is oily in nature hydrophobic tail come in contact with dirt and hydrophilic head of detergent molecule remain in water.

3. Slowly detergent surrounded dirt particles and pulling them out of substate
4. All the dirt particles removed from the substrate.

a)    Wetting agents: Wetting agent is a surfactant that when dissolved in water, lower the contact angle and reduce the surface tension of a liquid. According to the nature of the liquid and the solid, a drop of liquid placed on a solid surface will adhere to it or no. which is the wettability between liquids and solids. When the forces of adhesion are greater than the forces of cohesion, the liquid tends to wet the surface and vice versa.

To more clarification example of behavior of water and mercury in test tube can be taken. The surface of liquid water (meniscus) has a concave shape because water wets the surface and creeps up the side. The surface of Mercury has a convex shape it does not wet glass because the cohesive forces within the drops are stronger than the adhesive forces between the drops and glass.