Composition of Detergents

Composition of Detergents

As the vast majority of soilings are oily in nature, what is needed is a way to make those substances more soluble in water, so they can be washed away. To that end we introduce a detergent to water and the resulting solution is used to emulsify the oily soilings. An effective detergent must be capable of four basic cleaning functions:

  • Since most soiling is acidic in nature, the detergent must neutralise the acidic soiling
  • Emulsify oily soilings into suspended water-dispersible particles
  • Prevent flocculation of particulates like dust, clay and soot.
  • Maintain a stable suspension of soilings in water so that redisposition onto the surface doesn’t occur.

In order to achieve these functions, detergents are formulated with two fundamental components: Surfactants and Builders. The combinations of these two components are the basis of all detergent formulations. It must be pointed out that there are a number of other components in a detergent composition, like colourants, preservatives, biocides and inhibitors.

Surfactants

As we have already seen, water alone cannot clean oily soiling to a desirable level. That isn’t to say that water cannot clean, a water jet from a pressure washer relies on hydraulic pressure to remove soilings from a surface – with the hope that the water jet doesn’t damage the surface. Nevertheless, the cleaning ability of water can be vastly improved by the small addition of surfactants. The choice of surfactants used depends on the surface, the method of application and agitation and temperature of use.

sds moleculeThe word surfactant is a contraction of “Surface active agent” and as the name suggests, surfactants modify the surface of the liquid it is dissolved in (water in this case), reducing the surface tension and allowing oils and water to ‘mix’, forming an emulsion. This is a very basic mechanism for how surfactants work, for a more detailed study, download my paper on the mechanism of one well known surfactant, Sodium Dodecyl Sulphate (SDS).

There are hundreds of different surfactants, but they are broadly divided into four classes: Anionic, Cationic, Nonionic and Amphoteric. Anionic surfactants have a negative charge, Cationic surfactants have a positive charge, Nonionic surfactants have no charge (that is ionic charge) and Amphoteric surfactants can either be positive or negative charged.

Builders

Builders are added to cleaning products to enhance the performance of the surfactant mixture properties. They accomplish this through several ways:

  • Firstly, they soften water. That is, builders precipitate or sequester hard water ions, calcium and magnesium and prevent them from interfering with the surfactants.
  • Secondly, builders are alkaline and thus act as a buffer for acidic soils by maintaining a high pH in the cleaning solution. As the cleaning solution is used to dissolve and emulsify soils, the pH of the solution drops into the acidic region, reducing the effectiveness of the surfactant mixture.
  • Thirdly, builders help to break up and disperse particulates and large clumps of soils.
  • Fourthly, builders tend to prevent dispersed soils from redepositing or flocculating together. This is because builders impart an increased negative charge on the soils, which cause adjacent particulates to repel each other. Most surfaces carry a negative charge and so builders help to prevent redeposition of soils on the surface.