The Basic Principles of Detergents

The Basic Principles of Detergents

Water – The universal Solvent?

Water is a unique substance, with properties that sets it apart from virtually every other substance. The key property of interest to the science of cleaning is its polarity, which must not be confused with ionic charges normally associated with salts.

800px-Hydrogen-bonding-in-water-2DA water molecule is comprised of one oxygen atom and two hydrogen atoms, the oxygen atom is far bigger than the two hydrogen atoms and thus the larger oxygen nucleus exerts a larger attractive pull on the electrons in the molecule. This effect results in the oxygen atom being slightly negative (d-) and the hydrogen atoms being slightly positive (d+) – creating a polar (dipole) molecule.

Water: a polar molecule

The polar nature of the water molecule causes it to become electrostatically attracted to other water molecules as well as other ionic substances or charged solids. The electropositive hydrogen atoms on the water molecule will be attracted to the electronegative oxygen atoms on the adjacent water molecule. This form of attractive interaction is called hydrogen bonding and is the principal cause of the unusual properties of water and underlines the key physical mechanism to waters solvent ability, even though a hydrogen bond is only 10% of the strength of a covalent bond.

Without this phenomena occurring, life simply couldn’t exist, water wouldn’t be liquid at temperatures we are normally comfortable, it would be a gas. Also many biochemical reactions in our body can only place due to water’s solvent ability. 

Because of this effect, water acts as a solvent and can dissolve a wide range of ionic substances. An obvious example is table salt, Sodium Chloride – sodium +ve ions and Chlorine –ve ions. However, water cannot dissolve non-polar substances, substances like oils and greases, and in fact it appears to ‘repel’ all such substances.

Although not strictly accurate, the old adage “Like dissolves like” infers that only ionic and polar substances will dissolve in water. While this is largely true, it does imply that non-polar substances like oils, non-halogenated alkanes and long chain, saturated hydrocarbons are repelled; this is simply not the case. Very large molecules like oils possess very large distributions of electrons, which collectively can induce Van de Waals dispersion forces (Short range interactions) with adjacent non-polar molecule. However, it is energetically favourable for water molecules to form hydrogen bonds with each other, than it is to interact with the non-polar Van de Waals dispersion forces, therefore the oil phase in the water bulk phase separates.

Surface Tension

When an ionic or polar substance is added to water, water molecules surround the substance in such away that unlike charges form hydrogen bonds. Due to the relative small size of a water molecule, large numbers of water molecules surround it.

In the aqueous bulk phase, water molecules form hydrogen bonds in all three dimensions with neighbouring water molecules. This results in no overall attractive force or pull as its effects are ‘felt’ in all directions simultaneously and with the same magnitude.

surface tensionHowever, water molecules at the surface or phase boundary (interface) experience an imbalance of forces towards the bulk phase. This imbalance creates the appearance of a ‘film-like’ surface called surface tension, sometimes called interfacial tension. As the forces collectively pull the overall surface water molecules towards the bulk phase, the water will adopt a shape that minimises the surface area and therefore maximises the number of water molecules in the bulk. Geometrically the sphere has the smallest surface are per unit volume and so droplets of water will form spheres without any additional sources of external forces.

Wetting

The whole concept of surface tension is important to understand, but equally another concept called ‘wetting’ must be understood. Wetting is the interaction water, in the form of droplets, has with different solid materials. For example, a drop of water on a flat sheet of glass (figure 2) will have the tendency to ‘spread out’. By this we mean that the contact angle of the drop of water to the surface will approach zero. Whereas the same sized drop of water on a flat sheet of polypropylene will have the tendency to reduce the contact area, giving rise to a large contact angle.

The underlining explanation for this effect depends on whether the solid surface is polar or not. Glass is highly polar and so allows water to form hydrogen bonds with the silicon dioxide, whereas polypropylene is non-polar. Another way to view this is charged polar surfaces are high energy hydrophilic surfaces and non-polar surfaces are low energy hydrophobic surfaces.

Any substance, surface or compound that is non-polar is hydrophobic, like-wise any substance, surface or compound that is polar is hydrophilic. An unusual effect is observed when a hydrophobic substance is added to water, although it is strongly unfavourable, it is slightly favoured by enthalpy but strongly opposed by a large negative change in entropy (at STP).

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 (amongst others): Surfactants and Builders. The combinations of these two components are the basis of all detergent formulations.

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.

The 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.

Sodium Dodecyl Sulphate: A very widely used surfactant in detergents

sds moleculeThe surfactant sodium dodecyl sulphate belongs to one of four broad classes of surfactants called anionic surfactants. These molecules are composed of hydrophobic organic chains “tail” and a polar hydrophilic “head”. In the case of an anionic surfactant, the polar head (carboxyl and sulphonate groups) carries a negative charge which readily forms hydrogen bonds with water molecules in solution. The sodium ion, which fully ionises in solution is a mere spectator. Cationic surfactants typically carry a positive charged quaternary ammonium head and is an effective counterion for chlorine or bromine ions needed for disinfection. Nonionic surfactants do not fully ionise in solution, but rely on ester groups for enhanced water solubility through a process called ethoxylation; the greater the degree of ethoxylation, the greater the water solubility.

 

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.