Inorganic Components of Detergents
Builders and Other Additives
To use the detergents mentioned so far as end products, would be costly, and so
water is used to dilute these ingredients into the more consumer useable form. In powdered
detergents, sodium sulphate is used as a diluting agent, it is cheap, and is user
friendly. Although it has a slight negative effect on detergency, other products can be
added to over come this, CMC (carboxymethylcellulose) is one such additive.
Builders enhance the detergency action, and these may be relatively cheap, so
that less can be used of the more expensive detergents of high activity. The
builder can be inorganic or organic.
Inorganic builders can be put into 5 groups:
Oxygen Releasing Materials
There are two classes of phosphates - orthophosphates and complex phosphates.
Orthophosphates - trisodium phosphate, and disodium phosphate. Both can be anhydrons or
hydrated, e.g. Na3Po4 or Na3Po412H2O.
Trisodium phosphate is little used these days, because phosphates cause
eutrophication of water. It has the property of softening water by precipitating metallic
ions, as a gelatinous precipitate. It can also help in dispersing soil, and in the
saponification of fatty acids.
Disodium phosphate produces lower alkalinity and is rarely used.
Trisodium phosphate can co-crystalise with sodium hypochlorite (Na3Po412H2O)5
NaOCl. This then has the added advantage of a bleaching agent.
Complex (or condensed) Phosphates
These have a lower alkalinity than trisodium phosphate. The commonly used
complex phosphate are:
Tetrasodium pyrophosphate Na4P2O7
Sodium tripolyphosphate Na5P3O10
Sodium tetraphosphate Na6P4O13 (hygroscopic)
Sodium hexametaphosphate (NaPO3)6 (hygroscopic)
Tetrasodium pyrophosphate is the more alkaline, alkalinity reducing on
descending order. They produce metallic complexes with metallic ions. This can be written
ionically as Na2(MgP2O7). This, the magnesium ion in this instance is inactivated, or
sequestered. The product formed does not precipitate from solution.
Tetrasodium pyrophosphate is best for magnesium ions, and sodium
hexametaphosphate is best for calcium ions. Sodium tripolyphosphate lies in between.
All complex phosphates enhance the detergency of soap type detergents. If a
piece of cloth has imbedded in it some calcium in the form of an insoluble calcium soap,
the phosphate molecule re-dissolves the calcium, releases sodium which re-combines with
the soap molecule, and thereby regenerates useable soap. This is achieved by the sodium
combining with the fatty acid anion of the soap molecule.
This adding to the detergency is called
synergism. This means that if we have two ingredients:
1 gram/litre DDBS; or
1 gram/litre sodium tripolyphosphate; or
0.5 grams/litre of DDBS and sodium tripolyphosphate;
...it will be found that washing clothes in the third solution will be
cleaner and brighter, than if washed in either of the first two.
Complex phosphate can deflocculate and keep in suspension insoluble materials
like clay, and emulsify oily materials. Related to this property of deflocculation is the
property of petization, this being the property to keep finely divide particles (solids)
in suspension, and prevent them of recoagulating. One problem though, is the reversion of
complex phosphates back to orthophosphates. This is reasonably rapid for
hexametaphosphate, slower for the pyrophosphates. For liquid detergents, tetrapotassium
pyrophosphate is favoured, having a solubility of 60% in water.
Phosphate have become out of favour biologically because of eutrophication, but
suitable alternatives are hard to find that will completely replace them.
The addition of silicates to synthetic detergents has proved very beneficial.
Sodium silicate is made (via the following chemical equation) in an electric furnace.
Na2CO3+SiO2 = Na2SiO3+CO2.
There is also a wet process, whereby silica from sand is leached out under
pressure by concentrated caustic soda. SiO2+SiO2 = Na2SiO3+CO2. This produces crystalline
silicates, which contain water of crystallisation.
Silicates soften water by the formation of precipitates that can be easily
rinsed away. They tend not to deposit on the fibres of the cloth being washed, as they
have great suspending and anti-re-deposition qualities. They are used in dish-washing
powders, for their wetting and emulsifying properties. All silicates have excellent
buffering action against acidic compounds. This is important, because most soils in
laundering processes are acidic.
Silicates can inhibit the corrosion of stainless steel and aluminium by
synthetic detergents and complex phosphates. Sodium metasilicate is a commonly used
powdered silicate, and can be anhydrous or hydrated. Typical formula is Na2SiO3.
Another group, the colloidal silicates, are available in concentrated liquids,
and are known as water glass. They have varying ratios of Na2O:SiO2, from
1:1.6 to 1:3.75. The higher the proportion of silica present, the less soluble the
material becomes, and the lower the pH.
Soda ash (sodium carbonate) Na2CO3
Soda ash provides high alkalinity, only sodium hydroxide being higher on a w/v
basis. It softens water by precipitation of calcium and magnesium carbonates, provided the
pH of the solution is greater than pH9, and that this pH is maintained. Synthetic soda ash
(chemically produced) is of superior quality to mined natural soda ash.
Two grades are commonly used, light soda ash, and dense soda ash. Light soda ash
particularly, can absorb large amounts of liquid material onto its surface and still
remain dry to the touch, and keep its free flowing properties. It is also used as a
neutralising agent for the absorption of DDBSA (anionic surfactant).
Sodium bicarbonate is rarely used, and potassium carbonate is
used in polish manufacture as a source of alkalinity, because it is more soluble.
Oxygen Releasing Compounds
Sodium Perborate Na2B2O4(H2O2)26H2O or Na2B2O4(H2O2)2H2O
Sodium perborate releases nascent oxygen at elevated temperatures, and so acts as a
hydrogen peroxide bleach, and has been used in laundering as a bleach for many years.
Its main disadvantage is that the bleaching action only takes place at elevated
temperatures. To release its bleaching action at lower temperatures, an activator
must be added.
Typical are N1N1 tetra-acetyl methylene diamene, and tetra acetyl glycol uril
(TAGU). In certain parts of the world perborate is restricted because of its boron
salts effecting agriculture when effluent is sprayed onto pastoral land.
Sodium Percarbonate 2Na2CO3 3H2O2
This works in solution as if you had sodium carbonate and hydrogen peroxide added
separately. Beneficial is that percarbonate releases oxygen at a lower temperature, and is
effective as a laundry bleach.