Wetting, Adsorption, and Cleaning Processes - Surface and Colloid Chemistry: Principles and Applications - page 136

mechanical movement and agitation of the machines is coordinated with the soap/
detergent characteristics.
Typically, there are five different compositions of laundry detergents, shampoo,
and dishwashing powder:

laundry Shampoo dishwashing

Detergent (alkyl sulfate) 10-20 25 —

Soaps 5 — —

Nonionics 5-10 1-5

Inorganic salts (polyphosphate, silicates) 30-50 50 —

Optical brighteners <1 — —

It is worth noticing that the aim of detergents in these different formulations is dif-
ferent in each case. In other words, detergents are today tailor-made for each specific
application (Ruiz, 2008). The detergents in shampoo should give stable foam in order
to increase the cleaning effect. On the other hand, laundry detergents in dishwash-
ing should exhibit a lower surface tension but almost no foaming (because foaming
would reduce the cleaning effect). Hence, in dishwashing formulations, nonionics are
used, which are not very soluble in water and thus produce very little (or no) foam.
These are sometimes of type EOEOEOPOPOPO (ethylene oxide (EO)-propylene
oxide (PO)). The propylene group behaves as apolar, and the oxide group behaves
(through hydrogen bonding) as the polar part. These EOPO types can be tailormade
by combining various ratios of EO:PO in the surfactant molecule. In some cases,
even butylene-oxide groups have been used.
Additionally, in the case of shampoos, other criterion has to be strictly controlled.
Shampoos are used to wash hair and are designed to remove dirt without damaging
the hair. They should also have no skin or eye irritation effect. Baby shampoos are
particularly manufactured using surfactants that exhibit minimum eye irritation.
Soil consists mainly of particulate, greasy matter. Detergents are supposed to
keep the soil suspended in the solution and restrict redeposition. Tests also show that
detergents stabilize suspensions of carbon or other solids such as manganese oxide in
water. This suggests that detergents adsorb on the particles. Detergents add redeposi-
tion controllers such as carboxymethylcellulose.
In the early age of detergent usage during the 1960s, too much sewage treatment
showed foaming problems. Later, detergents were used with better degradation prop-
erties and better control. For example, straight-chain alkyls were more degradable
than branched alkyl chains.

6.3.2 W a T e r r e p e l l e n c y o f m a T e r I a l S

In many cases, such as with umbrellas and raincoats, the material used must be non-
wetting in water. As mentioned earlier, if the contact angle is

Larger than 90°, a nonwetting solid surface is present.
Less than 90°, a wetting solid is present.