Page 175 - Introduction to Colloid and Surface Chemistry

164 The solid-liquid interface

CH 2COOR CH 2OH RCOONa
i I
CHCOOR' + 3NaOH = CHOH + R'COONa
! I
CH 2COOR" CH 2OH R"COONa
fat glycerol soap

The potassium soaps tend to be softer and more soluble in water than
the corresponding sodium soaps. Soaps from unsaturated fatty acids
are softer than those from saturated fatty acids.
Soap is an excellent detergent but suffers from two main
drawbacks: (a) it does not function very well in acid solutions because
of the formation of insoluble fatty acid, and, (b) it forms insoluble
2 4 2+
precipitates and, hence, a scum with the Ca " " and Mg ions in hard
water. Additives such as sodium carbonate, phosphates, etc., help to
offset these effects. In the last few decades soap has been partly
superseded by the use of synthetic (soapless) detergents, which do
not suffer to the same extent from these disadvantages. The alkyl
sulphates, alkyl-aryl sulphonates and the non-ionic polyethylene
oxide derivatives are perhaps the most important. Until recently, the
+
alkyl-aryl sulphonate (CH 3)2CH[CH 2CH(CH3)] 3C 6H 4SO3Na has
found widespread use, but, on account of its non-biodegradability, it
has been withdrawn in many countries in favour of the more
biodegradable linear isomer and other 'softer' detergents.

Mechanisms of detergency

A satisfactory detergent must possess the following properties:

1. Good wetting characteristics in order that the detergent may come
into intimate contact with the surface to be cleaned.
2. Ability to remove or to help remove dirt into the bulk of the
liquid.
3. Ability to solubilise or to disperse removed dirt and to prevent it
from being redeposited on to the cleaned surface or from forming
a scum.

The solid substrate to be cleaned may be a hard surface (e.g. glass,
metal, plastics, ceramic), or fibrous (e.g. wool, cotton, synthetic