Chemistry, Third edition

SOLUBILITY 175

BOX 11.1

Soaps and detergents

What are soaps?

Soapsare salts of fatty acids. The commonest soap is sodium
stearate, made by reacting sodium hydroxide (caustic soda)
with stearic acid:

NaOHC 17 H 35 COOHC 17 H 35 CO 2 ,NaH 2 O

sodium stearic acid sodium stearate

hydroxide

The sodium stearate dissociates into sodium and stearate
ions in solution.

The C 17 H 35 – part of the soap consists of a long chain of
carbon atoms, with the COONaforming an ionic ‘end’ to the
molecule. The full structure of soap may be written as

or as

What are detergents?
Detergents are soap substitutes made in the oil industry. They
possess similar molecules to soaps, with the sulfonate group
(OSO 3 ) being the commonest ionic group. For example, the
detergent sodium lauryl sulfate (C 17 H 35 OSO 3 ,Na) has the
following structure:

The sodium lauryl sulfate dissociates into sodium and lauryl
sulfate ions in solution.

Cleaning action of soaps and detergents
Dirt is a greasy material in which waste solid particles get
stuck. Water cannot dislodge dirt (Fig. 11.3(a)). The cleaning
action of soap and detergents is identical and involves the
loosening of the solid particles from surfaces (normally of
clothes or some plates or dishes) and can be thought of as
taking place in three steps:

1.The hydrocarbon end (the hydrophobic end) of the stearate
or detergent ion is attracted to the grease in the dirt. The
ionic (hydrophilic end) of the soap or detergent is attracted
to the water (Fig. 11.3(b)).

2.The stearate or detergent ion reduces the stickiness (the
so-calledinterfacial tension) between the dirt and the
surfaces of the clothes or dishes. This is done by forming a
layer of stearate or detergent ions on the clothes or dishes
(Figs 11.3(c) and (d)). As a result, grease droplets are
encouraged to enter the water phase.

3.The stearate or detergent ions surround small droplets of

grease and form tiny clumps (containing typically 50–100

ions) called ionic micelles(Fig. 11.3(d)). The overall result

is a colloidal dispersion of oil droplets which, together with

the dirt particles, can be physically washed away.

OSO– 3 Na+

COO Na– +

CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 COO Na – +

CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2

Fig. 11.3The cleaning action of soap or detergents. The

stearate or detergent ion is symbolized as.