Surfactants (Soaps and Detergents) and Physicochemical Properties - Surface and Colloid Chemistry: Principles and Applications - page 48

It thus must be noted that, when a surfactant is needed for any application, its solu-
bility characteristics, besides other properties, which should conform to the experi-
mental conditions, should be considered. Therefore, a detergent manufacturer, for
example, must be in constant collaboration with the washing industry. The surfactants
available in the industry are characterized by their area of application. For instance,
anionic surfactants are used in shampoos and washing, while cationics are used in hair
conditioners. Hair surface has a negative (−) charge, and thus cationics strongly adsorb
there, leaving a smooth surface. The charged end is oriented toward the hair surface,
and the alkyl group is pointing away (as depicted here).

Cationic detergent + hair: -------alkyl group … polar group(+)hair(−)

3.2.1.2 micelle Formation (in aqueous media)
The surfactant aqueous solutions manifest two major forces that determine their
behavior. The alkyl part, being hydrophobic, tends to separate out as a distinct phase,
whereas the polar part tends to stay in solution. The difference between these two
opposing forces thus determines the properties of the solution. The factors to be
considered are the following:

a. Interaction of the alky group and water

b. Interaction of the alkyl hydrocarbon groups among themselves

c. Solvation (through hydrogen bonding and hydration with water) of the
polar groups

d. Interactions between the solvated polar groups

Below CMC, the detergent molecules are present as single monomers. Above CMC,
they are present as monomers, C mono , in equilibrium with micelles, C mice . The micelle
with aggregation number, N ag , is formed from monomers:

N ag monomer = micelle (3.1)

N ag monomers, which were surrounded by water, aggregate together above CMC and
form a micelle. In this process, the alkyl chains have transferred themselves from the
water phase to an alkane-like micelle interior. This occurs because the alkyl part has
a lower energy in micelle than in the water phase:

A. Water phase: Alkyl chain in water, surrounded by water

B. Micelle: Alkyl chain in contact with neighboring alkyl chains

Thus, in the case B, the repulsion between the alkyl chain and water has been removed.
Instead, the alkyl-alkyl attraction (B) is the driving force for micelle formation. The
surfactant molecule forms a micellar aggregate at a concentration higher than CMC
because it moves from the water phase to the micelle phase (lower energy). The
micelle reaches an equilibrium after a certain number of monomers have formed a
micelle. This means that there are both attractive and opposing forces involved in