Most
of us use shampoo daily to clean
our hair. When unwashed for a few days, our hair
becomes cased in a layer of grease, appearing
dull, unkempt and generally unpleasant. Further
prolonging of going
au naturel
often means itchy
scalps and eventually, matted hair. Once shampoo
is applied, our hair can return to its former shining
glory. What happens chemically, when we wash our
hair with shampoo and what exactly is in shampoo?
The human scalp contains sebaceous glands
that secrete a greasy, waxy substance called
sebum, made primarily of triglyceride oils and
wax. When sebum coats the cuticle or outer
keratin layer of the hair strand, it can protect
hair and hair follicles. However, when there is
an excess amount of sebum on your hair, the
hair strands tend to stick together, creating a
greasy and stringy look. Using water to clean
the sebum is essentially ineffective. Sebum
creates a hydrophobic barrier on the surface
of the hair, akin to a pseudo-waterproof
layer. Hydrophobic molecules are electrically
neutral and non-polar and water is
a polar molecule. This means that
sebum molecules tend to cluster
together when submerged in water
and therefore, insoluble, making
it very difficult to remove sebum
with water alone.
This is where shampoo comes
in. Shampoo contains a blend
of surfactants that can act as
detergents, foaming agents
or emulsifiers. The surfactants
i n s hampoo a r e o r gan i c
compounds that conta i n
both hydrophobic groups
and hyd roph i l i c g roups .
The hydrophob i c g roups,
conta i n i ng a fat t y cha i n,
are insoluble in water and
soluble in oil. In contrast, the
hydrophilic groups, containing
the polar head, are soluble to
water and insoluble in oil. The
hydrophobic groups attach to
the sebum coating of the hair and
other greasy substances, whilst the polar heads
attach to water molecules. When the hair is rinsed,
the detergent is washed away by the water, along
with sebum and other undesirables.
Aside from surfactants, modern shampoos also
include other ingredients such as sodium chloride,
which ser ves the roles of adjusting viscosity as
well as acting as a preservative. The surfactants
in shampoos organize themselves in structures
called micelles. Many shampoos contain anionic
surfactants, where there exists an overall charge
density. Positive sodium ions from
sodium chloride help to lower
the charge density of the surface
of the micelles, allowing a closer
packed structure, hence, a thicker solution.
This is, of course, a simplification of the
process, as the tolerance to salt varies in
different shampoo solutions.
Shampoo does i ndeed keep ou r
hair clean. However, the chemicals in
shampoo are specifically designed
to s t r i p away the natu ra l o i l s
in our hai r. Over-washing hai r
can i nduce t he s ebaceou s
glands to secrete more sebum
a n d ove r compe n s a t e f o r
t he s t r i pped o i l s . E x pe r t s
r e comme n d a s pe c t r um
o f d i f f e r i ng op i n i on s on
the opt imal f requency of
shampooing hai r, but the
general consensus seems to
indicate that shampooing
once ever y other day i s
enough to keep hair clean,
wh i l e not ha r m i ng t he
essential oils that keep our
hair strong and healthy.
Further Reading:
Dissecting
Shampoo
What does
shampoo do for
our hair?
By Oi Ying Wong
