Television would have us believe that shampoo will change our love life,
make us suave and sophisticated, or cure us of scratching our heads in
wonder and awe at how shiny our hair has become.
The purpose of a shampoo is to clean the hair. The shampoo must not
clean too well, or all of the protective oils in the hair would be
stripped out. It must not make the hair smell bad, despite the bad
smells of the detergents it is made of.
To sell well, the shampoo must look good, must feel thick or creamy
in the hands, and must produce a nice feeling lather. It must smell
nice, and not be too expensive.
Other selling points might be the herbal extracts currently in fashion,
or amino acids from exotic protein sources like silk or the milk of
The most common ingredient in shampoos is also the most common
in use in other products, a class of surfactants known as
straight-chain alkyl benzene sulfonates
. An example is
Ammonium Lauryl Sulfate
or its sodium relative, or the slightly larger related molecule
ammonium lauryl ether sulfate
, sometimes abbreviated as
ammonium laureth sulfate
These detergents work best in water that has little calcium and
magnesium, as these elements bind to the detergent and make an
insoluble scum. So
is used to
sequester the calcium and magnesium from the detergent, while keeping
them soluble, so they rinse away without scum.
(or MEA or TEA) is used
as a foaming agent, to make the lather. The other surfactants will
generate a certain amount of suds, but this foaming agent is added
to get the amount just right. Besides its foam stabilizing effects,
it is also a viscosity booster (it's thick).
Another foam stabilizing detergent is
which is a foam stabilizer, surfactant, and emulsifier.
is added for several of its special
properties. It is milder on the skin than the benzine sulfonates,
so adding it to the mix reduces the amount of the harsher detergents
needed. It is thicker than the other ingredients, so it can be
added to make the mix have the right viscosity. It has anti-static
properties, so the hair doesn't generate an electric charge and
jump to the plastic combs and brushes used when drying the hair.
It is a humectant, attracting moisture from the air, thus keeping
hair from drying out.
Lastly, it has antibiotic properties that can prevent spoiling of
is a hydrotrope
, a compound that makes it easier for
water to dissolve other molecules. It is added as a thickener,
and to help keep some of the odd ingredients added for marketing
effect in solution, including perfumes.
emulsifier used for this purpose.
is added to make shampoos opaque
and pearlescent. It has tiny flakes that mix well with surfactants,
and stay in solution. They also add shear-thinning qualities,
making liquid hand soaps pump out of the bottle easier.
Sodium chloride (table salt) is used to thicken the mixture if the
main surfactants are sodium lauryl sulfates. If the surfactants
are ammonium based, then ammonium chloride is used. Salt can make
the shampoo harsh and sting the eyes, so more expensive thickeners
are used to keep the salt levels low.
thickeners are often used, along with the surfactant based thickeners
is added as a humectant
(draws moisture from the air), as is
, which is also a
There are many additives put in shampoos and conditioners that appear
to be there mainly for marketing purposes. Honey, various herb
extracts, and similar items might add to the fragrance, but are
unlikely to have any effect in the concentrations used. Amino acids
can act as conditioners, but the source of the amino acid is not
important. Silk amino acids are no different from soy amino acids,
except in the proportions of which particular amino acids are
Two widely used preservatives,
are found in many shampoos, to prevent fungal and bacterial
spoilage. They release formaldehyde to kill germs.
Another broad-spectrum biocide is
and the related
preservative used in shampoos. It kills bacteria, fungi, and
yeasts, and works well in acidic mixtures.
Another bactericide used is
The surface of a strand of hair is covered with overlapping
sheets, somewhat like the scales on a fish, or the shingles on
a house. This surface is called the cuticle
Alkaline solutions raise these scales, so they stand up. This
makes the hair rougher, makes it look dull, and makes the hair
shafts stick together due to the rough texture.
Most shampoos are made slightly acidic, to keep the cuticle
smooth and lying flat on the hair shaft. Ingredients like
are added to acidify the shampoo.
As the shampoo mixes with the water in the shower or bath, or
mixes with dirt on the hair, it can become less acidic as the
acids mix with alkaline water or dirt. A compound that releases
more acidifying ions when the acidity gets low, or absorbs acid
when the acidity gets too high, is called a buffer
A typical buffering agent used in shampoo is
Since the goal is to keep the shampoo slightly acid, the term
"pH balanced" is actually a misnomer. We want the balance to be
tipped slightly to the acidic side.
Conditioners are compounds added to keep the hair cuticle smooth
Silicone oils such as
are used to make the hair shiny and slippery.
Humectants (moisturizers) like
help keep the cuticle moist, so that the scales do not stand up.
Long chain fatty alcohols like
lubricate the hair. One end of the molecule binds to the hair,
leaving the slippery fatty end on the outside to rub against
other strands of hair, or a comb.
Quaternary ammonium compounds are cationic surfactants that
bind well to anionic surfaces like the protien in hair.
The ammonium end sticks to the hair, leaving
the long fatty end of the molecule to act as a lubricant.
They are slightly conductive, so the reduce the buildup
of static electricity.
The "quats", as they are called, include compounds like
and they are all similar in form and function to
These compounds are also widely used as fabric softeners, for
all of the same reasons they make good hair conditioners. They
are also used to thicken the shampoo.
The emollient isopropyl palmitate
is used as a skin
softener, moisturizer, and as an anti-static agent.