Zwitterionic Surfactants: A Milder Alternative?

In recent months, we have taken an in-depth look at the structure and properties of cationic and nonionic surfactants.
Another interesting category of surfactants used in both hair and skin
care are the zwitterionic ones, those that naturally have two charges on
the molecule, both positive and negative. These are attractive to the
formulator due to their tendency to boost effects of other surfactants
in the solution, as well as an ability to ameliorate undesirable
properties of some surfactants, such as skin irritation and a tendency
to strip the hair and skin of too much moisture. One familiar surfactant
of this type is cocamidopropyl betaine, which is appreciated by many
curly-haired consumers for its gentle, yet effective, cleansing
capabilities.

What are they?

Zwitterionic surfactants (also: amphoteric surfactants) are
characterized by having two distinct and opposite charges on the
molecule at either adjacent or non-adjacent sites. The presence of both a
positive and negative charge renders the molecule overall
neutrally-charged at neutral pH. Some types of zwitterions are
susceptible to pH changes in a solution and may become completely
cationic or anionic in acidic or basic environments. The
positively-charged site is typically a quaternized ammonium ion, but can
also be a phosphonium ion, while the negatively-charged site can be one
of a variety of anionic groups, such as sulfate, carboxylate, or
sulfonate. There are several common categories of zwitterions used in
hair care formulations, such as the betaines and amphoacetates.

In general, amphoteric surfactants have been found to be compatible
with other surfactants and polymers, including silicones. The formation
of self-assembling complexes between amphoteric surfactants and polymers
or anionic surfactants has been observed and found to impart
interesting properties to solutions containing these molecules.
Amphoteric surfactants reduce static in hair by decreasing its surface
charge density. Since the interactions between hair and zwitterionic
surfactants are primarily physical rather than ionic, they are easily
rinsed and removed from the surface of the hair. They have been found to
minimize skin and eye irritation common to other surfactants,
especially sulfates. They also can boost the foaming performance of
anionic surfactant systems via a variety of mechanisms, by either
increasing the speed at which foam is formed (flashing), improving the
density and luxurious feel of the foam, or by increasing the foam
stability (longevity).

Types of amphoteric surfactants

 
The betaine family of zwitterions possesses the
positive-negative head group structure of trimethyl glycine (betaine),
an amino acid derived from sugar beets. The hydrophobic tail group can
be a straight chain alkyl group (such as in coco betaine), or can
contain an amido group, such as cocamidopropyl betaine. Other betaines
include lauramidopropyl betaine, oleamidopropyl betaine,
ricinoleamidopropyl betaine, cetyl betaine and dimer dilinoleamidopropyl
betaine. Additional variants are sulfobetaines, hydroxysulfobetaines
and sultaines. Betaines are more resistant to thickening via addition of
salts than their anionic cousins. For this reason, in order to achieve a
pleasingly thick product, addition of viscosity-boosting polymeric
additives may be necessary, which can increase the cost and complexity
of the formula.

Cocamidopropyl betaine is particularly valued for being an excellent
cosurfactant for sodium lauryl sulfate, but it is also a gentle cleanser
in its own right. Studies have found that it removes silicones from
hair very effectively, without drying out the hair. It has also been
shown to improve the solubility of sodium cocoyl isethionate, an
extremely gentle, creamy and emollient surfactant. The combination of
these two has the potential to create an extremely gentle and
moisturizing shampoo.

Betaines are more resistant to thickening via addition of salts than
their anionic cousins. For this reason, in order to achieve a pleasingly
thick product, addition of viscosity-boosting polymeric additives may
be necessary, which can increase the cost and complexity of the formula.

The specific betaine selected can have a significant impact upon the
viscosity, foaming behavior and detergency of the final product. In
order to choose the best betaine for her purpose, the formulator must be
familiar with the properties of each of her options and how each
interacts with the other ingredients in her formula. She will generally
have a goal in mind regarding the physical properties, cleansing
strength and cost of her formula and all of those will factor into the
decision. Fortunately, for consumers, the primary concern is how the
product feels on the hair and how the hair behaves afterward and the
differences should not be tremendous between the various betaines.
Currently, cocamidopropyl betaine is the one most often seen on labels
and in the proper formulation, and is quite gentle to hair and skin.

Other families of zwitterions are also used in formulations. They are
becoming more common as their interesting properties are explored and
as companies continue working to develop non-sulfate-based cleansing
platforms. Some familiar ones might be the imidazoline derivatives
alkylamphoacetates such as disodium lauramphoacetate, as well as
alkylamphopropionates. These materials share many of the beneficial and
gentle properties of the betaine family.

Overall, zwitterions are an interesting class of surfactants with the
potential for more growth in application, especially for curly hair,
which needs a more gentle approach to cleansing.

Consumer power

In recent years, there has been a growing demand for sulfate-free and
gentler shampoos and cleansing products, especially in the curly-haired
population, as we have learned the damage that can be done by harsh
surfactants. As formulators continue to respond to the push from
consumers for alternatives to sulfate-based cleansing systems, we can
expect to see a growing number of products relying upon the milder
cleansing properties of zwitterionic surfactants. They are less likely
to strip curly hair of its much-needed moisture and can impart a silky
feel to hair when used with other ingredients. A patent search reveals
that scientists are working to develop more polymer-zwitterion systems,
which should also eventually benefit the end product user by providing
performance-enhancing properties. In this demand-driven market, the
power consumers have to drive the research and development of new
products is quite remarkable and should not be underestimated.