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The rheology of matter
refers to its general ability to flow under applied forces. This is measured in
a lab using a rheometer. This works by applying stress to the matter, a
deformation then occurs and from this a value for viscosity can be calculated.
Within cosmetics, viscosity and rheology modifiers are a key part of the
chemist’s tool book to produce surfactant-shampoo products for consumers. 22

Rheology modifiers
are often referred to as thickening agents as this is their primary purpose
within cosmetics. The most common and cheapest method of thickening surfactant
micelle systems is using electrolyte salt (Sodium Chloride). Salt works by
reducing the micelle charge density, this results in the spherical micelles
being able to expand and instead form long rod-shaped aggregates. At this point
a solution may start to adopt sheer-thinning characteristics and experience
large increases in viscosity. This is because of the micelle aggregates
becoming so long they begin to randomly entangle with one another forming a
complex amorphous network. This will keep thickening up to a maximum where the
viscosity will then drop following further electrolyte addition. This is
believed to be consequence of branching that occurs between the micelles at
high ionic charge. The micelles are also believed to begin to break at a
quicker rate than what they can form resulting in a drop of viscosity. 23,24,26,41.

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It is also possible
to use amphoteric surfactants to aid the thickening process of anionic based
surfactant systems, this works on a similar premise to salt. They contain both
the anionic and cationic groups and thus can act by reducing the charge density
between the head groups.

Polymer based thickeners
are also available; these include hydrophilic molecules like Polyethylene
glycol. These works by forming inter linking bridges between the micelles
spheres. The micelles gradually become extremely interconnected with one
another eventually resulting in a very large increase in viscosity. 25   

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