The rheology of matterrefers to its general ability to flow under applied forces. This is measured ina lab using a rheometer. This works by applying stress to the matter, adeformation then occurs and from this a value for viscosity can be calculated.Within cosmetics, viscosity and rheology modifiers are a key part of thechemist’s tool book to produce surfactant-shampoo products for consumers. 22Rheology modifiersare often referred to as thickening agents as this is their primary purposewithin cosmetics. The most common and cheapest method of thickening surfactantmicelle systems is using electrolyte salt (Sodium Chloride).
Salt works byreducing the micelle charge density, this results in the spherical micellesbeing able to expand and instead form long rod-shaped aggregates. At this pointa solution may start to adopt sheer-thinning characteristics and experiencelarge increases in viscosity. This is because of the micelle aggregatesbecoming so long they begin to randomly entangle with one another forming acomplex amorphous network. This will keep thickening up to a maximum where theviscosity will then drop following further electrolyte addition.
This isbelieved to be consequence of branching that occurs between the micelles athigh ionic charge. The micelles are also believed to begin to break at aquicker rate than what they can form resulting in a drop of viscosity. 23,24,26,41.It is also possibleto use amphoteric surfactants to aid the thickening process of anionic basedsurfactant systems, this works on a similar premise to salt. They contain boththe anionic and cationic groups and thus can act by reducing the charge densitybetween the head groups.
Polymer based thickenersare also available; these include hydrophilic molecules like Polyethyleneglycol. These works by forming inter linking bridges between the micellesspheres. The micelles gradually become extremely interconnected with oneanother eventually resulting in a very large increase in viscosity. 25