Friday, February 8, 2008
Any woman who has used permanent colour on her hair can tell you that the colouring process changes the way her hair feels and behaves. It often feels rougher and drier and over time loses colour and shine to become dull in appearance.
However, scientists have been at a loss to explain all the changes found in colour-treated hair. When examining colour-treated hair using microscopy, the traditional way of evaluating hair damage, scientists often found the colour-treated hair fibre looked normal. Yet the sensory properties of the hair were clearly different. This required scientists to look beyond the visual changes in hair to the physiochemical changes resulting from hair colouring.
Color strips F-layer
Since the beginning of the last century, it has been known that the outside surface of a healthy hair cuticle is covered by a thin, protective, hydrophobic (water repellent) layer the thickness of only a single molecule. However, it wasn't until the 1980s that this layer was identified as 18-methyleicosanoic acid (18-MEA), which is covalently bound to the surface of the cuticle. Hair researchers have named this the F-layer. This permanently attached fatty acid layer makes the hair hydrophobic and contributes to hair's natural smooth and lubricious feel.
When hair is treated with a traditional permanent oxidative hair colourant, one unintended side effect is that the F-layer is stripped away from the cuticle surface. The permanent removal of the F-layer significantly changes the physiochemical nature of the hair surface. The most noticeable change is a shift in the hydrophobic nature of the hair, in fact completely reversing this property, leaving the hair hydrophilic in nature. The illustration shows the impact of changing the hydrophobicity of the hair, with water easily beading up on hair that has an intact F-layer, while a water droplet absorbs quickly into hair that has been colour treated and has lost its natural hydrophobic layer. Another important change is to increase the hair's friction potential by removing its natural lubricant. The result is hair that feels drier and rougher and is more susceptible to frictional damage.
New formulas and ingredients developed
P&G Beauty scientists have used this knowledge to direct the development of new conditioner formulae and ingredients. One specific new development is the use of cationic surfactants such as BAPDMA (behenyl amidopropyl dimethylamine glutamate). This molecule has a positively charged end group that helps it bond to the stripped cuticle surface. It has a long alkyl tail to mimic the lost fatty acid (18-MEA) of the F-layer. The result is restoration of the hydrophobic properties of hair and a smoother, more lubricious feel. In addition, an unexpected bonus of restoring the hair's own water repellency was the reduction in hair colour fading due to colour being leached away by water during shampooing and bathing.
"The good news is that by understanding the biology and physiochemical properties of hair and by careful choice of ingredients, researchers can reverse the well-known issues associated with colour-related hair damage from a regular rinse-off conditioner," says Dr Frauke Neuser, P&G Beauty Senior Scientist.
Work is now underway to apply this knowledge to the fundamental chemistry taking place during the colouring process that causes the F-layer to be removed. This information may also be used in the future to develop hair colouring products that actually preserve the F-layer and the hair's own natural look and feel after colouring.
