You step into the shower, only to discover that your shampoo bottle is practically empty. Argh! You fill the bottle partway with water, shake it up, and douse your wet hair with the diluted, sudsy shampoo. In seconds, you're lathered up, rinsing off, and ready to face your day.
As you turn off the water, you realize you've still got some shampoo solution left in your bottle. You're tempted to put the bottle back in your shower caddy and save the remainder for your next shampoo. After all, it seems a waste to just dump it, and you haven't got much time to go to the store to get a new bottle.
Resist temptation, brave shampooer. If you dilute your shampoo to eke another day's hairwashing out of a bottle, throw the rest away immediately afterward.
Why? According to a chemist friend of mine, once you've diluted shampoo, you've rendered the preservatives in it too weak to do their job. Your watered-down shampoo is chock full of nitrogenous compounds and carbohydrates that bacteria and fungi will find oh so tasty. So if you let this solution sit for a day (or more) and then dump it on your hair, you'll also be dumping on a nice homemade culture of various nasties that may find your scalp to be equally delicious. You could find yourself getting a case of dermatitis or an infection.
Common Shampoo Chemicals (And What They Do)
Acids: the innate alkalinity of soaps and detergents can make hair look dull because the hair cuticle swells and gets rough in an alkaline solution, so most shampoo manufacturers add acids to brighten hair's shine. The pH of most shampoos is usually 6.5 to 7.5.
Detergents: these remove oils from your hair and let them dissolve away in water.
Lather builders: suds improve shampoo's cleaning action.
Conditioners: these chemicals put a coating on the hair shafts to make them thicker, smoother, softer, easier to comb, and less prone to static; they also strengthen the hair's cuticle.
Thickeners: these add body to the shampoo and some also act as weak conditioners.
Preservatives: these keep down bacterial and fungal growth.
Sunscreens: ultraviolet radiation can damage your hair and scalp.
* Some thickeners and conditioners cloud shampoo and, in the absence of dyes to make the shampoo green or blue, also tend to make it look very much like semen. Additives like oils and proteins that have not been thoroughly emulsified will also cause this effect.
References: http://www.exploratorium.edu/exploring/hair/hair_3.html, http://www.chemistrystore.com/shampoo_formulas.htm