Life's Great Mysteries - Why is shampoo always clear and conditioner always opaque?

I noticed this long ago, and when hotels started putting these products in small, clear bottles, I even made the conclusion that the shampoo was made to look different from the conditioner simply to make it easier for people to grab the right bottle when they have soap in their eyes and they cannot read the ridiculously small print on the tiny bottles. This certainly makes sense to me.

But, as is often the case, my conclusion was mostly wrong (although I would still bet plenty of people use this characteristic to choose the right bottle). Conditioner is opaque because it is emulsified. An emulsion is a mixture of two or more liquids that do not normally mix… they are unmixable (I just wanted to use that word). Conditioner is partly water and partly oil. But water and oil do not mix, so chemists have to use an emulsifier to make them mix. When the emulsifier makes the water and oil mix, the mixture becomes opaque.

Now, shampoos on the other hand are partly water and partly a surfactant. A surfactant is something that reduces the surface tension of the liquid in which it is dissolved. In the case of shampoo, the surfactant acts as a detergent and a foaming agent (we all love our shampoo to foam up nicely, right?). Well, there is very little oil in shampoo, so there isn’t a need for an emulsifier, and therefore the shampoo remains transparent.

But what about those shampoos that are opaque? Yes, there are some, and it is usually because they have much higher oil content (or they are shampoo and conditioner combined).

Okay, seeing as how you are probably mesmerized by the exciting topic of shampoo, let’s look at a related question: why do we have to use shampoo before we use the conditioner? What if I’m feeling rebellious and want to reverse the order? Well, remember that shampoo has a surfactant that reduces surface tension, so the shampoo does not cling to your hair. Shampoo washes out completely, and therefore does not leave any conditioning coating to moisturize the hair. Conditioner clings to hair, because conditioners have positively charged ingredients that tend to adhere to hair, which is naturally negatively charged. This is why we use conditioner last—to leave a protective coating on the hair.

So, now you can wash your hair in a very knowledgeable way. And if you can’t read the bottle because you don’t wear your glasses in the shower, use the clear stuff first, then the opaque stuff.

Photo Credit:
- Man holding shampoo - DepositPhotos