I went on vacation to France recently and realized the natural shampoo that I bought well did not work as well. I had short hair so it was more noticeable when my girlfriend reported the same. I was wondering what the culprit and found it when I was making the morning coffee. On the inside of my kettle was white stuff at the bottom. This was not bacteria though it was calcium.
The water was hard!!! I found a French map of hard water areas with blue being very hard, followed by yellow, orange and the softest water being green. Hard water increases the surface tension of the water slightly as well as changes the properties.
In order for the detergent in shampoo to clean my hair the water has to be reduced so the water can spread out (also the water reduces its surfaces tension when heated) and soak into the surface of the hair. The shampoo reduces the surface tension of water and allow the water to mix with the dirt and grease (sebum) in your hair so it can be washed away. If the water is hard like mine in France determined how well (or poorly) my shampoo worked. The natural soap can formed a scum in the French water and did not rinse away easily. So I had to go to the store and buy some new shampoo with synthetic detergent. This synthetic detergent reacted less to the minerals in hard water but ended up stripping away all the natural oils from my girlfriend's hair. We did not really find a good solution for this. People can also use a vinegar rinse or use filtered water when they shower in places with hard water. Possibly detergent manufacturers might make better formulations for their detergents in different countries using tensiometers. These devices can help figure out the best amount of shampoo to use as well as change the shampoo for different regions in the world.