[nashalo]

If you use regular soap on your face it's because the soap water has a high pH whereas your skin has a slightly acidic pH of around 5.5. So the soap disrupts the acidic barrier of your skin which prompts the skin to produce more sebum (acidic) to restore said barrier. [this is a simplification]

This happens on all your skin but it's more egregious on the face because the skin is more delicate there.

No idea for the shampoo though.

[renw0rp]

Are there any negative implications of using baking soda in you arm pits instead of a deodorant? I think it has exactly same effect pH wise as you have described about soap. It seems to be quite effective and more natural way of neutralising under pits odor than regular deodorants though and I haven't personally noticed any side effects.