A Staff Report from the Straight Dope Science Advisory Board

Followup: Is pH-balanced soap impossible?

Dear Straight Dope:

How can a soap be pH balanced? Soaps supposedly work because they are basic. At least that's what my science teachers have always told me.

You must have been throwing a spitball at a crucial part of the explanation, Other--you've got this a little scrambled. For those who haven't taken chemistry, acids and bases are chemical opposites--mix them together and you get a substance that's neutral, neither acid nor basic. You need an alkali (a base) to make soap, but the soap doesn't work because it's basic; on the contrary, the finished product is more or less neutral (although see below).

Soap molecules work because they're got a water-loving end and a water-phobic end. The water-phobic end attaches itself to oily dirt, while the water-loving end attaches itself to the water. By and by the soap manages to wedge itself in between the dirt and whatever it is you were trying to clean, whereupon the dirt floats free and you can flush it down the drain.

The Procter & Gamble folks tell me some alkalinity is useful in soap to remove acidic soils, which casts some doubt on whether "balanced pH" is a desirable thing. Some chemists are skeptical of claims that balanced-pH soaps are somehow better. But nobody is saying that balanced (neutral) pH soap wouldn't work at all.

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