Posted 05 January 2012 - 08:37 PM
You're right, BP preparations are usually slightly basic because benzoyl peroxide is unacceptably unstable in acidic environments. But this instability is exactly what you'd want when benzoyl peroxide is on your face, so I wouldn't worry about that too much.
This is me thinking out loud:
The oxidative capacity is likely exhausted quite quickly once the chemical is absorbed into the skin, as it's also unstable in the presence of pretty much any organic material, particularly those full of catalytic enzymes like our skin. For evidence, you could experiment with how quickly a dollop of BP bleaches a colored cloth. Other indications come from the use of BP in tooth bleaching, where concentrated gels are usually kept in for no more than 15 minutes.
However, there's the interesting wrinkle that the Acne.org Regimen has brought up - large amounts of 2.5% benzoyl peroxide work better than smaller amounts of a more concentrated preparation. In addition, large amounts of 2.5% BP work better than smaller amounts of 2.5% BP.
There's never been a great explanation for this, as far as I know. One possibility I've entertained is that (when you use a large amount) much of the preparation remains relatively unabsorbed on the surface of the skin. As some BP diffuses into the skin and decomposes, the reservoir of BP above provides a constant stream of oxidation for some significant period of time. This long duration of low oxidative activity would preferentially attack the intercellular matrix of the stratum corneum, sebum outside the encapsulation of a human cell membrane, and the relatively weak cell membrane of P. acnes (which has no appreciable enzymatic defenses against free-radical oxygen). Something similar to this "wave" of biochemical action occurs with several other dermatological drugs, including alpha and beta hydroxy acids.
Meaning, when you use a lot of dilute BP on your face, it's effective because its kind of a "slow-release" over the entire day or night.
I know this theory soup doesn't help much, but its what my brain printed out today. If it helps, I know Dan suggests you don't wash your face again until 8 hours after you first applied BP - but this might just be to minimize irritation. In my personal experience, BP seems to exerts its effects within 4 hours at the very most.
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