Interesting, no?
From Skin Biology:

Skin pH and the Acid Mantle

"Skin pH" is a chemist's term meaning "Potential of Hydrogen" and is used to measure the degree of acidity or alkalinity in the outer layers of the skin. It is measured on a scale ranging from 0 to 14 where the center of the scale, 7, is neutral (neither acid nor alkaline). A reading below 7 indicates that the substance being measured is acid and above 7 is alkaline. The pH system works in 10-fold multiples and each pH unit represents a 10-fold difference in alkalinity. For example, a soap with a pH of 10.5 has 10-times the alkalinity of a soap of pH 9.5.

The acid mantle is a combination of sebum (oily fats) and perspiration that is constantly secreted to cover the skin's surface and maintain a proper skin pH. Normal skin pH is somewhat acidic in the range of 4.2. to 5.6. It varies from one part of the body to another and, in general, the pH of a man's skin is lower (more acidic) than that of a woman's.

The Acid Mantle - Anti-oxidant, Water Repellent, Bacterial Inhibitor, and Protein Hardener

The acid mantle protects skin in several ways:

Antioxidant - The lipids (fats) in the mantle are sacrificially oxidized to protect the underlying skin from excessive oxidation. This is why "whiteheads" - which are un-oxidized sebum in pores - turn into "blackheads" as the sebum is oxidized.

Water Repellent - The fats in the mantle repel water from the skin much as the oil on a duck's feathers repels water. This keeps water from loosening and damaging the skin's outer layers of hard protective proteins and renders the skin less vulnerable to damage and attack by environmental factors such as sun and wind and less prone to dehydration.

Bacterial Inhibition - The acid pH of the mantle inhibits bacterial growth on the skin. The acid pH especially inhibits the growth of foreign, pathogenic bacteria and fungi. Thus, the skin remains healthier, and has fewer blemishes.

Maintains Protein Hardness - The outer skin proteins are made of keratin, a very hard protein, that is also used in nature to make horns on animals. Keratin must be kept at an acid pH to maintain its hardness by keeping the protective proteins tightly bound together. More alkaline pHs soften and loosen the fibers of keratin and create gaps in the protective covering. This allows more allergens, irritants, bacteria and viruses to penetrate into the skin. Acne, skin allergies and other skin problems become more severe when the skin becomes more alkaline.

"Mild" soaps often damage skin

"Mild" soaps are often alkaline (pH 9.5-11), and remove the natural acid protection as well as extracting protective lipids (fats) from the skin. In addition, such soaps often have high levels of synthetic detergents which both strip away the mantle and loosen the protective keratin proteins. Irritated and eczematous skins tend to have a more alkaline pH, and washing with soap can increase this alkaline state and make the skin even more vulnerable to irritation and infection.