When combined with water, soap forms an alkaline solution with a pH value between 8 and 12. The pH value of 7 marks the neutral point, but the skin’s surface is naturally acidic with a pH of 5.5. When an alkaline soap is used to wash the skin, it destroys the skin’s acid mantle. The acid mantle does regenerate within a few hours, but in the meantime, the skin’s natural defense against pathogens, irritants, and dehydration is reduced. The chances of skin problems developing are also heightened because soap makes the skin swell up.
At first, your skin will feel soft and smooth when this happens, but the swelling leads to harmful environmental influences penetrating the skin, resulting in a loss of the skin's moisture. When soap reacts with lime in hard water, it reduces its effectiveness, so quite a lot of soap is required to cleanse the skin properly when washing with it. The large volume of alkaline solution is then very harmful to the skin. Soap-free cleansers with a pH value of 5.5 do not cause the skin to swell up, instead preserving its natural acid mantle. These products are therefore far more compatible with sensitive skin than soap is.