What characteristic of ingredients makes them more effective for absorption into the skin?

• A. Higher molecular weights
• B. Smaller molecular sizes
• C. Solid-state form
• D. Acidic pH levels

Answer: (B)

Smaller molecular sizes are more effective for absorption into the skin because the skin's barrier properties primarily limit the penetration of substances. The stratum corneum, which is the outermost layer of the skin, acts as a barrier that is best penetrated by smaller molecules. When ingredients have a smaller molecular size, they can more easily pass through this barrier and reach deeper layers of the skin, where they can exert their beneficial effects.

In contrast, ingredients with higher molecular weights typically cannot penetrate the skin as effectively, as their larger size inhibits their ability to diffuse through the various layers of the skin. Solid-state forms of ingredients may also have limitations in terms of absorption, as they might not be able to dissolve adequately in skin lipids or interstitial fluids. Lastly, while certain pH levels can affect the stability and activity of ingredients, they do not inherently correlate to the effectiveness of absorption as molecular size does. Therefore, smaller molecular sizes are the defining characteristic that enhances the likelihood of effective skin absorption.