For molecules to penetrate the cuticle layer, they typically need to be smaller than these gaps, which are on the scale of a few nanometers. Molecules with a molecular weight below 500 Daltons (Da) are generally considered favorable for hair penetration. This range includes substances such as small organic molecules, vitamins, amino acids, and certain cosmetic ingredients.

In terms of size, molecules with a diameter of less than 1-3 nm have a better chance of passing through the gaps between the cuticle scales and penetrating the hair shaft. Here are some examples of molecules and their sizes:

• Water molecule (H2O): Diameter of about 0.28 nm.
• Glucose molecule: Diameter of about 1 nm.
• Vitamin B molecule (pantothenic acid): Diameter of about 0.8 nm.
• Glycerin molecule: Diameter of about 0.7 nm.

These molecules fall within the size range, that allows them to potentially penetrate the hair shaft.

However, it’s important to note, that not all molecules can easily penetrate the hair shaft. The cuticle layer acts as a barrier, and the hair’s hydrophobic nature can further limit penetration. Researchers have developed various techniques, such as specialized delivery systems and conditioning agents, to enhance molecule penetration.

Ultimately, while smaller molecules generally have a better chance of penetrating the hair shaft, the ability to penetrate depends on multiple factors, including molecular weight, size, hair condition, and the presence of other barriers.