In hair care science, the terms absorption and adsorption often surface in discussions about how active cosmetic ingredients interact with hair fibers. While these words are frequently used interchangeably in consumer discussions and online content, they describe two fundamentally distinct processes with unique mechanisms.
To shed light on these differences, we turn to the expertise of a seasoned hair scientist and cosmetic formulator with a Ph.D. in Chemistry. Together, we’ll explore the science behind these two mechanisms, their significance in hair care, and how they apply to various active ingredients. Understanding these concepts can help you make more informed choices for your hair care routine.
Absorption vs. Adsorption
In chemistry, absorption refers to a process where molecules penetrate a substance and integrate into its internal structure, becoming part of its body. In contrast, adsorption occurs when molecules adhere to the surface of a material without penetrating or becoming part of its internal structure. In adsorption, the interaction is limited to the surface of the substrate, leaving the material’s core unaffected.
Absorption vs. Adsorption: Understanding the Core Differences in Hair Science
In the context of hair science, absorption refers to the process where molecules penetrate the hair cuticle and integrate into the hair’s internal structure, such as the cortex. This allows active ingredients to work from within, addressing the deeper needs of the hair strand.
In contrast, adsorption involves molecules adhering to the surface of the hair cuticle without penetrating it.1,2 These molecules remain on the exterior, often forming a protective layer or delivering immediate benefits like smoothing or reducing frizz. This surface-level interaction ensures that the hair’s internal structure remains unaffected while enhancing its external appearance and feel.
The mechanisms of absorption and adsorption are influenced by several factors, including pore size, the size and nature of the molecule, and its dosage. When it comes to absorption in hair, only molecules with the right size can pass through the cuticle’s pores and penetrate into the inner structure. Larger molecules are unable to do so and remain on the hair’s surface.
Microscopic studies have revealed that for a molecule to be absorbed, its size must be less than 10 angstroms (10 Å) in diameter. Molecules exceeding this size threshold are unable to pass through the cuticle and instead interact with the hair at the surface level, governed by adsorption. This size-dependent behavior highlights the importance of molecule design in creating effective hair care products.
In adsorption, larger molecules remain on the hair’s surface, attaching through weak chemical bonds such as Van der Waals interactions or ionic bonds. These molecules do not penetrate the hair but instead create surface-level effects.
For example, large polymers commonly used in hair care formulations form a coating on the hair’s surface without entering the cuticle. Similarly, cationic hair-softening agents bond to the negatively charged surface of hair via ionic interactions, smoothing and softening the cuticle. Silicones are another key example, adhering to the hair shaft through adsorption to provide lubrication, enhance shine, and reduce friction.3,4 These surface-level interactions play a crucial role in delivering immediate, visible benefits to the hair.
Does Hair Adsorb or Absorb Water Molecules?
Water, being a small molecule with an approximate diameter of just 2.75 angstroms, is significantly smaller than the average pore size of the hair cuticle. This allows water molecules to pass through the cuticle barrier with ease. As a result, water is absorbed into the hair fiber, penetrating into its inner structure rather than simply adhering to the surface. This absorption is a critical factor in maintaining the hair’s hydration and elasticity.
Why Water’s Unique Properties Matter for Hair Absorption
Water is a remarkable chemical compound with distinct physical and chemical properties. Its molecular structure consists of a single oxygen atom bonded to two hydrogen atoms. Due to oxygen’s high electronegativity and its lone pairs of electrons, water molecules are polar, allowing them to form hydrogen bonds with the protein sites in hair fibers.
This interaction is key to water absorption in hair. As water penetrates the cuticle and binds to these protein sites, the hair fiber absorbs it, causing the fiber to swell and its diameter to increase.5, 6 This swelling effect underscores water’s critical role in maintaining hair’s elasticity and hydration.
Key Takeaways
In hair science, absorption and adsorption are distinct mechanisms. Absorption involves molecules’ penetration into the hair fiber, while adsorption refers to molecules’ adhesion to the hair’s surface.
Both processes can occur in hair, depending on factors such as the nature of the active ingredient, its molecular size, and the condition of the hair fiber. Understanding these differences helps in selecting products tailored to either penetrate deeply for lasting benefits or work on the surface for immediate effects like smoothing and shine.
References
- Robbins, C. R., Chemical and Physical Behavior of Human Hair. Springer Berlin Heidelberg: 2012. ↩︎
- Zviak, C., The Science of Hair Care. Taylor & Francis: 1986. ↩︎
- Andre, V.; Nörenberg, R.; Hössel, P.; Pfau, A. In The role of polymer‐surfactant interactions in the adsorption process of hair‐conditioning polymers, Macromolecular Symposia, Wiley Online Library: 1999; pp 169-179. ↩︎
- Ran, G.; Zhang, Y.; Song, Q.; Wang, Y.; Cao, D., The adsorption behavior of cationic surfactant onto human hair fibers. Colloids and Surfaces B: Biointerfaces 2009, 68 (1), 106-110. ↩︎
- Barba, C.; Martí, M.; Manich, A.; Carilla, J.; Parra, J.; Coderch, L., Water absorption/desorption of human hair and nails. Thermochimica Acta 2010, 503, 33-39. ↩︎
- Stam, P. B.; Kratz, R. F.; White JR, H. J., The swelling of human hair in water and water vapor. Textile research journal 1952, 22 (7), 448-465. ↩︎
