A fundamental investigation to study the surface of hair by modeling the wetting behavior of oil on a flat keratin surface was conducted. This study provides a model to study the complex keratin surface in contact with surfactants used in shampoo and conditioner formulations. This study focused on using keratin films as a tool to evaluate the effectiveness of surfactants to wet and dewet oil on hair.
Keratin, a natural polymer is the major component of all hair types. Hair is highly crosslinked with disulfide bonds which were extracted from hair fibers by breaking peptide bonds.1,2,3 Extracted keratin has the ability to self-assemble and was casted into films. Two surfaces were investigated, silica glass and keratin films. The keratin was extracted from the hair by using the Shindai Method for protein extraction.1
Surfactants at their critical micelle concentration played an important role in the wetting of oils.4 Surface wetting profiles were studied to model the packing and arrangement of surfactant at the interface of keratin and oils. A model of a monolayer and possibly a bilayer of surfactants on the surface of keratin are suspected to influence wetting properties.5,6
This study demonstrated that keratin films can be used to model the surface of the hair fiber. Changes in contact angle as well as the physical profile of the oil droplets in contact with the keratin surface behave similarly to natural hair fibers. Differences in wetting profiles were distinguishable between anionic and cationic surfactants. The anionic surfactants produced high contact angle reading exhibiting a dewetting response. Wetting and deposition was improved by use cationic surfactants. Differences in wetting was also detectable between two cationic surfactants with different counterions using this modeling study. These results supports the claims of the dual processes of charge-driven adhesion and hydrophobic effects conducted on tassels of hair.6