The mitigation of damaged hair conditions involves the application and penetration of substances to stabilize broken bond sites, restore lipids and proteins, reinstate hydrophobicity, and recover hair mechanical properties. Vegetable oils, in general, exhibit a list of advantageous characteristics much desired by consumers, given the associated benefits for hair fibers. While coconut oil is highly popular in the hair care market and extensively studied for its ability to diffuse through the hair cortex, the effects of avocado and argan oil on the internal structure of hair and their potential benefits remain underexplored. Tensile and fatigue tests, as well as Raman spectroscopy, were carried out to investigate the interaction of these three oils with virgin and bleached Caucasian hair. The oils were applied in sufficient amounts directly to hair tresses and maintained for 24 h at 25 ◦C. Our results show that the three oils successfully diffused and interacted with the cortical region of the hairs. Their impact on hair mechanical properties depends on the level of damage and humidity conditions. In virgin hair, coconut and avocado oil reinforce the hydrophobic barrier of the cellular membrane complex, preventing water from causing intense perturbation of the mechanical properties, leading to increased stiffness and break stress. Meanwhile, due to the high degree of unsaturation of its fatty acid chains, argan oil increases water absorption, resulting in losses in hair resistance. When bleached, the hydrophilicity of the hair fiber increases, determining more affinity for argan oil. Consequently, the affinity with water is also elevated, causing increased fragility to mechanical stress. The analyzed vegetable oils are not always beneficial for hair care. Their specific chemical characteristics and hair conditions will influence the final results and should be taken into consideration in hair care product development.
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