Can hair really be hydrated?

As a reminder, each hair fiber is composed of three superimposed layers: the medulla, the innermost layer; the cortex, accounting for nearly 80% of the hair; and the cuticle, the external protective layer. The cortex consists of keratin and also contains melanin pigments, which give hair its color. It also serves as the primary water reservoir. The cuticle, in turn, is formed of flat cells that overlap like shingles, and their cohesion is essential for limiting water loss, as well as preventing water penetration. When these cells become disorganized, often due to an alteration of the lipid intercellular cement, the hair become more porous: they retain water less effectively and absorb it more readily.

That’s why the verb “hydrate” isn’t necessarily the most appropriate when discussing hair. The goal of hair care is not to make water penetrate into the fiber. When water infiltrates a porous hair strand, it causes a swelling of the cuticle, further disrupting the organization of its scales. This swelling and contracting cycle, which repeats with each wash and dry, weakens the fiber, increases breakage risk, and promotes split ends. That’s also why it’s inadvisable to let your hair air-dry, since moisture remains longer. The threat water poses to hair fibers is illustrated by the images below, taken with an atomic force microscope. They clearly show the lifting of the cuticle when the strand is immersed. Without applying a conditioner to smooth and reseal it, the cuticle stays lifted and vulnerable to external aggressors.

It is preferable to refer to "conditioning" one's hair or "protecting it from dehydration" rather than "hydrating" it.

Hydrating, or more precisely conditioning hair, involves preventing water from entering or leaving the fiber. To accomplish this, the cuticle scales must be properly smoothed. This specifically involves the systematic application of a conditioner after every wash. Indeed, using a conditioner flattens the cuticle scales along the hair shaft thanks to the cationic surfactants it contains. This treatment also increases the adhesion of the cuticular scales to the hair shaft, which enhances the hair’s light-reflecting properties and reduces friction between hair fibers.

It is also important to maintain the cuticle’s lipid barrier. This is where emollients and occlusive agents come into play. Vegetable oils, such as the coconut oil or the apricot oil, rich in fatty acids, can form a film on the fiber surface, thereby reducing water evaporation. Some studies have even shown that lauric acid, one of the main components of coconut oil, can penetrate the hair and mitigate both cuticle swelling caused by water and the stress this imposes on the hair fibers.

Other studies have examined the effect of nanoemulsions on the hydration of textured hair samples obtained from a woman. Some hair fibers underwent a bleaching process, with or without subsequent recoloring. For reference, a nanoemulsion is a mixture of two or more immiscible liquids in which one is finely dispersed in the other, forming droplets at the nanometer scale. Specifically, three nanoemulsions were studied: a control nanoemulsion, a version enriched with 7.5% coconut oil, and a third containing 7.5% coconut oil and 0.1% wine lees extract. These nanoemulsions were applied to the three hair types, and their hydration was then observed.