Double shampooing is beneficial because the first wash removes sebum and impurities (which limits lather, as the surfactants are occupied capturing the oil), while the second cleanses thoroughly, particularly in the case of hard water, thanks to the chelating agents in clarifying shampoos.

noli's read

Carla's explanation of the surfactant mechanism is entirely accurate and consistent with the principles of fluid physics described in formulation manuals in the Journal of Cosmetic Science. Surfactants first associate with fatty substances to emulsify them, and it is only once the hair is clean that they can freely trap air to form lather. Regarding the impact of hard water, an experimental study published in the International Journal of Dermatology confirms that minerals like calcium alter washing efficacy and that chelating agents help neutralize these residues. Furthermore, expert consensus in hair care validates the value of double washing to eliminate the buildup of styling products or excess sebum. It should simply be qualified by noting that this method may prove too harsh for very dry scalps if they do not have product buildup.

the sources

No named source held this one up — that absence is part of the verdict.