Analysis and quantification of the propensity of hair dyes to desorb from human hair fibre using an accelerated 48-wash method

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The durability of oxidative hair dyes is a critical factor in consumer satisfaction and hair colour product development. This study introduces an accelerated 48-wash analytical method for quantifying hair dye desorption, combining spectrophotometric colour strength measurements with a back-extraction protocol coupled to HPLC. The approach was first validated using a two-component model dye system comprising p -aminophenol and 2-methyl-5-aminophenol, forming a heterodimeric product that could be quantitatively monitored. Hair types studied included Natural White Hair and Bleached Blonde Hair, with dyeing performed using either ammonia or monoethanolamine as the alkalizing agent. Results demonstrate that most colour loss occurs during the first wash, and Bleached Blonde Hair treated with monoethanolamine exhibits the highest dye desorption, reflecting superficial dye deposition and structural vulnerability of chemically processed hair. In contrast, ammonia-based formulations promote deeper dye penetration and improved retention. Colour loss trends quantified by spectrophotometric measurement followed a logarithmic pattern over the accelerated 48 wash test, and strong correlations ( R 2 > 0.95) were observed between spectrophotometric data and HPLC quantification of dye removal. The robustness of the method was also demonstrated on multi-component dye formulations, and although direct HPLC quantification was challenging for these more complex systems, the spectrophotometric measures serve as a reliable proxy for quantification of actual dye loss, based on the calibration established with the model two-component system. This work provides a quantitative framework for understanding hair dye wash fastness, the influence of hair type and alkaliser, and the mechanisms of dye desorption. The method supports future product innovation by linking molecular-level dye behaviour to long-term colour performance.