On the use of fluorescein as a probe to monitor anionic surfactant rinsability from skin
K. P. Ananthapadmanabhan , K. K. Chan , X. Lei , M. P. Aronson

Synopsis

Recent studies (1) demonstrated that the intrinsic interactions between anionic surfactants and the stratum corneurh of skin could be monitored from the displacement of a competitive protein-binding probe. The method correlated with direct measurements of surfactant binding and with clinical mildness. However, the level of strongly bound surfactant that remained after rinsing did not agree with an earlier study by Wortzman et al. (2) in which the “rinsability” of cleansing surfactants from skin was measured using fluorescein as a marker dye for the surfactants in bar slurries. This prompted the present careful reexamination of the fluorescein assay.

It is shown that fluorescein does not track the binding of surfactants to skin and thus cannot measure any intrinsic interactions between a cleansing composition and the skin. Thus, the assay reported by Wortzman et al. actually measures the way fluorescein applied to skin from a solution or slurry is rinsed from the skin under unrealistic conditions employing a limited amount of ambient temperature water with no mechanical agitation. Not only do differences in fluorescein retention disappear under rinsing conditions that are more characteristic of everyday use, the differences between products primarily arise from differences in pH and counterion type and the consequent differences in fluorescein solubility. Thus, the assay measures fluorescein rinsability and does not measure the surfactant rinsability! Therefore, any conclusions reached about surfactant rinsability are erroneous, arise from an artifact of the test method, and have little to do with surfactant rinsability or mildness.

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