Water diffusion coefficients versus water activity in stratum corneum: a correlation and its implications
M. Stockdale

Synopsis

Literature reports of rates of trans-epidermal water loss through skin exposed to differing external relative humidities have been used to generate water diffusion coefficients by means of Pick's law. These have been related, within a given series, to the diffusion coefficient at a standard average water activity to give relative water diffusion coefficients. These have been combined, together with literature values of water diffusion coefficients obtained by kinetic experiments on stratum corneum exposed to various humidities, to yield a correlation between the average water activity of stratum corneum and the relative water diffusion coefficient over the full range of water contents. The asymptotic relationship derived shows that the diffusion coefficient increases rapidly as the water activity approaches its maximum. The implications of this relationship in terms of water activity profiles of stratum corneum are discussed. A theoretical analysis of the subject is presented and water activity profiles of model systems are derived.

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