J. Soc. Cosmetic Chem., 28, 481-484 (August 1977) Diffusion theory analysis of transepidermal water loss through occlusive films IRA WEIL and HENRICUS M. PRINCEN Lever Brothers Company, Research Center, Edgewater, NJ 07020. Re•edJanua• 25,1977. Synopsis It is shown that simple DIFFUSION THEORY dictates that the application of an OCCLUSIVE FILM on SKIN always results in a decrease in TRANSEPIDERMAL WATER LOSS (TWL), as expected intuitively. The basic inconsistency in a previous analysis, which seemed to predict the possibility of the opposite effect, is pointed out. INTRODUCTION In a recent paper in this Journal (1), a theoretical argument was advanced to demonstrate that an occlusive film, when applied to skin, may lead not only to increased hydradon of the stratum corneum but, contrary to expectation, to a concur- rent increase in the transepidermal water loss (TWL). The authors used their argument to explain some experimental data that seemed to indicate such an increased flux, and to caution against rejecting potential occlusive agents that exhibit this effect, since they may, nevertheless, be excellent in their ability to effect increased skin hydradon. The authors went so far as to state that "this increase in TWL is evidence of increased skin hydradon in the stratum corneum." Although the analysis seemed sound at first sight, we had the distinct impression that the conclusion had to be in violation of some basic principles in transport phenomena. If not, some curious paradoxes could be envisaged (for example, insulating one's house could conceivably lead to a greater heat loss in wintertime). The question to be asked is whether introduction of an extra resistance in a transport process can cause a change in the original resistance that will not only nullify, but overcompensate for, the intrinsic effect of the added resistance. We believe that such an effect is, in general, impossible, although we shall prove it only, in a very simple way, for the system of an occlusive film on skin. THEORETICAL Let us consider the concentration (or activity) profile of water in the stratum corneum in the absence and presence of an occlusive film (Fig. 1). In both cases, the concentra- 481

482 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS DERMIS X STRATUM CORNEUM Figure 1. Concentration profile of water in the stratum corneum: solid line, nonoccluded case dashed line, occluded case tion at the dermis side equals c 0, which is determined by the constant activity of water in the dermis. In the nonoccluded case, the concentration drops to c'• at the outer sur- face (x = 8), which is at equilibrium with the ambient atmosphere. Therefore, c'• is de- termined by the ambient water activity. In the occluded case, the concentration drops only to c• (c• c• ), since part of the overall activity drop has to occur across the occlu- sive film (not shown in Fig. 1). Thus, the water content of the stratum corneum is indeed raised by the presence of an occlusive film, namely by an amount equal to the area between the two curves in Fig. 1. It is an essential feature of Fig. 1 that the concentration profile is not linear, but curved as a result of the concentration dependence of the diffusion coefficient D(c). At any point in the corneum, the water flux is given by or upon integration, dc j = -D(c) •xx (1) fo J dx =- D(c) dc c o (2)