l0 JOURNAL OF COSMETIC SCIENCE The fluxes from the structured emulsions with steareth-2/-21 or triethanolamine stearate were significantly higher than that of the aqueous reference (t-test, p = 0.05%). The main barrier for the percutaneous absorption of almost all compounds is the stratum corneum, which is composed of keratinized cells embedded in lameliar lipid layers (27). Most intercellular lipids are in a gelified crystalline state, but some of them may be in a liquid crystalline state (28-30). It is generally accepted that these intercellular domains constitute the primary route for the passive permeation of many molecules through the skin. The effective barrier property of the stratum corneum intercellular lamellae has been attributed to the highly ordered bilayer structures in the intercellular spaces. One could hypothesize that surfactants that have formed liquid crystals in the emulsions, diffusing as single molecules through the intercellular lipid lamellae, are able to pref- erentially transform the gelified crystalline lipid packing into the liquid crystalline lipid packing and create a more fluid, permeable, membrane. French et al. (31) have shown that incorporation of surfactants (dodecyl ether ethoxylate type) into multilamellar vesicles of distearylphosphatidylcholine (model of stratum cor- neum lipids) disrupted the highly ordered packing of the lipid chains, causing increased fluidity within lipid bilayers. For the aqueous solution and for simple emulsions, benzophenone-4 partitioned between the stratum corneum and water, in which the diffusible permeant concentration can be modified by miceliar solubilization. For structured emulsions, the case could be more complex. Indeed, it has been demonstrated that when some suspensions of nonionic vesicles (32) or liposomes (33) (e.g., an aqueous compartment surrounded by lameliar layers) were put on the skin, vesicles aggregated and fused, thus depositing lameliar structures on the surface of the stratum corneum. If a similar phenomenon occurred with an oily droplet surrounded by lameliar layers or with lameliar layers in the aqueous phase, benzophenone-4 partitioning between water and stratum corneum could be re- placed by partitioning between lameliar liquid crystals and stratum corneum. If the partition between stratum corneum and lameliar liquid crystals were more favorable to the stratum corneum than water, permeation could be enhanced. REFERENCES (1) S. Friberg, Three-phase emulsions,J. Soc Cosmet. Chem., 30, 309-319 (1979). (2) G. Couarraze and J. Wepierre, "Topical Applications of Liposomes," in Liposomes, New Systems and New Trends in their Applications, F. Puisieux, P. Couvreur, J. Delattre, and J. P. Devissaguet, Eds. (Editions de sant•, Paris, 1995), pp. 615-644. (3) H. Komatsu, H. Okamoto, K. Miyagawa, M. Hashida, and H. Sezaki, Percutaneous absorption of butylparaben from liposomes in vitro, Chem. Pharm. Bull., 34, 3423-3430 (1986). (4) A. Vermoken, M. Hukkelhoven, A. Vermeesch-Markslag, C. Goos, P. Wirtz, and J. Ziegenmeyers, The use of liposomes in the topical application of steroids,J. Pharm. PharmacoL, 36, 334-336 (1984). (5) M. Mezei and V. Gulasekharam, Liposomes, a selective drug delivery system for the topical route of administration, (2) Gel dosage form, J. Pharm. Pharmacol., 34, 473-747 (1982). (6) C. B. Lalor, G. L. Flynn, and N. Weiner, Formulation factors affecting release of drugs from topical vehicles. II. Effect of solubilities on in vitro delivery of a series of n-alkyl p-aminobenzoates,J. Pharm. Sd., 84, 673-676 (1995). (7) P. Ashton, K. A. Walters, K. R. Brain, and J. Hadgraft, Surfactant effects in percutaneous absorption. I. Effects on transdermal flux of methyl nicotinate, Int. J. Pharm., 87, 261-264 (1992).

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