NEW APPROACHES TO STUDY PHASE PROPERTIES 167 ß They have general applicability. Both methods apply to a wide range of materials including liquid crystals, polymers, and nonaqueous solvents. There are some limitations to these methods which must be addressed: ß They require access to a high-intensity x-radiation source of the type available at a synchrotron radiation facility. ß They require a large amount of sample. However, this disadvantage is offset by the fact that both methods are nondestructive and the samples can be recovered for reuse. Furthermore, the lyotrope gradient method in its present configuration requires only a single sample representing about 40 mg lipid to construct a complete phase diagram. ß Lyotrope gradient samples ideally should be fluid. ß Since both methods are based upon x-ray diffraction, difficulties are encountered in distinguishing liquid phases. ß Phase diagrams are meant to indicate phase behavior at equilibrium. In these methods, a non-equilibrium or steady-state approach is used to obtain equilibrium information. Although this theoretically is a point of contention, experimental evidence shows that phase diagrams created with these methods agree with and extend the infor- mation contained in phase diagrams produced with conventional equilibrium methods. ACKNOWLEDGMENTS This work incorporates the undergraduate scientific writing project of M. T. Bywater and was supported in part by a Grant-in-Aid of Research from Sigma Xi, The Scientific Research Society, and The National Institutes of Health, Grant Number DK 36849, to M. Caffrey. REFERENCES (1) R. G. Laughlin, "Phase Equilibra and Mesophases in Surfactant Systems," in Surfactants, T. F. Tadros, Ed. (Academic Press, London, 1984), pp. 53-78. (2) M. Caffrey and F. S. Hing. A temperature gradient method for lipid phase diagram construction using time-resolved x-ray diffraction. Biophys. J., 51, 37-46 (1987). (3) M. Caffrey, A lyotrope gradient method for lipid mesomorphic phase diagram construction using time-resolved x-ray diffraction. Biophys. J., 51, 444a (1987). (4) M. Caffrey, Kinetics and mechanism of the lameIlar gel/lamellar liquid crystal and lameliar/inverted hexagonal phase transition in phosphatidylethanolamine: A real-time x-ray diffraction study using synchrotron radiation. Biochemistry, 24, 4826-4844 (1985). (5) M. Caffrey and D. H. Bilderback, Real-time x-ray diffraction using synchrotron radiation: System characterization and applications. Nucl. Instr. and Meth. Phys. Res., 208, 495-510 (1983). (6) M. Caffrey, Kinetics and mechanism of transitions involving the lameliar, cubic, inverted hexagonal and fluid isotropic phases of hydrated monoacylglycerides monitored by time-resolved x-ray diffrac- tion. Biochemistry, 26, 6349-6363 (1987). (7) M. Caffrey, X-radiation damage of hydrated lecithin membranes detected by real-time x-ray diffrac- tion using wiggler-enhanced synchrotron radiation as the ionizing radiation source. Nud. Instrum. and Meth. Phys. Res., 222, 329-338 (1984). (8) V. Luzzati, "X-Ray Diffraction Studies of Lipid-Water Systems," in Biological Membranes, Physical Fact and Function, D. Chapman, Ed. (Academic Press, New York), Vol. 1, pp. 71-123.

j. Soc. Cosmet. Chem., 39, 169-177 (May/June 1988) The influence of solubility, protein bindino, and percutaneous absorption on reservoir formation in skin SCOTT R. MISELNICKY, J. LEON LICHTIN, ADEL SAKR, and ROBERT L. BRONAUGH, Division of Toxicology, Food and Drug Administration, Washington, D.C. 20204 (S.R.M., R.L.B.), and College of Pharmacy, University of Cincinnati, Cincinnati, OH 45267 (J.L.L., A.S.). Synopsis Properties of a compound thought to influence reservoir formation in the skin during percutaneous absorp- tion have been examined and compared for a series of seventeen heterogenous compounds. Lipid (octanol) and water solubility were determined in addition to an octanol/water (pH 2) partition coefficient (O/W K). Protein binding to bovine serum albumin was measured using equilibrium dialysis. Percutaneous absorp- tion and reservoir formation were measured in rat skin mounted in flow-through diffusion cells. The reservoir was determined from epidermal levels of compound 72 hr following topical application. A nega- tive rank correlation was seen between reservoir formation and percutaneous absorption. When the five rapidly penetrating compounds were excluded, significant correlations were seen between reservoir forma- tion and three properties of the compounds--O/W K, binding affinity, and lack of water solubility. A multiple correlation analysis indicates that approximately equal contributions were made to reservoir for- mation by the protein binding and solubility properties of the chemicals. INTRODUCTION The stratum corneum is a lipid-enriched layer of keratinized cells that serves as the primary barrier to percutaneous absorption for most compounds. This layer can also serve as a depot for the storage of certain topically applied compounds. The existence of this reservoir effect has been well-documented for steroids. Malkinson and Ferguson (1) first postulated in 1955 that the skin functioned as a reservoir for topically applied steroids. In 1963, Vickers (2) demonstrated by means of the vasocon- strictor assay that a reservoir did in fact exist in the skin for potent fluorinated steroids. The site of reservoir formation was shown to be the stratum corneum since this effect was not seen after either removal of the stratum corneum by tape-stripping or bypassing the tissue with an intradermal injection. Since the vasoconstrictive ability of fluorinated steroids has provided an easy method for detecting reservoir formation, most studies of this depot effect have been performed in vivo with these steroids. Reservoir formation of several nonsteroidal compounds has been measured by determination of in vitro epidermal tissue levels (3,4). The properties of chemicals that promote reservoir formation in skin have received little 169