92 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS I. This modification of the triolein content is reasonable since about 12% triolein was found in the lipids of the outer stratum corneum by Lampe et alo (14). Glyceridacid and glycerol were added directly to the model lipid (32% water content) in constricted tubes, followed by the usual mixing procedure described previously (2,4). Lipid samples were examined by polarized light microscopy at 6% and 92% RH. Samples of the lipids (60-100 mg) were spread on microscope slides in uniform layers, 0.5-mm thick and 0.5-cm 2 and placed in constant-humidity chambers photomicro- graphs were taken at the initial time and after 6 h, 24 h, and 48 h of exposure to 6% RH or 92% RH. This technique detects both solid crystal and liquid crystalline phases. DSC studies were performed at 6% and 92% RH. 10-15-mg samples of the model lipid or model lipid plus additive were weighed accurately into DSC pans and placed in the constant-humidity chambers for 6, 24, and 48 hours. Samples were reweighed to determine water loss, the pans were hermetically sealed, and DSC was run at 5øC/min from 10-110øC using a DuPont 9900 differential scanning calorimeter. RESULTS AND DISCUSSION STUDIES WITH MODEL LIPID ALONE DSC of modified model stratum corneum lipids (32% hydrated) showed a broad en- dothermic transition, with an onset temperature of 30øC and a final temperature of 70øC (Figure la). (The DSC curve is the first heating run following sample preparation.) This corresponds to a transition from a liquid crystalline phase, observed by x-ray diffraction and microscopy (2), via the melting of lipid domains, to an unknown phase not deter- mined in the present study. The transition maximum is quite broad but is centered at about 52øC. This transition parallels the broad and overlapping transition of intercel- lular lipids observed at a maximum of 65øC for human as well as porcine stratum corneum (5-7). After incubation for 6, 24, and 48 h at 6% RH (Figures lb, lc, and ld), the DSC transitions show substantial changes, with emergence of some sharper transitions overlapping the broader transition. There was also an increase in coopera- tivity of the transition, as it sharpened and shifted to higher temperatures. These changes are consistent with dehydration of the lipids, as observed for phospholipids (8). In contrast, minor changes in the transition shape occurred after 6 h at 92% RH, while after 24 h, the transition sharpened, with a maximum at 52øC and with little change on further incubation (data not shown). Table II summarizes the corresponding enthalpy data, from triplicate measurements, for the model lipid, under different incubation times and RH. Despite the broadness of the transition, there was very good reproducibility for transition enthalpies. At 6% RH, the transition enthalpies were 15 --+ 1, 21 --- 1, 43 --- 1, and 52 --- 1 J/g after 0, 6, 24, and 48 h incubation (Table II), with corresponding water losses of 18.2 --- 0.1, 28.6 --- 0.5, and 29.5 --- 1.1% (Table Ill). The more than doubling of the enthalpy at longer incubation times indicates that a large fraction of the hydrocarbon chains in the lipid bilayers has crystallized into a more ordered state, such as in a solid crystalline form, resulting in a greater enthalpy for the transition. At about 29% dehydration, most of the lipids are fully dehydrated, resulting in increased enthalpy changes as the hydro- carbon chains are packed more closely for maximum Van der Waals interaction.

STRATUM CORNEUM LIPIDS 93 ! i I I ! ' I • I ' 0 20 40 60 80 100 Temperature ( ø C) Figure 1. Differential scanning calorimetry of model lipid incubated for 0 h (a), 6 h (b), 24 h (c), and 48 h (d) at 6% RH. Table II Enthalpy Values for Model Lipid, Model Lipid Plus 5%, 10%, and 15% Glyceridacid, and Model Lipid Plus 5% and 10% Glycerol, Incubated for Different Periods of Time at 6% and 92% Relative Humidities Enthalpy 0/g lipid or J/g (lipid + additive) Model lipid + Model lipid + Glyceridacid glycerol Time Model lipid 5 % 10% 15 % 5% 10% 0 h 15 + 1 13 11 8 14 13 6 h at 6% RH 21 - 1 15 15 12 17 20 24 h at 6% RH 43 - 1 21 22 17 38 30 48 h at 6% RH 52 - 1 35 28 23 45 35 6 h at 92% RH 20 - 2 15 13 13 15 14 24 h at 92% RH 20 - 1 17 15 15 -- 15 48 h at 92% RH 24 - 1 17 16 18 19 13 The transition enthalpies exhibited a small change from 15 +- 1 J/g lipid at 0 h, to 20 + 2, 20 + 1, and 24 + 1 J/g lipid after 6, 24, and 48 h of incubation at 92% RH (Table II). The corresponding water losses were 13.3 +- 0.5, 21.6 + 0.2 and 24.9 + 0.4%, respectively, after 6, 24 and 48 h (Table III). Thus, despite 25% of initial weight