Influence of oleic acid on the structure of a mixture of hydrated model stratum corneum fatty acids and their soaps

The phase behaviour of a hydrated (32% (w/w) water) mixture of saturated and unsaturated fatty acids and their soaps has been examined. By progressively increasing the proportion of oleic acid, it could be shown that this unsaturated component exists partly as a separate phase within the others. The unsaturated fatty acids form an H_II phase at 25°C, whereas the saturated fatty acids form a fatty acid—soap crystal under the conditions employed. The results help to explain the role played by the fatty acids (and also cholesterol) within the lipid fraction of human stratum corneum. A feature of this study is a comparison of the results obtained by X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy.