Dry skin condition remains one of the major dermatological problems and to tackle or prevent this condition, consumers turn to the use of moisturising skin creams. Dry skin manifests from reduced barrier performance of the upper layer of the skin, the stratum corneum. This reduced barrier leads to increased water loss from the skin, which is due to the depletion of orthogonally packed lipids in the stratum corneum. To improve the skin barrier and thus reduce dry skin, one method is to deliver lipids with such a dense chain packing to the skin via the skin cream. It is important therefore, to understand in depth, the properties of such lipids to make more informed formulation development decisions in industry as well as providing novel insights to the scientific community.
This thesis investigates the detailed structure of lipids used typically in dermatological formulations and their stability to temperature change and interactions with other components found in formulations. The combination of X-ray scattering and differential scanning calorimetry, DSC, techniques reveal information on the nanostructure and thermal behaviour of PCs and thus are employed as suitable analytical methods.
Three different systems are studied: (i) the influence of humectants on lipids; (ii) comparison of industrial lipid samples with high purity lipid samples; (iii) more complex multi-component formulations. Humectants are found commonly in skin cream formulations as they provide hydration to the skin by attracting water molecules. Thus, there is a need for understanding their effect on the overall lipid structure and in turn, the overall structure of the skin cream. Mixed lipid systems, both pure lipid mixtures and industrial lipid mixtures (containing impurities) are studied to understand how similar lipid mixtures behave compared to pure lipids. Finally, more complex formulations enable the formulator to understand and predict the structure of the final product formulation. This thesis reports new understandings on the influence of humectants on the structure of lipids at low concentrations (< 1M), as well as detailed structural analysis and evidence for phase separation in lipid mixtures, and existence of coexisting lamellar phases from the lipid and oil components of multi-component systems, for the first time.
The insights and knowledge generated from this thesis will contribute positively to the formulation industry and help the industry transform from being an art to science.
