Steady-State Diffusion in Complex Amphiphilic Films

Abstract: The relation between structure and diffusive transport at steady-state is investigated theoretically for amphiphilic systems. Amphiphilic systems typically show a large response to moderate changes in control parameters, such as temperature, osmotic pressure, and the presence of cosolvents and cosolutes. Such systems is therefore expected to show a local response in structure due to the transport process. The structure of the system is analysed in terms of a local equilibrium description. The main focus is on systems with the propensity of undergoing an internal phase separation. In this case there is a particularly strong coupling between the diffusion process(es) and the local structure of the system, which can lead to non-linear behaviour. Specific applications include diffusive transport through the outermost layer of human skin, the stratum corneum, where the possibility of a phase change of the stratum corneum lipids could explain experimental observations of a non-linear behaviour of the water transport. A model for the formation of a gradient in pH over the stratum corneum is also presented in terms of the diffusive transport of water and carbon dioxide. Another application is film formation at the air-liquid interface of surfactant-water systems undergoing evaporation. A study showing that the structure of the surfactant layer lining the alveoli is functionally beneficial for the diffusive transport of oxygen is also presented.