The molecular interaction of dipalmitoylphosphatidylcholine (DPPC) liposomes adsorbed on TiO2 to solutions containing the anionic surfactant, sodium dodecyl sulfate (SDS) was studied using Attenuated Total Reflection – Fourier Transform Infrared (ATR FTIR) spectroscopy. Specifically, the amount of SDS incorporated in the DPPC structure and the amount of DPPC expelled is measured as a function of contact time. At the same time, analysis of changes to the headgroup IR bands of both SDS and DPPC provides structural information on the nature of the SDS/DPPC assemblies. It is found that the incorporation of SDS follows two routes. The first path is the generally accepted process in which the SDS intercalates into the DPPC liposomes. At the same time, a small fraction of the DPPC is expelled from the liposome. At high SDS solution concentrations, this results in the disintegration of the liposomes leading to formation of mixed SDS/DPPC micelles on the TiO2. In the second route, the SDS binds electrostatically to the outer leaflet of the DPPC liposome. While both processes (formation of aggregated SDS structures on the liposome surface and intercalation of SDS into the liposome) occur simultaneously, it is also shown that the uptake of SDS occurs in a series of steps with contact time. These steps are mirrored by the loss of DPPC from the liposome and arise from the ejection of mixed DPPC/SDS micelles.
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