Phospholipid adsorption at oil in water versus water in oil interfaces: Implications for interfacial densities and bulk solubilities

The adsorption of the phospholipid (PL) 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) dissolved in the oil phase squalene or squalane was studied at the interface with water. Dynamic interfacial tension was measured by profile analysis tensiometry (PAT). Two different setups were considered, a buoyant oil drop surrounded by water or a pendant water drop in oil. For the proof of methodology the interfacial tensions of water in chloroform and chloroform in water interfaces were measured. From the equilibrium interfacial tensions of different PL concentrations, the Gibbs adsorption isotherm was determined. These isotherms allow the extraction of the minimal areas per lipid molecule as well as critical aggregation concentration (CAC). We found significant differences in the results obtained for both setups. By means of PAT, the CAC determined for water drops in oil was smaller than for oil drops in water. Photon correlation spectroscopy (PCS) and nuclear magnetic resonance (NMR) were used to verify the determined CACs. We concluded the setup-dependent differences to be caused by depletion effects when initially dissolving the surface active component in the small drop volume. We recommend the reversed setup where the surface active component is dissolved in the surrounding bulk phase to avoid corrections of bulk concentrations. (C) 2015 Elsevier B.V. All rights reserved.