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Phospholipids (PLs) are widely used in the pharma industry and a better understanding of their behavior under different conditions is helpful for applications such as their use as medical transporters. The transition temperature Tm affects the lipid conformation and the interfacial tension between perfluoroperhydrophenanthrene (PFP) and an aqueous suspension of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC), as well as a mixture of these PLs with cholesterol. Interfacial tensions were measured with the Du Noüy ring at quasi-equilibrium; the area per molecule was calculated according to the Gibbsian approach and a time-dependent tension gradient. Results show that the time tε to reach quasi-equilibrium was shorter when the temperature was above Tm, indicating a faster adsorption process (tε,DPPC,36 °C = 48 h, tε,DPPC,48 °C = 24 h) for PL in the liquid crystalline state than in the gel state (T < Tm). In addition, concentration-dependent results of the interfacial tension revealed that above the respective Tm and at all concentrations c > 0.1 mM, the average minimum interfacial tension for DPPC and DSPC (14.1 mN/m and 15.3 mN/m) does not differ significantly between those two lipids. Equilibrium between monolayers and bilayers shows that for T < Tm, surface pressures ∏ ≈ 31 mN/m are reached while for T > Tm, ∏ ≈ 41 mN/m. Mixtures with cholesterol only reach ∏ ≤ 31 mN/m Tm, with no significant difference between the two PLs. The higher interfacial tension of the mixture indicates stabilization of the liposomal conformation in the aqueous phase by the addition of cholesterol. The high diffusion coefficients show that adsorption is mainly based on liposomes. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]