Surface Potential of Lipid Interfaces Formed by Mixtures of Phosphatidylcholine of Different Chain Lengths

The dipole and ζ potentials of mixtures of dimyristoylphosphatidylcholine /dipalmitoylphosphatidylcholine (DMPC/DPPC), dioleoylphosphatidylcholine/ distearoylphosphatidylcholine (DOPC/DSPC), and dioleoylphosphatidylcholine /dimyristoylphosphatidylcholine (DOPC/DMPC) were measured as a function of the mole fractions at temperatures below and above the phase transition. The dipole potentials of lipid films were determined in monolayers by means of the constant-area method proposed by MacDonald and Simon ( Proc. Natl. Acad. Sci. USA 1987, 84 4089). The ζ potentials were obtained by measuring the electrophoretic mobility of MLVs for similar mixtures at the same temperature. Discontinuities in the dipole potential and the ζ potential for 0.33 and 0.66 molar fractions were observed for DPPC/DMPC mixtures in the gel state well above the experimental error. These singularities in the values of dipole and ζ potentials were less noticeable or absent in DSPC/DOPC and DMPC/DOPC. The results can be explained as a consequence of the organization of the polar head groups at the lipid interface for peculiar molar ratios when the lipids are miscible in the whole range of molar fractions. It is concluded that the organization of lipids in bilayers gives similar arrangements in the surface to those in monolayers. The surface properties seem to be a consequence of the type and phase state of the lipids in the mixture rather than of the supramolecular organization such as monolayer or bilayer. Thus, monolayers and bilayers are intrinsically similar model systems in regard to their surface properties.