Spatial variation in the molecular tilt orientational order within the solid domains of phase-separated, mixed dialkylphosphatidylcholine monolayers

The miscibility of the solid-phase-forming distearoylphosphatidylcholine (DSPC) and the fluid-phase-forming dilauroylphosphatidylcholine (DLPC) at the air/water interface was investigated by the Langmuir film balance. Surface pressure–area isotherms suggest that mixtures containing 25.0–62.5-mol% DLPC (range of composition investigated) are phase-separated. The lateral structure of the DSPC/DLPC monolayers was imaged by Brewster angle microscopy (BAM) as a function of the surface pressure. Quasi-circular condensed domains appeared at pressures between 0 and 0.5 mN m−1, and these structures were already fully developed at ∼1 mN m−1. Further compression of the monolayers above 1 mN m−1 merely brought the domains closer together. The mixed monolayers consisted of solid domains of DSPC, ∼3–20 μm in size, in a fluid matrix of DLPC. BAM and the phase contrast mode of intermittent-contact atomic force microscopy (AFM) revealed that the quasi-circular DSPC domains are divided into segments of different reflectivities (BAM) or phase shift (AFM) that arise from abrupt changes in the long-range orientational order of the tilted hydrocarbon chains. The DSPC domains in DSPC/DLPC internally exhibited star and cardioid textures that were heretofore only reported for single-component lipid monolayers in the phase coexistence region.