Miscibility and phase separation in mixed erucylphosphocholine–DPPC monolayers

Binary Langmuir monolayers composed of 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) and erucylphosphocholine (ErPC) – a new generation anti-cancer drug of phospholipids-like structure, have been studied with classical Langmuir technique complemented with Brewster angle microscopy (BAM) and Grazing Incidence X-ray Diffraction (GIXD). In the course of surface pressure (π)–area (A) isotherms for film containing mole fraction of ErPC (XErPC) equal to 0.1, 0.2 and 0.3, two collapses appear – the value of the first one is close to the collapse pressure observed for pure ErPC, while the second one occurs at the pressure similar to that for pure DPPC. Such a behavior could imply that the investigated system is immiscible – the expulsion of a component collapsing at a lower pressure (ErPC) from mixed monolayer occurs at the first transition while DPPC (collapsing at a higher pressure value) is expelled at the final collapse. To confirm this hypothesis further analysis has been performed. Namely, thermodynamic analysis based on the calculation of the excess free enthalpy of mixing (ΔGMexc) evidenced for mutual miscibility of DPPC and ErPC in the region of surface pressures below the first collapse, which has been additionally confirmed with BAM images. On the other hand, at high surface pressures, above the first collapse (50 mN/m) GIXD experiments complemented with BAM images proved phase separation of the investigated lipids and 2D crystalline domains formation. The interpretation of the X-ray scattering results enabled us to propose a possible model of molecular packing, according to which only condensed domains of the 1:1 lipids ratio are periodically ordered; whereas at the other proportions the lipids are located randomly within the domains.

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► Mixed Langmuir monolayers DPPC and erucylphosphocholine (ErPC) have been studied.
► The analysis of the isotherms suggested immiscibility between film components.
► Deeper analysis with BAM and GIXD proved phase separation at high pressures.
► A model of organization of lipid molecules in monolayer has been proposed.