Ellipsometric study of the physical states of phosphatidylcholines at the air-water interface

The ellipsometric isotherms of a homologous series of phosphatidylcholines at the air-water interface are reported for the first time. The ellipsometric angle, δΔ, has proven to be very sensitive to the nature of the physical state of the film as well as to the physical-state changes that occur during film compression. At 17 ± 1°C, two compounds of the series, i.e. L-α-dimyristoylphosphatidylcholine (DMPC) (C-14) and L-α-dipalmitoylphosphatidylcholine (DPPC) (C-16) exhibit a phase transition while L-α-distearoylphosphatidylcholine (DSPC) (C-18) and L-α-diarachidoylphosphatidylcholine (DAPC) (C-20) show only a solid-state behavior upon compression. This physical state is also observed for DPPC at 10°C. A rodlike model was used to depict the different molecular behaviors exhibited by the film-forming molecules of this series. Equations relating variations of refractive indices, film thickness, and molecular area to film organization were obtained so that the evolution of the molecular properties of the film can be followed during monolayer compression. Calculated refractive indices and film thicknesses were obtained from experimental δΔ. Results indicate that refractive indices are higher for a solid state than for a liquid expanded state. Moreover, in the same physical state, refractive indices slightly increase with chain length. It is observed that positive anisotropy is shown for the films in the solid state whereas negative anisotropy is exhibited by those in the liquid expanded state. These results strongly suggest that phase transition is made up of a mixture of bent conformers, which, by reduction of molecular area, favors all-trans conformation.