X-RAY-DIFFRACTION AND MOLECULAR-DYNAMICS STUDIES - STRUCTURAL-ANALYSIS OF PHASES IN DIGLYCERIDE MONOLAYERS

We report a detailed structural analysis of the phases of 1,2-sn-dipal mitoylglycerol Langmuir monolayers at room temperature. Pressure-induc ed transitions have been investigated by combination of molecular-dyna mics simulations and grazing-incidence x-ray diffraction (XRD). The di glyceride film undergoes two phase transitions occurring at 38.3 and 3 9.8 Angstrom(2)/molecule. Simulation indicates that the first transiti on involves a reorientation of the headgroups while simulation and XRD show that in the second transition the order parameter is the tilt an gle of the alkyl chains. A methodology for Fourier analysis of simulat ed Langmuir monolayers is presented. According to the simulation, in t he two states of higher surface pressure the alkyl chains are vertical and pack in a centered-rectangular (nearly hexagonal) lattice. In the second phase transition the alkyl chains start tilting; At the lowest pressure the tilt angle reaches approximate to 14 degrees in a direct ion close to a nearest neighbor direction. Both arrangements of the al kyl chains are confirmed by XRD. For higher order and fractional order Bragg peaks, simulations predict higher intensities than observed wit h XRD. This may indicate that in the simulated monolayer the finite si ze with periodic boundary conditions imposes a higher degree of order.