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-dipalmitoylglycerol Langmuir monolayers at room temperature. Pressure-induced transitions have been investigated by combination of molecular-dynamics simulations and grazing-incidence x-ray diffraction (XRD). The diglyceride film undergoes two phase transitions occurring at 38.3 and 39.8 Angstrom(2)/molecule. Simulation indicates that the first transition involves a reorientation of the headgroups while simulation and XRD show that in the second transition the order parameter is the tilt angle of the alkyl chains. A methodology for Fourier analysis of simulated Langmuir monolayers is presented. According to the simulation, in the 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 direction close to a nearest neighbor direction. Both arrangements of the alkyl chains are confirmed by XRD. For higher order and fractional order Bragg peaks, simulations predict higher intensities than observed with XRD. This may indicate that in the simulated monolayer the finite size with periodic boundary conditions imposes a higher degree of order.