We report on the characterization of high quality sapphire single crystals suitable for high-resolution X-ray optics at high energy. Investigations using rocking curve imaging reveal the crystals to be of uniformly good quality at the level of ∼10−4 in lattice parameter variations, δd/d. However, investigations using backscattering rocking curve imaging with a lattice spacing resolution of δd/d∼5×10−8 show very diverse quality maps for all crystals. Our results highlight nearly ideal areas with an edge length of 0.2–0.5 mm in most crystals, but a comparison of the back reflection peak positions shows that even neighboring ideal areas exhibit a relative difference in the lattice parameters on the order of δd/d=10–20×10−8; this is several times larger than the rocking curve width. Stress-strain analysis suggests that an extremely stringent limit on the strain at a level of ∼100 kPa in the growth process is required in order to produce crystals with large areas of the quality required for X-ray optics at high energy.
An integrated approach is applied to reveal fine changes in the surface-normal structure of 1,2-dimyristoyl-sn-glycero-3-phospho-Lserine (DMPS) monolayers at the air−lipid−water interface occurring in a liquid expanded (LE)−liquid condensed (LC) transition. The combination of the Langmuir monolayer technique, X-ray reflectometry, and molecular dynamics (MD) modeling provides new insight into the molecular nature of electrostatic phenomena in different stages of lipid compression. A homemade setup with a laboratory X-ray source (λ = 1.54 Å) offers a nondestructive way to reveal the structural difference between the LE and LC phases of the lipid. The electron density profile in the direction normal to the interface is recovered from the X-ray reflectivity data with the use of both model-independent and model-based approaches. MD simulations of the DMPS monolayer are performed for several areas per lipid using the all-atom force field. Using the conventional theory of capillary waves, a comparison is made between the electron density profiles reconstructed from the X-ray data and those calculated directly from MD modeling, which demonstrates remarkable agreement between the experiment and simulations for all selected lipid densities. This confirms the validity of the simulations and allows an analysis of the contributions of the hydrophobic tails and hydrated polar groups to the electron density profile and to the dipole component of the electric field at the interface. According to the MD data, the dependence of the Volta potential on the area per lipid in the monolayer has a different molecular nature below and above the phase transition. In the LE state of the monolayer, the potential is determined mostly by the oriented water molecules in the polar region of the lipid. In the LE−LC transition, these molecules are displaced to the bulk, and their effect on the Volta potential becomes insignificant compared with the contribution of the hydrophobic tails. The hydrophobic tails are highly ordered in the state of the liquid crystal so that their dipole moments entirely determine the growth of the potential upon compression up to the monolayer collapse.
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