RESUMO
The structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane emulsions was investigated by photon correlation spectroscopy and small-angle X-ray and neutron scattering, using emulsions with different neutron scattering contrasts. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the emulsion droplets. A monolayer, structurally similar to a half DMPC bilayer, with a thickness of 16 Å is found. Thereby, the phosphocholine headgroups arrange flat at the oil-water interface. A deep penetration of the tetracosane oil into the stabilizer layer can be ruled out.
RESUMO
Using photon correlation spectroscopy, transmission electron microscopy, microcalorimetry, wide-angle X-ray scattering (WAXS), and small-angle X-ray and neutron scattering (SAXS, SANS), the structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane suspensions was studied from the molecular level to the microscopic scale as a function of the temperature. The platelike nanocrystals exhibit for tetracosane an unusual orthorhombic low-temperature crystal structure. The corresponding WAXS pattern can be reproduced with a predicted orthorhombic unit cell (space group Pca21), which usually occurs only for much longer even-numbered n-alkanes. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the nanocrystals. Their structure was investigated by SAXS and SANS, using suspensions with different neutron scattering contrasts. As for the emulsions in Part I , the crystallized nanoparticles are covered by a DMPC monolayer. Their significant smaller thickness of 10.5 Å (for the emulsions in Part I : 16 Å) could be related to a more tilted orientation of the DMPC molecules to cover the expanded surface of the crystallized nanoparticles.
RESUMO
ZnO nanoparticles (NPs) have great potential for their use in, e.g., thin film solar cells due to their electro-optical properties adjustable on the nanoscale. Therefore, the production of well-defined NPs is of major interest. For a targeted production process, the knowledge of the stabilization layer of the NPs during and after their formation is of particular importance. For the study of the stabilizer layer of ZnO NPs prepared in a wet chemical synthesis from zinc acetate, only ex situ studies have been performed so far. An acetate layer bound to the surface of the dried NPs was found; however, an in situ study which addresses the stabilizing layer surrounding the NPs in a native dispersion was missing. By the combination of small angle scattering with neutrons and X-rays (SANS and SAXS) for the same sample, we are now able to observe the acetate shell in situ for the first time. In addition, the changes of this shell could be followed during the ripening process for different temperatures. With increasing size of the ZnO core (d(core)) the surrounding shell (d(shell)) becomes larger, and the acetate concentration within the shell is reduced. For all samples, the shell thickness was found to be larger than the maximum extension of an acetate molecule with acetate concentrations within the shell below 50 vol %. Thus, there is not a monolayer of acetate molecules that covers the NPs but rather a swollen shell of acetate ions. This shell is assumed to hinder the growth of the NPs to larger macrostructures. In addition, we found that the partition coefficient µ between acetate in the shell surrounding the NPs and the total amount of acetate in the solution is about 10% which is in good agreement with ex situ data determined by thermogravimetric analysis.
