P. Tippmann-Krayer

Phases of phosphatidyl ethanolamine monolayers studied by synchrotron x-ray scattering

For the first time, phospholid monolayers at the air/water interface have been studied by x-ray diffraction and reflection all along the isotherm from the laterally isotropic fluid (the so-called LE phase) to the ordered phases. The model used to analyze the data, and the accuracy of the parameters deduced, were tested by comparing the results obtained with two lipids having the same head group but different chain lengths. Compression of the fluid phase leads predominantly to a change of thickness of the hydrophobic moiety, much less of its density, with the head group extension remaining constant. The main transition involves a considerable increase (approximately 10%) of the electron density in the hydrophobic region, a dehydration of the head group and a positional ordering of the aliphatic tails, albeit with low coherence lengths (approximately 10 spacings). On further compression of the film, the ordered phase undergoes a continuous transition. This is characterized by an increase in positional ordering, a discontinuous decrease in lateral compressibility, a decrease in chain tilt angle with respect to the surface normal towards zero and probably also a head group dehydration and ordering.

Thickness and temperature dependent structure of Cd arachidate Langmuir-Blodgett films

Abstract The structure of monolayers of Cd arachidate on water and on solid support, and the thickness dependent changes when building up a multilayer via the LB technique are studied by means of grazing incidence diffraction of X-ray. In monolayers the perpendicularly oriented amphiphilic molecules are arranged in a hexagonal lattice, whereas for thicker layers (even for three layers) they crystallize in an orthorhombic unit cell with a reduced molecular are ( A = 18.2 A 2 ) compared to that of the monolayer ( A = 19.7 A 2 ). In-plane diffraction measurements with wave vector transfer perpendicular to the surface (rod scans) could prove for multilayers a maximum tilt angle of 2°. The data prove orientational as well as positional correlation between adjacent monolayers. For multilayers, a thermally induced phase transition accompanied by an expansion of the lattice from an orthorhombic ( A = 18.2 A 2 ) to a hexagonal ( A = 19.7 A 2 ) lattice is found. This structural change is reversible below 110 °C.

CEMS/XPS study of iron stearate Langmuir-Blodgett layers

SummaryLangmuir-Blodgett mono- and multilayers of ferric stearate have been formed on oxidized silicon wafers. Thermodesorption of these layers was investigated by conversion electron Mössbauer and photoelectron spectroscopy and some complementary methods. Heating of samples in air up to 523 K leads to a desorption of the fatty acid chains, while the ferric ions are left on the substrate surface. These ions do not cluster laterally like it was found for Cd ions. They form a rather homogeneous, closed oxidic layer. This well defined layer may be used afterwards for further studies of surface reactions as well as interface and intra-layer interactions. The surface iron ions were found to exhibit a reasonably high recoilless fraction. Therefore, Mössbauer spectroscopy allows to follow chemical, structural, and magnetic changes of the iron ions even if the surface is covered by less than one monolayer.

Comparative optical reflection and mass spectrometry analysis of thermodesorption of Langmuir-Blodgett films

Abstract Thermodesorption of cadmium arachidate multilayers is studied by optical surface analysis and by mass spectrometry measurements. The optical reflection technique has been improved to discriminate signal contributions from desorption and light scattering. The scattering arises from film heterogeneities that are also observed by Nomarsky microscopy. The assessment of these heterogeneities is important to understand mass spectrometry data. Both the optical technique and mass spectrometry are sensitive to observing the multilayer phase transition at 110 °C and the desorption near 200 °C (at the heating rate applied). The mass spectrometry analysis yields detailed information on the desorbing species and hence on the desorption mechanism, whereas the optical reflection technique, because of its ease of application, can be used for screening materials and processes. In addition, it can be used to calibrate the mass spectrometry signal as a rate of thickness changes.