Florencio Penacorada

Ellipsometric studies on uranyl arachidate Langmuir-Blodgett films

Abstract In the presence of counterions, the quality of fatty acid monolayers, prepared by the Langmuir-Blodgett (LB) technique is highly influenced by the subphase pH value. In this paper, we present ellipsometric studies on arachidic acid LB films, prepared at different subphase pH values, with uranyl (O U O) 2+ as the counterion. In order to obtain the refractive indices and thickness, independently, the films were prepared as steps with bilayer step size. Refractive index, real and imaginary part, monolayer thickness and surface roughness on a microscopic and macroscopic scale have been determined for three different pH values. The results show that films prepared at a pH value of 4.8 and 4.0 are of good optical quality, whereas those prepared at pH 2.5 are inhomogeneous.

Investigation of the vertical molecular exchange in a complex organic multilayer system

A vertically stacked Langmuir–Blodgett (LB) multilayer system composed of undeuterated and deuterated Cd-stearate bilayers being separated by bilayers of the amphiphilic copolymer MSA22 has been investigated. Specular and off-specular scattering curves were collected at room temperature and under thermal annealing up to 85°C. The neutron measurements have been performed on the new neutron reflectometer ADAM at the Institute Laue-Langevin at Grenoble for the first time. Already at room temperature the intermixing between deuterated and non-deuterated molecules amounts to 20% and increases during thermal annealing. In contrast to recent measurements of samples without copolymers the thermally induced interdiffusion is significantly reduced now, unfortunately at the cost of losing the lamellar order. The evaluated activation energy of molecular exchange confirms our recent results. X-ray and neutron specular reflectivity curves cannot be interpreted solely by the Parrat formalism. Under relaxed condition of experimental resolution the off-specular scattering dominates the reflectivity curves even for large qz. On the other hand, the evaluated qz-dependence of the resonant-diffuse scattering reveals that the height–height correlation of interfaces roughness is the main origin of diffuse scattering in such multilayers.

Temperature- and time-dependent investigations of cadmium stearate and uranyl stearate multilayers by means of neutron reflectivity measurements

Abstract Langmuir-Blodgett (LB) multilayers of cadmium and uranyl stearate salts, prepared by sequential transfer of deuterated and non-deuterated bilayers on a silicon support, have been investigated by neutron reflectivity measurements at the TAS8 beam line at Riso. Freshly prepared LB multilayers already show an intermixing of the bilayers which can be attributed to incomplete transfer of the monolayer from the water surface to the substrate. This becomes visible via the reduced intensity of that superlattice peak which probes the contrast in scattering length density between the deuterated and the non-deuterated monolayers. Further experiments reveal that the intermixing increases as a function of temperature. The kinetics of this process have been investigated by combined temperature- and time-dependent X-ray and neutron reflectivity measurements. For annealing temperatures below 55 °C cadmium stearate films show a dramatic decrease in intensities of the superstructure peaks, indicating an increase of intermixing. Taking into account the fact that the X-ray reflectivity patterns remain nearly unchanged, this intermixing is interpreted as discrete hopping of the fatty acid salt molecules between various bilayer sites. Its time constant decreases as a function of temperature and vanishes close to the melting point of the stearic acid phase at about 75 °C. The Arrhenius-like activation energy for the hopping process was estimated to be about 1.9 eV. For uranyl stearate samples we already observed complete intermixing at about 40 °C, which is interpreted in terms of the reduced lateral interaction among the hydrocarbon chains induced by the bigger counterions.

Small angle X-ray reflectivity study of langmuir-blodgett films of a peripherally substituted zinc phthalocyanine

Abstract Langmuir-Blodgett films of zinc 11,18,25-tri(tert-butyl)-4-sulfo-phthalocyanine (ZnPctSO3Na) have been deposited onto hydrophilic and hydrophobic silicon wafers. Y-type films were formed on both types of substrate, and the transfer ratio was very close to unity. The organization of the films on the molecular level was probed by X-ray specular reflectivity.

Structure of langmuir monolayers of substituted phthalocyanines

Langmuir floating layers of two phthalocyanine derivatives, ytterbium bisphthalocyanine and tetra-tert-butyl nickel phthalocyanine, were investigated by means of compression isotherms, surface potential kinetics and Brewster angle microscopy (BAM) in order to study the influence of peripheral substituents on the structure and stability of these films and on their suitability for a subsequent transfer onto solid substrates. Specific substitutions that may lead to amphiphilic molecular units seem to play a key role in the development of well organised thin films prepared with this technique.

Monolayers and multilayers of uranyl arachidate: in-plane structure of uranyl arachidate multilayers

Abstract The molecular in-plane structure of uranyl arachidate Langmuir-Blodgett (LB) films formed at different subphase pH values was analysed by means of X-ray grazing-incidence diffraction. For multilayers formed at low subphase pH a reorganisation of the arachidic acid film structure is confirmed. At appropriate subphase pH values, reorganisation of the film structure e.g. via the formation of three-dimensional crystallites, is prevented by the presence of the uranyl ions and by the subsequent introduction of conformational disorder (gauche defects) in the alkyl chains. The observation of a macroscopic flow-induced in-plane texture in these uranyl arachidate LB films has profound implications for the design of ordered, supramolecular structures by the Langmuir-Blodgett technique.

Structure and stability of Langmuir monolayers and Langmuir-Blodgett films of bisaroyl azide bolaamphiphiles

Abstract Langmuir-Blodgett (LB) films of bolaamphiphiles with reactive head groups can be used for the surface modification of composite membranes for gas-separation processes. The scope of our investigations was to gain a first insight into the monolayer behaviour and LB film structure of previously synthesized bisaroyl azide bolaamphiphiles. The layers have been analysed by means of surface potential measurements and Brewster angle microscopy. Furthermore, parameters for successful deposition of LB films were found. As expected for a molecule with two hydrophilic ends, the transfer ratio on upstroke was close to one and no transfer occurred on downstroke. The multilayer structure was analysed by scanning force microscopy (SFM) and X-ray reflectivity measurements. The SFM images revealed a periodic in-plane structure on the molecular level. Based on a combination of the X-ray data with the results of other methods, two possible models of the multilayer structure are presented.

Investigation of molecular diffusion across organic multilayers using neutron specular reflectivity

Abstract Langmuir–Blodgett (LB) multilayers of fatty acid salt molecules have been investigated by neutron specular reflectivity (NSR) at the reflectometer ADAM at the Institute Laue-Langevin in Grenoble, France. Due to the high flux at the sample site, the NSR curve could be recorded over more than six orders of magnitude, corresponding with q z ≤0.3 A −1 . Because of the incoherent hydrogen scattering, the detectable coherent NSR curve is limited to 10 −5 . The incoherent background of the multilayer systems could be determined for the first time by means of a spin-polarized NSR experiment. The observed intermixing at freshly prepared LB multilayers increases during thermal annealing below the phase transition temperature, whereas the vertical structure remains. This is explained by thermally induced vertical reorganisation of molecules across the multilayer system. The molecular exchange can be described as an analogue to an order–disorder transition. The evaluated activation energy of about 1.7 eV is related to the breaking of van der Waals bonds between one particular molecule and its hexagonal neighbourhood. Time-resolved measurements at particular temperatures allow us to estimate a vertical “diffusion constant” in the range of 10 −23 m 2 /s.