Monolayers and multilayers of chlorophyll [correction of chlorophyl] a on a mercury electrode

A novel experimental technique used to investigate chlorophyll films on a hanging mercury drop electrode is described. Two different procedures are employed to prepare self-assembled chlorophyll monolayers and multilayers on the mercury electrode. Upon illuminating the chlorophyll a (Chl)-coated mercury electrode with an appropriate light source, the photocurrents generated by the Chl aggregates are measured under short-circuit conditions in the absence of photoartefacts. The preliminary results obtained by this novel technique are presented.     

Subphase pH effect on composition and structure of Fe(III) arachidate monolayers

The arachidic acid monolayers on Fe(III) subphase surfaces with various pH values have been studied. The π–A isotherms of monolayers and Fourier transform infra-red (FTIR) spectra of Langmuir–Blodgett (LB) films show that the films obtained on different pH subphases with a concentration of Fe(III) 5×10⁻⁵ M have different compositions. At low pH (2.4), the film appears to be almost 100% acid. The content of Fe(III) arachidate increases with increasing pH, at pH 3.7, the film appears to be 100% salt; and the content of salt decreases with pH larger than 3.7. The transmission electron microscope (TEM) observations show that the monolayer structures changed from patch aggregates formed by Fe(III) arachidate at low pH to a network or dotted structures formed by hydrolysates of Fe(III) at high pH. The Brewster angle microscopy (BAM) experiments show the different monolayer behaviors at various pH. These results indicate that the subphase pH greatly influenced the arachidic acid monolayers.     

Polyelectrolyte/magnetite nanoparticle multilayers: preparation and structure characterization

Polyelectrolyte composite planar films containing a different number of iron oxide (Fe3O4) nanoparticle layers have been prepared using the layer-by-layer adsorption technique. The nanocomposite assemblies were characterized by ellipsometry, UV-vis spectroscopy, and AFM. Linear growth of the multilayer thickness with the increase of the layer number, N, up to 12 reflects an extensive character of this parameter in this range. A more complicated behavior of the refractive index is caused by changes in the multilayer structure, especially for the thicker nanocomposites. A quantitative analysis of the nanocomposite structure is provided comparing a classical and a modified effective medium approach taking into account the influence of light absorption by the Fe3O4 nanoparticles on the complex refractive index of the nanocomposite and contributions of all components to film thickness. Dominant influence of co-adsorbed water on their properties was found to be another interesting peculiarity of the nanocomposite film. This effect, as well as possible film property modulation by light, is discussed.     

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

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 Risø. 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.     

X-ray structure of dihydrazinium uranyl dioxalate monohydrate

The crystal structure of dihydrazinium uranyl dioxalate monohydrate, (N₂H₅)₂ [UO₂(C₂O₄)₂(H₂O)], has been determined by x-ray diffraction. The structure was solved by heavy-atom method and refined to an R value of 0–059 using 2312 reflections. The N₂H ₅ ⁺ ions are not coordinated to the metal. In the anion [UO₂(C₂O₄)₂(H₂O)]²⁻, the linear UO ₂ ²⁺ group is coordinated by two chelating bidentate oxalate oxygens and a water oxygen. The coordination polyhedron around the uranium atom is an approximate pentagonal bipyramid.     

18-Electron rule and structure of uranyl sulfate complexes

Characteristics of the Voronoi-Dirichlet polyhedra were used to perform the crystal-chemical analysis of all known sulfate-containing uranyl compounds. The effect of the type of coordination of sulfate ions by U(VI) atoms on the characteristics of S-O and U-O bonds in the crystal structure is considered. The electrondonor properties of sulfate ions were estimated quantitatively on the basis of the 18-electron rule. Some predictive properties of the stereoatomic crystal structure model were demonstrated in relation to tri-and tetrasulfatouranylates.     

Formation and structure of polyelectrolyte and nanoparticle multilayers: effect of particle characteristics

The formation and structure of multilayer films containing a cationic polyelectrolyte and anionic silica nanoparticles were studied by means of ellipsometry and atomic force microscopy. Three types of silica particles of different sizes were examined. The density and thickness of the films as well as the adsorption kinetics appear to be strongly dependent on the choice of particle; smaller particles favor the formation of smooth and dense films with a higher content of the inorganic component.     

Hydrothermal-induced structure transformation of polyelectrolyte multilayers: from nanotubes to capsules

The assembled polyelectrolyte nanotubes composed of poly(styrenesulfonate) and poly(allylamine hydrochloride) multilayers by using the layer-by-layer assembly combined with the porous template method can be transformed into capsules by a high-temperature treatment. Scanning electron microscopy and confocal laser scanning microscopy images revealed the whole transition process. The structure transformation of polyelectrolyte multilayers after annealing can be initiated by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged polyelectrolyte groups. The transition process from tubes to capsules is supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes.     

Synthesis and structure of uranyl complexes with malonic acid dianions

The uranyl complexes with malonic acid dianions [UO₂(C₃H₂O₄)(CO(NH₂)₂)]·H₂O (1), [UO₂(C₃H₂O₄)(CONH₂NMe₂)]·H₂O (2), and [UO₂(C₃H₂O₄)(MeCONMe₂)] (3) were synthesized and characterized by X-ray crystallography. The structural units [UO₂(C₃H₂O₄)L] in the crystals of 1–3 refer to the AK²¹M¹ crystal chemical group (A = UO₂ ²⁺, K²¹ = C₃H₂O₄ ^2?, M¹ = L) of uranyl complexes; the crystals of 1 have a framework structure and 2 and 3 have a chain structure. Some structural features of the [UO₂(C₃H₂O₄)L] complex groups are discussed.     

A DFT study of uranyl hydroxyl complexes: structure and stability of trimers and tetramers

A DFT study of U(VI) hydroxy complexes was performed with special attention paid to the [(UO₂)₃(OH)₅(H₂O)_4–7]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. It was established that the ionicity of the U=O bond increased when moving from [(UO₂)(H₂O)₅]²⁺, [(UO₂)₂(OH)(H₂O)₈]³⁺, [(UO₂)₂(OH)₂(H₂O)₆]²⁺, [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ to [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ species. In both [(UO₂)₃(OH)₅(H₂O)_4–6]⁺ and [(UO₂)₄(OH)₇(H₂O)_5–8]⁺ complexes, the U=O bond was observed to have a range of different lengths which depended on the composition of the first coordination sphere of UO₂ ²⁺. The cyclic structures of trimeric complexes were somewhat more stable than their linear structures, which was probably due to the steric effect.

     

The structure of self-assembled multilayers with polyoxometalate nanoclusters

Using electrostatic layer-by-layer self-assembly (ELSA), the formation of multilayers with polyelectrolytes and nanoscopic polyoxometalate (POM) clusters of different sizes and charges is investigated. The multilayers are characterized by UV-vis absorption spectroscopy, optical ellipsometry, cyclic voltammetry, and atomic force microscopy. In all cases, it is possible to find experimental conditions to achieve irreversible adsorption and regular multilayer deposition. Most importantly, the surface coverage is directly related to the total charge of the POM anion and can be controlled from submonolayer to multilayer coverage by adjusting the ionic strength of the dipping solutions. Imaging the interfaces after POM deposition by atomic force microscopy reveals a granular surface texture with nanometer-sized features. The average interfacial roughness amounts to approximately 1 nm. Cyclic voltammetry indicates that the electrochemical properties of the POM clusters are fully maintained in the polyelectrolyte matrix, which opens a route toward practical applications such as sensors or heterogeneous catalysts. Moreover, the permeability toward electrochemically active probe molecules can be tailored through the multilayer architecture and deposition conditions. Finally, we note that despite the low total charge and comparably small size of the discrete POM anions, the multilayers are remarkably stable. This work provides basic guidelines for the assembly of POM-containing ELSA multilayers and provides detailed insight into characteristic surface coverage, permeability, and electrochemical properties.     

Stability and structure of polyelectrolyte multilayers deposited from salt free solutions

Molecular-dynamics (MD) simulation results show that polyelectrolyte multilayers deposited from salt free solutions on charged planar surfaces are thermodynamically stable structures that form spontaneously regardless of the method of deposition. The simulation also shows that the polyelectrolyte multilayers are "fuzzy" in nature and molecules in one layer interpenetrate other layers. The influence of chain length, surface charge, and polymer charge is also investigated. Layer thickness was found to be independent of chain length. The ratio of surface to chain charge was found to influence the thickness of the first layer and the amount of polymer absorbed in the first few layers. The thickness of the subsequent layers was found to be independent of the charge ratio.     

Analysis of electronic states and energy level structure of uranyl in compounds

An effective operator Hamiltonian has been developed for evaluating the excited states of uranyl ion in compounds. Including free-ion and crystal-field interactions, the parametrized Hamiltonian is able to reproduce the energy level structure in good agreement with experimental measurements on uranyl in chloride compounds and nitride complexes. It is shown that the low-lying excited states belong to a nonbonding σf configuration (therefore, one Slater integral of G(3) is sufficient to account for the Coulomb electron exchange interaction) and that the spin-orbit coupling surpasses the electrostatic interactions in energy level splitting. In comparison with previously reported ab initio calculations, the present studies find that the Coulomb interactions in uranyl were excessively evaluated in the first-principle calculations.     

Predicting the redox properties of uranyl complexes using electronic structure calculations

A plethora of chemical reactions is redox driven processes. The conversion of toxic and highly soluble U(VI) complexes to nontoxic and insoluble U(IV) form are carried out through proton coupled electron transfer by iron containing cytochromes and mineral surfaces such as machinawite. This redox process takes place through the formation of U(V) species which is unstable and immediately undergo the disproportionation reaction. Thus, theoretical methods are extremely useful to understand the reduction process of U(VI) to U(V) species. We here have carried out the structures and reduction properties of several U(VI) to U(V) complexes using a variety of electronic structure methods. Due to the lack of experimental ionization energies for uranyl (UO₂(V)‐UO₂(VI)) couple, we have benchmarked the current and popularly used density functionals and cost effective ab initio methods against the experimental electron detachment energies of [UO₂F₄]^1‐/2‐ and [UO₂Cl₄]^1‐/2‐. We find that electron detachment energy of U(VI) predicted by RI‐MP2 level on the BP86 geometries correlate nicely with the experimental and CCSD(T) data. Based on our benchmark studies, we have predicted the structures and electron detachment energies of U(V) to U(VI) species for a series of uranium complexes at the RI‐MP2//BP86 level which are experimentally inaccessible till date. We find that the redox active molecular orbital is ligand centered for the oxidation of U(VI) species, where it is metal centered (primarily f‐orbital) for the oxidation of U(V) species. Finally, we have also calculated the detachment energies of a known uranyl [UO₂]¹⁺ complex whose X‐ray crystal structures of both oxidation states are available. The large bulky nature of the ligand stabilizing the uncommon U(V) species which cannot be routinely studied by present day CCSD(T) methods as the system size are more than 20–30 atoms. The success of our efficient computational strategy can be experimentally verified in the near future for the complex as the structures are stable in gas phase which can undergo oxidation.     

Crystal structure of uranyl tungstate Cs2U2WO10

A representative of uranyl tungstates Cs₂U₂WO₁₀ has been synthesized. A single crystal X-ray diffraction experiment has been performed for the compound prepared and its crystal structure has been solved. It is monoclinic, space group P2₁/n, a = 8.4611(5) Å, b = 28.691(2) Å, c = 9.6499(6) Å; β = 107.073(1)°, R ₁ = 0.086 for 4599 independent reflections with I > 2σ(I). The crystalline structure of the compound is formed by infinite negatively charged layers [U₂WO₁₀] _2∞ ^δ? interleaved with cesium cations which bond the layers. The compound has been studied with IR spectroscopy, and absorption bands of its spectrum have been assigned.