Inverse size exclusion chromatography (ISEC)

In this work, an improved protocol for inverse size-exclusion chromatography (ISEC) was established to assess important pore structural data of porous silicas as stationary phases in packed chromatographic columns. After the validity of the values generated by ISEC was checked by comparison with data obtained from traditional methods like nitrogen sorption at 77 K (Study A), the method could be successfully employed as valuable tool at the development of bonded poly(methacrylate)-coated silicas, while traditional methods generate partially incorrect pore structural information (Study B). Study A: Different mesoporous silicas were converted by a pseudomorphical transition into ordered MCM-41-type silica while maintaining the particle-size and -shape. The essential parameters like specific surface area, average pore diameter and specific pore volume, the pore connectivity from ISEC remained nearly the same which was reflected by the same course of the theoretical plate height vs. linear velocity curves. Study B: In the development of bonded poly(methacrylate)-coated silicas for the reversed phase separation of biopolymers, ISEC was the only method to generate valid pore structural information of the polymer-coated materials. Synthesis procedures were developed to obtain reproducibly covalently bonded poly(methacrylate) coatings with good thermal stability on different base materials, employing as well particulate and monolithic materials.

Sorption and direct speciation of neptunium(V) on aluminium oxide and montmorillonite

This study comprised batch experiments, direct speciation studies via EXAFS, and modelling with the 2SPNE SC/CE model to elucidate the mechanisms of Np(V) sorption on montmorillonite and, for reference, on γ-Al2O3. The sorption of pM 239Np(V) and µM 237Np(V) on montmorillonite (STx-1, 4 g/L) and γ-Al2O3 (0.5 g/L) was studied at room temperature in the presence and absence of ambient CO2 covering a pH-range from 2.5 (STx-1) or 5 (γ-Al2O3) to 10.5 with 0.01 or 0.1M NaClO4 as background electrolyte. The Np(V) uptake was determined by γ spectroscopy of the supernatants and calculated as percentage as well as distribution coefficient Kd. Sorption starts from pH ~6 and, under exclusion of CO2, increases continuously, while, in the presence of ambient air, it reaches a maximum at pH ~8.5 (γ-Al2O3: log Kd max ≈ 4 mL/g; STx-1: log Kd max ≈ 2.7 mL/g). Beyond that it decreases again due to the formation of queous neptunium carbonate complexes. Furthermore, neptunium sorption on montmorillonite is influenced by ionic strength at pH <6 through ion exchange processes pointing towards the formation of outer-sphere surface complexes there. Isotherms measured at the sorption maximum showed the precipitation of resumably neptunium carbonate complexes above 3∙10^-5 M under ambient air conditions. Additionally, they indicated progressive saturation of the sorption sites of γ-Al2O3. At selected pH (STx-1: 5.0, 7.0, 8.0, 8.5, 9.0, 9.5; γ-Al2O3: 8.5, 9.5) EXAFS samples were prepared as wet pastes with µM 237Np and measured at room temperature in fluorescence mode at ANKA and ESRF. Several spectra were averaged and analysed with EXAFSPAK and FEFF 8.20 employing models of NaNpO2(CO3) or soddyite, (UO2)2SiO4∙2(H2O). The shorter atomic distances of the neptunyl ion at pH 5 compared to the others hinted at the retention of the hydration shell and, thus, at outer-sphere sorption. On average the bond lengths for Np(V) sorbed on STx-1 at high pH were Oax ≈ 1.84 Å and Oeq ≈ 2.53 Å. At high pH, ternary neptunyl carbonate surface complexes could be identified for montmorillonite (C ≈ 3.00 Å), but not for γ-Al2O3, where an interaction of neptunium with the aluminium surface atoms according to the soddyite model gave better agreement with the experimental data. However, neither structure as suggested by the two models could be excluded for both systems rendering a combination most likely. Modelling of the sorption data provided further evidence for the existence of ternary neptunium carbonate surface complexes in both cases. The results of this study can aid environmental risk assessment for clay-based nuclear waste repositories by providing valuable input data for simulations of radionuclide migration from a final disposal site.

A mesoscopic simulation method for electrolyte solutions

This thesis deals with the development of a novel simulation technique for macromolecules in electrolyte solutions, with the aim of a performance improvement over current molecular-dynamics based simulation methods. In solutions containing charged macromolecules and salt ions, it is the complex interplay of electrostatic interactions and hydrodynamics that determines the equilibrium and non-equilibrium behavior. However, the treatment of the solvent and dissolved ions makes up the major part of the computational effort. Thus an efficient modeling of both components is essential for the performance of a method. With the novel method we approach the solvent in a coarse-grained fashion and replace the explicit-ion description by a dynamic mean-field treatment. Hence we combine particle- and field-based descriptions in a hybrid method and thereby effectively solve the electrokinetic equations. The developed algorithm is tested extensively in terms of accuracy and performance, and suitable parameter sets are determined. As a first application we study charged polymer solutions (polyelectrolytes) in shear flow with focus on their viscoelastic properties. Here we also include semidilute solutions, which are computationally demanding. Secondly we study the electro-osmotic flow on superhydrophobic surfaces, where we perform a detailed comparison to theoretical predictions.