Decyltrimethylammonium bromide micelles in acidic solutions:counterion binding, water structuring and micelle shape

Wide-angle neutron scattering experiments combined with Empirical Potential Structural Refinement modelling have been used to study the detailed structure of decyltrimethylammonium bromide micelles in the presence of acid solutions of HCl or HBr. These experiments demonstrate considerable variation in micelle structure and water structuring between micelles in the two acid solutions and in comparison with the same micelles in pure water. In the presence of the acids, the micelles are smaller, however in the presence of HCl the micelles are more loosely structured and disordered while in the presence of HBr the micelles are more compact and closer to spherical. Bromide ions bind strongly to the micelle surface in the HBr solution, while in HCl solutions, ion binding to the micelle is similar to that found in pure water. The hydration numbers of the anions and extent of counterion binding follow the predictions of the Hofmeister series for these species.</p

Decyltrimethylammonium bromide micelles in acidic solutions:counterion binding, water structuring and micelle shape

Wide-angle neutron scattering experiments combined with empirical potential structural refinement modeling have been used to study the detailed structure of decyl­trimethyl­ammonium bromide micelles in the presence of acid solutions of HCl or HBr. These experiments demonstrate considerable variation in micelle structure and water structuring between micelles in the two acid solutions and in comparison with the same micelles in pure water. In the presence of the acids, the micelles are smaller; however, in the presence of HCl the micelles are more loosely structured and disordered while in the presence of HBr the micelles are more compact and closer to spherical. Bromide ions bind strongly to the micelle surface in the HBr solution, while in HCl solutions, ion binding to the micelle is similar to that found in pure water. The hydration numbers of the anions and extent of counterion binding follow the predictions of the Hofmeister series for these species

A study of rare earth doped silicate and phosphate glasses

The complementary techniques of X-ray diffraction and EXAFS have been applied to silicate and phosphate glass systems containing varying quantities of rare earth elements. The silicate systems that have been studied are rare earth doped fibre optic preforms of interest to the optoelectronics and telecommunications industry. Techniques were developed to allow spatially resolved diffraction and EXAFS data to be taken from the small ~1mm diameter core region of the preforms. Absorption maps were made displaying the distribution of the rare earth ions as a function of radial position, and similarly for the germanium codopant incorporated in these systems. The diffraction results show as expected, that silica dominates the structure evident in the preforms whilst EXAFS measurements taken at the germanium K-edge, suggest that this codopant occupies an eightfold coordinated site surrounded by oxygen atoms at 1.7A. The phosphate glasses studied are rare earth metaphosphates of the composition R(PO3)3. X-ray diffraction measurements were taken showing that the network structure of these systems is essentially constructed from PO4 tetrahedra. The EXAFS experiments, performed on a range of glasses over the rare earth LIII edges, show a trend in the first shell distance rare earth-oxygen, consistent with the Lanthanide contraction, the rare earth ions occupy-ing sites with between six and eight-fold coordination of oxygen atoms about the rare earth ions

EXAFS studies of rare-earth metaphosphate glasses

An extended x-ray-absorption fine structure (EXAFS) study has been carried out on a range of rare-earth metaphosphate R(PO3)(3) glasses of growing interest in optical communications and laser technologies. Phosphate glasses modified using the rare-earth oxides Pr6O11, Nd2O3, Eu2O3, Gd2O3, Tb2O3, and Ho2O3, have been investigated using their respective rare-earth L(III) absorption edges. The data provide information on the local environment of the rare-earth ion within the phosphate glass matrix constructed from linked PO4 tetrahedra. The rare-earth ions occupy sites with an average coordination number in the range, 6 less than or equal to N less than or equal to 8, the surrounding atoms being oxygen. The first shell interatomic distance over the range of rare-earth ions establishes the rare-earth contraction of ionic radii with increasing atomic number in a series of glasses. There is also evidence for a rare-earth-phosphorus correlation between 2.7 and 3.6 Angstrom: and a further rare-earth-oxygen correlation at approximately 4 Angstrom. The EXAFS spectrum shows no evidence for R-R correlations within the short-range order, a result especially pertinent to the optical and magnetic properties of the glasses. The fractal dimensionality 4C(11)/B of these glasses, obtained from the elastic stiffnesses determined from ultrasonic wave velocities, ranges between 2.3 and 2.8, indicating that their connectivity tends towards having a three-dimensional character

Reverse Monte Carlo modelling of Eu and Tb metaphosphate glasses

It is possible to make phosphate glasses (R2O3)(x)(P2O5)(1-x) With contents of rare-earth elements, R, close to the metaphosphate composition, R(PO3)(3). These glasses are particularly stable to water, and have interesting optical, acoustic and magnetic properties. We have modelled Eu and Tt, metaphosphate glasses using the reverse Monte Carlo (RMC) technique with X-ray diffraction data. The model building process must include additional constraints, and we have derived these from knowledge of metaphosphate structures, including metaphosphate crystals. The model building procedure was first to fit structural constraints alone, and then to fit the diffraction data while maintaining the structural constraints. The models constructed consist of chains of phosphate tetrahedra with Eu (or Tb) 6-fold coordinated to non-bridging oxygens (NBOs), and are in good agreement with the diffraction data. Thus the models provide a demonstration of validity of this structural description of rare-earth metaphosphate glasses. In addition, two different values were used for minimum rare-earth separation, 3.9 and 5.4 Angstrom, based on two different metaphosphate crystals, and more satisfactory models were obtained when using the shorter distance

An X-ray absorption study of doped silicate glass, fibre optic preforms

Optical fibre preforms, which have their germanosilicate core regions doped with small quantities of the rare-earth element erbium, have been studied using Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS) at the germanium K absorption edge. These studies were performed using a circular X-ray beam of 100 mu m diameter, allowing information to be gathered as a radial function of position across the core region of the preform. This positioning was accomplished by means of a motorized pinhole collimator and sample stage developed for use on the focused X-ray beamline 8.1, at the SRS, Daresbury Laboratory, UK. The EXAFS results are consistent with the germanium sites coordinated to surrounding oxygen atoms at a mean distance of 0.17 nm. Absorption maps of the rare-earth and germanium distribution across the core region of the preforms have also been obtained, showing a correlation between the distributions of the two atom types

EXAFS Investigations Of High-Nuclearity Pd Clusters

Pd K-edge X-ray absorption spectra at several temperatures of Pd-561(phen)(36)O-200 and other high nuclearity ligand-stabilized Pd clusters are reported. Data-analysis performed in a wide k-range (up to 17.5 Angstrom(-1)), accounting for possible effects of asymmetry, allowed us to obtain very reliable results for the nearest neighbour Pd-Pd distribution. Mean bond length and variance are determined as a function of temperature. The data indicate the existence of regular FCC particles with a very weak relaxation with respect to the bulk ideal geometry and no asymmetry effects in the bond length distribution