Fluid self-diffusion in Scots pine sapwood tracheid cells

The self-diffusion coefficients of water and toluene in Scots pine sapwood was measured using low field pulsed field gradient nuclear magnetic resonance (PFG-NMR). Wood chips of 8 mm diameter were saturated with the respective liquids, and liquid self-diffusion was then traced in one dimension orthogonal to the tracheid cell walls in the wood's radial direction. The experimental echo attenuation curves were exponential, and characteristic self-diffusion coefficients were produced for diffusion times spanning from very short times to times on the order of magnitude of seconds. Observed self-diffusion coefficients were decaying asymptotically as a function of diffusion time, an effect which was ascribed to the cell walls' restriction on confined liquid diffusion. The observed self-diffusion behavior in Scots pine sapwood was compared to self-diffusion coefficients obtained from simulations of diffusion in a square. Principles of molecular displacements in confined geometries were used for elucidating the wood's cellular structure from the observed diffusion coefficients. The results were compared with a mathematical model for diffusion between parallel planes.     

Recombination and transport processes in dye-sensitized solar cells investigated under working conditions

The transport and recombination of electrons in dye-sensitized TiO(2) solar cells were studied by analysis of the current and voltage response to a small square-wave light-intensity modulation. Solar cells were studied under working conditions by using potentiostatic and galvanostatic conditions. An increase in applied voltage, that is, from 0 V toward open-circuit voltage, was found to lead to faster electron transport at low light intensities, while it slowed transport at higher light intensities. This observation seems to be conflicting with the multiple trapping model with diffusive transport. An effective diffusion length at the maximum power point was calculated, and it was shown that it decreases with increasing light intensity.     

Detailed in situ XRD and calorimetric study of the formation of silicate/mixed surfactant mesophases under alkaline conditions. Influence of surfactant chain length and synthesis temperature

The formation of mesoscopically ordered silica/surfactant composites under alkaline synthesis conditions has been studied by time-resolved in situ small-angle X-ray diffraction with synchrotron radiation. Alkyltrimethylammoniumbromide surfactants, C(n)()TAB, of different chain lengths (n = 14, 16, and 18) as well as mixtures thereof were used as structure directing agents and the measurements were carried out at two different temperatures. A linear relationship between the mean surfactant chain length and the d spacing of the hexagonal phase was observed, suggesting an ideal mixing of the surfactants in the supramolecular surfactant aggregates. It is shown that the formation of the hexagonal phase is kinetically controlled mainly by the rate of silicate condensation, while the effect of changes in the surfactant chain length on the kinetics is small under the studied conditions. Two concominant, albeit partly interlinked, processes, suggested being intra- and intermicellar condensation, followed by aggregate-aggregate condensation, govern the nucleation and growth of the hexagonal phase. The two-step mechanism is confirmed by a microcalorimetric study where the heat evolved during the hydrolysis-condensation reactions is followed as a function of time.     

The specific adsorption of sodium cations on less common metal oxides at high ionic strengths

Sodium cations adsorb specifically on metal oxides at high ionic strengths. This results in a shift in the isoelectric point (IEP) to higher pH values. When the critical concentration of electrolyte is exceeded there is no IEP at all and the electrokinetic potential is positive even at very high pH values. The critical NaI concentration is rather insensitive to the nature of the metal oxide (but silica behaves differently), and this suggests that the specific adsorption is chiefly due to ion-ion and ion-solvent interaction in solution. The experimental results obtained with indium and niobium oxides (critical concentrations of about 0.35 mol dm(-3)) confirmed this trend.     

Morphology of synthetic goethite particles

The specific surface area of synthetic goethite depends on the preparation: the Fe(III):OH ratio, the rate of base titration of Fe salt, and the temperature and time of crystallization. The crystals also have different morphologies as determined by SEM or TEM. Carbon coating is used to improve the quality of SEM images of nonconducting specimens. We show here that needle-like goethite particles become substantially thicker in the course of standard carbon coating, and the length-to-width ratio obtained for carbon-coated particles is lower than that for the original goethite particles. The morphology of the goethite particles was also studied by tapping mode AFM.     

Multiinstrument study of the electrophoretic mobility of quartz

The electrophoretic mobility of quartz in 0.01 mol dm(-3) NaCl and NaNO3 over the pH range 2-8 was studied using five different commercial instruments. The mobilities over the pH range 4-8 were relatively consistent, but the mobilities over the pH range 2-4 and the position of the isoelectric point IEP varied from one instrument to another. This result suggests that the discrepancies in the apparent IEP of quartz (and other silicas) reported in the literature are due to the instrument artifacts.     

Aqueous Amino Silane Modification of E-glass Surfaces

The majority of work available in the literature examines the effect of epoxy silane, gamma-aminopropyltrimethoxy silane, and methacrylate silane on E-glass surfaces. As alternatives to the most commonly used silanes, we investigated two novel silanes: gamma-ureidopropyltriethoxy silane and Nbeta(aminoethyl) gamma-aminotrimethoxy silane and additionally an amino-functional polysiloxane. The ureido silane-treated E-glass fibers demonstrated a zeta potential similar to that of the untreated E-glass fibers, which was independent of deposition solution pH over the pH range investigated. A moderately hydrophobic E-glass surface, which was silane concentration dependent, was noted as being due to condensed Si-O-Si bonds at the surface. The diamino silane demonstrated an extremely basic surface at the higher silane concentrations investigated. These outer surface layers were modified by changing the pH and the concentration of the deposition solution. The polysiloxane produced an increase in the hydrophobicity of the E-glass fiber, especially when deposited from extremely basic solutions. At the higher solution concentrations investigated, the silanes and the siloxane was initially deposited in patches and an outer surface or "skin" was formed over these patches, giving the appearance of fully coated fibers. Copyright 2001 Academic Press.