Cellulose nanocrystal submonolayers by spin coating

Dilute concentrations of cellulose nanocrystal solutions were spin coated onto different substrates to investigate the effect of the substrate on the nanocrystal submonolayers. Three substrates were probed: silica, titania, and amorphous cellulose. According to atomic force microscopy (AFM) images, anionic cellulose nanocrystals formed small aggregates on the anionic silica substrate, whereas a uniform two-dimensional distribution of nanocrystals was achieved on the cationic titania substrate. The uniform distribution of cellulose nanocrystal submonolayers on titania is an important factor when dimensional analysis of the nanocrystals is desired. Furthermore, the amount of nanocrystals deposited on titania was multifold in comparison to the amounts on silica, as revealed by AFM image analysis and X-ray photoelectron spectroscopy. Amorphous cellulose, the third substrate, resulted in a somewhat homogeneous distribution of the nanocrystal submonolayers, but the amounts were as low as those on the silica substrate. These differences in the cellulose nanocrystal deposition were attributed to electrostatic effects: anionic cellulose nanocrystals are adsorbed on cationic titania in addition to the normal spin coating deposition. The anionic silica surface, on the other hand, causes aggregation of the weakly anionic cellulose nanocrystals which are forced on the repulsive substrate by spin coating. The electrostatically driven adsorption also influences the film thickness of continuous ultrathin films of cellulose nanocrystals. The thicker films of charged nanocrystals on a substrate of opposite charge means that the film thickness is not independent of the substrate when spin coating cellulose nanocrystals in the ultrathin regime (<100 nm).     

Analysis of hydrogen peroxide and an organic hydroperoxide via the electrocatalytic Fenton reaction

A new concept for the electrochemical detection of hydrogen peroxide, and organic hydroperoxides is presented. One advantage of the significance of this technique is that it does not require chemical modification of the electrode or addition of enzymes. Direct electro-reduction of the peroxides was not observed on the carbon disk electrode as it is a kinetically slow process. Redox cycling of the iron complex is apparent as Fe^IIEDTA rapidly reduces the O-O bond of the peroxides (Fenton Reaction) upon its production by the kinetically facile electro-reduction of Fe^IIIEDTA. This provides an enhanced and steady-state reductive current as observed by cyclic voltammetry. These features are indicative of the electrocatalytic (EC′) mechanism. A calibration curve was constructed based on the chronoamperometric response at 32 s and a detection limit for H₂O₂ and t-butyl hydroperoxide was calculated to be 0.4 μM and 20 µM, respectively. This difference is attributable to the rate in which the iron(II) complex reduces the O-O bond, H₂O₂ (2.3 × 10⁵ M^− 1 s^− 1) being faster than for the organic peroxide (5.1 × 10⁴ M^− 1 s^− 1). The Fe^IIEDTA complex was observed to be unreactive toward dialkyl peroxides. This method may find use in the detection of peroxide-based explosives or in enzymatic assays as it is rapid, simple, inexpensive and should prove to be robust.     

Avoiding Carbothermal Reduction: Distillation of Alkoxysilanes from Biogenic,Green, and Sustainable Sources

The direct depolymerization of SiO₂ to distillable alkoxysilanes has been explored repeatedly without success for 85 years as an alternative to carbothermal reduction (1900 °C) to Si_met, followed by treatment with ROH. We report herein the base‐catalyzed depolymerization of SiO₂ with diols to form distillable spirocyclic alkoxysilanes and Si(OEt)₄. Thus, 2‐methyl‐2,4‐pentanediol, 2,2,4‐trimethyl‐1,3‐pentanediol, or ethylene glycol (EGH₂) react with silica sources, such as rice hull ash, in the presence of NaOH (10 %) to form H₂O and distillable spirocyclic alkoxysilanes [bis(2‐methyl‐2,4‐pentanediolato) silicate, bis(2,2,4‐trimethyl‐1,3‐pentanediolato) silicate or Si(eg)₂ polymer with 5–98 % conversion, as governed by surface area/crystallinity. Si(eg)₂ or bis(2‐methyl‐2,4‐pentanediolato) silicate reacted with EtOH and catalytic acid to give Si(OEt)₄ in 60 % yield, thus providing inexpensive routes to high‐purity precipitated or fumed silica and compounds with single Si?C bonds.     

Carboxymethyl cellulose on a fiber substrate: the interactions with cationic polyelectrolytes

High and low molecular weight (M_w) carboxymethyl celluloses (CMC) were adsorbed on a well-characterized fiber substrate (long fibers of a commercial bleached birch kraft pulp with the carboxylic acid groups in Na-form) to increase the charge of the fibers in a controlled fashion. The M_w played a role in the utilization of CMCs as a strength additive in paper sheets nearly doubling the tensile strength with the high M_w CMC. Swelling properties of the CMC treated fibers were measured with water retention value (WRV). The WRV increased more with the high M_w CMC. The swelling was further tuned by two highly cationic polyelectrolytes; high M_w poly(diallyldimethyl ammonium chloride) (PDADMAC) and low M_w polybrene (hexadimethrine bromide, [3,6]-ionene). They were chosen because of their known ability to neutralize the anionic charge either exclusively on the surface or in the whole fiber, respectively. Adsorption of PDADMAC could reduce WRV of the CMC pre-treated fibers to the level of the untreated reference, while polybrene adsorbed pulps with 3–10 times more cationic polyelectrolyte deswelled the fibers only slightly more than the surface neutralized fibers. These results indicated surface conformation differences with low and high M_w CMCs. While the conformation did play a role after physical alteration (drying and rewetting) of the fibers, the paper sheets produced from these fibers showed remarkable differences. In extreme cases, the strength of the paper could be retained after drying (low M_w CMC + PDADMAC) or paper, resistant to disintegration, could be achieved (CMC + polybrene).     

First steps toward spatially resolved OSL dating with electron multiplying charge-coupled devices (EMCCDs): System design and image analysis

Spatially resolved OSL (SR-OSL) measurements are made with a scanning or imaging system that records luminescence emissions from known sources within a sample. This work is a first step toward the development of an SR-OSL methodology based on the imaging of ultraviolet OSL emissions with an EMCCD camera. We present UV OSL images of natural quartz extracts, OSL/TL/IR images of sandstone, image-based OSL measurements from aluminum oxide grains, and dose recovery experiments on aluminum oxide and Risø calibration quartz. Camera sensitivity and quantitative reproducibility indicate that EMCCD-based OSL dating is feasible. Further experimentation is necessary to adequately define the error sources and measurement limitations of EMCCDs in the OSL regime.     

Further numerical methods for the Falkner-Skan equations: Shooting and continuation techniques

We consider in this paper the numerical solution of the Falkner-Skan differential equation, modelling under some similarity assumptions the boundary layer equation. We look for the extremal solution of this third order differential equation. The methods we use are basically the Newton method with a shooting process, which is coupled with a continuation method: they allow us to follow the solution arcs which contain regular and turning point solutions.     

The room temperature oxidation of porous silicon

The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p⁺- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH₄) was found in the gas. Traces of SiOCH₃ and SiOC₂H₅ groups were also found in FTIR spectra indicating Si---O---C_xH_y passivation of the surface.