The proper structure of cubic ice confined in mesopores

To examine the reason for the formation and the structure of cubic ice in a restricted space, we measured the powder x-ray diffraction patterns of cubic ice formed within the mesopores of porous silicas as a function of pore size (4-70 nm). The results strongly suggest that cubic ice formed in the mesopores does not take a cubic structure as envisaged by Konig. It may be actually composed of very small crystallites of hexagonal ice that contains a large amount of growth faults depending on the crystallite size, that is, ice with disordered stacking sequence. Suppression of crystal growth of ice in the mesopores seems to be a vital factor for the formation and the stability of cubic ice.     

The fluorescence properties of metal complexes of alkyl derivatives of aromatic schiff bases

Seventeen alkylated salicylidene-o-aminophenol derivatives were tested as fluorimetric reagents for aluminium, gallium, indium, scandium and beryllium. The aluminium, gallium and beryllium complexes are intensely fluorescent, and the scandium and indium complexes weakly fluorescent. The fluorescence properties of the aluminium, gallium and beryllium complexes were studied and conditions for the fluorimetric determination of these metals were established. 2-Hydroxy-4-methylaniline-N-2-hydroxy-4-methyl-benzylidene is a good reagent because of the reproducibility and sensitivity of the fluorescence. The optimal ranges for determination are 0.005–3 mg Al/25 ml, 0.1–7 μg Ga/25 ml and 0.02–7 μg Be/25 ml. In all cases, 1:1 metal—ligand complexes are formed. Optimal reaction conditions and interference studies are reported.     

Self-crosslinkable polymers. I. Copolymerization of glycidyl methacrylate with 2-vinylpyridine and 2-vinyl-5-ethylpyridine

A soluble and self-crosslinkable linear copolymer with pendant epoxy and pyridyl groups was obtained from glycidyl methacrylate (M₁) and 2-vinylpyridine (M₂) or 2-vinyl-5-ethylpyridine (M₂) by the action of azobisisobutyronitrile. The monomer reactivity ratios were determined in tetrahydrofuran at 60°C: r₁ = 0.510, r₂ = 0.620 with 2-vinylpyridine and r₁ = 0.57, r₂ = 0.62 with 2-vinyl-5-ethylpyridine. These were consistent with the calculated values with the reported Q and e values for these monomers. The intrinsic viscosities of the copolymers with 2-vinylpyridine and with 2-vinyl-5-ethylpyridine were found to be 0.17–0.19 and 0.26–0.38, respectively, in tetrahydrofuran at 30°C; they were independent of the copolymer composition. The copolymers were amorphous, had no clear melting points, and became insoluble crosslinked polymers under heating without further addition of any curing agents.     

The role of metallic matrix modifiers in graphite furnace atomic absorption spectrometry

The role of metallic matrix modifiers in AAS is considered; for elements reduced, together with modifier elements, during ashing and the beginning of the atomization process, the thermodynamic activity and melting points of the alloys formed are important. For Mg²⁺, prevention of the effects of halide ions is considered, as is also the behaviour of buffer and/or carrier in emission spectrometric analysis of the other alkali and alkaline earths.     

Numerical Analysis of Coupling between Two-Dimensional Conventional Dielectric Waveguides and Photonic Crystal Waveguide

A problem of coupling between conventional dielectric waveguides and a two-dimensional photonic crystal waveguide is analyzed, using the Fourier series expansion method combined with the absorbing boundary condition of a perfectly matched layer (PML). A novel structure for the input and output ends of the photonic crystal waveguide is proposed to increase the coupling efficiency. Numerical examples are demonstrated and discussed to validate the effectiveness of the proposed structure.     

Lower closure point of adsorption hysteresis in ordered mesoporous silicas

To examine the nature of the lower closure point of adsorption hysteresis in ordered mesoporous silicas, we measured the temperature dependence of the adsorption-desorption isotherm of nitrogen for three kinds of ordered silicas with cagelike pores and three kinds of ordered silicas with cylindrical pores. The lower closure point pressure of nitrogen in the cagelike pores with sufficiently small necks, that is, the cavitation pressure of a confined liquid, did not depend appreciably on the cage size in the temperature region far away from a hysteresis critical temperature (Tch) but its cage-size dependence was noticeable in the vicinity of Tch. The lower closure point in the cylindrical pores depended on the pore size, and its thermal behavior was totally different from that in the cagelike pores. Nevertheless, the hysteresis critical points of nitrogen in the ordered mesoporous silicas, which are defined as a threshold of temperatures (Tch) and pressure above which reversible capillary condensation takes place in a given size and shape of pores, fell on a common line in a temperature-pressure diagram regardless of the pore geometries. We consider this finding as evidence that capillary evaporation in the cylindrical pores follows a cavitation process in the vicinity of Tch in the same way as that in the cagelike pores and also that the low limit of the hysteresis loop that has been long recognized since 1965 is due to the occurrence of a vapor bubble in a stretched metastable liquid confined to the pores with decreasing pressure (cavitation).     

Accurate relations between pore size and the pressure of capillary condensation and the evaporation of nitrogen in cylindrical pores

To examine the theoretical and semiempirical relations between pore size and the pressure of capillary condensation or evaporation proposed so far, we constructed an accurate relation between the pore radius and the capillary condensation and evaporation pressure of nitrogen at 77 K for the cylindrical pores of the ordered mesoporous MCM-41 and SBA-15 silicas. Here, the pore size was determined from a comparison between the experimental and calculated X-ray diffraction patterns due to X-ray structural modeling recently developed. Among the many theoretical relations that differ from each other in the degree of theoretical improvements, a macroscopic thermodynamic approach based on Broekhoff-de Boer equations was found to be in fair agreement with the experimental relation obtained in the present study.     

Nature of adsorption and desorption branches in cylindrical pores

To examine the nature of the adsorption and desorption branches in hysteretic adsorption isotherms of gases on mesoporous materials, we measured the temperature dependence of the adsorption and desorption isotherms of argon, oxygen, and carbon dioxide onto MCM-41 with a pore diameter of 4.4 nm. The results clearly show that in the open-ended cylindrical pores of MCM-41, capillary condensation rather than evaporation takes place near a thermodynamical equilibrium transition, as opposed to the general statement that capillary evaporation can occur via a meniscus formed at the pore mouth, and, thus, takes place at equilibrium.     

EFFECT OF PERIODIC DEFECTS ON ELECTROMAGENTIC SCATTERING BY THE TWO-DIMENSIONAL PHOTONIC CRYSTALS

Electromagnetic scattering from two-dimensional photonic crystals with two-dimensionally periodic defects is analyzed using a model of multilayered periodic arrays of parallel circular cylinders. The reflectance of the photonic crystals of finite thickness, which are free-standing or embedded in a dielectric slab, is obtained in terms of the lattice-sums, the T-matrix of a circular cylinder, and the generalized reflection matrix for a layered system. Numerical examples demonstrate that the refection and transmission bands of the crystals are reformed by introducing the periodic defects and their band natures are very sensitive to the polarization of excitation and the relative position of the defect element within a unit cell.