Rotational diffusion of tungstic acid colloids in microgravity as studied by aircraft experiments

 Rotational relaxation times (τ) of anisotropic tungstic acid colloids (3.24 μm in major axis) in aqueous suspension are measured in microgravity (0G), normal gravity (1G) and at 2G. The measurements at 0G and 2G are achieved by parabolic and circular flights, respectively. The limiting slopes of the relaxation curves in the plots of the transmitted light intensity against time are close to zero at 0G irrespective of the flow directions in the flow cell, whereas those at 1G and especially at 2G depend on the flow direction by the convection of the suspension and particle sedimentation. Experimental errors at the τ values at 0G are small compared with those at 1G and 2G, which is ascribed to the lack of movement of impurities in the suspension such as quite small air bubbles, which cannot be recognized with the naked eye, and the convection of the suspension in microgravity. More reliable rotational relaxation times are obtained in microgravity; however, the relaxation times themselves are quite insensitive to gravity. Theτ values observed are larger than those calculated from the particle size, which indicates the important contribution of the electrical double layers formed around the colloidal particles. Received: 22 February 2001 Accepted: 13 June 2001     

Ionic photodissociation of some polymers quenched with electron donor or acceptor in solution

Ionic photodissociation processes of vinyl polymers with pendant carbazolyl groups in solution have been studied by nonsecond laser photolysis. The ionic dissociation yield decreases in the order of monomer, oligomers, and polymers.     

5. Makromolekulare Systeme

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5. Makromolekulare Systeme     

Asymmetric conjugate reduction of alpha,beta-unsaturated ketones and esters with chiral rhodium(2,6-bisoxazolinylphenyl) catalysts

New asymmetric conjugate reduction of beta,beta-disubstituted alpha,beta-unsaturated ketones and esters was accomplished with alkoxylhydrosilanes in the presence of chiral rhodium(2,6-bisoxazolinylphenyl) complexes in high yields and high enantioselectivity. (E)-4-Phenyl-3-penten-2-one and (E)-4-phenyl-4-isopropyl-3-penten-2-one were readily reduced at 60 degrees C in 95 % ee and 98 % ee, respectively, by 1 mol % of catalyst loading. (EtO)2MeSiH proved to be the best hydrogen donor of choice. tert-Butyl (E)-beta-methylcinnamate and beta-isopropylcinnamate could also be reduced to the corresponding dihydrocinnamate derivatives up to 98 % ee.     

Synthesis of 4,8-dimethyl-6-phenyl-5,6,7,8-tetrahydro-4H-1,3,6-dioxazocines

Five 6-phenyl-5,6,7,8-tetrahydro-4H-1,3,6-dioxazocines were synthesized by the reaction of the corresponding N,N-bis(2-hydroxypropyl)- and N,N-bis(2-hydroxyethyl)chloroanilines with paraformaldehyde in the presence of p-toluenesulfonic acid.     

Rheological properties and orientational distributions of dilute ferromagnetic spherocylinder particle dispersions. Part II. Analysis for the two typical magnetic field directions

We have investigated the orientational distributions and rheological properties of dilute colloidal dispersions, which consist of ferromagnetic spherocylinder particles. First, the governing equation of the orientational distribution function has been derived for the typical two cases of magnetic field directions: the direction parallel to the shear flow and the direction parallel to the angular velocity vector of the shear flow. The equation has been solved approximately by Galerkin's method. With these numerical solutions we have obtained the results of the orientational distribution and viscosity. The results obtained for the magnetic field in the shear flow direction are summarized as follows. In the case of a weak magnetic field, the particle tends to orient nearly toward the shear flow direction and its opposite direction. As the magnetic field increases, the orientation of the particle is restricted and the viscosity increases significantly. As the influence of the magnetic field becomes dominant, an overshoot in the viscosity curve appears. This is due to the fact that there is a maximum deviation of the averaged particle direction from the magnetic field direction. When the strength of the magnetic field increases significantly, the particle inclines close to the magnetic field direction and the viscosity converges to a constant value. Particles with a larger aspect ratio give rise to a larger increment in the viscosity since such elongated particles induce larger resistance in a flow field. We also have obtained results for the case of the magnetic field in the direction parallel to the angular velocity vector of the shear flow. When the flow field is dominant over both the rotational Brownian motion and the magnetic interaction, the particle rotates in the plane nearly perpendicular to the magnetic field direction. As the magnetic field increases, the particle inclines toward the magnetic direction. For this direction of field, the viscosity is independent of the magnetic field and is always zero.     

Electron spin resonance study on the vanadium adsorption by persimmon tannin gel

The persimmon tannin gel can adsorb vanadium highly effectively from aqueous solutions containing VOCl(2) and NH(4)VO(3), respectively. The adsorption of vanadium from the VOCl(2) solution had a broad maximum at around pH 5-6, while that from the NH(4)VO(3) solution, a sharp maximum at around pH 3.75 and a broad one at around pH 5-6. The adsorption of vanadium by the gel from both VOCl(2) and NH(4)VO(3) solutions was rapid, and was obeyed the Langmuir adsorption isotherm. The ESR spectrum of VO(2+) in the persimmon tannin gel showed a typical powder pattern with g-anisotropy and anisotropic hyperfine structure (I=7/2), with g(//)=1.937, g( perpendicular)=2.005, mid R:A(//)mid R:=552 MHz, and mid R:A( perpendicular)mid R:=168 MHz, suggesting a square pyramidal coordination structure of VO(2+)-persimmon tannin complex. The ESR analysis of vanadium adsorption from the NH(4)VO(3) solution (pH 6) indicated the reduction of VO(3)(-) to VO(2+). The high vanadium-adsorption ability of the persimmon tannin gel from the VOCl(2) solution was attributed to the stable complex formation of VO(2+) with catechol and pyrogallol groups in the gel, while the vanadium adsorption from the NH(4)VO(3) solution can be explained as the combination of H(3)VO(4) and VO(2+) adsorptions.