Comparison of micro- and mesoporous inorganic materials in the uptake and release of the drug model fluorescein and its analogues

The uptake of the three species of the drug model fluorescein (fluorescein sodium salt (FNa), fluorescein free acid (F), and fluorescein diacetate (FDA)) by zeolite NaX and the mesoporous zeotype MCM-41 was investigated as well as their release rates into solutions at pH 7 and pH 4.5. UV/Vis analysis was carried out at a wavelength of 490 nm. Uptakes of the sodium salt of 9 % for zeolite X and 14 % for MCM suggest little penetration of the pores. The use of ethanol as the loading solvent for F resulted in little uptake for both zeolitic materials due to the successful competition of the ethanol for binding sites. Use of acetone (weaker proton acceptor) as loading solvent significantly improved the uptake of F to 17 % and 12 % for zeolite X and MCM, respectively, whilst the uptake of FDA in acetone increased still further to 22 % and 17 % for zeolite X and MCM, respectively. Generally there was a large initial release of the fluorescein analogues from the surface of the zeolites with very little further increase over time. The prescence of an esterase enzyme in the release medium of FDA tripled the release from MCM to 15 % but left the release from zeolite X unaffected at 6 %. The results obtained show that uptake of fluorescein and its analogues is dependent on the loading solvent used, the amount released is influenced by not only the solvent but the pH and the presence of enzymes in the release medium.     

Comparative study of 8-hydroxyquinoline derivatives as chelating reagents for flow-injection preconcentration of cobalt in a knotted reactor

8-Hydroxyquinoline (HQ), 2-methyl-8-hydroxyquinoline (CH₃-HQ), 5,7-dichloro-2-methyl-8-hydroxyquinoline (Cl₂-CH₃-HQ), 5,7-dibromo-8-hydroxyquinoline (Br₂-HQ), 5-sulfo-7-iodo-8-hydroxyquinoline (ferron) and 5-sulfo-8-hydroxyquinoline (SO₃H-HQ) were compared as chelating reagents for on-line sorption preconcentration of cobalt in a knotted reactor (KR) precoated with the reagent. The results obtained with the different HQ derivatives reveal those properties of the chelating reagent responsible for the processes taking place in the KR. The influence of hydrophobicity, acidity, stability of the cobalt chelate and type of substituents in the HQ ring system on the separate steps of the flow injection (FI) preconcentration procedure are discussed. According to the performance characteristics of the different HQ derivatives, the most important parameters for on-line preconcentration in a KR are the hydrophobicity of the reagent and the stability of the chelate complex with the analyte.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 Solution

The oxidation of polycrystalline platinum in 0.5 M H₂SO₄ is studied by cyclic voltammetry at potential scan rates of 5–500 mV s^–1 while varying the potential cycling range. The scheme, which is proposed for explaining the observed acceleration and deceleration of oxygen sorption at 0.75–1.0 V, accounts for the presence of oxygen in the subsurface layers of platinum (O_ss) and the formation of a barrier layer comprising complexes O_ss–Pt _n –SO₄. Cycling platinum secures certain steady-state contents of O_ss at 0.01–1.35 V. In an anodic scan, O_ss accumulates at E > 0.85 V (slow post-electrochemical stage) due to exchange of platinum and oxygen atom sites. In a cathodic scan, the desorption of most oxygen gives way to the adsorption of anions, which prevent residual O_ss from appearing on the surface. The residual O_ss disappears at E < 0.1 V after a sufficiently complete desorption of anions and the destruction of stable complexes O_ss–Pt _n –SO₄. Varying the potential cyclic limit leads, after a delay, to other steady-state O_ss contents.     

Study of physical and thermochemical properties of modified zeolites

The treatment of zeolite with a solution of NaOH at different concentrations creates modified forms of natural zeolites.These modified zeolites exhibit an increased uptake of caesium and a satisfactory uptake of cobalt. The structural changes in the modified zeolites were studied by XRD analysis, which reveals the mineral compositions of the studied materials. The thermodynamic properties of the modified zeolites were studied by means of DTA and thermogravimetry, and the chemical composition too was determined.     

Sorption of petrochemicals from aqueous solutions on thermally activated schungite rock

Thermal oxidative treatment enhances the activity of a schungite rock in the sorption of petrochemicals from aqueous solutions. The effect of the thermal oxidative activation on the content and distribution of carbon in the rock and the dependence of the sorption activity on the texture of sorbents were studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2233–2238, October, 2005.     

Electrosorption of hydrogen into palladium-gold alloys

Hydrogen electrosorption into Pd-Au alloys has been studied in acidic solutions (1 M H₂SO₄) using cyclic voltammetry. Pd-Au electrodes with limited volume were prepared by electrochemical co-deposition. It was found that the maximum H/(Pd+Au) ratios decrease monotonically with increasing gold content and reach zero at ca. 70 at% Au. Similarly to the case of Pd limited volume electrodes, two peaks in the hydrogen region, corresponding to two types of sorbed hydrogen, are observed on voltammograms for Pd-rich alloys. The hydrogen capacity, H/(Pd+Au), measured electrochemically, depends on the sweep rate in the cyclic voltammetry experiments, which suggests that two different mechanisms for hydrogen desorption from the Pd-Au alloy are possible. After a strong decrease of Pd concentration at the electrode surface, caused by long cyclic polarization to sufficiently anodic potentials, the amount of absorbed hydrogen is still significant for alloys initially rich in Pd. The results obtained from CO adsorption experiments suggest that only Pd atoms are active in the hydrogen absorption/desorption process. Electronic Publication     

Strukturuntersuchungen an naßersponnenen Polyacrylnitrilfasern nach der Gefriertrocknung

Zusammenfassung An unterschiedlich verstreckten und hydrothermisch nachbehandelten, danach gefriergetrockneten Polyacrylnitril-Gelfasern (PAN) wurden Messungen der Röntgenkleinwinkelstreuung (RKWS), Röntgenweitwinkelstreuung (RWWS) und Tieftemperatur-Gassorption (TTGA) durchgeführt. Die RKWS-Messungen zeigen, daß sowohl in den isotropen als auch in den orientierten Proben eine exponentialstatistische Verteilung der Form und Größe der Inhomogenitäten (Hohlräume, Cluster) vorliegt, welche durch die Kohärenzlängel _c bestimmt wird. Eine erhöhte Röntgenabsorption wurde bei den verstreckten Proben festgestellt. Die Analyse der aus der Kohärenzlänge und den Volumsanteilen der Phasen berechneten charakteristischen RK WS-Parameter (z.B. innere Oberfläche, Durchschußlänge) ergibt, daß jede Behandlung der Gelfasern eine Zu- oder Abnahme der etwa gleich großen Hohlräume verursacht (nucleotic change). Das relative Ausmaß dieser Veränderungen wird besprochen.Der Vergleich mit den Ergebnissen der TTGA an den gleichen Fasern zeigt, daß die Tendenzen bei der spezifischen Oberfläche und dem Porenvolumen (Zu- bzw. Abnahme) bei beiden Methoden annähernd gleich sind. Die Absolutwerte des Hohlraumvolumens unterscheiden sich aber z.T. um den Faktor 40. Dies führt zur Schlußfolgerung, daß mit der RK WS Faserbereiche als Hohlräume erfaßt werden, die bei der TTGA nicht angezeigt werden, weil sie für das Meßgas unzugänglich sind.

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Structural studies on wet-spun freeze-dried polyacrylonitrile fibers

Summary Polyacrylonitrile fibers (PAN) were drawn to different extents, hydrothermally aftertreated, and at last freeze-dried. The products were studied by small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), and low temperature gassorption (LTGS). The SAXS data show that the shape and the size of the inhomogeneities (voids, clusters) have an exponential statistical distribution determined by the correlation lengthl _c, both in the isotropic fibers and in the fiber-oriented ones. It has also been observed that stretching always causes a higher X-ray absorption. An analysis of the characteristic SAXS parameters (e.g., internal surface, mean chord length), which were calculated from the correlation length and the volume fractions of the phases, evidences that in all samples an augmentation or diminution of the number of the voids with nearly the same shape and size takes place (nucleotic change). Its relative number has been pointed out.Comparison with the results of the LTGS investigations at the same fibers shows that the tendencies of the specific surface and porosity (increase, decrease) are approximately the same with both methods. However, the absolute values of void volume differ in some cases by the factor 40. This leads to the conclusion that the SAXS method records domains of these fibers as voids which are not registered by LTGS (voids not accessible for the gas).

Herrn em.o.Univ. Prof. Dr.Josef Schurz zum 70. Geburtstag gewidmet     

Swelling equilibrium and preferential sorption of crosslinked poly(methyl methacrylate) in the mixture acetonitrile + n-butanol

Measurements of preferential sorption,( ₃), determined by refractometry, and swelling equilibrium, ₃ ^–1, of PMMA networks have been carried out in the cosolvent mixture MeCN+BuOH at 25 and 49 C. With an intermediate mixture composition, ₃ ^–1 passes through a maximum at both temperatures. At 25 C MeCN is preferentially adsorbed by the network over most of the composition range, but a small inversion is detected. At 49 C MeCN is preferentially adsorbed over all the composition range.The behavior of the system crosslinked PMMA/MeCN + BuOH is compared with the results obtained for solutions of linear PMMA in mixtures formed by the same two solvents, MeCN and BuOH.     

Possibilities of nitric acid preparation of powder cellulose forms

For preparation of various modified forms of microcrystalline cellulose, an optional combination is proposed of hydrolytic activity with other properties of nitric acid such as, adduct forming ability (manifested as Knecht compound formation), oxidizing, bleaching and nitrating. The manifestation of a chosen acid function is reached by variation of the reaction conditions: concentration, temperature, time, etc. The sole-reagent continuous procedure involving several operations made in a predetermined sequence allows the use of a more simple technological scheme and saves energy, time and materials. The features of transformation of fibrous and microcrystalline forms of native cellulose into cellulose-II through the Knecht compound are discussed. Purely acidic transformation schemes of native cellulose into microcrystalline cellulose hydrate and amorphous low-nitrate powder cellulose according to the above scheme are considered. Morphological features as well as distinguishing properties — enhanced hydrophilicity, sorption activity and degree of whiteness — of the prepared materials are pointed out. The possibility of obtaining uniformly oxidized forms of powder cellulose according to this scheme and the outlook for the use of nitrogen(IV) oxide as a multifunctional reagent are discussed.     

Specific Sorption Sites for Nitrogen in Zeolites NaLSX and LiLSX

Specific sorption sites for nitrogen, N₂, in NaLSX and LiLSX zeolites were investigated using a DRIFT spectroscopic method. Sorption of molecular hydrogen, H₂, by NaLSX or LiLSX zeolite at 77 K with DRIFT control of perturbation of sorbed molecules allowed to discriminate two or three different types of specific sorption sites in the respective zeolites. Their H–H stretching frequencies are 4077 and 4081 cm^–1 for NaLSX, and 4061, 4084 and 4129 cm^–1 for LiLSX. With reference to an independent investigation by methods of both sorption thermodynamics and molecular modeling for N₂ sorption on LiLSX, the first two of the corresponding bands were ascribed to H₂ sorption on lithium cations, Li⁺, localized in supercages of the faujasite, FAU, zeolite framework at sites SIII and SIII, while the latter band most likely belongs to H₂ sorption on Li⁺ cations at sites SII, and on hydroxyl groups, OH. Sorption of N₂ by Li⁺ cations at sites SIII and SIII is the strongest, resulting in a decrease of intensity of the corresponding DRIFT bands that stem from subsequent H₂ sorption. Nitrogen sorption by Li⁺ cations at sites SII is much weaker. Sorption of N₂ on Na⁺ cations at sites SIII in NaLSX zeolite is also stronger than by Na⁺ cations at sites SII.