A. Hakanen

F-Centre luminescence from oxide-covered aluminium cathode induced by two-step reduction of peroxydisulfate anions

Light emission from aluminium oxide during cathodic pulse-polarisation of oxide-covered aluminium in aqueous solution was observed to be strongly enhanced in the presence of peroxydisulfate ions. The spectrum of the light emission had a broad maximum between 400 and 450 nm being attributed to F-centre luminescence of aluminium oxide. The mechanism of the luminescence is associated with the two-step reduction of peroxydisulfate anions near the oxide-covered cathode where the first one-electronic reduction step occurs either (i) by tunnel-emission generated hydrated electrons or (ii) by trickling down the surface states to the energy level of peroxydisulfate ion or (iii) by direct heterogeneous electron transfer from the bottom of the aluminium oxide conduction band to peroxydisulfate ions during strong downward band bending induced by cathodic pulse-polarisation. The second step occurs by electrons from F- or F + -centres at the oxide/electrolyte interface. Transitions of aluminium oxide conduction band electrons to fill the sulfate radical-emptied electron trapping sites (oxygen vacancies) produces analogous F- and F + -centre luminescence to that occurring during photoluminescence and thermoluminescence of aluminium oxide. No enhancement of light emission was observed in the presence of hydrogen peroxide which is also reduced in a two-step process at oxide-covered aluminium electrode. This can be explained by the fact that the energy level of hydroxyl radical under the present conditions lies ca. 1 eV above, whereas the energy level of sulfate radical lies somewhat below the colour-centre sub-band of aluminium oxide. Therefore, the sulfate radical is a sufficiently strong oxidant but the hydroxyl radical is too weak an oxidant to abstract electrons from F- and F + -centres.

Acoustic Emission during powder Compaction and its Frequency Spectral Analysis

AbstractAcoustic emission was detected during compaction of three different pharmaceutical materials: lactose monohydrate, microcrystalline cellulose and maize starch varying compressive forces between 0 and 60 kN. Acoustic emission signals were recorded on magnetic tape with a microphone and transformed to frequency spectra by using FFT-analysis.After rough identification of frequency components each spectrum was divided into three bands. By using integrated band powers the acoustic activity on these bands could be compared quantitatively. Many frequency peaks were observed, too, and many of them could be identified.

Acoustic Characterization of a Microcrystalline Cellulose Powder During and After Its Compression

AbstractAcoustic emissions were detected, both during the roller compaction of the microcrystalline cellulose powder and from single tablets after compaction by a single-punch tablet machine, via air using a microphone with a flat frequency response up to 20 kHz. Both of the compaction units were instrumented for the measurement of applied compressive force. The microcrystalline cellulose roller compacted using compressive forces below 30 kN showed a quite normal compaction behaviour but the product compacted at this force split into two and turned to yellow by its edges. This “capping” phenomenon was indicated by an enhancement of acoustic emission in the region of about 17-23 kHz. Acoustic emissions from single tablets after compaction by a single-punch tablet machine seemed to appear as wave packets consisting in very many frequency components that may, in addition, be time-varying. However, some small peaks were found probably being characteristic of these transient sounds.

Monitoring the acoustic activity of a pharmaceutical powder during roller compaction

Abstract Acoustic relaxation emissions (ARE) from microcrystalline cellulose (MC) and maize starch (MS) were detected during roller compaction using a microphone with flat frequency response up to 20 kHz and a tunable high pass filter the limiting frequency of which was set at 15 kHz. The noise from the compactor itself was found to appear mainly below 15 kHz. The ARE intensity of MC was observed to increase as a function of applied compressive force up to 45 kN, while the ARE intensity–force curve of MS had a maximum at 50 kN. A Gaussian-shaped function fitted reasonably to the data in both cases.

X-ray irradiation-induced optical luminescence of terbium(III) chelates in aqueous solutions

Abstract Hydrated Tb(III) can be excited in aqueous solutions by various pathways during X-ray irradiation. The dominating pathway is probably due to chemical excitation, induced by oxidation of Tb(III) by ionized water molecules, followed by immediate reduction of the resulting Tb(IV) to Tb(III) by hydrated electrons or atomic hydrogen. The high enthalpy of this reduction reaction produces Tb(III) in its highly excited states resulting in typical 5 D 4 → 7 F i multiplet peak emissions. Other possible excitation pathways are direct excitation of Tb(III) by an impact of X-ray photons or photoelectrons, photoluminescent UV excitation due to the solid state emission of the cell materials, and/or an energy transfer from the excited water molecules. The same excitation mechanisms are also valid for chelated Tb(III), but the chelates with aromatic moieties produce luminescence predominantly by a sensitized mechanism where the aromatic moiety is first chemically excited by radical reactions, followed by an energy transfer to the central ion. Radiative relaxation of chelated Tb(III) results in an emission spectrum similar to that of hydrated Tb(III), but with a radioluminescence intensity considerably stronger than that of hydrated Tb(III).

Sonoluminescence of chelated terbium(III) in aqueous solution

TbIII chelates containing aromatic moieties show sonoluminescence in aqueous solutions during the sonolysis of water. The observed 5D4→7FJ transitions of TbIII are due to the excitation of ligand, followed by an intramolecular energy transfer from the ligand to the central ion, which finally emits. No sonoluminescence of hydrated or EDTA-chelated TbIII ion could be observed. Ligand excitation can be based either on an energy transfer from the intrinsic emission centres of the sonolysis of water to the aromatic ligand, or on redox reactions between the ligand and hydroxyl radicals and hydrogen atoms produced by the sonolysis of water. Experimental results give greater support to the latter, chemiluminescent, excitation pathway.

Characterization of two terfenadine polymorphs and a methanol solvate: kinetic study of the thermal rearrangement of terfenadine from the methanol solvate to the lower melting polymorph

Abstract Terfenadine was crystallized from solutions of the drug in ethanol, acetone, and methanol. Two polymorphic forms and a methanol solvate were obtained and characterized using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and thermogravimetry (TG). Kinetic parameters were determined for a structural change that the methanol solvate went through during the desolvation reaction which was found to be completed by heating. On the basis of the X-ray diffractogram and the DSC scan the desolvated product was identified as the lower melting polymorph.

Mechanism of terbium(III)-enhanced lyoluminescence of x-ray irradiated sodium chloride

Abstract Dissolution of x-ray irradiated sodium chloride in aqueous solution produces a short-lived dynamic solid/solution interface which is able to raise the hydrated terbium(III) cation to its lowest excited state ( 5 D 4 ) and thus to induce the well-known 5 D 4 → 7 F-multiplet peak emissions of terbium(III). This contribution points out that the lowest excited state 5 D 4 of terbium(III) is attained by an intermolecular energy transfer from a lowest excited singlet state of a solid/solution interface-bound chloride anion which is generated by the recombination of a hydrated electron with the solid/solution interface-bound chlorine atom.

Intrinsic and 1-aminonaphthalene-4-sulfonate-specific extrinsic lyoluminescences of X-ray irradiated sodium chloride

Abstract 1-Aminonaphthalene-4-sulfonate (ANS)-specific extrinsic lyoluminescence (LL) of X-ray irradiated sodium chloride is observed at 425 nm when the irradiated salt is dissolved in an aqueous solution of ANS. The paper discusses, in detail, the mechanism of the ANS-specific LL and its analytical applicability. Also, the intrinsic LL of X-ray irradiated sodium chloride is studied. Hydrated electron as well as hole scavenger experiments support the proposal that in the case of the intrinsic LL of X-ray irradiated sodium chloride, trapped electrons (mainly F-center electrons) are released and hydrated whereas trapped holes (V-centers) remain surface-bound and are only partially hydrated before recombination occurs. These hydrated electrons and dissolving solid surface-bound hole centers, which are probably only partially hydrated, are able to act as reducing and oxidizing agents, respectively, in the luminophore oxidation-initiated reductive excitation pathway of ANS. Solution additives (halides and pseudohalides) show that in the chemiluminescence processes in question, oxidizing agents will follow the Marcus theory of electron transfer reactions. The LL method described allows the determination of ANS in the concentration range ≈10 −11 − 10 −7 M. This suggests that aminonaphthalene derivatives can be used as label molecules in high sensitivity lyoluminobioaffinity assays.

Quenching of cathodic electrogenerated F-center luminescence of aluminium oxide by lanthanide cations at the electrode/electrolyte interface

Abstract Energy transfer between aluminium oxide F-center and lanthanide cations at an oxide-covered aluminium electrode during the cathodic pulse-polarization of the electrode is investigated by means of Stern-Volmer luminescence quenching kinetics. Terbium III -specific extrinsic luminescence is observed while some other lanthanides are observed to quench the F-center luminescence. Different quenching efficiencies of the lanthanides are discussed to be dependent on the different energy acceptor characteristics of the tri- or divalent lanthanides.