Interfacial chemistry and adhesive durability

The improved understanding of adsorption chemistry which has arisen from the development of new surface analytical techniques during the last twenty years has had a major effect on the understanding of catalytic reaction mechanisms. However, there are many other areas of technology where Interfacial chemistry has a determining role but where the understanding is much less developed. In this paper examples are presented of the role of interfacial chemistry in adhesion. In particular, It Is shown that modification of a surface to the extent of just a few atomic layers in depth can have dramatic effects on the performance of adhesive bonds, particularly when exposed to hostile environmental conditions. These examples will be used to highlight a need for greater fundamental understanding of the interfacial chemistry of adhesion and also other technologies where interface effects are dominant.     

Energy dissipation via radiative recombination and vibronic relaxation in σ-delocalized silicon backbone network polymers

The nature of ultrafast energy dissipation in poly(n-hexylsilyne), a prototypical σ-delocalized alkysilicon network polymer, is explored. This disordered silicon backbone material exhibits strong near-UV to visible band-edge absorption and a high quantum yield of visible emission. The time evolution of the emission band is studied over four decades of time using time-resolved luminescence as a probe. The data indicate that while there is an ‘intrinsic’ Stokes shift after photoexcitation due to kinetic energy relaxation (<10ps), thermalization within a dense band of vibronic states via intramolecular phonon-assisted hopping on a nanosecond timescale is the dominant mechanism for excited-state decay. The data can be understood in terms of theoretical predictions for energy relaxation in disordered materials.     

Unexpected relations between matrix elements of the three-body scalar operators ti in the f shell

It has been known for some time that crystal-field matrix elements (i.e., matrix elements of sums over spherical harmonics involving the coordinates of the individual electrons) are often unexpectedly proportional to one another in the f shell. To see whether similar relations hold for more complicated operators than those provided by the crystal field, we examined the matrix elements of the three-electron scalar operators t_i for all configurations f^N, as calculated by W. T. Carnall on the basis of the computer program of Hannah Crosswhite. These operators are widely used to take configuration interaction into account, and we found a surprising number of proportionalities that go beyond what would be expected on a straightforward application of the Wigner-Eckart theorem, as applied to the irreducible representations of the classic groups SO(7), G₂ and SO(3) used by Racah in defining the f-electron states. A listing of such relations is provided.     

Analysis of the fluorescence of the ion Eu in fluoroborate glasses containing silver particles

We have studied theoretically and experimentally the fluorescence enhancement in fluoroborate glasses, doped with the ion Eu³⁺, containing small silver particles. The model developed shows that the gain coefficient has quenching and enhancement regions. The absorption peak of the metallic particles occurs at 425 nm, which is not in resonance with the ⁵L₆ level of the ion Eu³⁺. For this reason it is not possible to make a complete comparison between theoretical and experimental results, but the structure of the absorption spectra allows us to check some crucial aspects of the problem, such as energy transfer, enhancement of the local field and the high absorption by the particle system. Results of electronic microscopy have revealed non-uniformity of the particle distribution with average radii of approximately 20 Å and 35 Å and a measured localized mean filling factor of 0.02.     

Absorption spectra of solid rare earth complexes obtained from the inner filter effect on luminescence

The inner filter effect is observed in luminescence whenever a second substance is present that has absorptions which overlap the luminescence. This results in a diminution of the luminescence at those wavelengths and is generally observed in solution. In the present communication we show that this effect can be observed when a solid rare earth complex is physically mixed with a solid luminescent organic matrix with which it does not interact. The resulting luminescence is reduced in emission intensity at those wavelengths corresponding to the absorptions of the rare earth ion. This is illustrated with a mixture of Na₃[Ho(ODA)₃] ·2NaClO₄·6H₂O and pyrene at 77 K, and it is shown that the absorption spectrum of the holmium complex can be extracted from the luminescence spectrum of pyrene. This absorption spectrum is in good agreement with that obtained by conventional spectroscopy with a single crystal.     

Kinetics of the inhibition of ascorbic acid browning by sulphite.

Despite differences in the structures of aldoses and ascorbic acid, ASA, the non-enzymic browning of the latter involves intermediates similar to those found in Maillard browning. The kinetics of the sulphite-inhibited browning of ASA suggest that, under anaerobic conditions, the rate of reaction of sulphite species, S(IV), is of first order with respect to S(IV). The possibility that S(IV) could catalyse the hydrolysis of the lactone ring of ASA is considered by reference to D-glucono-delta-lactone. Evidence is presented to suggest that, under aerobic conditions, autoxidation of ASA leads to the oxidation of S(IV). The composition of melanoidins from ASA-glycine mixtures is compared with that from arabinose-glycine; the former contain 2 x 5 times more ASA-derived residues per glycine molecule than arabinose-derived residues per glycine molecule in the latter. The implications of these findings to the mechanism of ASA browning are discussed.     

In vitro model for the evaluation of toxicity and antinutritional effects of sulphites.

The food preservatives, sulphur dioxide and its salts, are known to present some toxic, mutagenic and antinutritional effects; in fact they interact with a number of nutrients, e.g. some vitamins, notably thiamine (Th) and folic acid (FA). The effect of different concentrations of sodium bisulphite in cell culture media has been studied in vitro on a human cell line, HEp-2, deriving from a carcinoma of the larynx. Moreover, the sulphites have been tested with different levels of Th and FA with the aim of elucidating how much the cellular response depended on either the anti-nutritional effect or the toxicity of sulphites. Cell growth has been taken as an index of cytotoxicity and measured both as total protein content and as colony-forming ability. With no Th and FA in the culture medium, a clear decrease of cell growth was observed either with or without addition of sodium bisulphite. A dose-dependent reduction of protein content was detected in cells treated with 10, 50, 100, 200, 250 or 500 microM sodium bisulphite. Moreover, when the cells were treated with 10 or 100 microM of this compound, the colony-forming ability was reduced both in number and colony size. As far as the interaction of the two vitamins with sodium bisulphite is concerned, when these nutrients were present in the medium at 0.5, 1.0, 1.5, 2.0 or 2.5 mg/l, a similar growth profile, determined from their concentration, was observed in treated and control cells, the growth levels being affected by the sodium bisulphite contents. At higher levels of Th and FA, the growth index was still increasing only in treated cells, this phenomenon being particularly evident in cultures treated with 200 microM sodium bisulphite. The colony-forming ability was reduced in controls but still increased in treated cells at the highest concentration of vitamins.