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The adsorption of lumiflavin (LF) at a mercury electrode surface has been studied by means of cyclic voltammetry in phosphate buffer solutions of pH 6, 7 and 8. At low to medium coverages, LF, which is redox-active, is suggested to adsorb in an orientation parallel to the electrode surface, independent of potential. At higher coverages, concerted re-orientations of the LF monolayer occur as the potential is changed. At potentials positive of ca. -0.35 V and negative of -1.0 V, adsorbed LF is likely in the parallel configuration, while two perpendicular orientations exist at potentials between these limits, consistent with higher attainable coverages. One of these is particularly stabilized in pH 6 solutions, when reduced LF is known to be uncharged, while it is likely to be anionic (and hence de-stabilized) at pH 7 and 8. A second set of small redox peaks observed just negative of the main redox peaks have been assigned to lumiflavin dimers, formed when the molecules are oriented in the perpendicular orientation in the negative potential range. The electrochemical behaviour of LF shows striking differences from that of FAD, although the common electrochemically active moiety is the isoalloxazine ring system.

The photochemistry of adsorbed long-chain a-diazo ketones, azidoformates, and a-azido ketones was explored as a method of improving the adhesion between polyethylene and polar inorganic fillers. A GC-MS study of the photoreactions of monolayers of intermediate chain length compounds adsorbed on alumina established that grafting reactions predominated and that cyclization and interchain reactions were much less pronounced than analogous photoreactions in cyclohexane solution. This result was attributed to the decreased mobility of the surface-active compounds at the interface. The effects of surface modification of alumina and silica gel of large particle size and fumed silica of 17-20-nm particle size on the tensile properties of blends with linear low-density polyethylene were investigated. Monofunctional grafting agents such as octadecanoyl azide resulted in no improvement in elongation at break in comparison to untreated filler, a result indicating that simple wetting of the filler surface was insufficient to promote adhesion between polymer and filler in this system. With bifunctional grafting agents containing both a photochemically and a thermally active group, such as 12-azido-l-diazo-2dodecanone and l-diazo-ll-dodecen-2-one, significant improvements in elongation at break were observed over the untreated filler. Evidence for improved adhesion between polymer and surface-modified filler was provided by scanning electron microscopy of fracture surfaces.

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