Surface heating and energy transfer in pulsed microwave catalytic systems: A microwave-induced acoustic study

Since the early demonstrations of our concept of pulsed microwave catalysis in chemistry, we have witnessed the coming of age of the technique as an enabling technology in the industrial world, despite the inadequate understanding of the basic mechanisms of how microwaves interact with metallic surface sites and the subsequent energy transfer. We shall present here some very recent developments of the microwave-induced acoustic technique and its applications to a better understanding of the basic surface heating and energy transfer phenomena in pulsed microwave catalytic systems.     

Model calculations of interface segregation in dilute systems

Using a discrete model for the segregation kinetics based on the Darken theory of diffusion and on the regular solution model (RSM), one and two dimensional simulations of interface segregation in binary and ternary systems have been performed. The aim of the calculations was to investigate the kinetics of segregation sequences in ternary systems, the segregation kinetics in discrete systems during the initial phase and finally the kinetics of surface segregation in the region of a grain boundary intersected by the surface.     

Study of the liquid crystalline phases of benzopurpurin in water

We report the NMR study of the liquid crystalline phases of aqueous solutions of the dye benzopurpurin (BP-4B). Upon changing dye concentration, the system exhibits a phase transition between two ordered phases at about 3·5 wt %. The structure of these phases was not determined but the evidence suggests that they consist of columns or helically twisted columns of stacked dye molecules which are randomly oriented, similar to nematic phases.     

Liquid crystalline behaviour from binary mixtures of non-mesogenic 4-alkyl-N-(4-cyanophenyl) piperidines [1]

A relatively rare phenomenon, the induction of thermotropic mesophases from combinations of homologues of non-mesogenic compounds, is examined in some detail. Thus, monotropic nematic phases have been obtained from binary mixtures of 4-alkyl-N-(4-cyanophenyl)piperidines (I). Except for the hexyl homologue, the compounds I with butyl to nonyl as alkyl groups are non-mesomorphic as neat materials. From phase diagrams, the nematic clearing temperatures of the binary mixtures are found to vary linearly with composition and to increase as the molecular lengths of the components are increased. By introducing a methyl group at the 1-position of the alkyl chain, the melting points of the neat components are depressed, and the nematic clearing temperatures of the binary mixtures are lowered. Factors associated with the induction of this mesomorphism and with the 'kinetic' and thermodynamic stabilities of the monotropic nematic phases are discussed. Among these is the orientation between the rings of the core groups in I and in the more common mesogens, the trans-4-alkyl-1-(4-cyanophenyl) cyclohexanes, II.     

Working Methods Paper: Critical considerations with respect to the identification of tin species in the environment

Biogeochemical pathways of tin species in the environment are still controversial, e.g. with regard to methylation and transmethylation phenomena, owing to the fact that the identification of methylated tin-compounds is often difficult. The previous tentative identification of a mixed methylbutyltin compound in sediment and biological samples by GC/AAS after hydride generation gave an illustration of this problem. This compound was previously identified in sediments by other authors and also suspected to occur in a contaminated sediment sample from the Boyardville Marina, France. The retention time obtained by GC/AAS corresponded to the actual retention time of a mixed methylbutyltin calibrant. However, additional checks demonstrated that the compound detected was actually monophenyltin. This evidence was produced by a thorough analysis of a selected sediment sample by alternative techniques such as GC/AAS and GC/AES after pentylation, GC/FPD and GC/MS. The results presented highlight the need for a full identification of compounds to avoid mis-interpretation.     

General base catalysis in the decomposition of N-Cl-Valine in aqueous solution

This article analyzes the kinetics of the decomposition of N-Cl-Valine in aqueous solution, which is formed rapidly by chlorination of Valine with sodium hypochlorite. A general-base catalyzed process not yet described is reported. The experimental evidence shows two competitive decomposition paths: an unimolecular concerted fragmentation process (k = (1.8 ± 0.1) · 10^?4 s^?1 at 298 K) and the other one is an E2 elimination process whose importance increases with pH and depends on the nature and the concentration of the bases present in the medium. © 1994 John Wiley & Sons, Inc.