Chemical nature and water/oil emulsifying properties of asphaltenes

Asphaltenes which differ in their abilities to form water/oil emulsions were analysed by acidimetry, infrared and nuclear magnetic resonance spectroscopies. Asphaltenes which easily form such emulsions are loosely packed molecules and exhibit a marked acid/base imbalance.     

NUMERICAL ANALYSIS OF HEAT TRANSFER BY NATURAL CONVECTION AND RADIATION IN PARTICIPATING FLUIDS ENCLOSED IN SQUARE CAVITIES

The influence of thermal radiation on natural convection in a participating fluid contained in a square cavity is studied numerically. The radiative transfer process is solved from the PI approximation. The Navier-Stokes equations are solved by a finite difference scheme integrated over control volumes. A numerical study of the so-called window problem (thermally driven cavity) shows the influence of thermal radiation on this reference problem for Rayleigh numbers in the range of 10³-10⁷ and Planck numbers varying from 1 to 0.05. The isotherms, streamlines, and heat lines show an increase of the dynamical effects in the central part of the cavity and a significant modification of the boundary layers. Results obtained from the simulation of an isotropically scattering medium are given.     

Comparison of two headspace sampling techniques for the analysis of off-flavour volatiles from oat based products

Two different headspace sampling techniques were compared for analysis of aroma volatiles from freshly produced and aged plain oatcakes. Solid phase microextraction (SPME) using a Carboxen–Polydimethylsiloxane (PDMS) fibre and entrainment on Tenax TA within an adsorbent tube were used for collection of volatiles. The effects of variation in the sampling method were also considered using SPME. The data obtained using both techniques were processed by multivariate statistical analysis (PCA). Both techniques showed similar capacities to discriminate between the samples at different ages. Discrimination between fresh and rancid samples could be made on the basis of changes in the relative abundances of 14–15 of the constituents in the volatile profiles. A significant effect on the detection level of volatile compounds was observed when samples were crushed and analysed by SPME–GC–MS, in comparison to undisturbed product. The applicability and cost effectiveness of both methods were considered.     

Spectroscopic Evidences of Charge Transfer Phenomena and Stabilization of Unusual Phases at Iron Oxide Monolayers Grown on Pt(111)

Strong electrostatic coupling between the local structure of metal-supported oxide ultra-thin films and the charge state of adsorbed species (such as Au atoms and O₂ molecules) is at the origin of a co-existence of alternative adsorption configurations, with drastically different structural and electronic characteristics. This effect may contribute also to the stabilization of oxide phases of unusual stoichiometries, where the ion valences are compensated by an electron exchange with the metal substrate. We report an analysis of the spectroscopic signatures of such differently charged adsorbates and unusual oxide phases, including valence and core electrons spectroscopies and vibrational characteristics. We show that the adsorbate charge states can be identified by their signature in the electronic states around the Fermi level and by the modification of the vibrational frequencies of adsorbed probe molecules. Moreover, core level analysis may contribute to the identification of the oxygen-rich phase, formed at high oxygen exposure. The detailed theoretical investigation of the spectroscopic signatures of alternative adsorption configurations and adsorbate charge states supplies a set of data, which may help the experimental identification of such species.     

Estérification de l'acide stéarique par des alcools légers: Catalyse homogène par les titanates

Stearic acid estérification with low boiling alcohols using alkyl titanate compounds is performed in a semi-continuous reactor. The kinetic study of the reaction at high temperature (for which titanates are more efficient) requires to include the uncatalyzed reaction. The significance of the catalyst is specified; the nature of the active species, depending on rapid exchange reactions is also discussed.