Gilberte Dosseh

Adsorption and Structure of Benzene on Silica Surfaces and in Nanopores

Grand canonical Monte Carlo simulations are used to study the adsorption of benzene on atomistic silica surfaces and in cylindrical nanopores. The effect of temperature and surface chemistry is addressed by studying the adsorption of benzene at 293 and 323 K on both fully and partially hydroxylated silica surfaces or nanopores. We also consider the adsorption of benzene in a cylindrical nanopore of diameter D=3.6 nm and compare our results with those obtained for planar surfaces. The structure of benzene in the vicinity of the planar surface and confined in the cylindrical nanopore is determined by calculating orientational order parameters and examining positional pair correlation functions. The density profiles of adsorbed benzene reveal the strong layering of the adsorbate, which decays with the distance from the silica surface. At a given temperature and at low pressures, the film adsorbed at the fully hydroxylated silica surface is larger than that for the partially hydroxylated silica surface. This result is due to an increase in the density of silanol groups that induces an increase in the polarity of the silica surface, which becomes more attractive for the adsorbate. Our results also suggest that the benzene molecules prefer an orientation in which their ring is nearly perpendicular to the surface when fully hydroxylated surfaces are considered. When partially hydroxylated surfaces are considered, a second preferential orientation is observed where the benzene ring forms an angle of approximately 50 degrees with the silica surface. In this case, the average orientation of the benzene molecules appears disordered as in the bulk phase. These results suggest that determining the experimental orientation of benzene in the vicinity of a silica surface is a difficult task even when the surface chemistry is known. Capillary condensation in the nanopores involves a transition from a partially filled pore (a thin film adsorbed at the pore surface) to a completely filled pore configuration where the confined liquid coexists at equilibrium with the external gas phase. The disordered orientation of the adsorbed benzene molecules in the case of the partially hydroxylated surface favors the condensation of benzene molecules (the condensation pressure for this substrate is lower than that for the fully hydroxylated surface). Finally, these results are consistent with the structural analysis, showing that (1) benzene tends to relax its liquid structure a little in order to optimize its molecular arrangement near the pore wall and (2) the disordering of the liquid structure induced by the surface becomes stronger as the interaction with the pore wall increases.

Identification of aromatic molecules in intermediate boiling crude oil fractions by 2D n.m.r. spectroscopy

Abstract This paper reports the full identification of the aromatic components of four intermediate boiling (175.5 ⩽ Tb ⩽ 254 °C) crude oil fractions analysed by short- and long-range heteronuclear shift correlation 2D n.m.r. spectroscopy. The method is time-consuming but provides reliable data which are needed for the thermodynamic simulation of crude oils in the boiling temperature range where the more usual 1D methods alone are not entirely satisfactory.

Glass transition, freezing and melting of liquids confined in the mesoporous silicate MCM-41

Abstract MCM-41 is a recently discovered mesoporous silicate that consists of a hexagonal arrangement of cylindrical pores. Because the pores are very well defined in size and shape, MCM-41 can be considered as a model material in comparison with previous types of porous glass and has therefore been used in the present study. In particular, we have focused on the properties of confined liquids and related phase transitions using an MCM-41 with a pore diameter of 4 nm. We discuss here the first stage of a study where the thermodynamics of a series of molecular liquids (water, cyclohexane, benzene, toluene, o-terphenyl and m-toluidine) are analysed by differential scanning calorimetry. Only a moderate decrease in the melting point of organie liquids has been observed. Moreover, analysing the freezing process, we have established thermal treatments that provide temperature ranges where confined liquids can be studied below the melting temperatures of both the bulk and the confined phases. Surprisingly, no si...

Thermodynamic and Structural Properties of Fragile Glass-Forming Toluene and Meta-Xylene: Experiments and Monte-Carlo Simulations

Abstract We report a characterization of isobaric and isothermal glass transition of two fragile liquids, toluene and meta-xylene, by elastic neutron scattering, calorimetry, and Monte-Carlo simulations. We studied thermodynamic and structural properties in the supercooled and overcompressed liquid up to the glass transition. The simulated thermodynamic or structural properties are in good agreement with experimental ones. From the Prigogine-Defay ratio, the simulated glass transition seems associated to one order parameter. The structural changes in the metastable regime are attributed to density variations. No structural heterogeneity appears on the experimental or simulated neutron spectra.

From Homogeneous to Segregated Structure of Poly(dimethylsiloxane)/Cellulose Derivative Mixed Langmuir Films Depending on Composition: An in Situ Neutron Reflectivity Study.

The mixing behavior of deuterated polydimethylsiloxane (PDMSd) and cellulose acetate butyrate (CAB) spread as Langmuir films at the air-water interface was studied by means of surface pressure-area isotherms, Brewster angle microscopy (BAM) observations, and in situ neutron reflectivity. The contrast variation method was used with different D2O/H2O mixtures as subphase, allowing contrast matching to either CAB, PDMSd, or PDMSd/CAB mixed film if homogeneous. At PDMSd volume fractions Φ lower than 0.6, the mixed film is a homogeneous monolayer throughout the film compression, in agreement with the monophasic film observed by BAM and the attractive interactions between PDMSd and CAB evidenced from the isotherm measurements. In contrast, at PDMSd volume fractions Φ higher than 0.6, a vertically segregated structure of the mixed film is highlighted. Indeed, whatever the surface pressure, a bilayer structure is observed with a PDMSd layer in contact with the air over a thin CAB layer in contact with the subphase. These results show that the structure of the film is mainly driven by the PDMSd volume fraction which allows obtaining either a homogeneous membrane which composition can be tuned or a vertically segregated system. In contrast, only the thickness of the layers varies with the surface pressure while the structure of the film is not affected.

The Sites of Premelting in Organic Compounds

Abstract Bartis recently proposed a theory which should allow to determine whether impurities or defects are responsible for the premelting phenomena in crystalline solids. We have tested this model on several orientationally disordered molecular crystals studied by proton NMR spectroscopy. This showed that extended defects, and more likely grain boundaries are at the origin of the premelting phenomena in these crystals.