Orientation Of Benzene Molecules Research Articles

Computer Simulation of Benzene–Water Mixture Adsorption in Graphitic Slit Pores

The adsorption of mixtures of benzene–water in graphitic slit pores has been studied using Monte Carlo simulation. It is shown that the adsorption of the mixture starts with the adsorption of benzene molecules on the graphite surface which then act as anchors for water molecules to adsorb. High loadings of water in a slit pore significantly affect the arrangement and orientation of benzene molecules. When the water density in the pore approaches its bulk liquid density, water molecules displace some benzene molecules from the surface, and the benzene sorption process switches from adsorption to an absorption mechanism. We have also studied the influence of the external vapor composition on the adsorption isotherms of benzene and water. It is observed that a high concentration of benzene in the vapor phase facilitates the adsorption of both benzene and water in such a manner that the condensation pressure of benzene and the onset pressure of water adsorption shift to lower values. At high pressures, the adsorption of water is dominant at the expense of benzene adsorption. This interesting finding calls for a review of the concept of graphite surface hydrophobicity when water is coadsorbed with hydrocarbons.

Multilayer adsorption of benzene on graphitised thermal carbon black—The importance of quadrupole and explicit hydrogen in the potential model

Grand Canonical Monte Carlo (GCMC) simulation was used to study multilayer adsorption of benzene on graphitised thermal carbon black over a range of temperature between 273 K and 373 K. Three potential models for benzene were compared: TraPPE-UA-9, OPLS-AA and TraPPE-EH, in order to study their capability to correctly describe the adsorption isotherms and isosteric heats. In the sub-monolayer region, there is no difference between the simulation results obtained with the three models, which all describe the experimental data well. However, in the multilayer region only the TraPPE-EH model, that includes the quadrupole moment and explicit modelling of the hydrogen, is able to describe the experimental data accurately; the other two models significantly under-predict the data. The TraPPE-EH model was then used to investigate the microscopic behaviour of the adsorbed phase: the local density distribution and the orientation of various layers, particularly the change in orientation of benzene in the first layer with increase in loading from sub-monolayer to multi-layer. It was found that the orientation of benzene molecules in the first layer is affected by the presence of molecules in the higher layers, but that most benzene molecules remain in an orientation parallel to the surface, which is favoured by the interaction with the surface, while a smaller population takes 60° and verical orientations to the surface which maximises the entropy. The ratio of the number of molecules having parallel orientation to that having 60° and vertical orientations decreases in higher layers due to the weaker influence of the adsorbate–adsorbent interaction. Detailed study of the orientation shows that most slant and vertical molecules have two of their hydrogen atoms closer to the surface.

THE ORIENTATION OF BENZENE ON BIMETALLIC SURFACES

Angle-resolved UV photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation is used to determine the orientation of benzene molecules on two modifications of the pseudomorphic Ni/Cu(111) layer, the Ni-terminated adlayer and the Cu-terminated Ni sublayer. It is found that the molecules adsorb with their C–H bonds 30° off the close-packed substrate rows (σv orientation) on the Ni adlayer whereas no preferential orientation was found for the Cu-terminated Ni sublayer. For these and other close-packed mono- and bimetallic surfaces involving Cu, Ni and Ru, the correlation between adsorption geometry and reactivity of benzene is explored in connection with temperature-programmed desorption (TPD). Like for the Ni/Cu(111) sublayer, no preferential lateral orientation and a weak bond is found on most other Cu-terminated surfaces at 80 K. This goes along with the absence of any dissociation on these surfaces. Only on the stretched 1 ML Cu/Ru(0001) surface is the substrate–benzene bond strong enough to induce a clear preferential orientation (σd with C–H bonds parallel to the close-packed rows) but still too weak to induce dissociation. The same orientation was found for the saturated benzene layer on Ni(111) where the molecules also do not dissociate. Dissociation is, however, observed for the saturation coverage of benzene on the Ni/Cu(111) adlayer and for low benzene coverages on Ni(111). On these surfaces the azimuthal orientation of the benzene molecules is σv, i.e. rotated by 30° with respect to the close-packed rows.