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