Fundamental study of adsorption of non-polar and polar fluids on hydrophobic ordered solids

Abstract

Adsorption of gases on solids has been a subject of interest due to its increasing use as an economical process for separation and purification in chemical and biochemical industries. To optimise these processes, a good fundamental understanding of adsorption is required. With the advancement of technology, a better understanding of the microscopic mechanism of various adsorption phenomena can be achieved with molecular simulation. This thesis aims to achieve a better understanding of the adsorption of non-polar and polar adsorbates on hydrophobic ordered solids with a combination of Monte Carlo simulations and high resolution experiments. The thesis starts with the adsorption study of simple adsorbates (e.g. neon, argon, krypton, xenon and nitrogen), which serve as a reference as well as a baseline for the understanding of the behaviour of complex adsorbates. It is then followed with the study of polar molecules (e.g. methanol and water), whose adsorption behaviours are of industrial interest and still remain challenging for adsorption scientists.The adsorption/desorption hysteresis of krypton on a highly graphitised thermal carbon black (GTCB) was examined because krypton is an important adsorbate to probe low surface area solids, especially graphite used in nuclear industries. Hysteresis is commonly assumed to only occur in mesopores, but experimental evidence supports the view that it can occur in the Kr/graphite system. With the combined study of high-resolution adsorption experiments and molecular simulations, it was revealed that the origin of hysteresis is attributed to the continuous densification and ordering of the adsorbate as adsorption proceeds in higherlayers.One of the key findings in this thesis is the new model for GTCB, which goes beyond the homogeneous model and gives a better description of experimental data. The new model takes into account: (1) the energetic corrugation, (2) the difference between the polarizabilities of a carbon atom in the direction vertical to the graphene surface and the direction parallel to the surface, (3) the difference in the interlayer spacing between graphene layers, and (4) the smaller collision diameter and the greater well depth of interaction energy for a carbon atom in the outermost graphene layer than those in the underneath layers. This graphite model particularly gives an improved description of commensurate (C) and incommensurate (IC) packing. It was found that N2 can form C packed layers because its collision diameter is close to the commensurate graphite lattice spacing λ of 0.426 nm. Xenon, on the other hand, exhibits IC packing because its molecular size is greater than λ . Neon does not exhibit C packing because the gain in the intermolecular potential interactions in the incommensurate IC packing when moleculesmove away from carbon hexagon centres does not compensate for the increase in the solidfluid potential energy.Another key component of this thesis is the effects of temperature on the transition from non-wetting to wetting, which were investigated with extensive simulations of argon adsorption on substrates of different strength. The appropriate parameter to account for the role of temperature in the wetting transition is the ratio of the isosteric heat to the heat of condensation, which is a measure of the relative strength between adsorption and cluster formation. For temperatures greater than wetting temperature TW, the adsorption mechanism is layer-by-layer wetting if the wetting temperature is lower than the roughening temperature TR. On the other hand, if the wetting temperature is greater than TR the mechanism is either a continuous wetting or a pre-wetting, which then transitions to continuous wetting when the temperature is greater than the critical pre-wetting temperature.Having achieved an improved understanding of simple gases adsorption, we extend the study to methanol, a representative of polar adsorbate. Our experimental result of methanol adsorption on GTCB for temperatures in the range 175K-298K shows a distinct transition from non-wetting to incomplete wetting and finally to complete wetting as the temperature is increased, a phenomenon that has not been observed in the literature. These wetting transitions are a result of the interplay between the intermolecular interaction, the adsorbateadsorbent interaction and the temperature (entropy). It was also found that functionalgroups do not affect the adsorption behaviour of methanol beyond monolayer coverage.Finally, the adsorption of water in the confined space between a graphitic cylinder and a planar graphite surface was investigated to understand the role of confined space topology on the clustering and filling of water. Functional groups are often invoked as the strong interaction sites required to induce the nucleation of water embryos, which develop into clusters and fill the spaces of micropores. Nevertheless, it was found that graphitic pores without functional groups can induce pore-filling because of the enhancement in the adsorbent-adsorbate potential resulted from the overlapping of the potential fields exertedby opposite pore walls. It is shown that water can adsorb in a confined graphitic space at ambient temperatures, provided that the enhancement is sufficient to compete with the intermolecular interaction of water.