Gases adsorbed on graphite

Table I. Twodimensional triple points Tt and critical temperatures To of the first layer of noble gases adsorbed on graphite, BN and MgO.

The two-dimensional approximation also applies to Lennard-Jones gases adsorbed on smooth surfaces such as those presented by the basal plane of graphite. Thus, the classic experiments of Thoiny and coworkers [76,77] show coexistence and critical points for several simple gases adsorbed on graphite that appear to be in good agreement with theory. 

To show the main aspects of the phase diagram for rare gases adsorbed on graphite. The richness of the phases that are found and the differences between the phase diagrams for each rare gas have been the subject of many experimental, theoretical, and computer simulation studies. 

To show the validity of the Cuadros-Mulero equation of state for two-dimensional Lennard-Jones fluids as the simplest expression to be able to use Steele s theory to reproduce experimental results for nonquantum rare gases adsorbed on graphite. 

The above studies were complemented with a great number of studies devoted to phase transitions for monolayers of rare gases adsorbed on graphite. Before detailing these studies, however, we shall summarize other results obtained with rare gases or mixtures of rare gases on different surfaces or for multilayer adsorption on graphite. 

An incommensurate solid to a reentrant fluid transition and a reentrant fluid to commensurate transition, both being of second order. In particular, the existence of a novel reentrant fluid phase separating the commensurate and incommensurate phases constitutes clearly one of the most spectacular and interesting phenomena observed in the study of rare gases adsorbed on graphite. Another important result is that there is no multicritical (commensurate-incommensurate fluid) point at high temperatures. 

Our results show that Steele s model for monolayer adsorption is a good representation of real systems such as rare gases adsorbed on graphite. Deviation with respect to experimental results with argon or krypton by using the RO or CM... 

No truly two-dimensional systems exist in a three-dimensional world. However monolayers absorbed on crystalline or fluid surfaces offer an approximation to two-dimensional behaviour. Chan and coworkers [31] have measured the coexistence curve for methane adsorbed on graphite by an ingenious method of determining the maximum in the heat capacity at various coverages. The coexistence curve (figure A2.5.29) is fitted to p = 0.127, very close to the theoretical 1/8. A 1992 review [32] summarizes the properties of rare gases on graphite. 

Studies based on the introduction of interaction potentials that are sufficiently realistic to produce results comparable with experimental data. In the adsorption case, these have been rare gases adsorbed on the graphite basal plane. The agreement with experiment, while of theoretical interest, primarily serves to validate the model potentials used. 

Physical adsorption of inert gases on metals is studied at temperatures between 10 and 78 K. At too high a temperature the adsorbed layer boils off the surface. At too low a temperature an adsorbed gas molecule does not migrate on the surface after striking it the surface structure is random and does not anneal to reflect the energetics of adsorbate-substrate interaction. In the temperature range for which surface equilibration occurs, the structure of the surface layer is independent of the inert gas adsorbed and of the metal surface exposed. Whether Xe is adsorbed on graphite,... 

The virial coefficient theory can also be applied to mixtures (e.g., rare gases adsorbed on another rare gas plated on graphite [193,194]). Thus, Brown arid Hsue [194] have reported... 

It is noted in Sections XVII-10 and 11 that phase transformations may occur, especially in the case of simple gases on uniform surfaces. Such transformations show up in q plots, as illustrated in Fig. XVU-22 for Kr adsorbed on a graphitized carbon black. The two plots are obtained from data just below and just above the limit of stability of a solid phase that is in registry with the graphite lattice [131]. 

Another cause of growth which is of equal importance with graphitisation is the penetration of oxides into the metal along the graphite flakes. This presumably takes place because oxidising gases can be adsorbed on to the... 

The case of triangular lattice is particularly interesting since it corresponds to adsorption on graphite and on the (111) plane of several fee metal crystals [15,102,103,135]. The distance between adjacent potential minima for the graphite basal plane is equal to 2.46A and hence is too small to allow for their mutual occupation by even very small atoms of light noble gases. The same is true for adsorption on metals. Experimental studies have demonstrated that for rare gas atoms and simple molecules adsorbed on the graphite basal plane as well as on the (111) faces of fee crystals the ordered state corresponds to either the /3 X %/3 [102] or to the 2x2 phase [103,136,137] shown in Fig. 8. 

Polar gases on graphite will be nonwetting if the dipolar interactions in the adsorbed phase are sufficiently strong. Water on graphitized carbon black is a... 

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