On the Heat of Adsorption at Layering Transitions in Adsorption of Noble Gases and Nitrogen on Graphite

The isosteric heat versus loading of simple fluids (noble gases and nitrogen) adsorbed on a graphitic surface at a range of temperatures is investigated with GCMC simulations. At low temperatures (usually below the bulk triple point) the adsorption isotherms show a distinct 2D-phase transition in the first layer and possibly in higher layers and then has a more complex pattern. For densities less than the gaseous 2D-density, the heat increases linearly with loading. For loadings across the 2D-transition, the isosteric heat is constant, and when loadings are greater than the “liquid-like” density of the 2D-transition the isosteric heat again increases linearly with loading up to monolayer concentration, beyond which the isosteric heat decreases sharply, indicating the onset of adsorption into the second layer. Grand canonical Monte Carlo simulation shows that the constant heat observed in the 2D-transition is because of the increase in the number of clusters of similar size, rather than the increase in the cluster size. At very low temperature, a dense layer forms before the onset of the next layer and there is no heat spike in the plot of the isosteric heat versus loading because there are no gaps large enough in the dense layer for further addition of a particle. Moreover, there is no heat spike in these plots at high temperatures because of the increase in thermal motion of the molecules. Therefore, there is a narrow range of temperature from which we can see an isosteric heat spike, resulting from the squeezing of molecules into the lower layers, which increases both the solid−fluid interaction and the fluid−fluid interaction. We have found that the 2D-critical temperatures of the first three layers correlate well with the well depth of the fluid−fluid interaction, and interestingly the 2D-critical temperature of the third layer is between those of the first and second layers.