Effects of Surface Curvature and Surface Strength on Argon Adsorption in Carbon Nanotubes at Temperatures below the Triple Point

Abstract

This paper describes an investigation of argon adsorption into carbon nanotubes at temperatures below the triple point, using a grand canonical Monte Carlo simulation to study the effects of confinement and surface strength on the 2D transition. In large pores, it was found that 2D transitions can occur in more than one layer, but are absent in higher layers for small pores. The 2D critical temperature of the first layer for a small pore (R = 1.2 nm) was found to be ca. 66 K (MWCNT) and 65 K (SWCNT), compared to 55-59 K observed experimentally for a flat graphite surface. This is because of the overlapping effects due to the surface curvature or the confinement in a carbon nanotube. Assuming a weaker carbon surface by reducing the graphene surface strength by 40% for SWCNT, the 2D critical temperature was only modestly reduced to 63 K. This suggests that the experimental data at 59 K might be attributed to other factors, other than confinement effects. An imperfect surface is suggested and, employing 5% defects on this surface, the 2D critical temperature has been determined as 58 K which is in better agreement with the experimental value of 59 K.

On the condensation/evaporation pressures and isosteric heats for argon adsorption in pores of different cross-sections

“…Previous studies of the effects of pore shape on condensation and evaporation include a density functional investigation of adsorption in single pores with cylindrical, square and triangular cross sections by Malanoski and van Swol [8]; simulations of nitrogen adsorption in hexagonal and cylindrical pores of MCM-41 by Ustinov [9], and a simulation study of the effects of surface curvature and adsorption strength on 2D transitions of argon adsorbed in carbon nanotubes by Liu et al [10]. Triangular pores have also been studied in earlier work [11][12][13][14] as a kernel for the characterization of activated carbon.…”

Effects of confinement on the molar enthalpy of argon adsorption in graphitic cylindrical pores: A grand canonical Monte Carlo (GCMC) simulation study

“…As the temperature is increased (77 and 87.3 K), the 2D transition disappears because the 2D critical temperature of the first layer was found to be ca. 66 K [40], but the isotherms still exhibit a layering mechanism, followed by a capillary condensation in the core. When the temperature is increased to 120 K, the plateau of the first layer becomes less clear because of the increasing molecular thermal motion.…”

“…At temperatures well below the triple point (83.8 K), for example at 57.5 K, we observe a 2D transition in the first two layers, followed by capillary condensation in the core. Detailed study of argon adsorption for temperatures well below the triple point has been presented in Liu et al [40]. As the temperature is increased (77 and 87.3 K), the 2D transition disappears because the 2D critical temperature of the first layer was found to be ca.…”