On the Nonwetting/Wetting Behavior of Carbon Dioxide on Graphite

Extensive simulations of CO adsorption on graphite were carried out over a range of temperature to investigate the effects of temperature on the nonwetting/wetting transition. CO adsorbed on graphite is nonwetting below the bulk triple-point temperature (T = 215 K) but wetting above this temperature and differs from noble gases, which form an adsorbed film at all temperatures. Our simulation results confirm that the CO /graphite system is nonwetting at temperatures below 90 K, as reported experimentally by Terlain, A.; et al. [Phase Diagrams of Films of Linear Molecules with Large Quadrupole Moments (CO , N O, C N ) Adsorbed on Graphite. Surf. Sci. 1983, 125, 304-311] and by Morishige, K. [The Structure of a Monolayer Film of Carbon Dioxide Adsorbed on Graphite. Mol. Phys. 1993, 78, 1203-1209]. For temperatures between the wetting temperature of 90 K and the bulk triple point, incomplete wetting occurs, and the adsorbed film has a finite thickness at the bulk coexistence pressure. This is a consequence of the fact that the isosteric heat of adsorption at zero loading, (0), is lower than the heat of sublimation of bulk CO . On the other hand, at temperatures greater than T , the adsorption isotherm exhibits continuous wetting as the pressure approaches the bulk coexistence pressure because qs(0) is greater than the bulk heat of condensation. We support these findings with detailed analysis of the molecular configurations along the canonical isotherms, the isosteric heat as a function of loading, and the local orientation-density distributions. The two-dimensional critical temperature of the first adsorbate layer was determined as 130 K, in excellent agreement with 127.5 K estimated experimentally by Terlain, A.; et al. [Phase Diagrams of Films of Linear Molecules with Large Quadrupole Moments (CO , N O, C N ) Adsorbed on Graphite. Surf. Sci. 1983, 125, 304-311]. In the final section, we present a parametric map showing regions of nonwetting, wetting, and incomplete wetting for CO adsorption on the surface of adsorbents of different strengths