Wetting transitions for the Ar-CO2interface: Modified-hypernetted-chain and density-functional-theory results

A model of fluid-argon–solid-${\mathrm{CO}}_2$ interface is investigated by means of density-functional (DF) -theory and modified-hypernetted-chain (MHNC) calculations. Both theoretical approaches predict complete wetting of the ${\mathrm{CO}}_2$ wall from the bulk Ar vapor when this approaches the coexistence region. At a bulk vapor density $n_B$ ${\sigma }^3$=0.08 and for an assigned wall-fluid coupling strength, the DF and MHNC estimates of the temperature for which complete wetting occurs are very close to each other ($T^{\mathrm{*}}$≃1.16), and to the coexistence temperature of the homogeneous bulk phase obtained through independent DF and MHNC calculations. The DF results also show the existence of a prewetting transition, very close to the coexistence line, and of a ‘‘surface spinodal line,’’ that is a set of points near and inside the coexistence region, at which transverse correlations in the fluid near the wall diverge, while the adsorption or coverage remains finite. The MHNC does not have convergent solutions in regions too close to coexistence so that it is hard to detect the very short prewetting line; however, the extrapolated behavior of the transverse correlations and coverage, which both show a diverging trend on approaching a well-defined temperature, and of the density profile whose features, far from the wall, faster and faster herald in the approach to coexistence, all point to the occurrence of complete wetting. Moreover, in agreement with DF results, transverse correlations also show a diverging trend inside the coexistence region while the coverage remains finite, thus confirming the existence of a surface spinodal line.