Molecular Anisotropy and Angular Redistribution in Layering Adsorption of Nitrogen in Model Carbon Slit Pores

Abstract

Molecular orientation is commonly averaged out in adsorption models, yet anisotropic adsorbates near regular interfaces can exhibit orientation-dependent stabilization at low coverage. Here we examine how the explicit diatomic representation of nitrogen affects layering adsorption at 77.4 K in ideal slit-shaped carbon pores (1.01–7.24 nm). Three descriptions are compared: grand canonical Monte Carlo (GCMC) with a rigid diatomic nitrogen model (D-GCMC), GCMC with a single-site spherical Lennard–Jones (LJ) model (LJ-GCMC), and isotropic weighted density functional theory (WDFT) based on the same LJ fluid. Adsorption isotherms, density profiles, orientational statistics, and apparent angular free-energy profiles were analyzed. Relative to spherical LJ nitrogen, diatomic nitrogen shows a shift of the first-layer onset to lower relative pressure (RP), together with broader layer-growth intervals. In a 7.24 nm slit pore, the onset occurs at RP = 1.7 × 10–6 for D-GCMC and 3.7 × 10–5 for LJ-GCMC. These differences are associated with near-wall orientation-dependent energetic stabilization at low coverage and subsequent angular redistribution during layer densification. The wall-induced angular energy anisotropy is strongest in the first layer, with a maximum bias of about 2.5 kJ·mol–1 (∼3.9 kBT), and becomes negligible in higher layers. Although isotropic WDFT neglects explicit molecular orientation, it approaches the D-GCMC behavior beyond the first layer, where surface-induced orientational constraints are weak and orientational effects are effectively averaged. These results show that explicit molecular anisotropy can systematically affect adsorption onset and layer-growth width at regular interfaces, while isotropic descriptions remain appropriate when orientational signatures are averaged in multilayer or heterogeneous regimes.