Adsorption of Volatiles on Dust Grains in Protoplanetary Disks

Abstract The adsorption of volatile molecules onto dust grain surfaces fundamentally influences dust-related processes, including condensation of gas-phase molecules, dust coagulation, and planet formation in protoplanetary disks. Using advanced ab initio density functional theory with r 2 SCAN+rVV10 van der Waals functionals, we calculate adsorption energies of H 2 , H 2 O, and CO on carbonaceous (graphene, amorphous carbon) and silicate (MgSiO 3 ) surfaces. Results reveal fundamentally different adsorption mechanisms: weak physisorption on carbonaceous surfaces (∣Δ ϵ ad ∣ ∼ 0.1−0.2 eV) versus strong chemisorption on silicates (∣Δ ϵ ad ∣ ∼ 0.5−1.5 eV) via coordination bonds. Kinetic Monte Carlo simulations incorporating these energies demonstrate divergent surface evolution: carbonaceous grains exhibit distinct condensation radius compared to silicates, while the cocrystal of H 2 O and CO significantly increases the desorption temperature of CO. The actual radii of gas-phase molecular depletion could thus be a comprehensive result of temperatures, chemical compositions, and even evolution tracks. Meanwhile, silicates maintain chemisorbed molecular coatings throughout most disk regions. Such dichotomy in surface coverage could also provide a natural mechanism for carbon depletion in inner planetary systems.