Adsorption of volatiles on dust grains in protoplanetary disks

TL;DR

This work quantifies how volatile molecules adsorb on dust grains in protoplanetary disks using vdW-corrected DFT with $r^2$SCAN+$rVV10$ to compute adsorption energies for $H_2O$, $CO$, and $H_2$ on carbonaceous and silicate surfaces. It reveals a fundamental dichotomy: carbonaceous grains exhibit weak physisorption ($|\Delta \epsilon_{ad}|\sim0.1{-}0.2$ eV) while silicates chemisorb strongly ($|\Delta \epsilon_{ad}|\sim0.5{-}1.5$ eV) via coordination bonds; this drives divergent surface evolution as shown by kinetic Monte Carlo simulations. The study uncovers a CO–$H_2O$ cocrystal that significantly raises CO desorption temperature and shifts CO surface snowlines inward, contingent on multi-species adsorption and evolutionary history. These findings imply that inner-disk carbon depletion patterns, CO gas masses, and JWST-accessible chemistry depend sensitively on grain composition, thermal history, and surface coatings, highlighting the need for chemo-dynamical models that couple microscopic adsorption physics to disk evolution.

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 ($|Δε_{\rm ad}|\sim 0.1-0.2~{\rm eV}$) versus strong chemisorption on silicates ($|Δε_{\rm ad}|\sim 0.5-1.5~{\rm eV}$) 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 molecule 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.

Figures (5)

• Figure 1: Top-down views of the model carbonaceous substrates studied in this article, including graphene (upper panel) and amorphous carbon (lower panel). The supercells used in the DFT calculations are illustrated by black solid lines in each panel, respectively.
• Figure 2: Left column: Adsorption energy as functions of molecular orientations and distances to the substrates for $\mathrm{CO}$ (upper row), $\mathrm{H_2O}$ (middle row), and $\mathrm{H_2}$ (lower row) molecules over graphene substrates. Right column: Illustrations of the molecular orientation notations. The depicted molecules correspond to the energy curve panels in the left column.
• Figure 3: Example of $\mathrm{H_2O}$ single-molecule adsorption onto carbonaceous surface under various situations, placed above: (a) "doped" oxygen atom, (b) doped nitrogen atom, (c) hydrogen confined in the bulk substrate, and (d) surface not covered by hydrogen.
• Figure 4: Configurations of adsorbed $\mathrm{CO}$ molecules (upper row) and $\mathrm{H_2O}$ molecules (lower row) over different crystal surfaces (indicated above each column) of the model silicate $\mathrm{MgSiO_3}$. It should be noted that adsorption of $\mathrm{H_2O}$ over the $(100)$ surface results in molecular dissociation.
• Figure 5: Surface coverage conditions of grains in the mid-plane of a Solar-system-equivalent PPD model. Panel (a) illustrates the coating conditions of amorphous carbon grains given by converged KMC calculations, showing a sample surface with $64\times 64$ site (each site separated by $3~\text{\AA}$, the typical separation between adsorption sites). Panel (b) plots the surface occupancy conditions for amorphous carbon and the $(001)^+$ surface of $\mathrm{MgSiO_3}$.