Molecular dynamics investigations of liquid–vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores

Abstract

Formaldehyde exposure has been associated with several human cancers, including leukemia and nasopharyngeal carcinoma, motivating the present investigation on the microscopic adsorption behaviors of formaldehyde in multi-component-mixture-filled micropores. Molecular dynamics (MD) simulation was used to investigate the liquid–vapor interaction and adsorption of formaldehyde, oxocarbons, and water in graphitic slit pores. The effects of the slit width, system temperature, concentration, and the constituent ratio of the mixture on the diffusion and adsorption properties are studied. As a result of interactions between the components, the z-directional self-diffusivity (D_z) in the mixture substantially decreased by about one order of magnitude as compared with that of pure (single-constituent) adsorbates. When the concentration exceeds a certain threshold, the D_z values dramatically decrease due to over-saturation inducing barriers to diffusion. The binding energy between the adsorbate and graphite at the first adsorption monolayer is calculated to be 3.99, 2.01, 3.49, and 2.67 kcal mol⁻¹ for CO₂, CO, CH₂O, and H₂O, respectively. These values agree well with those calculated using the density functional theory coupled cluster method and experimental results. A low solubility of CO₂ in water and water preferring to react with CH₂O, forming hydrated methanediol clusters, are observed. Because the cohesion in a hydrated methanediol cluster is much higher than the adhesion between clusters and the graphitic surface, the hydrated methanediol clusters were hydrophobic, exhibiting a large contact angle on graphite.