CO2 Capture by Hyperbranched Poly(alkylene imine) Thin Films: A Molecular Dynamics Simulation Approach

Direct air capture (DAC) of CO2 using solid amine sorbents requires a molecular-level understanding of interfacial phenomena under ultradilute conditions. In this study, we employ density functional theory (DFT)-calibrated force fields and molecular dynamics (MD) simulations to investigate CO2 capture behavior in thin films of hyperbranched poly(ethylene imine) (HB-PEI) and poly(propylene imine) (HB-PPI) using explicit slab geometries. The inclusion of a polymer-air interface induces significant structural changes, increasing the fractional free volume (FFV) from 19.1% to 25.0% for HB-PEI and from 19.9% to 29.9% for HB-PPI, along with increases in specific surface area to 0.29 and 0.34 Å-1, respectively. Despite these structural enhancements in HB-PPI, transport analysis reveals that HB-PEI consistently exhibits higher CO2 diffusivity, with diffusion coefficients decreasing from 0.937 × 10-5 to 0.049 × 10-5 cm2/s as CO2 and H2O loading increases, whereas HB-PPI shows lower values, ranging from 0.321 × 10-5 to 0.014 × 10-5 cm2/s. This indicates that CO2 transport is governed by polymer-gas interactions and segmental dynamics rather than free volume alone. Coordination number analysis further shows that HB-PEI exhibits strong CO2 affinity, with primary amine coordination decreasing from 0.655 to 0.464 under hydration, while HB-PPI displays an increase from 0.600 to 0.716, reflecting distinct interaction mechanisms. Additionally, pair correlation analysis reveals preferential CO2 binding to secondary amines in HB-PEI and primary amines in HB-PPI. Hydration introduces a critical limitation by forming hydrogen-bonded networks and competing for amine sites, leading to up to ∼95% reduction in CO2 diffusivity. These results demonstrate that interfacial heterogeneity, amine distribution, and hydration effects collectively govern CO2 capture behavior, underscoring the necessity of incorporating interfacial models for predictive design of DAC materials.