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Interfacial interactions of nonionic and ionic forms of poly(acrylic acid) and polyethyleneimine with activated biocarbon surface – molecular modeling
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DOI：10.1016/j.ijadhadh.2026.104348.png)
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Abstract
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Activated carbons derived from plant biomass have shown great potential as effective adsorbents for water-soluble polymers, due to their porous structure and surface functionalities. To understand the adsorption mechanisms at the molecular level, computational modelling was performed for aqueous solutions of two polymers: poly(acrylic acid) (PAA) and polyethyleneimine (PEI), occurring in their nonionic and ionic forms. The study involved modeling interactions occurring both between the oligomeric segments of the polymers and between these segments and a graphene-like surface modified with nitrogen and oxygen atoms, typical functionalities found on biomass-based activated carbon surfaces. The simulations were performed using the MOPAC2016 program and the semi-empirical PM6-D3H4 method, which, in addition to covalent interactions, also allows for the consideration of hydrogen bonds and much weaker dispersion forces between atoms. The presence of an aqueous environment containing PAA or PEI was modeled using the COSMO continuous polarization method (Conductive Screening Model). It was shown that the interfacial behavior of the nonionic form of PEI is determined only by the dispersion interactions, whereas in the case of the nonionic form of PAA, the hydrogen bonds can be additionally formed. The adsorption energy per segment for the nonionic forms of polymers is approximately -21 kJ/mol for PAA and -16 kJ/mol for PEI. The increase in ionization of both polymers with changes in the solution pH (for PAA – with increasing pH and for PEI – with decreasing pH) makes the adsorption energetically less favorable, with the adsorption energy changing from -102.1 and -74.9 kJ/mol for an uncharged system of PAA and PEI, respectively, to +48.8 kJ/mol for PAA5- anion and +47.5 kJ/mol for PEI5+ cation.
Keywords:
activated carbon
poly(acrylic acid)
polyethyleneimine
adsorption mechanism
molecular modeling
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