Interplay between Electrostatic and Hydrophobic Interactions in the pH-Dependent Adsorption of Ibuprofen onto Acid-Functionalized Multiwalled Carbon Nanotubes

The pH-dependent adsorption of ibuprofen (IBP) onto acid-functionalized multiwalled carbon nanotubes (CNT-COOH) was investigated by batch adsorption experiments and quantum mechanics (QM) and molecular mechanics (MM) simulation. A force field was developed and applied for monitoring interactions between IBP and CNT-COOH. Our simulations relied on the behavior of the pH-sensitive CNT-COOH functional groups and IBP molecules in solution, which occur as neutral or deprotonated species. QM and MM calculations were performed to determine the pH-dependent binding energy (Ea(BE)(pH)), and molecular dynamics (MD) simulations were performed to calculate the number of IBP species that interact with the CNT, surfaces at different pH conditions. The adsorption experiments demonstrated the strong affinity of IBP for CNT-COOH in acidic conditions, but a much decreased adsorption under basic conditions. The QM and MM results agreed with the experiments and further demonstrated that the Ea(BE)(pH) values were attractive in acidic conditions, but repulsive in neutral and basic conditions. The MD results revealed that the largest number of IBP species interacted with CNT-COOH in acidic conditions, where hydrogen bonds were observed. In basic conditions, the deprotonated species of both the CNTs and IBP bind via sodium ion mediation. Hydrogen bonding and ion mediation accounted for a small fraction of the total binding, whereas hydrophobic interactions were responsible for most of the adsorption across the pH range. There is an increase in electrostatic repulsion as the system becomes more basic when the greatest numbers of negatively charged deprotonated groups on both the CNTs and IBP occur. Our study provides valuable insight into pH-dependent binding and will aid in the design of more efficient nanomaterials for water remediation and drug delivery.