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 (E
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 E
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.