New insight into adsorption mechanism of ionizable compounds on carbon nanotubes

We studied the pH-dependent adsorption of benzoic acid (BA), phthalic acid (PA), and 2,6-dichloro-4-nitrophenol (DCNP) by hydroxylated, carboxylated, and graphitized carbon nanotubes (CNTs). Adsorption is contributed by formation of a negative charge-assisted H-bond (-)CAHB between a carboxyl group on the solute and a phenolate or carboxylate group on the surface having a comparable pKa. This exceptionally strong H-bond is depicted as (RCO2···H···O-CNTs)(-). Over a limited pH range the free anion undergoes proton exchange with water concurrent with adsorption, releasing hydroxide ion in a stoichiometry of up to 1.0 for BA, 1.7 for PA, and 0.5 for DCNP. Little hydroxide is released upon adsorption by the O-sparse graphitized CNTs. Anion exchange and ligand exchange reactions as a source of hydroxide release were ruled out. The higher stoichiometry for PA indicates involvement of both carboxyl groups with adjacent surface oxyl groups. The lower stoichiometry for DCNP is consistent with steric inhibition of H-bonding by the ortho chlorines. Formation of (-)CAHB helps overcome the unfavorable free energy of proton exchange with water, and results in an upward shift in the pKa in the adsorbed state compared to the dissolved state from 0.9 to 3.1 units. The proposed mechanism is further supported by additional structure-activity considerations. The findings provide new understanding of the interactions between ionizable organic compounds and carbonaceous surfaces, which has implications for noncovalent derivatization of CNTs, fate of ionizable pollutants, and associations of natural organic matter with CNTs and other carbonaceous materials in the environment.