Competitive Sorption Used To Probe Strong Hydrogen Bonding Sites for Weak Organic Acids on Carbon Nanotubes
Li, Xiaoyun | Gámiz, Beatriz | Wang, Yiquan | Pignatello, Joseph J. | Xing, Baoshan
We recently proposed that weak acids (AH) adsorb to partially oxidized carbonaceous materials in part by forming strong hydrogen bonds with acidic surface groups, depicted by (A···H···O–surf)⁻, known as negative charge-assisted hydrogen bonds, (−)CAHBs. Here we use competition experiments to show that sorption of AH on carbon nanotubes (CNTs) can be described conceptually by a dual specific/nonspecific domain model, where one domain involves (−)CAHB sites that can become saturated. The trends observed in single-solute adsorption, including the stoichiometric release of hydroxide upon sorption of carboxyate or phenolate anions, were consistent with trends in the previous studies and pointed to the formation of (−)CAHB. 3,4-Dinitrophenolate formed (−)CAHBs more efficiently than did 2,6-dichloro-4-nitrophenolate because of alleviation of steric hindrance to approach by the ortho chlorines. Competition against a (−)CAHB-capable target compound was greater when the competitor was also (−)CAHB-capable than when it was not (e.g., benzoate as target vs 3,4-dinitrophenolate or nitrobenzene as competitor; mono-n-butyl phthalate as target vs methyl benzoate or p-tolyl acetate as competitor). Experiments also revealed competition between the nitroaromatic species for π–π electron donor–acceptor sites. The findings will contribute to a better understanding of the adsorption mechanism of ionizable compounds on carbonaceous materials.
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