The sorption of ionizable organic compounds to pyrogenic carbonaceous materials (char, soot, biochar, activated carbon, nanotubes, etc.) is central to their behavior in technological processes and applications and is important to their environment fate. This talk will describe a heretofore-overlooked interaction between weak acids (RCO2H, ArOH, ArSO2NHR') and carboxyl/hydroxyl groups on carbonaceous surfaces, the formation of an exceptionally-strong ('low barrier') hydrogen bond, that contributes to their sorption. The bond can be written: (-X���H���Y-)-, where X and Y are O or N atoms. The bond forms for pairs of interacting species where the difference in the aqueous pKa between the H-bond donor group (-XH = -X-  +  H+) and the H-bond acceptor group (-YH = -Y-  +  H+) is small.  The proton in the low-barrier H-bond is shared nearly equally, making the bond much stronger than ordinary H-bonds. The reaction is demonstrated to occur on biochar and functionalized carbon nanotube surfaces populated with carboxyl, aryloxyl and similar functional groups. The reaction is important for carboxylic acids, phenols with electron-withdrawing substituents, aryl sulfonamides and other compounds with pKa values between ~3 and ~9. Formation of the bond can result in an increase in the intrinsic pKa of the adsorbed species.

Formation of exceptionally strong "low-barrier" hydrogen bonds between weak acid adsorbates and carboxyl/hydroxyl groups on pyrogenic carbonaceous surfaces

248th National Meeting (2014)

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