Direct Spectroscopic Evidence for Charge-Assisted Hydrogen-Bond Formation between Ionizable Organic Chemicals and Carbonaceous Materials

The direct evidence for the formation of charge-assisted hydrogen bond (CAHB) between the charged groups of ionizable organic chemicals (IOCs) and carbonaceous materials with similar proton affinity remains elusive. We therefore selected three pharmaceutical contaminants (PCs) as representative IOCs to provide the direct evidence of CAHB formation between IOCs and functionalized carbon nanotubes (CNTs) and its intensity/contribution to PC sorption on CNTs by NMR, FTIR, and DFT analyses. Sorption of PCs on functionalized CNTs resulted in the FTIR characteristic peak that appeared at a higher frequency (3787 cm^-1) and the ¹H NMR characteristic peak that emerged at an extremely low-field region (<18.0 ppm), which can be used as the direct spectroscopic evidence for CAHB formation. Both homonuclear CAHB (HM-CAHB, e.g., [O-H···O]^-) and heteronuclear CAHB (HT-CAHB, e.g., [N⁺-H···O^-]/[O-H···N]⁺) exhibited a much higher sorption energy (|E_ads| ≥ 56.24 kJ/mol) than ordinary hydrogen bond (OHB, |E_ads| ≤ 6.136 kJ/mol), leading to a greater sorption contribution (HM-/HT-CAHB ≥ 42.3%, OHB ≤ 36.5%) and irreversibility (hysteresis index: HM-/HT-CAHB ≥ 1.69, OHB ≤ 0.43) of PCs on CNTs. This work presents the direct evidence for CAHB formation between IOCs and CNTs, which is significant for understanding and predicting the environmental fate and risk of IOCs, thus providing new insights for controlling their pollution using specifically designed carbonaceous materials.