Specific F− binding to phenyl ring of aromatic polymers

Most of the hydrogels deswell more remarkably in F− containing solutions than in other monovalent anion containing solutions. However, significant deswelling followed by abnormal reswelling of polymer gel in KF solutions with increasing F− concentration was observed in a series of polymer gels consisted of phenyl rings, for instance, poly(styrene sulfonic acid) (PSSA), hydroxypropyl methylcellulose phthalate (HPMCP) and poly(4-vinyl phenol) (P4VPh) gel. Driving force of this phenomenon was studied to reveal the specific interactions involved in the aqueous systems of aromatic polymers. Elemental analysis and XPS results suggest that F− is embedded to the gel by the physical adsorption of KF, as well as the interactions between phenyl ring and F−. Further theoretical calculations revealed that the interaction may be (phenyl)CH⋯F−(H2O)n interaction, which is stronger than (phenyl)CH⋯(H2O)n hydrogen bond. This kind of interaction decreases with the increasing water number and it is invalid when the surrounding water number is more than 5 for the phenol–F−(H2O)n system. Therefore, we conclude that F− could bind to phenyl ring via such (phenyl)CH⋯F−(H2O)n interaction in solutions with low hydrophilicity. The strong polarization effect of F− and (phenyl)CH⋯F−(H2O)n interaction are two important driving forces for the reswelling of gels.