Counterion specificity in the phase behavior of tetradecyldimethylamine oxides at different degrees of protonation

Effects of counterion species on the aggregate structure of tetradecyldimethylamine oxide (C14DMAO) accompanying the protonation were examined by viscoelastic properties and phase behavior observations. Different extents of the synergetic behavior between protonated and non-protonated amine oxides were observed depending on the counterion species, which follow the Hofmeister, or the lyotropic, series. The efficacy of monovalent anions with respect to the increasing the surfactant packing parameter was as follows: salicylate > perchlorate > nitrate > bromide > chloride, formate. Vesicle formation was found in the case of salicylate and perchlorate anions and elongation of rodlike micelles was observed for nitrate, bromide, sulfate, tartronate and tartrate anions but little effects in the case of chloride and formate anions. The observed ionic specificity is well correlated with the free energy of hydration of counterions.

Graphical abstractNote that a 20% protonation with HCl on a 100 mM C14DMAO solution leads to a decrease of the viscosity while the same protonation with HSCN leads to an increase of the viscosity by almost a factor of 1000.Figure optionsDownload full-size imageDownload as PowerPoint slide