Unusual aqueous-phase behavior of cationic amphiphiles with hydrogen-bonding headgroups

Two cationic surfactants with hydroxyl and carbamate hydrogen-bonding sites at their headgroups were synthesized. Both surfactants are ionic liquids (one of them at room temperature). Samples are isotropic solutions over the entire 0-100% concentration range, which is highly unusual for ionic surfactants. Surface tension, NMR, and conductivity measurements indicate classical micelle formation in aqueous solutions with CMCs below 10 mM. Pulse-gradient spin-echo (PGSE) NMR confirms micelle formation and provides micellar hydrodynamic radii of about 3.8 nm. Because this value is larger than the length of the extended surfactant molecules, about 2.7 nm, it appears that hydrogen-bonded water of hydration contributes substantially to the effective micelle size. At higher concentrations (above 25 wt %), surfactant solutions become viscous, but line broadening in the NMR is small relative to that found with a conventional cationic surfactant (CTAB). Thus, long rod formation, the source of line broadening in the latter, is absent with the new surfactants. Finally, PGSE NMR data show a 5-fold decrease in the diffusion coefficient between 5 and 20 wt %, above which the diffusion coefficients remain constant. The results are best explained by micelle clustering that is likely aided by intermicellar hydrogen bonding. The possibility of an isotropic liquid crystal (LC) phase with cubic symmetry is discussed and dismissed, demonstrating that LC formation of ionic surfactants at high concentrations, the usual behavior in past work, need not occur. Nor is there a definite connection between ionic liquid behavior and isotropic morphology.