Resiliently Spherical Micelles of Alkyltrimethylammonium Surfactants with Multivalent, Hydrolyzable Counterions

A series of C12–C16 alkyltrimethylammonium surfactants with hydrolyzable phosphate (PO4 3–, HPO4 2–, and H2PO4 –), oxalate (HC2O4 – and C2O4 2–), and carbonate (HCO3 – and CO3 2–) counterions have been prepared, and their micellar solution behavior has been characterized. Critical micelle concentrations were measured using electrical conductivity and were found to depend on both the counterion and its hydrolysis state. All monovalent counterions bind less strongly to the micelle surface than does bromide or chloride, whereas multivalent species bind more strongly. Small-angle neutron scattering reveals that, unlike alkyltrimethylammonium bromides and chlorides, micelles are small and spherical in the presence of hydrolyzable counterions of all valences and remain spherical even in the presence of added electrolyte. This is consistent with the strong solvation of even strongly bound hydrolyzable counterions, which prevents the screening of repulsions between adjacent headgroups necessary for sphere–cylinder transformations. Salts of multivalent hydrolyzable counterions could thus be used to control the micelle structure in novel ways.