Nuclear Magnetic Resonance and Small-Angle Neutron Scattering Studies of Anionic Surfactants with Macrocounterions: Tetramethylammonium Dodecyl Sulfate

Abstract

Micellar solutions of tetramethylammonium dodecyl sulfate have been studied to determine the degree of counterion binding. Tetramethylammonium chloride was added over a wide range of surfactant concentrations such that the total concentration of tetramethylammonium ions in solution remained constant. Small angle neutron scattering experiments showed a constancy in aggregation number across this series, consistent with the constant C_aq concept of Bales et al. (J. Phys. Chem. B2001, 105, 6798). Pulsed-field gradient and electrophoretic NMR experiments were used to determine the degree of counterion dissociation, α, which was found to be 0.33. This value is in contrast to the value from conductivity measurements (α = 0.2), but supports the concept of an aggregation number based definition of α.