Toroidal micelles formed in viscoelastic aqueous solutions of a double-tailed surfactant with two quaternary ammonium head groups

Abstract

Innovation in surfactant structures is an effective way to prepare new soft materials with novel applications. In this study, we synthesized a double-tailed surfactant containing two quaternary ammonium head groups (Di-C12-N2). The Di-C12-N2 solution behavior was investigated by surface tension, fluorescence, rheology, and cryo-TEM methods. Although Di-C12-N2 contained a large double-tailed hydrophobic group, the solubility of Di-C12-N2 was ∼90 mmol L⁻¹ at 25 °C with a Krafft temperature of ∼1 °C. The increase in Di-C12-N2 concentration in the solutions led to the formation of various aggregates, including spherical micelles, worm-like micelles, multi-layered vesicles, and a rare type of small toroidal micelles. The two quaternary ammonium head groups in Di-C12-N2 led to strong electrostatic interactions between molecules, which was critical for the formation of toroidal micelles. Moreover, with an added NaCl concentration of 40 mmol L⁻¹, the viscosity of the 5 mmol L⁻¹Di-C12-N2 solution increased by ∼1000 times compared to the pure 5 mmol L⁻¹Di-C12-N2 solution, revealing the high sensitivity of the unique head groups to ionic strength. This study enriches the research on the self-assembly principles of surfactants and brings new potential applications for new soft materials.