Viscoelastic behavior of wormlike micellar solutions formed by an aspartame-based bicephalous anionic surfactant

Innovation in the molecular design of surfactants holds great potential for developing novel soft materials with unique properties. A surfactant with a long alkyl tail is expected to form giant aggregates with intriguing behavior. However, molecules with a large hydrophobic group often suffer from poor solubility in solutions, inhibiting the aggregation process. Herein, a new aspartame-based bicephalous anionic surfactant, disodium stearoyl-L-aspartyl-phenylalanine (C18-AP-2Na), has been synthesized. C18-AP-2Na showed excellent compatibility with the cationic surfactant cetyltrimethylammonium bromide (CTAB) and resulted in transparent viscoelastic mixed systems over a wide range of molar ratios and concentrations. Moreover, when CTAB and C18-AP-2Na were mixed with an equimolar charge ratio, the viscosity increased consistently from 61 mPa s (0.13 wt%) to an astonishing 14 000 Pa s (13.2 wt%). Cryo-TEM images revealed a network of extensively entangled wormlike micelles with cross-sectional diameters of 4-5 nm and lengths extending up to several micrometers at a C18-AP-2Na/CTAB molar ratio of 5 mM : 10 mM. The presence of long alkyl tails is the origin of wormlike micelle elongation. Different from other conventional anionic surfactants, C18-AP-2Na is distinguished by the amino acid unit near the head group. The extended molecular structure is not linear shaped. C18-AP-2Na is able to combine with CTAB molecules through electrostatic attractions while avoiding the too close contact of the alkyl tails. In this way, the formed ionic pairs remain hydrated in solutions instead of being precipitated. In addition, due to the strong attractions between the head groups of C18-AP-2Na and CTAB, the inorganic ions are ineffective to shield the head group charges. The viscosity of the mixed solutions remained nearly unchanged even with NaCl concentrations of up to 5%, demonstrating significant salt resistance. This work utilizes the advantages of the amino acid and develops stable cationic/anionic mixed solutions with strong viscoelasticity. The excellent compatibility as well as the strong salt resistance make the formulations promising for applications in oil recovery, cosmetic formulations, and the creation of smart materials. The self-assembly principles of surfactants demonstrated here also offer valuable insights for designing new viscoelastic systems and molecular structures.