Pyrophosphate-Based Gemini Surfactants

Abstract

Two series of pyrophosphate-based gemini surfactants [diimidazolium (1) and disodium P,P'-dialkyl pyrophosphate (2) (a, R = 1-octyl; b, R = 1-dodecyl; c, R = 2-dodecyl)] were synthesized, along with a series of imidazolium monoalkyl phosphate surfactants (3a−c) for comparison. The surfactants were characterized by measurement of their Krafft temperatures (Tk) and critical aggregation concentrations (cac's). The Tk and cac values of the pyrophosphate surfactants are lower than those of their conventional phosphate surfactant counterparts. Aggregated surfactants 1 and 3 in D2O were characterized by 1H and 31P NMR spectroscopy, and solutions of 1c and 3c in H2O were characterized by phase-contrast optical microscopy and cryogenic high-resolution scanning electron microscopy (cryo-HRSEM). By optical microscopy, fiberlike aggregates were observed for gemini surfactant 1c, and both fibers and rod/tubule-like aggregates were observed for its single-chain counterpart 3c. By cryo-HRSEM, lamellar ice was observed for both 1c and 3c. Also, a single-crystal X-ray diffraction study of 1b was performed, and its cleavable nature was demonstrated.