A Sponge Morphology in an Elementary Coacervate

A zwitterionic gemini surfactant forms a coacervate which is “elementary” in the sense that it consists of a single solute as opposed to the multicomponents (e.g., cetylpyridinium chloride/hexanol/water/NaCl) common in the coacervate literature. The gemini dissolves in water but then quickly separates as oily droplets which, despite the high water content of 83 wt %, are immiscible with water. Cryogenic temperature high-resolution scanning electron microscopy (cryo-HRSEM) examination of the droplets shows a distinct “sponge” structure. Although previously proposed for coacervate phases, a sponge morphology has never before been clearly depicted. The absence of previous electron microscopy (EM) pictures of the coacervate network is attributed to artifacts associated with transmission electron microscopy (TEM) methods, to the fragility of the coacervate toward physical perturbations, and to possible compositional changes with complex mixtures during sample preparation. When the gemini coacervate was exposed to mild shear, the honeycomb structure disappeared and was replaced by a lamellar phase.