Unique Aggregation Behavior of a Carboxylate Gemini Surfactant with a Long Rigid Spacer in Aqueous Solution

A new gemini surfactant with a long and rigid spacer, <i>O</i>,<i>O</i>′-bis­(sodium 2-dodecylcarboxylate)-<i>p</i>-dibenzenediol (referred to as C₁₂ϕ₂C₁₂), has been synthesized. Its aggregation in aqueous solution has been studied using static and dynamic light scattering measurements. The homologue <i>O</i>,<i>O</i>′-bis­(sodium 2-dodecylcarboxylate)-<i>p</i>-benzenediol (C₁₂ϕC₁₂) whose spacer only contains a single phenyl group was also examined for comparison. Dynamic light scattering (DLS) revealed the unexpected existence of large aggregates in the solution of C₁₂ϕ₂C₁₂. However, C₁₂ϕC₁₂ showed rather normal aggregation behavior. Both the results of intrinsic viscosity and light scattering demonstrated a loose structure for the large aggregates of C₁₂ϕ₂C₁₂. This behavior was attributed to an extending configuration of C₁₂ϕ₂C₁₂ with the two alkyl tails stretching toward the solution due to the rigidity of the long spacer. The large network-like aggregate formation was an inevitable outcome of spontaneously reducing the energy of the system. Freeze-fracture transmission electron microscopy (FF-TEM) images and ¹H NMR measurements supported this speculation. Due to the columnar-like molecular geometry, the large network-like aggregates were directly transformed into rodlike micelles with increasing surfactant concentration. Depending on further micellar growth, the wormlike micelles were finally formed as confirmed by rheological measurements.