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, O,O'-bis(sodium 2-dodecylcarboxylate)-p-dibenzenediol (referred to as C-12 phi C-2(12)), has been synthesized. Its aggregation in aqueous solution has been studied using static and dynamic light scattering measurements. The homologue O,O'-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C-12 phi C-12) 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 C-12 phi C-2(12). However, C-12 phi C-2(12) showed rather normal aggregation behavior. Both the results of intrinsic viscosity and light scattering demonstrated a loose structure for the large aggregates of C-12 phi C-2(12). This behavior was attributed to an extending configuration of C-12 phi C-2(12) 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 H-1 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 theological measurements.