Enhanced Surfactant Supramicellar Assembly by Hydrophobic Dopants

Abstract

The light scattering peak (LS peak) near the surfactant critical micelle concentration (cmc) demonstrates that low hydrophobic dopant levels can cause supramolecular assembly of surfactants into metastable structures much more massive than dopant-free micelles. These supramicellar assemblies (SA) exist over the entire LS peak region, which extends from above the cmc down to well below the cmc. Dodecanol (D) was the dopant, and sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (DTABr) were the surfactants in this work. Well above the cmc dopant is solubilized by micelles. The SA appear above the cmc, dependent on dopant level and solution ionic strength, as seen by an abrupt increase in LS. Hence, the micelles do not simply release their hydrophobic payload at the cmc; rather, the dopant causes a morphological change from normal micelles above the LS peak concentration regime into SA as the LS regime is entered from above the cmc. Below the cmc the LS peak has a cutoff concentration corresponding to the solubility limit of dodecanol in water, which can be termed a "critical supramicellar assembly concentration" (csac). A three-component model is proposed that self-consistently yields maximum micellar dopant loading, SA mass, and dopant solubility in solution. The constancy of SA molar mass under widely different ionic strength and dopant levels conditions is surprising and not currently understood.