Interaction of small ions with the micellar surface of an ultralong chain zwitterionic surfactant

In earlier work a detailed examination has been made of the structure of micelles formed from the ultralong chain zwitterionic surfactant (eicosyldimethylammonio)hexanoate. In the present work the behavior of micelles has been investigated in D2O solutions of electrolytes. The electrolytes used included sodium chloride, potassium bromide, potassium iodide, and sodium sulfate. As the electrolyte concentration increased, the micellar surface potential increased, leading to strong electrostatic interactions between the micelles. The latter depended on the anion used and indicated that binding to the micellar surface occurred in the order I- > Br- > Cl-. However, sulfate ions appeared to have little affinity for the micelle surface. Electrostatic interaction also increased with decrease of pH until at a pH below ca. 3.5 precipitation of the cationic surfactant salt occurred.