Micelle Formation in Sodium Dodecyl Sulfate and Dodecyltrimethylammonium Bromide at Different Temperatures

In an aqueous medium, both pure and mixed surfactants form micelles after reaching a concentration called the critical micellar concentration (CMC), whose determination has considerable practical importance normally to understand the self-organizing behaviors of surfactants in exact and detailed ways. Many studies have been devoted to the elucidation of micellar structures under different conditions1−5. Some early investigators observed fairly abrupt changes in a number of physicochemical properties at or near the CMC and concluded that micelle formation had at least some of the properties of a phase change. A phase-separation or two-phase model was often used to describe the thermodynamics of micelle formation6−8. The two-phase model predicts a homogeneous monodisperse micellar phase and a constant monomer concentration above the CMC. Careful experimental measurements using highly purified systems revealed that somewhat gradual and continuous changes in physicochemical properties occurred near the CMC10−13, that micelles appeared to be polydisperse, and that monomer activities changed above the CMC. For example, gradual and continuous decreases in the equivalent conductance of sodium dodecyl sulfate (SDS) in water and changes in the heat of micelle formation for alkyl carboxylates in water