Comparative Studies on the Crystalline to Fluid Phase Transitions of Two Equimolar Cationic/Anionic Surfactant Mixtures Containing Dodecylsulfonate and Dodecylsulfate

In this work, a cationic surfactant, dodecyltrimethylammonium bromide (DTAB), and an anionic surfactant, sodium dodecylsulfonate (SDSO3) or sodium dodecylsulfate (SDSO4), were mixed in an equimolar ratio to prepare SDSO3-DTAB and SDSO4-DTAB binary mixtures. The phase behavior, structure, and morphology of these two surfactant mixtures were investigated by differential scanning calorimetry, synchrotron X-ray scattering, freeze-fracture electron microscopy, and Fourier transform infrared spectroscopy. It was found that upon heating, both of the two systems transform from multilamellar crystalline phase to liquid crystalline (or fluid) phase. It is interesting to find that, although SDSO3 has a lower molecular weight, the crystalline phase of SDSO3-DTAB shows much higher thermostability as compared with that of SDSO4-DTAB. Other than this, we observed a large difference in the repeat distances of the two crystalline phases. More interestingly, at 60 degrees C in the fluid phases, cylindrical micelles formed in the SDSO3-DTAB system, while spherical micelles were observed in the SDSO4-DTAB system. Our present work demonstrates that a subtle difference in the headgroup structure of the anionic component markedly affects the thermostability, packing structure, and morphology of the surfactant mixtures, which suggests the importance of the match of the head-head and tail-tail interactions between the cationic and anionic surfactants.