Phase behavior and characterization of micellar and cubic phases in the nonionic surfactant C_〈17〉E_〈84〉/water system. A PFG NMR, SAXS, Cryo-TEM, and fluorescence study

An extensive study of the diffusion behavior of the C_〈17〉E_〈84〉/water system is presented. The surfactant, when mixed with water, forms a micellar phase below ≈13 wt % and a cubic phase between ≈20-60 wt % (at 25 °C). In addition to the pulsed field gradient (PFG) NMR technique (used to determine the self-diffusion coefficients), the system has been investigated by small-angle X-ray scattering (SAXS), cryotransmission electron microscopy (cryo-TEM), and time-resolved fluorescence quenching (TRFQ). The Self-diffusion coefficient (D) and the transverse relaxation time (T₂) of the surfactant molecules decrease significantly when going from the micellar to the cubic phase. The results of the PFG NMR, SAXS, cryo-TEM, and TRFQ experiments show that the cubic phase is composed of discrete aggregates. On the basis of the rapid transverse ¹H NMR relaxation in the cubic phase, it is argued that the micellar building blocks of the cubic phase are nonspherical. Furthermore, the SAXS data for four different concentrations of the surfactant (25, 35, 45, and 55 wt %) in the cubic phase can be indexed to the space group Im3m. If the data from the NMR and the SAXS measurements are combined, a lifetime of the surfactant monomers in the micelles of 8 and 7 ms was obtained at 25 and 35 wt % C_〈17〉E_〈84〉, respectively.