Wormlike Micelles of Polyoxyethylene Dodecyl C12Ej and Heptaoxyethylene Alkyl CiE7 Ethers. Hydrophobic and Hydrophilic Chain Length Dependence of the Micellar Characteristics

The micelles formed with pentaoxyethylene C12E5 and heptaoxyethylene C12E7 dodecyl ethers and heptaoxyethylene tetradecyl ether C14E7 in dilute aqueous solutions were characterized by static (SLS) and dynamic light scattering (DLS) experiments at several temperatures T below the critical points. The SLS results were successfully analyzed by using a thermodynamic theory formulated with wormlike spherocylinder model for SLS of micelle solutions, thereby yielding the molar mass Mw of the micelles as a function of c along with the cross-sectional diameter d. The radius of gyration 〈S2〉1/2 and the hydrodynamic radius RH of the micelles as functions of Mw were found to be also well described by the corresponding theories for the wormlike spherocylinder or wormlike chain models. Combining the present and previous results for various CiEj (polyoxyethylene alkyl ether) micelles, effects of the hydrophobic (alkyl) i and hydrophilic (oxyethylene) chain length j on the micellar characteristics have been clarified as: (i) The micellar length L becomes larger for larger i and smaller j. (ii) The d values do not significantly vary with the values of i and j. (iii) The stiffness parameter λ−1 decreases with increasing i at fixed j and increases with increasing j at fixed i. (iv) The spacing s between adjacent oxyethylene tails of the surfactant molecule on the micellar surface gradually increases with increasing i and j.