Light-scattering characterization of the wormlike micelles of hexaoxyethylene dodecyl C12E6 and hexaoxyethylene tetradecyl C14E6 ethers in dilute aqueous solution

The size, shape, and flexibility of hexaoxyethylene dodecyl ether C12E6 micelles and hexaoxyethylene tetradecyl ether C14E6 micelles in dilute aqueous solutions were determined at finite surfactant concentrations, c, by static (SI,S) and dynamic light scattering (DLS) experiments at several temperatures T below the critical points. The SLS results were successfully analyzed with the aid of the thermodynamic theory for SLS from micelle solutions formulated with the wormlike spherocylinder model. The analysis yielded the molar mass M-w of the micelle as a function of c and the cross-sectional diameter d of 2.3 nm for the C12E6 micelles and 2.4 nm for the C14E6 micelles. It has been found that micelles grow in size with increasing c and T, following the relation M-w proportional to c(1/2). The hydrodynamic radius R-H and the radius of gyration <S-2>(1/2) of the micelles as functions of M-w were found also to be well described by the corresponding theories for the wormlike spherocylinder and wormlike chain models, respectively. The results of the stiffness parameter lambda(-1) have shown that both micelles are far from rigid rods but rather stiff compared with typical flexible polymers. It has been revealed that the C14E6 micelles grow in size to a much greater length than the C12E6 micelles, presumably because of the thermodynamic reason that attractive interactions among C14E6 molecules are much stronger than those among C12E6 molecules as embodied by the values of the free-energy parameter g(2), reflecting the difference in the hydrophobic chain length. The values of d and the spacings s between two adjacent hexaoxyethylene chains on the micellar surface were found to be substantially the same for both of the micelles.