Effects of hydrophobic chain length on the characteristics of the micelles of octaoxyethylene tetradecyl C14E8, hexadecyl C16E8, and octadecyl C18E8 ethers

Size, shape, and flexibility of micelles of octaoxyethylene tetradecyl C14E8, hexadecyl C16E8, and octadecyl C18E8 ethers in dilute aqueous solutions were determined at finite surfactant concentrations c by static light scattering (SLS) and dynamic light scattering experiments at several temperatures T below the critical points. The SLS results were successfully analyzed with the aid of the thermodynamic theory formulated with wormlike spherocylinder model for SLS of micelle solutions. The analysis yielded the molar mass M-w of the micelles as a function of c and the cross-sectional diameter d. The hydrodynamic radius R-H and the radius of gyration < S2 >(1/2) of the micelles as functions of M-w, were found to be also well-described by the corresponding theories for the wormlike spherocylinder or wormlike chain models. The results of the stiffness parameter lambda(-1) have revealed that the micelles are far from rigid rods but rather stiff compared with typical flexible polymers and they grow in size with increasing T to greater length for longer hydrophobic chains, i.e., alkyl groups of the surfactants. As the alkyl group becomes longer, the d value increased, while the spacings s between adjacent hexaoxyethylene chains on the micellar surface were found to remain substantially constant.