Effects of hydrophobic chain length on the micelles of heptaoxyethylene hexadecyl C16E7 and octadecyl C18E7 ethers

Abstract

Micelles of heptaoxyethylene hexadecyl C16E7 and octadecyl C18E7 ethers in dilute aqueous solutions were characterized at finite surfactant concentrations c by static (SLS) and dynamic light scattering (DLS) experiments at several temperatures T below the critical points, in order to examine the effects of hydrophobic chain length on size, shape, structure, etc., of the micelles. The SLS results were successfully analyzed by the thermodynamic theory formulated with wormlike spherocylinder model for SLS of micelle solutions, thereby yielding the molar mass M w of the micelles as a function of c and the cross-sectional diameter d, indicating that the micelles assume a flexible spherocylindrical shape. The radius of gyration (S2)1/2 and hydrodynamic radius R H 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. The micelles grow in size with increasing T to greater length for longer hydrophobic chain, i.e., alkyl group of the surfactants. They were rather flexible compared to the micelles formed with other polyoxyethylen alkyl ethers. The cross-sectional diameter d and spacings s between adjacent hexaoxyethylene chains on the micellar surface were similar in the C 16E7 and C18E7 micelles.