STRUCTURE OF ZWITTERIONIC SURFACTANT MICELLES - MICELLAR SIZE AND INTERMICELLAR INTERACTIONS

An elastic and quasielastic light-scattering study of micelles made of zwitterionic phosphobetaine surfactants dissolved in water has provided their size and information about intermicellar interactions, as a function of surfactant molecular structure. Attention was focused on the influence of the length of the intercharge polymethylene bridge linking the two opposite electrical charges of the zwitterionic headgroup. The micelles are small, the size of most of them allowing a spherical shape. As the length of the intercharge arm increases, micelles get smaller, which is consistent with the concomitant increase of dipole moment and bulkiness of the zwitterionic headgroup. For the short-intercharge-arm surfactant molecules, the behavior is consistent with that of monodisperse hard spheres, and an additional attractive potential is observed for molecules with longer intercharge arm length. For 1 compound with an intercharge arm of 10 methylenes, this intermicellar attraction is so strong that a demixion is observed upon heating (clouding phenomenon). The shape of the lower consolute boundary agrees with what is expected for a solution of small micelles attracting each other.