PHASE-BEHAVIOR AND MICROSTRUCTURE OF POLYOXYETHYLENE TRISILOXANE SURFACTANTS IN AQUEOUS-SOLUTION

Small-angle X-ray and neutron scattering, wide-angle X-ray scattering, cryo-transmission electron microscopy, and video-enhanced optical mic roscopy are used in this work to determine the phase diagrams and micr ostructures of several trisiloxane polyoxyethylene surfactants (M(D'E( n))M) in water. Similar to the phase behavior of hydrocarbon polyoxyet hylene surfactants (C(m)E(n)), the phase behavior of the siloxane surf actants depends strongly on the size of the polyoxyethylene (E(n)) hea d group. As n rises from 5 to 8, 12, 16, and 18, the hydrophilicity in creases and the surfactant microstructures tend toward higher curvatur e structures. For example, in the comparable range of temperature and concentration in water, M(D'E,8) M forms hexagonally packed cylindrica l micelles and M(D'E8)M forms lamellar bilayers. Typical colloidal pha ses of hydrocarbon surfactants reported-the isotropic water-rich micel lar phase (L1), the isotropic surfactant-rich inverse micellar phase ( L2), as well as the normal lamellar and hexagonal liquid crystal phase s (L(alpha) and H-1)-are all found in the trisiloxane surfactant/water systems. Trisiloxane surfactants with shorter E(n) chains, such as M( D'E5)M and M(D'E8)M, in water form the isotropic sponge-like bicontinu ous phase (L3) SiMilar to that found in some C(m)E(n)/water binary sys tems. Surfactants with longer E(n) chains such as M(D'E12)M do not for m the L3 phase. Instead, the water-rich isotropic micellar phase (L1) dominates at low surfactant concentration. The phase behavior and micr ostructures reported in this paper also shed light on the ''supersprea ding'' behavior1,2 of M(D'E5)M and M(D'E8)M water dispersions.