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 microscopy are used in this work to determine the phase diagrams and microstructures of several trisiloxane polyoxyethylene surfactants (M(D'E(n))M) in water. Similar to the phase behavior of hydrocarbon polyoxyethylene surfactants (C(m)E(n)), the phase behavior of the siloxane surfactants depends strongly on the size of the polyoxyethylene (E(n)) head group. As n rises from 5 to 8, 12, 16, and 18, the hydrophilicity increases and the surfactant microstructures tend toward higher curvature structures. For example, in the comparable range of temperature and concentration in water, M(D'E,8) M forms hexagonally packed cylindrical micelles and M(D'E8)M forms lamellar bilayers. Typical colloidal phases of hydrocarbon surfactants reported-the isotropic water-rich micellar phase (L1), the isotropic surfactant-rich inverse micellar phase (L2), as well as the normal lamellar and hexagonal liquid crystal phases (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 bicontinuous phase (L3) SiMilar to that found in some C(m)E(n)/water binary systems. Surfactants with longer E(n) chains such as M(D'E12)M do not form the L3 phase. Instead, the water-rich isotropic micellar phase (L1) dominates at low surfactant concentration. The phase behavior and microstructures reported in this paper also shed light on the ''superspreading'' behavior1,2 of M(D'E5)M and M(D'E8)M water dispersions.