Formation and disruption of viscoelastic wormlike micellar networks in the mixed surfactant systems of sucrose alkanoate and polyoxyethylene alkyl ether

Addition of lipophilic nonionic amphiphiles, such as short EO-chain polyoxyethylene dodecyl ether (C12EOn, n = 1-4) and monolaurin (ML), to the dilute micellar solution of sucrose hexadecanoate (P1695) induces a sharp increase in viscosity that is attributed to the one-dimensional micellar growth. With successive addition of the nonionics, the viscosity gradually decreases and, eventually, a phase separation occurs. In the maximum-viscosity region of the mixed systems, a viscoelastic solution with a rheological behavior typical of the interwoven network of wormlike micelles is obtained. The oscillatory shear behavior of the systems can be described by the Maxwellian model in the region of low shear frequency. The ability of the lipophilic nonionics to induce micellar growth in the mixed system decreases in the following order: ML approximate to C12EO1 approximate to C12EO2 < C12EO3 < C12EO4. A similar effect of the hydropilic group is observed in the sucrose dodecanoate (SM1200)/C16EOn aqueous system, although the micellar growth is comparatively less favorable, and a sharp increase in viscosity occurs near the boundary of the micellar phase. The microstructural change with surfactant mixing ratio in the representative P1695/C12EO4 system, based on the results of dynamic light scattering and rheological measurements, is discussed. A change from spherical to wormlike micelles, followed by branching and further separation of branching points as disklike micelles, precursors of the lamellar liquid crystal phase, is proposed.