MICROSTRUCTURE AND RHEOLOGY OF LAMELLAR LIQUID-CRYSTALLINE PHASES

We have investigated the microstructure and rheological properties of ternary surfactant mixtures in a salt solution. The surfactants were 6 % sodium 4-dodecylbenzenesulfonate. 3% C13-16 ethoxylated alcohol with seven ethylene oxide (EO) units and 1% C13-15 ethoxylated alcohol wit h 2, 4, 7, 9, 11, 14, 20, or 25 EO units. The salt solution was 10% ni trilotriacetate.H2O. Microstructural investigations (electron microsco py, light microscopy, confocal laser microscopy, conductivity measurem ents, and centrifugation) show that at rest the samples containing the surfactant with 2 EO to 9 EO units are dispersions of lamellar drople ts (curved surfactant bilayers). The samples containing the surfactant with 11 EO to 25 EO units show a continuous lamellar structure (sheet s of surfactant bilayers) with a small amount of lamellar droplets pre sent. The change in several rheological parameters reflects this chang e in microstructure. The power law index from flow experiments at low shear rates changes from 0.1 for the lamellar dispersions to 0.4 for t he continuous lamellar phases. Similar changes are observed in shear m odulus and in the limiting strain for linear viscoelastic behavior. Th e continuous lamellar phase is converted to droplets by shearing at ra tes above 1 s(-1). The continuous lamellar structures will recover in about a week when the samples are allowed to relax. The nature of the droplets is highly dynamic. Confocal laser microscopy shows small fluc tuations in droplet shape on a time scale of about 100 s. This time co incides with a characteristic time of around 100 s pertaining to a (sh allow) peak in G ''.