COMPARISON OF THE LIQUID-CRYSTAL PHASE-BEHAVIOR OF 4 TRISILOXANE SUPERWETTER SURFACTANTS

The patterns of aqueous phase behavior of four trisiloxane superwetter surfactants are generally similar, but there are also substantial differences. All four surfactants readily form bilayer aggregates including vesicles, sponge phase (L3), and lamellar liquid crystals. At low concentrations (in the range in which they are used as wetting agents) mixtures are turbid. Both lamellar phase and small particles of uncertain identity are present. Dilute dispersions of these trisiloxane surfactants in water exhibit an unusual macroscopic birefringence. Features of the phase behavior at higher concentration and temperature are similar to the behavior of alkyl ethoxylate organic surfactants. The small particles are the common feature of these four surfactants, indicating a possible link to superwetting. Several lines of evidence indicate that the small particles are bilayer microstructures, but cryo transmission electron microscopy has not yet imaged any recognizable bilayer structures. Changing the surfactant structure from branched to linear has little effect on either the wetting or the phase behavior, demonstrating that the ''T'' shape of the molecule is not responsible for superwetting. End-capping groups have a significantly greater impact on the phase behavior than does the structure of the siloxane hydrophobe; changing the end cap of the poly(oxyethylene) group from -OH to -OMe or -OAc shifts phase boundaries to substantially lower concentrations and temperatures. The end-cap effect can be attributed to modification of the attractive potential between the surfactant particles.