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 dif ferences. All four surfactants readily form bilayer aggregates includi ng vesicles, sponge phase (L3), and lamellar liquid crystals. At low c oncentrations (in the range in which they are used as wetting agents) mixtures are turbid. Both lamellar phase and small particles of uncert ain identity are present. Dilute dispersions of these trisiloxane surf actants in water exhibit an unusual macroscopic birefringence. Feature s of the phase behavior at higher concentration and temperature are si milar to the behavior of alkyl ethoxylate organic surfactants. The sma ll particles are the common feature of these four surfactants, indicat ing a possible link to superwetting. Several lines of evidence indicat e that the small particles are bilayer microstructures, but cryo trans mission electron microscopy has not yet imaged any recognizable bilaye r structures. Changing the surfactant structure from branched to linea r has little effect on either the wetting or the phase behavior, demon strating that the ''T'' shape of the molecule is not responsible for s uperwetting. 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 modificatio n of the attractive potential between the surfactant particles.