HIGHLY ORDERED PARABOLIC FOCAL CONICS IN LYOTROPIC SYSTEMS

Parabolic focal conics develop at 10-20 wt % surfactant in the two-pha se lamellar-isotropic regions of various systems (SDS-hexanol-decane-w ater, alkylpolyglucoside APG600-fatty alcohols pentanol, hexanol, deca nol, dodecanol, tetradecanol, oleyl alcohol). Extended regular quadrat ic two-dimensional lattices are formed in microslides (0.05-0.4 mm thi ckness) when only a small amount (10-15 wt %) of the isotropic phase i s present. Five different focal planes can be visualized with the pola rization microscope. The midplane focus gives pictures which look like primitive two-dimensional lattices of equally sized vesicle droplets. The 1:1 mixtures give irregular pattern formation. Removal of the iso tropic phase by a 20 min centrifugation at 3000g leads to ordinary foc al conics. Parabolic focal conics seem to be formed when the isotropic phase and the coexisting lamellar phase have comparable compositions, and this leads to the assumption that both phases should be in a near -critical state. The formation of parabolic focal conics looks more li ke a spinodal demixing than a crystallization process with nucleation. The very regular parabolic focal conics are transient states which ch ange to mosaic textures, which look very similar to hexagonal fan text ures, whereas polycrystalline parabolic focal conics transform to ordi nary focal conics after several days or weeks.