Isooriented Lyotropic Lamellar Phase in the C12E6/D2O System.

Aq. solns. of a nonionic surfactant, n-dodecylhexaoxyethylene glycol [C12H25(OCH2CH2)6OH], termed C12E6, were studied in the mole ratio range W0 = 5.6-14.9, where W0 = [D2O]/[C12E6]. There, below a given temp., a smectic lamellar phase exists. As shown by 1H and 2H NMR spectra, cooling induces the isotropic phase to become an almost completely isooriented lamellar phase when the magnetic field strength is high (14.1 T). In presence of high magnetic fields, the phase diagram is slightly different with respect to the originally reported one, and the coexistence of an isotropic phase and of the isooriented lamellar one can be obsd. In a wide temp. range, only the isooriented lamellar phase is present; it is stable out of the magnetic field for several months. A careful study of the 2H splitting as a function of temp. and concn. indicates the onset of fluctuating cylinder inside the lamellar phase and defines their onset and region of existence. Optical polarizing microscopy expts. confirm the above observation, showing the occurrence of fine grained, mosaic optical textures.