X-ray-structure studies of lyotropic liquid crystals in the binary system hexadecyl dimethylethyl ammonium bromide/water

The phase diagram of the binary system hexadecyl dimethylethyl ammonium bro mide (C(16)Me(2)EABr)/water was fixed using polarizing microscopy. The lyot ropic mesophases were identified by texture observation. With increasing su rfactant concentrations the following isothermic polymorphisms were found: At T = 305 K: S <-> N-C <-> H-alpha <-> G <-> C and at T = 340 K: S <-> N-C <-> H-alpha <-> M-alpha <-> Q(alpha) <-> L-alpha <- > L-beta (S = isotropic micellar phase, N-C = rod-like lyotropic nematic phase, H-al pha = hexagonal phase chains fluid; M-alpha = two-dimensional monoclinic ph ase; Q(alpha) = cubic phase; L-alpha = lamellar phase, chains fluid; L-beta = lamellar phase, chains rigid; C = gel phase; C = crystalline phase). Studies of the N-C phase by X-ray diffraction led to the result that the av erage rod lengths of the micellar subunits in the N-C phase are not uniform . The average cylinder diameter has the magnitude of d(M) approximate to 54 Angstrom. In the H-alpha phase the cylinder diameters are dependent on the surfactant concentration. The Q(alpha) phase with the space group Pn3 is l ocated between the phase region of the N-C phase and the L-alpha phase. Thi s bicontinuous cubic phase consists of two separate rod-like labyrinths. The phase region of the lamellar phase is divided into two parts: the regio n of the L-alpha and that of L-beta phase. Mixtures of a surfactant concent ration of c(S)>94%w/w C(16)Me(2)EABr show the phase transition L-beta <-> L -alpha. The melting of the rigid chains of the L-beta phase was correlated with the decrease of intermicellar distances and of the area per surfactant molecule in the bilayer. The appearance of the G phase correlates with sha rp equidistance reflexes in the small angle region of the scattering curves .