Improper orientational ordering regime and tricritical behavior in a lyotropic liquid crystal

Optical microscopy, deuteron quadrupolar NMR, x-ray diffraction, and electr ical conductivity techniques have been used to map the phase diagram and to measure the nematic order parameter variation (S) in ternary mixtures of p entadecafluorooctanoate acid ammonium salt (APFO), heavy water (D2O), and a mmonium chloride (NH4Cl). A phenomenological model with an isotropic parent phase and a single primary segregation order parameter is elaborated on to describe in consistent terms the linear dependence of S(T) through the who le nematic stability region as well as the existence of a tricritical point at the nematic-to-lamellar phase transition line. [S1063-651X(99)06708-2].