Time-dependent phase behavior of 5CB-DDAB-water microemulsions monitored by means of dielectric spectroscopy.

The phase behavior of an inverse microemulsion composed of 4-cyano-4-n-pentylbiphenyl (5CB), didodecyl dimethyl ammonium bromide, and water was intensively studied recently, but controversy still remains on the existence of a "transparent nematic" (TN) phase. Previously, we studied the temperature-dependent dielectric behavior of this microemulsion [Z. Chen and R. Nozaki, J. Chem. Phys. 134, 034505 (2011)]. The results suggested that a superstructure composed of water droplets and confined 5CB molecules exists in the coexistence phase region, which may correspond to the notion of a TN phase. In this study, the time-dependent dielectric behaviors of this microemulsion in the isotropic, coexistence, and supercooled phase regions were investigated. In the coexistence and supercooled phase regions, the dielectric behaviors showed strong dependence on time, due to the hysteresis of phase transition and the upward diffusion of the water-droplet-rich isotropic phase. After the isotropic phase has been totally excluded out of the effective measurement area, the slow process was still observed in these two phase regions, which suggests that water droplets can stably exist in the liquid crystalline phase. A local microscopic phase evolution was observed in the coexistence phase region, which is similar to the temperature-induced phase evolution previously proposed by us. The dielectric behavior of the slow process during the phase evolution also suggests the existence of the superstructure. In addition, the mechanism of the slow process in different phase regions proposed in our previous study is also confirmed.