Phase equilibrium and dynamics in mixture of 5CB/dimethyl phthalate isotropic one-phase state

Phase equilibrium and dynamics were examined for a mixture of 5CB with dilute dimethyl phthalate (DMP; w_DMP = 3.1 wt%). The mixture was in an isotropic one-phase state at temperatures T above a critical temperature T_IN (~ 27.0°C). This T_IN was described well by a simple model of free energy contributed from a Flory-Huggins type mixing entropy and a Landau-de Gennes type nematic interaction, suggesting that the phase separation in the mixture was triggered by the nematic transition of 5CB and thus the orientation fluctuation of 5CB molecules is coupled with the composition fluctuation. This coupling affected the dielectric relaxation time τ_ε reflecting the orientation fluctuation of 5CB molecules: In a high-T asymptote (T > T_IN +10°C), τ_ε of the mixture was close to that of pure 5CB, which suggested no significant effect of the above coupling on the 5CB dynamics in the mixture at such high T. However, in a significantly wide range of T between T_IN and T_IN +10°C where the mixture was still in the isotropic one-phase state, τ_ε increased on cooling much more significantly compared to τ_ε in that high-T asymptote. The kinematic viscosity ν of the mixture exhibited a qualitatively similar increase in the same range of T, but this increase was weaker than that of τ_ε. This difference between τ_ε and ν is discussed on site in relation to the coupling of the orientation and the composition fluctuations.