Investigation Of The Deuterium Isotope Effects On The Critical Behaviour Of Binary Solutions

The deuterium isotope effects on the critical behaviour of liquid-liquid coexistence curves for binary solutions have been studied.The critical compositions and the critical temperatures of systems(2,6-dimethylpyridine+water/heavy water) and (2,6-dimethylpiperidine+water/heavy water) have been determined.The coexistence curves of(T,n)(T and n are temperature and refractive index,respectively) for all systems have been determined by measurements of refractive indices of coexisting phases.The coexistence curves of(T,n) were converted to the coexistence curves of (T,x) and(T,φ)(x andφare mole fraction and volume fraction,respectively) through the standard curves.The experimental data were analyzed to obtain the critical exponentsβ,the leading critical amplitudes B,the correction amplitudes B₁ and parameters for diametersρ_d of coexistence curves.It was found that when H₂O was substituted by D₂O in binary systems,the critical temperatures decreased by 5.9K and 3.2K for the system of 2,6-dimethylpyridine+water and of 2,6-dimethylpiperidine+ water,respectively,and the coexistence curves of the system of 2,6-dimethylpyridine +water changed more obviously comparing with the other one,but their critical compositions kept unchanged and the critical exponentsβof four systems were consistent with the three-dimensional Ising model(3D-Ising) value(0.327) near the critical point.The critical crossover behaviour of the system of 2,6-dimethylpyridine+heavy water also have been studied.The result showed that the effective critical exponentsβ_eff^sig exhibited nonmonotonic crossover from 3D-Ising to mean-field critical behaviour after the regular termατis subtracted,which was consistent with the system of 2,6-dimethylpyridine+water's crossover behaviour.