Temperature dependence of vapor/liquid interfacial tensions in a binary liquid mixture with a miscibility gap near its critical point: Determination of universal surface amplitude ratios

Abstract

The inverted pendant drop method (rising bubble method) is used to measure the temperature dependence of the surface tensions σ α,βγ , σ α,β and σ α,γ of a 2,6-dimethyl pyridine (abbreviated 2,6-DMP)/water mixture of critical composition in a temperature range −8.373 K < ( T − T c ) < +5.634K. The experiments are carried out near the lower liquid/liquid critical point of the system. α is the vapor phase and β,γ the homogeneous liquid phases. At temperatures above the liquid/liquid critical temperature a 2,6-DMP rich liquid phase β coexists with a water rich liquid phase γ. The temperature dependence of the liquid/liquid interfacial tension σ β, γ of that system is known. The surface tension data are used to calculate the material specific values of the critical amplitudes B α,βγ , B α,β and B α,γ of the corresponding surface tensions using four different fitting procedures. They differ in the number of free parameters of the fit. In three of the four fits the values of the critical amplitudes B α,β , B α,γ and B β,γ are consistent with Antonow's rule. The critical amplitudes are used to calculate the universal surface amplitude ratios Q, P +, P, P − which are theoretically expected to have material independent values. The signs of the calculated amplitude ratios agree with the theoretical predictions. In three of the four fits the sequence of the amplitude ratios ordered in accordance with their magnitude agrees with the corresponding sequence of the theoretical values. The experimental values are close to the theoretical values. Visual observations confirm the results of the measurements of the temperature dependence of the surface tensions: The α, γ interface is completely wet by phase β in the range of temperatures studied (( T − T c ) < 5.6 K). A wetting transition is not observed.