The vapor-liquid equilibrium of an ethanol-water solution held in small pores (pore size range, Aim; median pore size, 13.5 µ ) inside a porous sintered stainless steel plate was determined at 50 °C by using a static method. The vapor pressure of ethanol and of water held in these small pores was found to differ greatly from the values obtained from the Antoine equation. By a treatment of the experimental data, it was shown that the ratio of the vapor pressure of pure ethanol to that of pure water in the capillary plate is 3.7876. The ratio of the vapor pressure of pure ethanol and water as predicted from the Antoine equation is 2.3092. Thus, the relative volatility of ethanol and water held in the capillary plate is about 1.58 times the normal relative volatility in the bulk phase. Under the normal vapor-liquid equilibrium in the bulk phase, the value of the relative volatility of the ethanol-water system is unity at Xx = 0.9060 at 50 °C. In the experimental study of the same system in the capillary plate, the relative volatility does not become unity when Xx is less than 0.99. A further treatment of the experimental data by a method suggested by Hirata confirmed the absence of the formation of any azeotrope at Xx < 0.99. The consistency test and thermodynamic test were performed on the experimental data, showing that the data are very consistent and satisfy the rigorous thermodynamic relationships. These tests also displayed the absence of systematic errors in the experiment. In view of the experimental findings made in this study, it is strongly recommended that further experimental studies be conducted on the vapor-liquid equilibrium in small capillaries, for both ideal and nonideal liquid solutions. With the advent of capillary distillation,3,10 these data will be important for the understanding and implementation of the new technique to separate nonideal azeotropic mixtures.

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