Temperature Dependence of the Liquid-Vapor Isotopic Fractionation Factors in CD₃H-CH₄ and CD₃F-CH₃F Mixtures

Abstract

zum 60. Geburtstag gewidmetThe liquid-vapor isotope fractionation factors, LVIFF, a = CD3X/CH3X)j/(CD3X/CH3X)v, of CD3H-CH4 and CD3F CH3F mixtures have been measured from 102 160 K and 182 254 K, respectively. The results are in good agreement with previous vapor pressure ratios, VPIE, which cover the ranges 90 112 K and 132 211 K, respectively. Over the temperature ranges of the LVIFF measurements, lna is negative for the CD3H-CH4 mixture and positive for the CD3F-CH3F mixture. There is significant curvature in a plot of T2 In a vs. T for the CD3H-CH4 mixture. The curvature is related to the densities of the liquid and vapor phases. It is shown that an expansion of the pair correlation function up to powers of the square of the densities of the liquid and vapor phases accounts for the T2 In a vs. T curvature and leads to In a equal to zero at the critical temperature. Correction of the LVIFF values of T2 In a for the density effect gives values for the contributions of translation, rotation and internal vibration in good agreement with previous cell model calculations. Similar treatment of the LVIFF data on the CD3F-CH3F mixture leads to the conclusion that the internal zero point energy effect on the differences in the heats of vaporization of CH3F and CD3F makes a small contribution to the overall difference in free energies of vapor ization.