Physical Properties of Fluid CH4 and CD4: Theory

Abstract

Experimental data for the differences in the physical properties of CH₄ and CD₄ are subjected to a theoretical analysis. The theory is presented in a form which represents the total effects as sums of terms which are due to differences in the intermolecular potential functions plus other terms due to quantum effects on the translational, rotational, and (internal) vibrational degrees of freedom of the molecules. It is shown that the differences in liquid molar volumes, isothermal compressibilities, and critical constants are almost entirely accounted for if it is assumed that the ${\mathrm{CD}}_4$ molecule is 0.4% smaller than CH₄; that the depth of the intermolecular potential well is 0.9% smaller for CD₄ than for CH₄ and that other contributions to the net changes in these properties are small. The vapor‐pressure differences are due only in part to the differences in potential function; in addition, the various quantum contributions to this property are estimated, and it is concluded that the translational and vibrational effects are important for the CH₄–CD₄ pair but that the rotational quantum effect can be neglected. It is shown that the experimental vapor pressures and heat of vaporization differences can be accounted for if the vibrational zero‐point energy change upon vaporization is chosen to fit the data at one point.