Refractivity virial coefficients of gaseous CH4, C2H4, C2H6, CO2, SF6, H2, N2, He, and Ar

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Abstract
Accurate values of the second and third refractivity virial coefficients BR and CR of gaseous CH4, C2H4, C2H6, CO2, SF6, H2, N2, He (only BR is given), and Ar have been measured using the recently improved differential‐interferometric technique. This device basically consists of two coupled grating interferometers. During the overflow experiment of the gas one interferometer, with two similar cells in series, measures differentially the higher‐order effects of density, and the second interferometer, with two similar cells in parallel, measures at the same time the absolute value of the refractive index. The second interferometer is also used for an accurate control of the decompression during the overflow experiment. It is thus possible to extend the overflow experiments to high values of pressure. The overflow experiments were carried out up to pressures of 35 MPa. The density range extends up to two times the critical density. The third refractivity virial coefficient CR becomes significant for all the gases examined, except for He, when the measurements approach approximately the value 0.3 of the reduced density ρ/ρc. The fourth refractivity virial coefficient DR becomes significant for C2H4, C2H6, and SF6 when ρ/ρc approach approximately the value of one. The agreement between the classically calculated values of BR, based on the dipole induced dipole model, and our experimental BR values is very good for CH4, C2H4, C2H6, SF6, and Ar, but poor for CO2, H2, N2, and He. For He however, the quantum mechanical calculation of BR obtained by including both the long‐range and the short‐range effects on the polarizability is in remarkably good agreement with our experimental BR value. Accurate values of the first refractivity virial coefficients AR were determined independently by making absolute measurements of the refractive index as a function of pressure.