The velocity of sound and heat capacity in methane from near-critical to subcritical conditions and equation-of-state implications

Abstract

Values of the velocity of sound and derived values of the constant volume heat capacity in methane were determined as functions of temperature and pressure along the phase boundaries, at 298.15 K, and at densities from 0–11.6 mol dm⁻³ for temperatures from 113.5–193.05 K. Relative values of the density were also determined. Velocity dispersion arising from vibrational relaxation, relaxation near the critical point, and relaxation near the triple point were characterized. The results were tested by use of extended thermodynamic scaling representations for the chemical potential μ. Within the limits of possible computational errors, the results indicate that along the critical isochore, (∂²μ/∂T²)_p is finite, nonzero, and has a jump discontinuity at the critical temperature which implies that ε=2, (β+α) = (1/2), and α? (1/7), where α, β, and ε are scaling exponents.