The Nearly Classical Behavior of a Pure Fluid on the Critical Isochore Very Near the Critical Point Under the Influence of Gravity

Comprehensive measurements of the isothermal compressibility along the critical isochore in the critical region of pure sulfur hexafluoride (SF6) and pure carbon dioxide (CO2) were carried out. All measurements were performed with a multicell apparatus, especially designed for ppT measurements in the critical region. The height of the measuring cells was either 30 or 11 mm. Independent of the cell height, we found for both fluids nearly "classical" values for the critical exponent y in the limiting approach to the critical point. However, at a certain distance from the critical point {( T- Tc) ~ 90 mK or t ~ 2.82 x 10- 4 for SF6 and (T- Tc) ~ 55 mK or r ~1.81 x 10- 4 for CO2}, we observed a transition to values of the critical exponents which nearly meet the predictions of the renormalization-group theory. Since in the fitting range ( T — T c > 1 mK) the correlation length (Z < 0.5 um) is very much smaller than the geometrical dimensions of the measuring cells, we conclude that the reason for the different behavior is an explicit gravity effect governing the inner critical region. The two different loci of the transition points for sulfur hexafluoride and carbon dioxide can be attributed to the different gravity impact on the fluid corresponding to the different critical densities of the two substances.