X-ray transmission technique to study carbon dioxide in the critical region

摘要

An x-ray transmission technique, which utilized a diverging x-ray beam and a position sensitive detector, was used to determine density profiles for carbon dioxide in the critical region. This x-ray technique, which does not depend upon the determination of the dielectric constant, offered a means by which previous values of the critical exponent for the order parameter, the law of rectilinear diameter, and the isothermal compressibility for carbon dioxide could be verified. Results from this procedure were compared with the best available data obtained by optical methods. Results from the analysis obtained via this technique were in close agreement with values for the critical exponent and the scaling law for coexisting phases obtained by an optical method. However, analysis of the coexistence curve diameter showed a marked divergence from rectilinear behavior in the reduced temperature region t<10-4, which was not indicated by density measurements obtained by optical techniques. Due to the statistical error associated with the density profile data, it was impossible to confirm the predicted discontinuity of the isothermal compressibility across the coexistence curve.