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The critical behaviour of ethylene and hydrogen

University of British Columbia

Optical techniques have been used to study the behaviour of ethylene and hydrogen near their liquid-vapour critical points. From measurements of the coexistence curve of ethylene over the reduced temperature range 1.5 x 10⁻⁶ < t < 4.5 x 10⁻², where t — (Tc — T)/Tc and Tc is the critical temperature, we find the critical exponent β = 0.327±.002 and the corrections-to-scaling exponent ∆ = 0.46±.02. Similar measurements for hydrogen over the range 3.2 x 10⁻⁵ < t < 7.0 x 10⁻² give β = 0.326 ± .002 and ∆ = 0.46 ± .02. Measurements of the compressibility of hydrogen give the critical exponent [Formula Omitted] = 1.19 ± .05 and the critical amplitude ratio [Formula Omitted] = 5.2 ± .4. With the exception of ∆, which is slightly lower than its predicted value of 0.5, the results for these universal quantities are in agreement with theoretical predictions. The leading coexistence curve amplitude for hydrogen, B₀ = 1.19±.03, is lower than the corresponding values for ethylene, B₀ = 1.56 ± .03, and for other room-temperature fluids. This decrease is in qualitative agreement with the predictions of a theory of quantum effects on critical behaviour. Measurements of the coexistence curve diameter for both fluids show an anomaly near the critical point having a form consistent with the predicted t¹⁻α temperature dependence. These results are in agreement with a recent theory of the effects of many-body forces on the diameter; the hydrogen data indicate that these forces are attractive in that fluid. This suggests that quantum mechanical exchange interactions are important near the critical point of hydrogen.

Text

2010-09-24

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http://hdl.handle.net/2429/28661

Physics

University of British Columbia

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