Self‐Diffusion of Gaseous Carbon Dioxide in the Critical Region

Abstract

The self-diffusion coefficient, Dll, of gaseous C 0 2 has been measured by monitoring the transport of C i 4 0 2 along a horizontal capillary. The main aim was to study the behaviour of the coefficient in the critical region, but before doing this its variation with density was measured outside the region so as to establish a 'background' against which critical region behaviour could be compared. The non-critical variation of D l l with density has been found to conform closely to Enskog's classical dense-gas theory, a result which resolves disagreement between earlier tracer studies of CO, but appears to conflict with NMR spin-echo measurements, on other substances, which have found no density dependence. In the critical region a large positive divergence, similar to that found for thermal conductivity, has been observed. This again contrasts with the spin-echo studies, in which no critical anomaly whatsoever has been found. Because of the inherent experimental uncertainties the critical region findings, although interesting, are regarded as preliminary only, and in need of corroboration.Der Selbstdiffusionskoeffizient D , von gasfirmigem C 0 2 ist durch die Beobachtung des Transports von C1402 langs einer horizontalen Kapillaren gemessen worden. Das Hauptziel war das Studium des Verhaltens dieses Koefizienten im kritischen Bereich. Seine Abhangigkeit von der Dichte auBerhalb des kritischen Gebietes wurde vorher untersucht, um einen ,,background" zu bekommen, mit dem das Verhalten innerhalb des kritischen Gebietes verglichen werden konnte. Die nicht-kritische Anderung von D , , mit der Dichte geht mit Enskog's klassischer ,,dense-gas"-Theorie konform. Dieses Ergebnis zeigt Unterschiede zu friiheren Untersuchungen mit der Tracermethode und steht auBerdem im Gegensatz zu Ergebnissen von NMR-Spinecho-Untersuchungen an anderen Substanzen, bei denen keine Abhangigkeit von der Dichte gefunden wurde. Im kritischen Gebiet wurde eine groBe positive Divergenz, ahnlich der der Warmeleitfahigkeit, beobachtet. Dieses steht wiederum im Gegensatz zu Spinecho-Untersuchungen, bei denen keine kritische Anomalitat oder dergleichen gefunden wurde. Wegen der experimentellen Unsicherheiten werden die Ergebnisse, obwohl sie interessant sind, als vorlaufig betrachtet und bedurfen noch weiterer Bestatigung.

Self-Diffusion in Molecular Fluids and Noble Gases: Available Data

“…Equation is generally considered valid, and almost all the points at low density in the Supporting Information include this correction. ,,,,,− It has been applied even to dense fluids. ,− We have maintained this mass correction in the Supporting Information, except for the methane of Mueller and Cahill and the hydrogen isotopes of Amdur and Mason, , since the former authors converted their data to CH 4 instead to CD 4 and the others transformed the mixtures D 2 + T 2 and D 2 + TH to H 2 instead of to D 2 . In the case of liquids, the theoretical knowledge is lower, and although some of the cited references deal with the matter for benzene and cyclohexane − and a nuclear magnetic resonance study suggested that eq could be applicable too, no definitive relation has been accepted and the authors reported the values of 11 without correction.…”

“…Previous studies , have rejected the methane of Cini Castagnoli et al and the carbon dioxide of Duffield and Harris since these authors were the only ones to see an anomaly of the self-diffusion (similar to that of viscosity and thermal conductivity) in the vicinity of the critical point. Other researchers , have considered erroneous the methane and carbon dioxide of Drickamer and co-workers ,,, as well, but because their points were 50 % lower than the rest of the references.…”

Molecular dynamics simulations of diffusion and clustering along critical isotherms of medium-chain n-alkanes

“…(14). A typical plot of the correlation function I (k, t) over time is depicted in Figure 5 for nC 5 showing that the relaxation times of the three length scales are less than 2 × 10 −11 s.…”

“…Experimentally, Cini-Castagnoli et al 4 data from capillary tube measurements show that D s of methane decreases by (80 ± 20)% at the critical point. Duffield and Harris 5 data from a horizontal diffusion cell, where the critical temperature was approached isochorically, show a peak in D s a) Author to whom correspondence should be addressed. Electronic mail:…”

“…A sketch (not to scale) of the (a) atomic structure, (b) united atom (UA) model, and (c) anisotropic united atom (AUA) model for nC5 . In the UA model (b), the non-bonded interaction site is centered on the carbon nuclei r i , while in the AUA model (c), the interaction site is shifted from r i to R i by a distance δ whose magnitude is different for the methyl and methylene groups.…”