Self-diffusion of gaseous carbon dioxide in the critical region

The self-diffusion coefficient, D11, of gaseous CO2 has been measured by monitoring the transport of C14O2 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 D11 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 CO2 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 the 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. (orig.)