Carbon dioxide at its critical state

The June 10, 1968 issue of Chemical & Engineering News published an excellent feature article on the Critical State. The authors were Jan V. Sengers of the University of Maryland, and Anneke L. Sengers of the National Bureau of Standards. The remarkable illustration shown here appeared on the front page of that issue. A slightly edited extract from the text describing the pictures is reproduced below.

S.K. Lower - Simon Fraser University

A fluid near its critical point exhibits anomalously large light scattering because of large fluctuations in the refractive index. The illustration shows a glass bulb constructed by E.F. Mueller and his coworkers at the National Bureau of Standards [now the NTIS]. It has been used since the 1940's to demonstrate critical opalescence.

The glass bulb is filled with carbon dioxide with an average density close to the critical density; the pressure is more than 1000 psi (7000 kPa). The cell contains three balls with densities smaller than, approximately equal to, and larger than the critical density. The positions of these balls indicate the density distribution within the fluid. The four successive stages correspond to states of temperatures from slightly above to well below the critical point. In the first stage, at far left, carbon dioxide is in the gaseous phase at a temperature somewhat higher than the critical. The scattered light is sufficiently intense to be visible, the beginning of critical opalescence. The density is still rather uniform, thus the three balls are well separated, and the ball with density closest to critical floats in the center.

When the temperature is lowered to just above critical, as in the second illustration, opalescence becomes very intense. Since the expansion coefficient is large, the density distribution is very sensitive to temperature gradients, thus the middle ball isn't in the center of the cell. As soon as the temperature passes through the critical temperature a meniscus develops separating the gaseous and the liquid phases. This meniscus is apparent in the third stage at the center of the middle ball. Since the density in the upper half of the cell corresponding to the gaseous state has decreased considerably, the lighter ball originally at the top of the cell has fallen to the meniscus. The local fluctuations of the density in both the gaseous and liquid phases are still very large and critical opalescence persists below the critical point.

In the last stage, shown at the far right, the temperature is well below critical. The density of the liquid state has increased, and the three balls are floating on the surface of the liquid. The fluctuations are now small, and the light scattering is no longer visible.

In the critical region, gases become so compressible that gravity alone can set up appreciable density gradients. For example, in carbon dioxide with a critical pressure of 1000 psi, a pressure of only 1 mm of mercury (1 part in 50,000) causes a density increase of more than 10% when the gas is at its critical point. Since the critical density of carbon dioxide is about half that of water, a density increase of 10% will alrady be present in a column of gas only an inch (2.5 cm) high, simply because of the weight of the gas. An experiment close to the critical point in a vessel of finite height averages over a whole collection of states of the system so that details of critical behavior are obscured.