Supercritical Phase Transitions at Very High Pressure Kevin M. Scholsky The Louis Laboratory, S. C. Johnson & Son, Inc., Racine, WI 53403
In recent years numerous articles have appeared in the literature describing both the general behavior of supercritical fluids as well as a multitude of applications including separation of complex mixtures, extraction, polymer, and biomass processing, and others. Often such discussions ( 1 4 ) begin with a pressure-temperature phase diagram as depicted in Figure 1. To those unfamiliar with phase diagrams of substances at high compression, Figure 1 would lead one to believe that above the critical temperature (T,) and pressure (PC),in what is commonly referred to as the supercritical fluid region, the suhstance exists only as a fluid phase. While this condition holds true for the majority of high pressure-temperature regions cited in the literature, there are cases at ultrahigh pressure where Figure 1is an inadequate representation. For example, Figure 2 shows a phase diagrain of a pure substance at ultrahigh compression (5).At sufficiently high pressures (greater than lo8 Pa) it can he seen that the freezine curve can rise uninterruoted into the su~ercriticalregion.'l'hus both solid and fluid phases can exist at temperatures and oressures exceeding P and T,..The reason for the existence of a liquid-solid ph&ehound&y above T,,in con-
trast with the liquid-gas situation, is due to the fact that the liquid and solid phases differ in symmetry, while the gas and liquid phases do not (6). Knowledge of such phase hehavior is important in certain ultrahigh pressure reactions such as those described by Anderson et al. (7),who polymerized various olefins at pressures as high as 6 GPa and temperatures as high as 350 ' C . They found that numerous polymers incapable of polymerizing under normal conditions showed significant conversion at these elevated temperatures and pressures. Interestingly their research indicated that below 200 ' C cyclopentene (T, = 233 ' C , PC= 4767 Wa) did not polymerize even at pressures as high as 6 GPa. At 200 'C a low yield of a low-molecular-weightmaterial was obtained. Additional conversion of monomer was ohsewed at 250 O C , hut good conversion was only obtained at 300-350 O C . Spectroscopic analysis of the polymer indicated the following structure:
-
These researchers indicated that the exact state of the monomer during the reaction was unknown. However, the pronounced enhancement in polymerization rate upon rais-
Supercritical Fluid
Liquid
/
:ritical Point
Temperature ("C)
Figure 1. Ressurs-temperature phase dlagram commonly depicted in discus slons about supercritical fluids.
Figure 2. Pressure-temperatwe p b s e diagram tor a substance at very high compression (5).
Volume 66 Number 12 December 1989
989
ing the temperature from 250 to 350 O C may have resulted from meatlv increased molecular motion caused by a supercriticil solikfluid phase transition of the monomh. similar solid-fluid transitions might also be expected for other types of chemical reactions performed a t ultrahigh pressure. In conclusion, the author felt it would he useful to clarify that under certain conditions more than one type of phase can exist in the supercritical region. The chemical reactivity of a substance in this region should be strongly influenced by the actual type of phase behavior which occurs.
990
Journal of Chemical Education
Literature Clted
::~,";,"~;hbwc,";~;~&;,~;$;~m.
1984.88. 882-887. 3. Ely, J. F.;Baker, J. K. 1983, NBS ~ e p o r order t NO. ~ ~ 8 4 . 1 6 1 8 9 PP. 2,~ 4. Paul, D. F.: Wire, W. 8.. Eds. The PIinciplea of CasEitroction; Mills& Boon: London, 1971. z LeNeindro, 6. I" n i g h . ~JQ,,s,J., t ~ ~ chemistry rind ~ i ~ ~ hvan~~ i ~d i kiR.; ~ d s ~;e i d e ~~ s: r d ~ ~ cHh O t . I I ~1987: ~ ~ P, 52. 6. BmY. R. S.: Ri-, S. A.: Ross, J. Phvsicol Chemistry: Wiley: New York, 19Q pp 862863. 7. A n d e ~ n B.C.:Hoover, . C.:vogi. o . ~ o e r o r n o ~ a c u 1979.12. ~es 222.233.
;
