Ralph H. Petrucci Western Reserve University Cleveland, Ohio
A Vapor Phase Mechanism of Eutectic Fusion
For many years the results of phase rule studies have been used to solve practical problemproblems arising in the preparation, separation, and purification of chemical substances. A similar interest, it would seem, has not always been extended to some of the more fundamental aspects of heterogeneous equilibrium. The chemistry teacher becomes especially aware of the lack of a complete understanding of certain basic ideas when he is asked to answer such a question as, "How does a mixture of two solid substances, solids that are completely immiscible in each other in the solid state, commence to melt a t a temperature below the melting point of either pure substance?" The purpose of this article is to present one possible answer to this intriguing question through a vapor phase mechanism of eutectic fusion. General Description of the Vapor Phase Mechanism
A necessary condition for the establishment of equilibrium in a heterogeneous system is that matter be transported across phase boundaries to an extent such that the partial molar free energy of each component be equal in every phase in which that component is found. In the eutectic fusion process in a binary system, A-B, matter must be transported from the two pure solid phases, A and B, into a homogeneous liquid phase, the eutectic liquid. Let us consider how this transport can be achieved via the vapor phase. No matter how one chooses to carry out the eutectic fusion of a mixture of two solids, it is almost certain that a vapor phase will exist in equilibrium with the solid components, even if the volatility of the solids is extremely low. At temperatures below the eutectic value each solid vaporizes independently and a mixed vapor is produced. At the eutectic temperature this vapor phase comes into equilibrium with a liquid phase, the eutectic liquid. Thus the eutectic temperature is the lowest temperature a t which four-phase equilibrium, solid A+ solid B e vapor eutectic liquid, can exist in a binary system of the eutectic type. The first trace of eutectic can result from the phase reaction, solid A solid B vapor -+ eutectic liquid. Once the first trace of liquid has been produced the additional phase solid B eutectic liquid, can occur. reaction, solid A
-
+
+
-
+
Experimental Evidence
It is quite probable that many previous investigators have conceived of the role of the vapor phase in eutectic fusion; yet, there seems to he little discussion of this vapor phase mechanism, either speculative or based on experimental fact, in the published literature. In view of some of the lecture demonstrations they devised to illustrate the eutectic fusion process, perhaps
Kurnakov and Efremov (1) were the first persons to recognize the importance of the vapor phase in eutectic fusion. (A modification of one of their demonstrations is described in the next section.) In a t least two recent studies the vapor phase mechanism of eutectic fusion in a large number of binary and ternary organic systems has been studied and found to be of considerable importance. I n an invcstigation by Sorum and Durand (%) it was shown that two solids can be caused to melt a t their eutectic temperature even if the solids are not in physical catact. In a second investigation by Petmcci and Sorum (3) it was found that hquids obtained at the eutectic temperature by this "non-contact" fusion process-a process in which mixing of components occurs in the vapor phase and the first eutectic liquid is produced by condensation from the v a p o r 4 0 indeed possess the eutectic composition. Classroom Instruction
Through Demastr.ations: The process of %oncontact" fusion can be demonstrated as follows: a small beaker of camphor is placed in a desiccator; a watch glass containing somep-nitrophenol is supported above the beaker on a desiccator plate; and the desiccator is sealed. Within a few days' time the first droplets of liquid will be seen on the watch glass; ~vith increased time more liquid will appear. The eutectic temperature for the system, p-nitrophenol-camphor, is about -2'C. Thus if the desiccator is maintained a t room temperature, spontaneous melting ought to occur to produce a liquid phasd. Iu this demonstration the composition of the liquid phase is not necessarilv that of the eutectic but the manner in which it is produced is the same as outlined in the general description of the vapor phase mechanism, i.e., via the phase reaction, solid A solid B -t vapor liquid. Other pairs of substances can be used provided the desiccator is maintained a t a temperature above the eutectic temperature for the chosen system. Through Phase Models and Diagrams: Any attempt to illustrate the role of the vapor phase in eutectic fusion must take cognizance of solid, liquid, and vapor phases. The complete representation of phase behavior in a two component system of the eutectic type requires a three-dimensional model similar to those shown by Findlay, Campbell, and Smith (4) and Ricci (5). Although such three-dimensional diagrams can be drawn or constructed, perhaps the most convenient IT-ay to illustrate the vapor phase mechanism diagrammatically is to use a projection of the three-dimensional diagram onto the temperature-composition plane, i.e., a T/c projection of the P/T/c diagram. A temperature-
+
Volume 36, Number 12, December 1959
+
/
603
composition projection for a typical bmary eutectic system is shown in the figure. I n this diagram, 0, and 0, represent the melting points of the pnre solids A and B. The addition of component B to pure liquid A causes a depression of the freezing-point of A , with the curves 0,E and OAF representing, respectively, the variation of the composition of the liquid and vapor phases in equilibrium with pure solid A. The addition of component A t o pnre liquid B also results in a freezing-point depression, with O,E and 0,F representing the variation of the composition of the liquid and vapor phases in equilibrinm with pnre solid B. Any point within the area 0,EFG represents three-phase equilibrinm involving pure solid A , liquid, and vapor, with the compositions of these phases being determined as indicated by the representative tie-line. I n the region O,FEH, the three-phase equilibrinm involves a liquid, a vapor, and
\
,s '
COMPOSITION Figure 1.
T/c proiection of the space model for a binory system.
pure solid B. At the temperature described by the tie-line GFEH (the eutectic temperature), four-phases are in equilibrium, pure solids A and B, eutectic liquid of composition E , and entectic vapor of composition F. It is seen that the entectic temperature is the lowest temperature a t which a stable liquid phase can exist. At temperatures below the eutectic value a three-phase equilibrium exists, with the two pure solids A and B in equilibrium with a mixed vapor whose composition is represented by a point on the curve FS. If a mixture of pure solid A and pure solid B (of any overall composition provided that both solids are present in sufficient amounts to saturate the vapor phase) is heated, the vapor composition will vary along the curve S F . The exact manner in which the composition of the vapor changes with temperature can be easily determined if one knows the vapor pressure variation with temperature for each pure solid and if one assumes the mixture of A and B in the vapor is ideal. When the eutectic temperature is reached, vapor of composition F is in equilibrium with liquid mixture E in addition to being in eqnilibrium with the solids A and B; and eutectic fusion is free to proceed. The first 604
/
Journal of Chemical Education
- -
trace of eutectic liquid is formed by the phase reaction, solid B vapor eutectic liqnid. After solid A this first trace of liquid is formed, fusion can proceed by the more familiar phase reaction, solid A solid B eutectic liquid. "Non-contact" fusion can also be explained in this manner. I n eutectic type binary systems each solid component occurs as a pure phase and the vapor composition above the two solids is not dependent upon whether the solids happen to be mixed. The equilibrium between vapor F and liquid E a t the entectic temperatnre does not depend upon the source of the vapor F. It might be worthwhile to consider why this vaporphase mechanism of eutectic fusion has not received more widespread adoption. To this author it seems that misconceptions associated with the phase-rule concept of "condensed systems" have been largely responsible for this lack of an appreciation of the role of the vapor phase in eutectic fusion. The significance of the "condensed system" has been discussed very nicely by Ricci (6). In essence a "condensed system" is one that exists in the presence of its own vapor, but for which neither the composition of the vapor nor the pressure exerted by the vapor phase is recorded. The fact that in a "condensed system" neither vapor composition nor total pressure are recorded does not mean that the vapor phase is absent. All too often the impression is given that if a heterogeneous mixture is considered as a "condensed system" only the condensed phases, liquid and solid, are present. With this disregard for the presence of the vapor phase in a "condensed system" it is understandable that the mechanism of entectic fusion has proved to be a puzzling one. Finally, it should be pointed out that the vapor phase mechanism proposed here is one possible mechanism of eutectic fusion but that there are undoubtedly other mechanisms that might be involved. A brief discussion of other aspects of the eutectic fusion process as well as a complete rCsum6 of the subject, "Eutexia," is presented elsewhere in this journal by Copley (7).
+
-
+
Acknowledgment
Acknowledgments should be made to Professor C. H. Sorum of the University of Wisconsin whose inspiring teaching led to the author's interest in heterogeneous equilibrium; to the author's colleague, Dr. R. F. Firestone, for helpful discussions concerning this manuscript; and to G. N. Copley, College of Technology, Liverpool, for stimulating correspondence on the subject of entectics. Literature Cited
(1) KURXAKOV, N. S., AND EFREMOV, K . X., J. RUSS.Phys. Chem. So?., 44, 1992 (1931). E. A., J . Am. Chem. Soc., 74, (2) SORUM, C. H., AND DURAND, 11171 - ilQF.2) ,.".,. (3) PETRUCCI, R. H., AND SORUM, C. H., Can. J. Chem., 34, 629 (1956). (4) FINDLAY, A,, CAMPBELL, A. N., AND SMITH,N. O., "The Phase Rule and Its Application," 9th ed., Dover Publications, Inc., New York, 1951, p. 232. (5) RICCI,JOHNE., "The Phase Rule and Heterogeneous Equilibrium," D. Van Nostrand Ca., Inc., New York, 1951, n. 84. =.
(6) Ibid., p. 63. (7) COPLEY, G. N., J. CHEM.EDUC., 36,596 (1959)
