96
8.CANTOR, R. F. NEWTON,W. R. GRIMESAND F. F. BLANKENSHIP
In addition the thermal histories of the resins influence their structures as indicated by a comparison of the curves for membranes M5K-1, M5K-2 and M5K-3. In this order the severity of heat treatment of these membranes increases. Paralleling this is a regular shift of the second halo to larger Bragg spacings. Thus the shift of the second halo of M5K-1 to that of M5K-3 represents a change of 0.44 A. This is explained readily if the halo is the result of a disposition.of t,he copolymer chains to assume interchain distances grouped about a distance corresponding to sin 8 for the second halo. As heat treatment increases in severity the chains tend to move further apart and are quenched into a distended arrangement by an abrupt lowering of the temperature. The most pronounced influence of the solvent upon resins R2, R3, R4, R5 and R7 is that which results in the disappearance of the third halo when membranes are cast. Small-angle X-ray scattering patterns were taken on each of the resins before casting into membranes and all showed some scattering power with that of resin R1 being the greatest. Thus in spite of the amorphous character of these resins there are ordered areas within the resins which constitute discontinuities in electron density that enable some small-angle scattering. Guinier, Fournet, Walker and Yudovitch4 have shown that a difference in electron densities between two areas in a material are necessary for small-angle scattering. However, small-angle scattering experiments on membranes M2T, M3T, M5K-1 and M7K all show no scattering. This is (4) A. Guinier, G. Fournet. C. B. Walker and K. L. Yudovitcb, "Small-Angle Scattering of X-Rays," John Wiley and Sons, Inc., New York, N. Y., 1955.
Vol- 62
coincident with high-angle patterns of these membranes which show no third halo. It seems that the solution and bake-out treatment of these resins dissolves and does not allow re-establishment of certain ordered areas within the resins which before dissolution have an electron density different from the remainder of the material. The casting of membranes thus constitutes a fractionation process of a sort. An attempt to fractionate resin R5 by a solvent fractionation method produced seven experimental fractions. All fractions precipitated easily but only fractions one through three settled out without centrifugation. These three fractions when allowed to dry produced films in the bottom of the containers. Fractions four through six required centrifugation but when dried out remained as white powders. Fraction seven could not be centrifuged out and thus represents the residue obtained when all solvent was evaporated from the tails of the process. When baked-out and exposed to X-rays, fractions one through three showed identical diffraction patterns having only first and second halos. Fractions five and seven showed identical diffraction patterns having three rings with relative intensities as that of resin R5. However fraction six showed a quite different pattern in which the third halo had a much higher intensity than that observed for the second halo (see Fig. 3). A small-angle scattering pattern of this sixth fraction showed as much scattering as did resin R1. Acknowledgments.-We wish to express our appreciation to the Dow Chemical Company for supplying the resins used in this study and to Mrs. Dave Penny of our laboratory in her help with casting membranes.
VAPOR PRESSURES AND DERIVED THERMODYNAMIC INFORMATION FOR THE SYSTEM RbF-ZrF4 BY S. CANTOR, R. F. NEWTON,W. R. GRIMESAND F. F. BLANKENSHIP Chemistry Division, Oalc Ridge National L~bOratOTy,Oalc Ridge, Tennessee Received August 30, 1967
The vapor pressures of ZrFl from molten RbF-ZrF, mixtures have been measured to obtain information on the change in ZrFd activity with composition in this system. The results provide an example of the pronounced negative deviations from Raoult's law resulting from interactions between univalent and tetravalent cations and a common anion in a molten electrolyte. Eleven compositions, ranging from 35 to 100 mole % ZrF4 were studied. In this range the R b F is non-volatile in comparison with ZrF4, and hence approximate activities could be derived directly from total pressure measurements. A pressure range of 10 to 250 mm. (710-1265').was convenient for the quasi-static method used to measure the vapor pressures. At 912", the melting point of ZrF4, the activity coefficient of ZrF4, Y Z ~ F , , as a function of composition can bearepresented log Y Z ~ F , = -N2Rt,F (2.05 6 . 4 0 N ~ bwhere ~ ) N is mole fraction and application of the Gibbs-Duhem equat!on gives log (11.65 6.40Nzrp,). The negative deviations are associated with a large positive excess partial molal enYRbF = tropy of solution of Z r S . This. entropy behavjor can bq interpreted qualitatively as an effect of the substitution of nonbridging fluorides for fluoride bridges between zirconium ions.
-
+
Introduction Except for some fused salt systems with industrial applications, most of the investigations of binary molten electrolyte mixtures have dealt with relatively ideal solutions of univalent compounds. Binary systems which show marked deviations from ideality have received little attention, but are also of interest from the standpoint of solution theory. Pronounced negative deviations have
been found to result from the interaction between univalent and tetravalent cations and a common anion as, for example, in the NaF-ZrFd system.' The present investigation of the RbF-ZrFa system was undertaken in order t o supplement the thermodynamic information regarding mixtures of ZrF4with alkali fluorides. (1) K. A. Sense, C. A. Alexander, R. E. Bowman and R. B. Filbert, Jr.. TEIEJOURNAL, 61,337 (1957).
r)
1
,
t
VAPORPRESSURE OF THE SYSTEM RbF-ZrFd
Jan., 1958
was obtained by selecting the best crystals from a slowly cooled melt made from RbF supplied by the General Chemical Division, Allied Chemical Corporation. Analyses showed that other alkali metal ions were present in the following weight percentages: Li, 0.023; Na, 0.005; K, 0.16; Cs, 0.24. ZrR, low in hafnium, was obtained at Oak Ridge National Laboratory and further purified by subliming at 720" under vacuum. Translucent crystals were selected from the sublimate. Spectrochemical analyses showed these impurities in parts per million: Hf, 40; Si, 200; Al, less than 100: . Fe.. less than 100: Ni, less than 300. The purified components were iveighed and mixed thoroughly to give samples of about 80 g. for charging the vapor pressure apparatus. All manipulations with the purified salts were carried out in an argon-filled dry-box. Vapor Pressure Measurements.-The technique employed for the vapor pressure measurements was devised by Rodebush and Dixon.2 I n this quasi-static method the melt is contained in a cell provided with two vertical tubes. Vapor is allowed to reflux in the lower end of the tubes. One tubeleads to a stopcock which can be opened to vacuum while the other is attached to a manometer which measures inert gas pressure. A differential manometer indicates the difference in pres-
711 714 719 720 721 727 729 730 734 739 742 748 753 756
10.2 12.0 12.7 12.7 13.2 14.8 15.8 16.5 16.3 19.8 20.7 23.2 30.9 30.7
9.94 10.8 12.3 12.8 13.0 15.4 15.9 16.6 18.2 20.8 22.1 25.8 2'3.0 32.1
500
m _
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L
I
I
~
-
-
-
1
750
-7
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2
I 100
P E
1
I I
I7
1
i
g 2
E=
769 777 778 782 785 788 792 793
45.1 54.9 56.9 59.3 60.6 66.1 74.3 76.8
797
88.9
802 804 805 808
95.2 101 101 109
50
20 -
10
42.8 52.8 54.3 60.1 63.6 68.5 75.3 76.9 84.3 94.3 99.8
101 108
The differential manometer, containing dibutyl phthalate, was read with a magnifier originally designed for reading thermometers. Pressure differences of less than 0.02 mm. (2)
1 1
97
w.H. Rodebush and A. L. Dixon, Phus. Rev., 26, 851 (1925).
bration;
