COMMUNICATIONS TO THE EDITOR
2578 ence of oxygen in the system also suggests small quantities of the species OSiCN, but since the m/e values for the more abundant Si&+-ion overlap, this is not certain. (This uncertainty also explains the absence of measurements for the obvious equilibrium, SizN(g) 2C(graph) '/2Nz(g) = 2SiCN(g)). The ion intensities corresponding to Si+, Nz+, and SiCN+ were measured as a function of temperature and were used to calculate the equilibrium constants for reactions I and 11. From JANAF'O free energy functions for Si(g) and Nz(g),estimated values for SiCN(g) (71 cal deg-l mol-' at 1700"K, ref 298") and the equilibrium constants one derives the corresponding heats of reaction by the third-law method. These results are given in Table I. From a second-law treatment for
+
+
Table I : Equilibrium Data for SiCN (g) Formation
Reaction
+
SiC(hex) 1/2Nz(g) = SiCN(g)
Temp,
-log
OK
KP
1656 1680 1701 1721 1752
7.4 7.1 7.0 6.5 6.6
-A[(CTo H o dI / T ,
AH"aB8,
cal deg-1 mol-1
koa1 mol-1
(32) (32) (32) (32) (32)
109 108 109 106 109
Av 108
1780 1792 1803 1820 1853
1.2 1.0 0.9 0.9 0.8
(5.5) (5.5) (5.5)
-20 -18
-17
(5.5)
-18 -17
(5.5)
AV -18
reaction I osnealso obtains the value AH'298 3 kcal mol-,'.
=
113
Using the average of the second- and third-law heats for reaction I, AH'ZSS= 110.5 f 4 ltcal mol-' and the heat of formation of hexagonal silicon carbide, = - 18 f 4 kcal one calculates AHr' [SiCN(g)] = 92.0 f 7 kcal mol-'. Similar treatment for reaction I1 and the heat of sublimation of silicon, AH~'298 = 108.4 f 3 kcal gives aHr0[SiCN(g)] = 90.3 f 4 kcal mol-l, in good agreement with that obtained using the data for reaction I. Values for the heat of atomization of SiCN(g), as calculated from the heats of equilibria I and 11, the dissociation energy of Na(g), DO' = 226 f 2 kcal mol-',l0 the heat of sublimation of carbon, AHSo2g8= 170.9 f 0.5 kcal and the heat for the reaction, Si(g) C(graph) = SiC(hex), AH'298 = -125 f 3 kcal mol-'," are AHatoms[SiCN(g)] = 296 f 8 and 300 f 4 kcal mol-', respectively. Since fewer auxiliary thermodynamic data are required for computation using reaction 11, the latter value with smaller error limits is favored, and one finally chooses for the heat of atomization, the average [SiCN(g)] = 298 f 6 kcal weighted-value, AHostoms mol-'. This value is comparable to atomization energies given for similar molecules BC2 (294 f 6),la Sic2 (303 f 6),13RCN (301 f 5),6c and AlCN (297 A 5 ) kcal mol-l.sb Infrared matrix isolation studies are being conducted to obtain structural information on the species SiCN as well as AlCN and BCN.
+
Acknowledgment. This work was supported by the United States Atomic Energy Commission. (10) "JANAF Thermochemical Tables," Dow Chemical Co., Midland, Mich., 1967. (11) J. Drowart and G. DeMaria, Silicon Carbide; High Temp. Semicond. Proc. Conf., 16 (1960). (12) H. L. Shick, "Thermodynamics of Certain Refractory Compounds," Vol. 1, Academic Press, Inc , New York, N. Y . ,1966, p 158. (13) G. Verhaegen, F. E. Stafford, and J. Drowart, J . Chem. Phys., 40, 1622 (1964).
C O M M U N I C A T I O N S TO T H E E D I T O R
Ion Exchange between Solids Sir: Ion exchange normally occurs through the medium of aliquid phase. The exchanging ions may be dissolved in water, various nonaqueous solvents, mixtures of solvents, or they may be present in fused salts. It has now been found that certain types of ion exchangers, T h e Journal of Physical Chemistry, Val. 74, N o . 1% 1970
such as the various zirconium phosphates, have the ability to exchange ions directly with solids or gases. The exchanger, in the hydrogen form, is heated with an anhydrous metal salt which on exchange forms a volatile acid. The exchange reaction proceeds by continuous removal of the volatile acid. The details are given in the text which follows.
COMMUNICATIONS TO THE EDITOR Zirconium phosphate crystals and gels were prepared as described previously. For the qualitative ion-exchange experiments weighed amounts of dried zirconium phosphate and the anhydrous metal salt were ground together in an agate mortar. The mixtures were then transferred to a platinum dish and heated in a furnace a t several temperatures. Temperature control was good to *2%. X-Ray diffraction patterns of the cooled products were then obtained with a recording Norelco wide angle diff ractometer. For the quantitative weight loss experiments, weighed amounts of finely divided zirconium phosphate and metal salt were dispersed in dry benzene and then quantitatively transferred to a crucible. The solvent was then removed at reduced pressure. The dry solid mixture and crucible were weighed and the weight losses at various temperatures determined. The reactions described below may be represented by the equation Zr(HP04)2.H20
+ z//,MCL,
-+
J
Zr(JO~,z(F'Od2
2HC1
+ HzO
(1)
The results of reaction of a number of salts with a-zirconium phosphate crystals2 are summarized in Table I. The lithium and sodium exchanged phases were identified by comparison with the X-ray patterns of phases formed by the uptake of these ions from aqueoous solution followed by dehydration.2-4 The 7.41-A phase has not previously been reported. That it represents a partially exchanged phase was shown in the following way. If copper(I1) chloride reacts as in eq 1, the total weight loss for complete exchange should be 20.8%.
Table I : Results of Heating a-Zirconium Phosphate with Several Metal Halides
Salt
Reactant mole ratio,
Temp,
Time,
M/a-ZrP
O C
hr
LiCl
2
130
I
NaCl
2
125
1
NaCl
2
375
4
ZnClz ZnCl2
1 1
115 370
24 20
MnClz COClZ AlCl,
1 1
160 160 126
12 48
130 200
24 24
AIC1, HfCh
"8
'/a '/2
1
Produot(s)a
Zr(LiP04)z"20, phase H Mixture of 3 partially exchanged phases Zr(NaPO&, phase G 7.41 A phase 7.87 7.41 b; phases 7 , 4 1 phase 7.41 b; phase 7.41 b; 8.04 phases 7.41 phase 7.41 phase
+
+
a Numbers in angstrom units refer to the first interplanar spacing observed in the X-ray powder patterns. X-Ray patterns for phase H and G are given in ref 2.
Weight loss vs. temperature data for a 1 : 1 mol ratio of copper(I1) chloride and a-zirconium phosphate are shown in Table 11. Traces of a second phase (7.87 11.) were evident a t a total weight loss of only 6.53%. When zinc chloride or hafnium chloride y e r e exchanged instead of Cu(I1) chloride, the 7.41-A phase persisted up of the total capacity of the cation. to about Table 11: Reaction of CuClz with a-Zirconium Phosphate %wt
O C
Total timeat temp, hr
loss
Color
X-Ray results
125
3
4.91
Brown
125 200
21 48
5.41 6.53
260 260 330 360 400
24 48 12 4 8
15.66 17.15 20.8 21.1 21.1
Brown Brownish green Blue Blue Blue Blue Blue
u-Zirconium phosphate CUClZ 7.41 phase CuClz 7.41 A phase trace of 7.87 phase CuClz 7.87 d phase 7.87 d phase 7.87 A phase 7.87 d phase 7.87 b; phase
Temp
+
+ + +
These wide ranges of metal content forming the same structure indicate that solid solutions of cation within the crystal lattice are forming. At cation loadings of from 30 to 100% the 7-87-11.phase was obtained. This phase must also represent a range of solid solution compositions. The fact that a variety of cation types give the same phases (almost identical interplanar spacings but different intensities) is indicative that the crystal lattice remains rigid with the cations occupying similar exchange sites. This is unlike the behavior of a-zirconium phosphate exchanging ions in aqueous electrolyte solutions where the lattice expands by movement of the a-zirconium phosphate layers to accommodate hydrated cations.2 It would rather be expected that the phase changes might resemble those observed with fused saltsa6 That the phenomena we are observing is indeed ion exchange is shown by the fact that the cations may be eluted with dilute acid solutions. The cations may also be removed by contacting the exchanged phases with gaseous hydrogen chloride. In some cases the original a-zirconium phosphate, minus its mole of water, was obtained. In other cases the structure was altered as in yet undetermined ways. Ion-exchange separations may also be affected by the dry method. A solution containing equal parts of lith(1) A. Clearfield (1964).
and J. A. Stynes, J .
I n o r g . Nucl. Chem.,
26, 117
(2) A. Clearfield, W. L. Duax, A. 8. Medina, G. D. Smith, and J. R. Thomas, J . Phys. Chem., 73,3424 (1969). (3) A. Clearfield and J. M. Troup, ibid., 74, 314 (1970). (4) E. Torracca, J . Inorg. Nucl. Chem., 31, 1189 (1969). ( 5 ) G. Alberti, S. Allulli, and G. Cardini, J . Chromatogr., 45, 298 (1969).
The Journal of Physical Chemistry, Vol. 74,No. 19,1970
COMMUNICATIONS TO THE EDITOR
2580 ium and cesium chlorides was evaporated to dryness and the dry salt mixture ground together with a-zirconium phosphate crystals. On heating the mixture at 125” for several hours the lithium ion exchanged leaving the cesium chloride behind. This behavior is in accord with the known sieving properties of a-ZrP crystals and the idea of a rigid lattice proposed above. The crystals contain cavities which are quite large but the entranceways into the cavityoare only large enough to permit a cation of about 2.6 A diameter to enter.2p6 Thus, cesium ion should be excluded as was in fact observed. The phenomena described here seem to be a general property of ion exchangers in the hydrogen form. We have observed exchange in the dry state to occur with titanium, thorium, and cerium phosphate (both crystals and gels), zeolites, and Dowex-50. I n the latter case exchange was determined from the amount of HC1 evolved. Work aimed toward establishing selectivity series for different exchangers in the dry state and the exploration of the fuller implications of these phenomena is underway.
FILM
-
==t LIGHT
3 __..
DEPARTMENT OF CHEMISTRY CLIPPINGER GRADUATE RESEARCH LABORATORIES OHIOUNIVERSITY ATHENS,OHIO RECEIVED JANUARY 23, 1970
B0 R DER
4
-Ty
Figure 1. Cross section of vertical soap film joining a plateau border rising from the bulk liquid,
determination of the contact angle, as shown in Figure 3 of ref 3. The angular position of the maxima and minima of the diffraction fringes can be observed with good accuracy. These features can be used to estimate the contact angle by the following quantitative, though simplified, argument. The thickness of the border increases with y, the vertical coordinate measured downward from the cusp, as
T (6) A . Clearfield and G. D. Smith, Inorg. Chem., 8,341 (1969).
I&:
G L A S S R O D -+
where
K
= 2ey
+
K;y2
y > o
(1)
is the curvature of the surface immediately
A. CLEARFIELD below the cusp. Higher powers of 8 and y are neJ. M. TROUP glected. The major effect on the light of traversing the border is to delay it in phase by the angle (n - 1)kT where k = 2n/h. Thus its amplitude after that traversal is proportional to A ( y ) = exp[(n - l)kT].
Contact Angles and Diffraction by a Plateau Border
Sir: A vertical soap film, pulled up from a bulk solution, lifts a “plateau border,”’ each surface of which has the shape of a meniscus against a plane wetted wall (Figure 1). These two surfaces meet at a cusp above which the film rises. For the present purpose the film will be regarded as having negligible thickness. The angle, 8, at which each surface meets the plane of the film is the “contact angle” and is a sensitive indicator of certain film propertiesa2 Light directed normal to the plane of the film and traversing the uppermost part of the border is refracted downward by the liquid of the border acting as a prism of wedge angle 28 and refractive index n. Since the surfaces of this prism are not planar but concave outward (with curvature proportional to height above the flat horizontal surface), the lower the light passes through the border, the more it is deflected so that the light emerging from the border forms a “fan.” On the basis of geometrical optics, the contact angle can be determined from the angle of deflection, a , of the top of this fan.a Unfortunately, diffraction effects cause a disturbance of this simple picture, and lead to fringes which complicate the The Journal of Phyeical Chemistry, Vol. 74, N o . 12, 1970
The wave front of this distribution may be resolved by Fourier analysis into components of various angles of deflection. The component of (downward) deflection a (higher powers of which are neglected) is proportional to
c
S-+:
A ( y ) e - i k a Y dy ei/c(bti”Pti) dy ( 2 ) B(a) = where p = a - 2(n - 1 ) B and b = (n - 1 ) ~ .The contribution to B ( a )from negative y, representing light passing above the cusp, is small except for very small a and has been discarded. After making the substitution kb(y - p / 2 b ) 2 = st2/2, one obtains
The magnitude of this expression, hence of the intensity of the deflected light, has maxima and minima for values of p/2/xb that are very close to
_-P
-
4x6-
- 2(n - 1)8=
-
4%
(
4m
)
-1
‘1’
m = 1, 2, 3,
. . . (4)
(1) K . J. Mysels, K . Shinoda, and S. Frankel, “Soap Films,” Pergamon Press, New York, N. Y., 1959. (2) F. Huisman and K. J. Mysels, J . Phys. Chem., 73, 489 (1969); A . Scheludko, B. Radoev, and T. Kolarov, Trans. Faraday Soe., 64, 2213 (1968); T. Kolarov, A. Scheludko, and D. Exerowa, ibid., 64, 2864 (1968). (3) H. M. Princen, J . Phys. Chem., 72, 3342 (1968).
