SURFACE TENSION OF LIQUIDCADMIUM CHLORIDE-ALKALI CHLORIDE SYSTEMS this type has been observed in the flame studies of Bonne, et ~ 1 . ~ 6
Conclusions Studies of hydrocarbon pyrolysis in flow systems have much technological utility, but they will yield fundamental information on homogeneous reaction rates only with great difficulty. Other types of reactions, including reactions involving hydrocarbons, can be effectively studied in flow systems if the enormous complication presented by sooting phenomena can be avoided. The present study of methane pyrolysis shows it to be an example of a system that is very sensitive to the
353
effect of nucleation. The accumulation of evidence from this and other studies, particularly the valuable contribution of Eisenberg and Bliss, had led us to the conclusion that the controversy over the rate constant for the first-order homogeneous decomposition of methane is a consequence of the kinetic complications in flow- and static-system studies. On this basis, we believe that eq 2 must be chosen as the best available representation of the rate constant for the homogeneous first step in the decomposition mechanism. (16) U. Bonne, K.H. Homann, and H. Gg. Wagner, “Tenth Symposium (International) on Combustion,” The Combustion Institute, Pittsburgh, Pa., 1965,p 503.
Surface Tension of Liquid Cadmium Chloride-Alkali Chloride Systems by G. Bertozzil and G. Soldani’ Materials Department, Electrochemistry Group, Euratom CCR, Petten, Holland Accepted and Transmitted by The Faraday Society
(March 13, 1967)
Surface tension measurements have been carried out on four cadmium-alkali metal chloride systems, from the melting point up to 650°, and the results are given in the form of linear equations for each mixture. All the surface tension isotherms are S shaped. The behavior of the systems is discussed in terms of associated entities in pure CdC12 and complex anions in the mixtures. The effect of temperature is also taken into account.
Introduction The study of molten cadmium halide-alkali halide liquid mixtures is not only interesting because of the association phenomena which occur in them, but also because it could provide indications on the structure of pure liquid CdC12. The structure of pure liquid cadmium chloride is not yet well understood and is still open to discussion. In Bues’ opinion,2 the Raman spectrum of liquid CdClz maintains the peak of the solid polymer up to 100” above the melting point, thus indicating the presence in the liquid of an appreciable degree of polymerization. However, its low viscosity and high vapor pressure do not agree with this point of view; they seem rather to suggest a molecular structure similar to that of the mercuric halides, but with a much higher degree of ionization. In fact, the electrical conductivity of molten CdCl2 is similar to that of the alkaline earth halides, while that of HgClz is lo4 times lower. The dominant ionic species present must be the monatomic iona, since the frequencies corresponding to
the two possible complex ions (CdCl)+ and (CdCl8)do not appear in the vibrational spectrum of molten CdC12.a The principal equilibria which follow could be (CdC12),
JrnCdClz
CdCl2 _r Cd2+
+ 2C1-
the associated entities (CdCL), being so small and their concentration so low that no effect on the viscosity results. A structure of this type was proposed by Bockris and Angell.4 On addition of C1- ions, a different ioniaation process takes place (1) Address correspondence t o the authors a t Euratom COR, Ispra, Italy. (2) W. Bues, 2. Anorg. Allgem. Chem., 279, 104 (1955). (3) D. W. James in “Molten Salt Chemistry,” M. Blander, Ed.,
1964,p 515. (4) J. 0.M . Bockris and C . A. Angel], Electrochim. Acta, 1, 308 (1969). Volume 72, Numbw 1 January 1068
G. BERTOZZI AND G. SOLDANI
354 CdClz
+ nC1- JI(CdCl,+z)*-
which results in the formation of complex anions. Many authors have studied the mixtures of CdClz with the alkali metal chlorides using various techniques.6-9 The results have confirmed the presence of the complex ions (CdCl,+2)n-, where n is 1, 2, or 4, following the composition. Unfortunately, however, only the systems with potassium and sodium salts have been extensively investigated, and the effect of a gradual change in the alkali metal size has not been emphasized. As a rule, we observe that the stability of a complex, C,(BA,) (where C is an alkali or alkaline earth metal ion, B is a metal of the Ib or IIb group, and A is a simple anion) increases with increasing the size and decreasing the charge of the cation C; i.e., the covalent character of the B-A interaction increases when the polarizing power of the cation C decreases. This trend already has been pointed out by Thurmond,'O in connection with (Ag-Li)Br and (Ag-Rb)Br mixtures, and so, it should be possible to predict a priori that the stability of cadmium complexes will be low with lithium and sodium salts, and will increase in the order
mole yo were investigated. Results were reproducible within 1%.
Results The surface tension of pure CdC12is a linear function of the temperature in the range from the melting point up to 650"; the corresponding equation is =
99.1
- 0.023t
This result agrees within 1% with the data of Ellis, et aZ.16 We emphasize the fact that the temperature coefficient is particularly low in comparison with the values of 0.06-0.08 dyne/(cm deg), which it has normally for molten salts. The results of the surface tension measurements are given in Table I in the form of constants for linear equations of the type y = A - Bt. We notice that small amounts of alkali chloride are sufficient to raise the temperature coefficient, which
K + < Rb+ < Cs+ Since complex ions or other molecular species are likely to be surface active, the surface properties should be suitable for investigating their presence in melts. In the present work, the results of measurements on the surface tension of cadmium chloride-alkali chloride systems are discussed, and the effect of changing the alkali metal radius is outlined.
\
\
Experimental Section The surface tension measurements were performed by the Wilhelmy slide method, previously tested by us in a wide series of research on surface properties of ionic melts."-14 Analytical grade salts were dried by heating in air up to 400" for about 4 hr. The mixtures were prepared by weighing the predetermined quantities of the components in a platinum crucible, which was then placed in the furnace inside a small cylindrical cell of alumina. The lid of the cell had a small bore through which the suspension wire passed, so the liquid-vapor equilibrium was easily retained and the CdClz losses were negligible. However, the high vapor pressure of CdClz forced us to limit the working temperature at about 650°, so that the measurements were not carried out in a wide temperature range; this fact also hindered the study of mixtures in the range 80-100 mole % of alkali chloride. The surface tension isotherms of these systems do not have a simple shape. Several experimental points were, therefore, necessary in order to draw the curves correctly; all the compositions 10, 20, 30, . . . 80 The Journal of Physical Chemktry
:d C11
25
so
75 Mol.%-
D
Figure 1. Surface tension isotherms at 600' for cadmium chloride-alkali chloride systems.
(6) (a) M.F. Lantratov and A. F. Alabyshev, J . A p p l . Chem. USSR, 26, 321 (1953); (b) H. Bloom and E. Heymann, Proc. Roy. Soc. (London), A188, 392 (1947). (6) N.K.Boardman, A. R. Palmer, and E. Heymann, Trans. Faraday Soc., 51,277 (1955). (7) M. Tanaka and K. Balasubramanyam and J. 0. M. Bockris, Electrochim. Acta, 8, 621 (1963). (8) R.B. Ellis and A. C. Freeman, J . Phys. Chem., 69, 1443 (1966). (9) H. Bloom and S. B. Tricklebank, Australian J . Chem., 19, 187 (1966). (10) C.D.Thurmond, J . Am. Chem. SOC.,7 5 , 3928 (1963). (11) G.Bertozzi and G. Sternheim, J . Phys. Chen., 68,2908 (1964). (12) G.Bertozzi, ibid., 69, 2606 (1965). (13) G. Bertoazi and G. Soldani, ibid., 70, 1838 (1966). (14) G.Bertozzi and G . Soldeni, ibid., 71, 1636 (1967). (16) R. B. Ellis, J. E. Smith, W. 8.Wilcox, and E. H. Crook, $bid., 65, 1186 (1961).
SURFACE TENSION OF LIQUIDCADMIUM CHLORIDE-ALKALI CHLORIDE SYSTEMS
355
Table I Alkali chloride, mole %
10 20 30 40 50 60 70 80
---(Cd-Na)Cl-A
113.9 124.0 128.5 137.4 142.8 144.0 144.8
...
B
0.038 0.048 0.050 0.060 0.063 0,059 0.054
...
--(Cd-K)Cl--A
114.0 125.2 131.8 134.5 137.7 135.5 137.8
...
Cd-Rb) GI----
T---(
r-(Cd-Cs) A
B
A
B
0.040 0.054 0.062 0.065 0.070 0.064 0.063
120.5 128.2 132.1 131.4 131.1 130.8 128.8
0.050 0.060 0.066 0.068 0,068 0.066 0.060
reaches a maximum a t an approximately equimolar composition, then decreases again. Surface tension isotherms at 600" are shown in Figure 1. The dotted parts of the curves correspond to the region in which the melting point of the mixture is higher than the working temperature; likewise, the surface tensions of pure alkali chlorides at 600" have been calculated by linear extrapolation below the melting points. Although destitute of any real signification, these values enable us to draw the isotherms in the whole range of compositions. All of the four isotherms are S shaped, with a positive deviation from linearity in the CdClz-rich region and a negative deviation in the remaining region. The extent of the deviations increases with the size of the alkali metal ion. The isotherm for the (CdNa)C1 system deviates only slightly from linearity, and the inflexion is not very marked. The other three curves exhibit large deviations from linearity. I n all of them, a minimum and a maximum are present. The larger t h e radius of the alkaline ion, the more the maximum is shifted toward high CdC12 concentrations.
Discussion 1. Efect of Alkali Chloride Additions. Let us take into account, for comparison, the lead chloride-alkali chloride systems. They are known to contain the pyramidal PbC13- ions, similar to the CdCL-, as it has been recently shown by Ranian spectra investigation~.~~'~ The surface tensions of the lead chloride-alkali chloride systems were investigated by Dahl and Duke;" the isotherms exhibit a minimum, whose deepness increases when the size of the alkali metal ion increases (Figure 2). We want to emphasize that no positive deviations appear; these exist only in the cadmium halide systems, and are a peculiar feature of them. Positive deviations of the activity of CdClz a t low alkali chloride concentrations (z = 0.1) were found by Lantratov and Alabyshev by emf inverJtigations.6 It is quite generally accepted that small additions of alkali chloride to liquid CdClz result in the breakdown
...
122.1 125.9 127.0 128.3 126.1 126.5 124.7 125.5
...
.*.
C1--B
0.055 0.062 0.067 0.072
0.070 0.070 0.066 0.062
t '"I
I
(
P
b
-
d
/
Mole 'A-
Figure 2. Surface tension isotherms for lead chloride-alkali chloride systems (from data of ref 17).
of the associated (CdClz),. These entities in pure CdClz are likely to be surface active because of their large size and covalent character. They may be responsible for the low surface tension and its temperature coefficient; thus, their dissociation may account for the rapid increase of the surface tension upon dilution with alkali chlorides, so leading to positive deviations from linearity . No similar associated entities exist in liquid lead chloride, which has a much more ionic character; no further dissociation can result in it when alkali chlorides are added. An excess of C1- will remove the Pb2+ ions by fixing them in the complex PbC13-; this latter, being surface active, will decrease the surface tension. Let us consider the isotherm of the (Cd-Cs)C1 system (Figure 1). The surface tension of CdClz is increased by small additions of CsCl following the mechanism we have illustrated, until a maximum is reached a t 12 mole % ' of CsC1. Then, the complex ions CdC13-, which gradually form at the expense of Cd2+and C1-, make the surface tension decrease. After reaching a minimum a t an equimolar composition, the surface tension increases again, up to pure CsC1. (16) K. Balambrahmanyam and L. Nanis, J. Chem. Phys., 40, 2657 (1964). (17) J. L. Dahl and F. R. Duke, USAEC Report ISC-923, 1958. Volume 76,Number 1
January 1968
NOTES
356
L
0
25
50 Cd
75 C12Mole%-
I
100
Figure 3. Surface tension isotherms for the (Cd-Cs)Cl system a t various temperatures.
When the size of the alkali metal ion is varied by replacing cesium with rubidium, the shape of the isotherm changes slightly; the minimum becomes less deep, and the maximum is shifted toward higher alkali chloride concentrations. This means that-the alkali chloride concentration being equal-the CdCI8- concentration is lower with Rb+ than with Cs+; in other words, replacing cesium with rubidium causes the complex anion to become less stable. On going from rubidium to potassium, the isotherm changes further in the same way; the maximum and minimum are no longer distinct,, but overlap in a horizontal inflexion a t the composition of an approximately equimolar mixtures. At least, in the (Cd-Na)C1 system, maximum and
minimum have disappeared, and a slight inflexion only is present. We may therefore point out that really the stability of the complex anions decreases gradually from cesium to sodium, in agreement with the considerations outlined in the Introduction. By observing the phase diagrams of these systems,ls we may notice that the larger the alkali metal is, the higher and sharper the peak of the congruent IleCdCL compound. If this may be regarded as a rough indication of the stability of the complex, we clearly find a parallelism between this trend and our previous conclusions. 2. E$ect of Temperature. In Figure 3, the surface tension isotherms of the (Cd-Cs)C1 system a t various temperatures are shown. It appears that with increasing temperature the maximum flattens. We may understand this behavior by making the assumption that pure cadmium chloride contains some associated entities, which are likely to be thermally dissociated, so that their concentration diminishes by increasing temperature. Thus, the dissociating effect of the alkali chloride will be less and less remarkable as the associated entities disappear upon heating. Probably, if it were possible to work at higher temperatures, pure CdClz would no longer contain any associated group, and no positive deviation would appear in the surfacre tension isotherms; these would have the same shape as those of the PbCL systems. (18) E.Degurnov, Dokl. Akad. Nauk SSSR, 64, 517 (1949).
NOTES
Desorption of Cumene from Silica-Alumina Catalysts
by Yutaka Kubokawa and Hisashi RiIiyata Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka, Japan (Received March $8, 1967)
The kinetics of cumene cracking on silica-alumina catalysts have been investigated by many workers. As for the chemisorption of cumene on silica-alumina, there seem to have been no studies made of the heat of adsorption. Only the heat values on catalytically active sites have been estimated approximately from the kinetics of the reaction.' I n a previous work,2 it The Journal of Physical Chemistry
has been shown that desorption rate measurements can give unambiguous information on the heat of adsorption over a wide range for a given chemisorption system. In the present work similar measurements have been carried out for cumene adsorbed on silica-alumina.
Experimental Section Materials. A silica-alumina catalyst containing 13% alumina was obtained from the Shokubaikasei Go. It has a BET surface area of 448 m2/g. Cumene (1) C. D.Prater and R. M.Lago, Advan. Catalysis, 8, 298 (1956); W.B. Horton and R. W. Maatman, J . Catalysis, 3, 113 (1964). (2) Y. Kubokawa, Bull. Chem. SOC.Japan, 33, 546, 550, 555, 739, 747, 936 (1960); J . Phys. Chem., 67, 769 (1963); 69, 2676 (1965);
Y. Kubokawa and 0. Toyama, Bull. Chem. SOC.Japan, 64, 1407 (1962); Y. Kubokawa, S. Takashima, and 0. Toyama, J . P h y s . Chem., 68, 1244 (1964).
