COMMUNICATIONS TO THE EDITOR
Double-Layer Effect on “External Transport Numbers” in Molten Lead Chloride’
4 157
of the chloride ions passing through, while the effect of positive potentials is consistent with this trend a t +5 v, but shows a change in direction upon reaching +10 v.
Sir: The use of porous electrode-compartment separators as velocity reference in defining “external transport numbers” of the ions in molten salts has become commonplace in recent years.2 The significance of such quantities has yet to be adequately e l a b ~ r a t e d ,but ~ most workers have assumed them to be intrinsic properties of the molten salt. Although their measurement requires interaction of the salt with the porous separator, evidence h:ts been presented in the case of PbClz to indicate independence of experimental results on the nature of the ~ e p a r a t o r . ~In that study the transference number of C1- was found to be within 1% of 0.75 for each of three different separators: an “ultrafine” porosity Pyrex frit (max pore diam, 0.9 M ) ; a piece of unglazed porcelain; and a wad of asbestos fibers packed tightly between ‘*coarse” porosity Pyrex frits. Subsequent conceptual analysis of these experiments has convinced us that they represent a form of electroosmosis in which the value of the “transference number” observed is determined inside the pores of the separator, within a few ionic diameters of the interface between salt and pore ~ v d l . ~ Assuming the variations of separator material studied previously were insufficient for those transport number experiments to reflect corresponding differences in the properties of the interfacial region, we have been seeking a suitably porous material that is chemically inert to the melt but conducts electronically. Used as a separator of the electrode compartments, such a material could be charged to any potential between that at which lead deposits and that of chlorine evolution, a span of more than 1 v, without any electrochemical reaction occurring within its pores. If our hypothesis is correct, the corresponding effect on the structure of the electrical double layer might be expected to produce a noticeable dependence of the transference numbers on the potential. As yet unable to produce a workable cell containing such a frit, we have nevertheless succeeded in demonstrating the proposed effect by means of a brute-force technique. Wrapping a platinum wire around the outside of the tube containing an ultrafine Pyrex frit, we applied potentials ranging from - 10 to + 10 v (relative to One Of the lea‘’ carryat this point, ing on normal trmsference runs by a method similar to that of Fischer and Klemm.5 A pronounced effect of potentia’ On the numbers is evident in the Rsults sulnlnarized in Table 1. It is Seen that negative frit potf?ntialsenhance the tra11SpOI’t numbers
Table I: Effect of Externally Applied Potential on Transference Numbers of PbC12 in Ultrafine Pyrex Frits a t 565’ Applied potential, v
0 -5 - 10
+.i
+10
Av tobsd
0.762 0.802 0.858 0.716 0.760
No. of
Std dev
runs
=to.025
18 9 10 7 8
h0.037 AO. 040 f0.037 3Z0.034
Although glass does not conduct electronically, it is reasonable to expect the high voltage on the external wire to have some effect on the double layer a t the saltglass interface. There is actually a detectable current flowing through this interface when the potential is applied to the wire, presumably carried by cations entering or leaving the glass. This current, initially greater than 1 ma, rapidly decays to a steady-state value less than 0.5 ma, which is negligible compared with the electrolysis current of around 50 ma. It was independently established that the “frit current” produces no detectable volume changes in the absence of electrolysis. We attribute the observed effect on ta t negative potentials to an increase in the concentration of Pb2+ immobilized a t the glass interface by increased electrostatic attraction. This results in enhancement of the concentration of mobile chloride ions in the salt immediately adjacent to this layer. It is the ions in this region that determine the motion of all the salt in the pore relative to its wall, so tthat we observe an apparent increase in the relative mobility of c1-. The effect at +10 v is difficult to explain, but may be an indication that some Na+ ions, rather than Pbz+, are emerging from the glass in spite of our efforts to supply an excess of the latter during initial runs a t negative potentials. It is known that K a + has a far (1) This work is Part of a program supported by the U. S. Atomic Energy Commission. (2) For a summary of work prior to 1964, see the chapter by A. Klemm in “Molten Salt Chemistry,” M. Blander, Ed., Academic New N. y., 1964. (3) R. W. Laity in ”Encyclopedia of Electrochemistry,” by C. A. Hampel, Ed., Reinhold Publishing Co., New York, N. Y., 1964, p 653. (4) F. R. Duke and R. W. Laity, J . Phys. Chem., 59, 549 (1955). ( 5 ) n-. Fischer and A. Klemm, 2. 1\r~tUTfo~5Ch., 16a, 563 (1961).
Volume 71. Number 13
November 1967
COMMUNICATIONS TO THE EDITOR
4158
greater mobility than any doubly charged ion in glass, and its presence on the salt side of the double layer could introduce complicating structural modifications. SCHOOL O F CHEMISTRY RUTGERS UNIVERSITY NEW BRUNSWICK, NEWJERSEY 08903
RICHARD W. LAITY CARLAXELSJOBLOM
RECEIVED JULY 13, 1967
New Electron Spin Resonance Spectra in 5000 G
y-Irradiated Alkyl Halides at 77"KI 4900 G
Sir: The reported esr spectra of most alkyl halide glasses which have been y irradiated at 77°K are attributable to the alkyl radical formed byrupture of the C-X b ~ n d , ~ indicating -~ that other paramagnetic species (such as X, Xz-, RXaX, RX+, RX-) postulated to account for portions of the optical spectra5ss are present at very low concentrations or are obscured by line-broadening effects. Relatively complex spectra observed from some polycrystalline alkyl iodides4*'and some alkyl bromide^^!^ have been attributed to spinorbit coupling involving the halogen nucleus in halogencontaining radicals. Using esr sensitivities ranging up to several hundred times that required to give full-scale deflection for alkyl radicals produced by radiolysis of glassy alkyl halides, we have found that all ?-irradiated glassy and polycrystalline alkyl iodides and bromides tested give esr spectra at much lower and higher magnetic fields than previously observed. The low-field spectra fall into four types corresponding to spectra A, B, C, and D of Figure 1: (A) polycrystalline CzH51,(B) polycrystalline n-C4HJ, (C) polycrystalline n-CXH71, n-C5H91, n-CeHJ, n-C7H15I, and (D) all glassy and polycrystalline n-alkyl bromides from Cz through C,. The low-field spectra of alkyl iodide glasses are similar to the type C spectra after some change in the relative peak heights of the latter during standing at 77°K. All of the alkyl iodides, in either the glassy Or polycrystallil'e give highspectra to E. The high-fie1d spectra Of the bromides are all similar to F. ?-Irradiated polycrystalline n-alkyl chlorides from Cz through C6 give no esr spectra at fields more than 250 gauss below or above the free electron g value, although some Structure above and below the alkyl radical signal is present in every case. Glassy chlorides, when obtainable, give similar results. For tested the features (in the region of 3260 gauss) decay at 77°K. The Journal of' Physical Chemistry
Figure 1. Low-field (A, B, C, D) and high-field (E, F) esr spectra of 7-irradiated polycrystalline alkyl iodides and bromides a t 77°K: (A) CtHJ, signal level 400; (B) n-C4&,1, signal level 400; (C) n-C5H111, signal level 400; (D) n-C4HgBr, the three sections of the spectrum being measured a t different signal levels as indicated; (E) C2HJ, signal level 2500; (F) n-C5HllBr, signal level 1000. I n all cases the modulation amplitude was about 12 gauss and the y dose was 4 x 1019 ev g-1. The magnetic field corresponding t o the free-electron g value is ca. 3260 gauss.
However, some portions of the low- and high-field spectra grow while others decay, the growth in CzH51 glass being as much as fourfold in 2 days. The low- and high-field spectra must be caused by halogen-containing species. Consistent with this conclusion are the differences between the spectra of iodides, bromides, and chlorides. The low- and highfield spectra of polycrystalline and glassy C2D51are identical with those of Cz&,I,indicating that the observed line widths and line splittings are not controlled by hydrogen. (1) This work was supported in part by the United States Atomic Energy Commission under Contract AT(ll-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) B. Smaller and M. S. Matheson, J. Chem. Phys.. 28, 1169 (1958). (3) P. B. Ayscough and C. Thompson, Trans. Faraday Soc., 1477 (1962). (4) H. W. Fenrick, S. V. Filseth, A. L. Hanson, and J. E. Willard, J . Am. Chem. Soc., 85, 3731 (1963). (5) (a) E. P. Bertin and W. H. Hamill, ibid., 86, 1301 (1964); (b) J. P. Mittal and W. H. Hamill, ibid., 89,5749 (1967). (6) R. F. c. Claridge and J. E. Willard. ibid., 88, 2404 (1966). (7) H. W. Fenrick and J. E. Willard, ibid., 88, 412 (1966). (8) F. W. Mitchell, B. C. Green, and J. W. T. Spinks, J. Chem. Phy8., 36, 1095 (1962). (9) R. M. A. Hahne and J. E. Willard, J. Phys. Chem., 68, 2582 (1964).
