2408 eq 1 then a plot of DID + against molality should yield a straight line of unit intercept and slope 0.036 [(D+/DOh 1. Using the values of D and D + from Table I there results when a = 3.6 A a value of h = 12.8 (using D* = 2.44 X lov5 (ref 16c). This value of h is in good agreement with the value of 12 determined for a similar salt, cadmium sulfate, from compressibility measurements.16d I t is difficult, however, to reconcile such a large hydration number with the small ion-size parameter 3.64 8. Using the same procedure with a = G.0 A yields values of DID+ which are so scattered as to render meaningless any value of h determined from them. With the inclusion of the relative viscosity term the calculations give h = 3.8 when u = 3.64 A; with u = G.0 A the D I D + values are again considerably scattered and also suggest a negative value of h. It is therefore difficult to justify the use of eq 1 for CuSOl solutions. Equation 1 was derived from Hart!ey-Crank's17 description of diffusion by regarding D+ (eq 2) as an intrinsic diffusion coefficient. Bearmanls and Mills19have pointed out that the usual assumptions involved in applying that description to diffusion measurements makes it strictly applicable only to regular solutions. Equation 2 has a sounder theoretical basis although it has been suggested (ref 13, p 323) that it omits a correction for an electrophoretic-type effect operating between the diffusing ion pairs and water molecules and their neighbors. The hydration model of diffusion used by Stokes and coworkers has recently been criticized by Rasaiah and Friedman,20who have used a theoretical model which does not rely on hydration t o predict thermodynamic (but not transport) properties of 1: 1 electrolyte solutions. (17) G. S. Hartley and J. Crank, Trans. Faraday Soc., 45,801 (1949). (18) R. J. Bearman, J . Phys. Chem., 6 5 , 1961 (1961). (19) R. Mills, ibid., 67, GOO (1963). (20) J. C. Rasaiah and H. L. Friedman, J . Chem. Phys., 48, 2742 (1968); 50,3965 (1969).
Thin Layer Direct Current Conductivity of Benzene Solutions of Quaternary Ammonium Salts1&
by Alfred Prock and William A. LaVallee Chemistry Department, Boston University, Boston, Massachusetts 06816 (Received October 81, 1060)
This note describes measurements of dc conductivity of solutions of quaternary ammonium salts in benzene a t room temperature, where electrode spacing lies within the range 0.002-0.060 cm. Electrolyte concentration was sufficiently low to produce specific conductances in a decade wide range with center around The Journal of Physical Chemistry, Vol. 74, No. 11, 1070
NOTES
Figure 1. Diagram of apparatus. Cell alignment and spacing are determined through interferometry.
10-l2 ohm-' cm-l. The electrolytes used were tetraisoamylammonium picrate and tetraisoamylammonium tetraisoamyl boride. The latter is useful because anion and cation radii in solution can be assumed equal within a small error.lb A great simplification is afforded in the mathematical treatment of conduction in this case. The interfacial double layer thicknesses of such solutions span a range around lod3cmp2which is within the lower range of electrode half-spacing. Under these conditions it was possible to test the theory proposed by Gavis,3 which predicts that apparent specific conductance should increase without limit as electrode spacing decreases. The relationship derived by Gavis is xspp = xg coth L/24 where L/2 is the half-spacing distance and 6 is diffuse double layer thickness. I n some cases our results can be fitted reasonably well to the Gavis theory, but the conductivity increase a t small spacing is shown to be a result of metal ion injection at the anode.
Apparatus Figure 1 outlines the apparatus. The electrometer is a Keithley Model 610B and the recorder is a Texas Instruments Model Servomriter 11,and the voltage source is Zener diode regulated. The solution cell contains a pair of optical flats with evaporated metal films (Au or Ag) over a base layer of stannous oxide. The reflection of the films at the wavelength of Na D is high enough to provide sharp interference fringes which are then employed to align electrodes and to measure spacing through the angular variation of the fringes. The method used for electrode support and alignment is similar to that shown in ref 2, Figure 3, except that both electrodes are optical flats and both are immersed into the solution. The measurement cell is supported on an (1) (a) Research supported by NSF Grant GP 7094; (b) J. F. Coetzee and G. P. Cunningham, J . Amer. Chem. Soc., 86, 3403 (1964); ibid., 87, 2529 (1965); TIATIB was prepared by John Reardon of this department. (2) A. Prock, Rev. Sci. Instrum., 36, 949 (1965). (3) J. Gavis, J. Chem. Phys., 41, 3787 (1964).
2409
NOTES
8.0
P
-
3,O .-
8
2
5
--
2 2.0 -
6.0
-
4.0
-
-*
0.
X
r?
2.
t >
0
5
X
>-
-
-
0 0.2 V applied
I-
Li
1.0
0.5 V 1*2 v
0
m
0 0.2 Vapplied
0.5 V
W fn
W
a 2.0
-
I
I
12 1.2
1.6
-LOG
2.0
2.4
-
I
2.4
-
1,6
-
0
X
>-
t
2
5
iWii K
I 2,4
ELECTRODE S P A C I N G
analysis is in reasonable agreement with expectation4 Anal. Calcd for TIATIB: C, 80.88; 0, 14.94, N, 2.36. Found: C, 81.08; 0, 15.00; N, 2.52. Stock solutions were diluted on the rack by distillation of fresh benzene.
Results and Discussion
5 , O m
2.0
Figure 4. Apparent specific resistivity of T I A T I B ((1 f 0.3) X lo-' M ) in benzene as a function of electrode spacing with evaporated gold electrodes. Temperature is 25".
I
R
I
1 1.6
-LOG
E L E C T R O D E SPACING
Figure 2 . Apparent specific resistivity of T I A P ((6 f 2 ) x 10-7 M ) in benzene as a function of electrode spacing. Electrodes are evaporated gold. Spacing is expressed in centimeters. Temperature is 25".
E c
\\
O
E
-
o 0 , 2 V applled 0.5V 0
I 12
1.2 v
1
1
I
1,6
2.0
2;4
-LOG ELECTRODE SPACING
Figure 3. Same conditions as in Figure 2, but electrodes are evaporated silver.
Figures 2, 3, and 4 show typical results of specific resistance vs. electrode spacing for TIAP in benzene with Au and Ag electrodes, and for TIATIB in benzene with Au electrodes, respectively, under conditions of different applied voltages. The points were fit to Gavis' equation by least squares and show a reasonable fit, particularly at small electrode spacing. However, two discrepancies exist between experiment and theory. First, the dependence on applied voltage is opposite to what is expected. As shown by accounting for the missing term in the Gavis t h e ~ r ythe , ~ diffuse double layer thickness, 6, decreases with increasing applied voltage according to the expression 6
aluminum base weighted with lead and held by extensions springs, and solution is led in and out through Teflon tubing. This mounting has a natural frequency of only 2-3 Hz, well below the usual building noise frequencies. Interference patterns are thereby stabilized, and electrical noise is kept very low. Benzene was prepared by distillation of spectroquality material over LiAIH4 in a nitrogen atmosphere. Tetraisoamylammonium picrate (TIAP) was prepared by a standard method,2 and tetraisoamylammonium tetraisoamylboride (TIATIB) was purified by chromatography over silica gel with benzene as eluent, after the method of Sangster and Ervinej4 and melted at 242-244'. This is higher than that given in the literature1 but its
where 6 is the limiting low voltage value. As an example, if the large spacing specific resistance is 1OI2 ohm cm, electrode spacing is 0.010 cm, and applied voltage is 0.20 V, then 6 is 1.5 X cm, and 61 is 0.76. This leads one to expect to observe a decrease in specific resistance at smaller spacing when higher voltage is applied. Experimentally, the opposite effect is observed. Second, the values of 6 as determined by the fit to Gavis' (4) R. C. Sangster and J. S. Ervine, Jr., J. Phys. Chem,, 24, 670 (1956). (6) A. Prook and M. Djibelian, ;bid., 73, 4398 (1969).
The J O U Tof~Physical Chemistry, Vol. 74, No. 11, 1070
2410
NOTES
G 1.5
2.5
2.0
- L O G ELECTRODE SPACING
P
1 100
I
Figure 6. Bpparent specific resistivity of T I A P in benzene as a function of electrode spacing. Electxodes are stannous oxide coated flats without evaporated metal films.
20
V O L T A G E A P P L I E D , mV
Figure 5 . Ohm's law behavior shown extended to low values of applied voltage for T I A P ((6 f: 2 ) X lo-' M ) in benzene with evaporated gold electrodes. Temperature is 250.
equation are too large. For example, the data of graph 3 yield for 0.5 V applied a value of 6 of 11.7 X cm, whereas measurement2 indicates a value of 2.7 x lo-* cm . On the basis of the results shown in Figure 5 , which illustrates Ohm's law behavior of TIAP in benzene at large spacing down to 20 mV applied potential, it was suggested6 that a reaction involving an injection of ions from the metallic anode might be involved. Figure 6 shows experimental results for TIAP in benzene obtained using a cell of different design in which both electrodes were stannous oxide coated optical flats. The cell diagram is given as Figure 3, ref 7. The result is to be contrasted with Figure 3 of this paper. I n the present case there is no corresponding decrease in resistance as electrode spacing decreases, actually some rise is evident in harmony with the predictions of Silver's theory.*,o It would appear that a metal film a t the electrode is required in order that the enhancement of apparent conductance be observed. In an attempt to obtain a visual proof of migration of metal ions, experiments were ptrformed with Ag and Au evaporated films of about 100 A on a base of evaporated stannous oxide, with use of solutions of much higher salt concentration so that currents of 10-7-10-s A could be made to flow under voltages of less than half a volt. After several days of current passage it was clearly seen that metal had been stripped from the anode and deposited on the cathode. I n one case, a silver film was totally stripped from the anode under applied voltage of 0.2 V after several days of passage of lO-7-lO-* A. Although only qualitative, these experiments clearly corroborate the theory that metal ions are injected into the solution. It can be shown theoretically that metal ion injection is able to account qualitatively for all of the features observed in this conductivity study. If, for the present, bulk conductance is disregarded and if diffusion is neglected, the equations to be satisfied for injection a t The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
the anode in the case of plane parallel electrodes are
J+ = n+ep+E Div J+ = 0
(1) (2)
a
Div E = 4 - p
(3)
E
-kdx
=
V
(4)
where symbols have the usual meaning. At the inj ecting electrode the boundary condition is that the ratio of real field to geometric field is f. The result is
(1 1-
g(f' 4 - 4f +
;))l7$
As a check it is noted that when f = 0, the expression reduces to the familiar space charge limited (SCL) form 9 V2 J + = --F+E 32a
5
and when f = 1, J+ vanishes. This corresponds to a n unperturbed field with no injection. It is not possible at this stage to evaluate the ratio, f, which is determined by the rate of oxidation of metal a t the electrode, but for purposes of discussion it is assumed to be close to unity, so that the total field is only slightly perturbed. Under this assumption, the bulk current can be added without modification to J+ in order to give the total current. The result of such a calculation is shown in Figure 7 for two different large-spacing specific resistances. Mobilities of all ions were assumed to be equal and were obtained from a Stokes' l a y calculation with the ionic radius taken as being 3 A. The ratio, f, was chosen to be the constant, 0.90, at all electrode spacings. The result is qualitatively similar to those of (6) Professor J. J. Lingane, Harvard University, private communication. (7) A . Prock and R. Zahradnik, J . Chem. Phys., 49, 3204 (1968). (8) M. Silver, J . Chem. Phys., 42, 1011 (1965). (9) G. Briere, Chem. Phys. Lett., 1, 706 (1968).
2411
COMMUNICATIONS TO THE EDITOR
t
q J.o
0
2,o
X
1
I
1.4
2.0
-
2.6
LOG ELECTRODE SPACING
Figure 7 . Calculated apparent specific resistivity vs. electrode spacing. See text for details.
graphs 2, 3, and 4. For fixed applied voltage the curve shape at small spacings is a result of the enhancement of current through the L-3 dependence of injected cur-
rent, and at large spacing as a result of the disappearance of injection. The apparent double layer thickness, 6, increases with increasing voltage owing also to the V2/L3 dependence of the injected current component. This means that for f close to unity, the ratio of injected current to bulk current is V / L 2and an increase in applied voltage requires an increase in spacing in order to keep the ratio, hence apparent resistivity, constant, e.g., at 50% of its large spacing (bulk) value. The graph also indicates the proper dependence of the effect on the bulk resistivity. For lower values of bulk resistivity, then a t fixed applied voltage a closer spacing is required to produce an injected current which matches bulk current and thereby reduces apparent resistivity to 50% of its large spacing value. This behavior is found experimentally, as was also anticipated by the Gavis theory on the basis that 6 decreases as ionic concentration increases. In conclusion, the systems studied do indeed show an increase in apparent conductivity with decrease in electrode spacing, but not as described by the Gavis theory. Rather it is due to the augmenting effect of ion injection from the anode.
C O M M U N I C A T I O N S TO T H E E D I T O R
Fluorescence of p-Dioxane
Xir: Fluorescence has recently been reported from a wide variety of saturated hydrocarbons implying the existence of a t least one excited electronic state in these systems which is either bound or a t least sufficiently stable to permit the development of an observable emission.lB2 I n this communication we report a fluorescence from the saturated cyclic ether, p-dioxane. However, as will be demonstrated in what follows, its emission characteristics differ importantly from those of the saturated hydrocarbons. For excitation of p-dioxane as neai liquid (at 25") in its first absorption system a t 1849 A, a structureless fluorescence is observed with A,, = 2470 A and quantum yield +f = 0.029.3j4 Upon dilution with isooctane,6 the emission yield is strongly reduced and the emission spectrum blue-shifted, as shown in Figure 1. It will be noted that both +f and,,,A continue to decrease strongly even in solutions which are already predominantly isooctane. An effect of the solvent to
simply perturb the emitting species appears inadequate to explain this behavior. No fluorescence (+f < 10-6) has been observed from the vapor (at 25 Torr) when excited at 1849 A. Since this excitation is only 1340 cm-l above what has been assigned as the first 0-0 (1) F. Hirayama and 8. Lipsky, J . Chem. Phys., 51, 3616 (1969). (2) F. Hirayama, W. Rothman, and S. Lipsky, Chem. Phya. Lett., in press. (3) ,Matheson Coleman and Bell pdioxane (Spectroquality) was purified immediately before use by refluxing for 24 hr over sodium and then distilling under nitrogen atmosphere. The experimental technique for fluorescence measurements has been previously described (see ref l l ) . All quantum yields have been determined . ultimately relative to a fluorescence quantum yield of 1.0 for 2537 & excitation of 9,lO-diphenylanthraceFe (2 X 10-8 in cyclohexane; degassed). Comparison of 1849-A and 2537-A excitation was achieved through the use of oxygenated pxylene whose internal conversion efficiency has been determined to be unity (ref 11). (4) During the course of this work, a similar fluorescence has been reported following high-energy pulsed electron irradiation of pdioxane liquid. [J. H. Baxendale, D. Beaumond, and M. A. J. Rodgers, Chem. Phys. Lett., 4, 3 (1969)l. (5) Isooctane (Matheson Coleman and Bell, Spectroquality) was chosen as diluent since, unlike most other saturated hydrocarbons, it is negligibly fluorescent (see ref 2).
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
