4216
ROBERTL. KAYAND D. FENNELL EVANS
The Conductance of the Tetraalkylammonium Halides in Deuterium Oxide Solutions at 250
by Robert L. Kay and D. Fennel1 Evans Mellon Institute, Pittsburgh, Pennsylvania
16213 (Received June 28, 1966)
~~
~
Precise conductance measurements were carried out on dilute solutions of Me4NBr, Et4NBr, Pr4NBr, Bu4NBr, Me4NI, and BukWI in essentially pure DzO at 25”. The change in the ratio of the Walden product for DzO solutions relative to HzO solutions is explained by the increased structure in DZO solutions. The concentration dependence of conductance for these salts in dilute solution is almost identical with that found for H20 solutions in that unrealistically low ion size parameters are obtained which decrease with the increasing anion size. As was the case with HzO solutions, the data for Bu4NI analyzed for a small amount of association.
Introduction The increases in such properties as the heat capacity, the melting point, and the viscosity of DzO over that of HzO in the liquid state have been attributed to more structural order in liquid DzO.l This subject has been reviewed in detail recently.2 The increase in structural order has been attributed to an increase in the strength of the hydrogen bonding in DzO solutions as compared to HzO solutions. I n concentrated HzO solutions, the properties of the tetraalkylammonium ions are known to be affected by structural order in the solvent,a and even at infinite dilution4 it has been shown that, as the hydrocarbon portion of these electrolytes increases in length, water structure enforcement about these chains affects the ionic mobility. In this paper, we wish to investigate the same effect for these ions in DzO solutions. At the same time, we wish to determine if the greater structure present in liquid DzO has any effect on the concentration dependence of the conductance for these ions by comparison with known results for HzO solution^.^ Very few investigations of the transport properties of electrolytes in DzO have been reported. Most of the early work was carried out at a time when DzO was both scarce and extremely expensive. Only very small quantities of solution could be prepared and, for that reason, many of the data were restricted in scope and not too precise, Recently, precise conductance measurements for the alkali halides in pure DzO have been The Journal of Physical Chemistry
reported.s These data indicate that both the alkal and the halide ions behave in a similar manner in DzO as in H20, and the main effect is the decrease in mobility due to the 23% increase in viscosity. The quaternary ammonium salts have never been systematically investigated in DzO.
Experimental Section The conductance bridge, conductance cells, method
of procedure, and purification of salts have been amply described elsewhere and will not be repeated in detail here, except where the procedure was changed owing to the particular properties of DzO. A precision of 0.01% was sought in all the quantities measured. The resistance measurements were carried out on a calibrated Dike-Jones bridge. The 500 ml. Erlenmeyertype conductance cell was fitted with the cup dropping which enabled the salt samples to be added to the solvent without exposing the cell and its contents to the atmosphere. The salt samples themselves were weighed in small Pyrex cups on a microbalance, and all (1) G . NQmethyand H. A. Scheraga, J . Chem. Phys., 41, 680 (1964). (2) J. L.Kavanau, “Water and Solute-Water Interaction,” HoldenDay, Inc., San Francisco, Calif., 1964. (3) W. Y. Wen and S. Saitio, J . Phys. Chern., 68, 2639 (1964). (4) D. F. Evans and R. L. Kay, ibid., in press. ( 5 ) . C. G. Swain and D. F. Evans, to be published. (6) J. L. Hawes and R. L. Kay, J . Phys. Chm., 69,2420 (1965). (7) D. F. Evans, C. Zawoyski, and R. L. Kay, ibid., 69,3878 (1965).
CONDUCTANCE OF TETRAALKYLAMMONIUM HALIDES
solutions were made up by weight and vacuum corrected, The solutions were stirred continuously while the resistance measurements were being made. The constant-temperature oil bath was maintained a t 25 Irt 0.002" by means of' a mercury in glass thermoregulator, and the absolute temperature was determined by a calibrated platinum resistance thermometer and a Mueller bridge. The preparation of the salts has been described in detail elsewhere.' The unpurified DzO (Atomic Energy Commission, normal 018) content, was found to have a conductance of about 10-6 mho cm.-1 after distillation. This small conductance was due not to COZ but rather to some solid dissolved impurity. The conductance of the DzO was reduced further to 0.7-1.4 X lo-' mho cm.-l by passage through a 33-cm. column containing 100 ml. of mixed-bed, ion-exchange resins that had been equilibrated with DzO for more than 6 months and was the same sample as was used previouslys to prepare a t least 10 1. of conductance grade DzO. The density of the final product was found to be 1.1044 g. ml.-l. The pure product i s considered to have a density of 1.10451.9 The isotopic content is difficult to obtain f r o p density measurements alone, owing to an unknown but small Ols content. All manipulations involving the DzO were carried out in a drybox or in a closed system. Before each run, 60 ml. of the DzO was passed through the resin and used to rinse the conductivity cell. The cdls were thoroughly flushed with dry nitrogen and filled with 500 ml. of D20 by connecting the cells directly to the ion-exchange colurnn with catalyst-free polyethylene tubing while dry nitrogen flowed through the cell. The prolonged contact of the DzO with the glass walls of the cell and all of the manipulations of the DzO appeared to have no measurable effect on the isotopic content as verified by the fact that redistillation of a used solution in a Vigreux column gave a product with no change in density when compared to the value quoted above. The density of DzO was measured in a single-capillary pycnometer which contained 25 d.of liquid. The viscosity measurements were carried out in a Ubbelhode suspended level type of viscometer with a flow time of 540 sec. for water at 25' (0.008903 poise). The viscosity of D20 rneasured at 25" was 0.01094 poise, which agrees favorably with the 0.010963 poise obtained by Hardy and Cottington.1O A dielectric constant of 78.06 was measuredll for DzO a t 25" using the Cole-Gross transformer bridge and the completely guarded and shielded dielectric cells of Vidulich and Kay.12 These cells were developed for absolute meas-
4217
urements and have been shown to introduce less than 0.1% error in the determination of dielectric constants as high as 80. The value obtained here for D20 at 25" is somewhat larger than 77.94, which has been reported by Malmberg and Mayott.13 The same salt samples were used here as have been shown to give excellent checks with transference data for aqueous4and methanol s01utions.l~
Results The measured equivalent conductances, molar conTable I: Equivalent Conductances in D20 a t 25'
104c
A
104c
Ah
-BudNBr,
-MeaNBr, 8.613 21.582 36.502 51.094 65.636 87.052 103.260 120.777
107~0= 99.111 97.802 96,756 95.924 95.235 94.333 93.740 93.153
--EtrNBr, 4.479 13.735 26.643 38.162 50.767 65.855 81.006 91.912
~ O ? K O= 1.189.569 -0.009 88,440 0.030 87.319 -0.015 86.577 -0.005 85.869 -0.009 0.001 85.146 84.492 0.003 84.059 0.004
-Me4NI, 6.348 16.957 28.126 41.038 55.321 71.206 82.820 106.423
---l?r4NBr, 4.001 12.655 22.359 33.321 45.982 62.725 73.632 91.250
1 0 7 ~=~ 1.482.087 -0.003 80.973 0.017 80.081 -0.008 79.296 -0.012 78.545 -0.001 77.682 0.004 77.174 0.002 76.425 0.000
-Bu~NI,
0.830.022 -0.004 -0.007 -0.017 - 0.003 - 0.007 0.004 0.012
3.344 10.284 20.504 30.350 40.578 52.792 64.869 75.857
5.325 13.432 23.365 33.644 44.371 58.329 72.674 85.406
A
AA
1 0 7 ~=~ 0 . 9 1 7 78.996 -0.010 78.018 0 017 77.014 0.002 76.257 -0.006 75.593 -0.003 74.889 -0.004 74.269 0.000 73.747 0.004 =
98.545 97.319 96.393 95.528 94.731 93.963 93.452 92.528
10% = 77.701 76.550 75.550 74.663 73.831 72.910 72.056 71.365
0.700.012 0.008 -0.003 -0.015 -0.011 -0.004 -0.002 0,015
1.O-0.050 -0.003 -0.001 -0.019 -0.049 -0.028 0.003 0.048
(8) We are indebted to Dr. C. G. Swain of the Massachusetts Institute of Technology for donation of this equilibrated resin which had been used in the preparation of conductance grade DgO for the alkali halides. (See ref. 5 . ) The removal of all exchangeable protons was followed by density measurements. (9) I. Kirshenbaum, "Physical Properties and Analysis of Heavy Water," McGraw-Hill Book Go., Inc., New York, N. Y., 1951, p. 14. (10) H. C. Hardy and R. L. Cottington, J. Res. Natl. Bur. Std., 42, 573 (1949). (11) G.A. Vidulich, D. I?. Evans, and R. L. Kay, to be published. (12) G. A. Vidulich and R. L. Kay, J . Phys. Chem., 66,383 (1962). (13) C. G. Malmberg and A. A. Mayott, J . Res. Natl. Bur. Std., 56, 1 (1956). (14) R. L. Kay, C. Zawoyski, and D. F. Evans, J . Phys. Chem., 69, 4208 (1965).
Volume 69, Number 12 December 1966
ROBERTL. KAYAND D. FENNELL EVANS
4218
centrations, and solvent conductances, K ~ in , mho cm.-l are given in Table I along with Ah, the difference between the measured conductances and those calculated from eq. 1 below. Solution concentrations were calculated with the aid of densities obtained from the expression d = do A%, where % is the concentration in moles per kilogram of solution. The values of A given in Table I1 were the result of density measuresolutions i of the salts in DzO. ments on 0.05 @
+
Table 11: Density Increments A and Viscosity B Coefficients for DzO Solutions a t 25" Salt
A
B
MeeNBr EtdNBr PrrNBr BudNBr MedNI BurNI
0,032 0.019
0.08 0.33 0.79 1.26 0.04 1.21
0.005 0.000 0.069 0,028
+
The conductance parameters given in Table I11 were obtained by applying the Fuoss-Onsager conductance equation16in the form A = A, - SC''2
+ EC log C + (J - B&)C
and takes into account the rather large change of viscosity with concentration encountered with solutions of these large cations. The values of B given in Table I1 were obtained from direct measurement of the viscosities of concentrated solutions. l7 The leastsquares computer programs for eq. 1 and 2 are described in detail elsewherea6J8 The conductance parameters reported here in Table I11 were computed using unweighted values of A since the measurements were carried out at approximately equal intervals in C. Included in Table I11 are the standard deviations for each parameter along with the standard deviations of the individual points, uA. The values of a, ,B, El, and E2 for DzO solutions at 25" are 0.2310, 49.38, 0.5374, and 16.82, respectively, where S and E in eq. 1 and 2 are given by X = a& p and E = Elno- Ez. Limiting cation conductances and Walden products of the tetraalkylammoniurn ions are given in Table IV, along with R, the ratio of the Walden product in D20 to its value in H20,both at 25" ; that is
(1)
I n the case of Bu4NI, some association was suspected owing t o the exceedingly low d obtained from J .
The limiting conductances were computed from Longsworth's transference datalg for KC1 and NaCl and Ao(KBr) = 126.07 and ho(KC1) = 124.23, as reported by Evans5 for DgO a t 25". Since the transference data were measured a t rather high concentrations,
Table 111: Conductance Parameters for DsO Solutions a t 25" Salt
101.27i0.009 91.1OIt:O.Ol 83.50i0.006 80.29&0.006 100.42f0.008 79.34 zk 0 02
MerNBr EtrNBr PrrNBr
Bu4NBr MerNI
Bu~NI
KA
d
Ao
79.423=0.02"
1.64 i 0.02 1.60 Z!Z 0.04 1.71 d= 0.03 1.94 f 0.03 1.10 4 0.02 0.08 zk 0.02 3 . 8 AO,'7'
UA
0.01 0.01 0 009 0.009 0.01 I
0.04
4*lQ
0.01
J
69.6 63.5 66.0 74.5 39.3
-48.1 149 0 I
' From eq. 2.
Consequently, the conductance parameters for BuANI were obtained as well by means of the equation for associated electrolytes A =
A0
they were re-extrapolated5 using the Fuoss-Onsager evaluation of the electrophoretic effect which has been shown to give the correct concentration dependence
+ E C y log Cy +
-
(J
- B A J C y - KaCyAf2
I n eq. 3 and 2, B is the Jones-Dole16 viscosity B efficient given by (7
-
V0)/110
=
The Journal of Physical Chemistry
+
4c1Ia BC
(2) CO-
(3)
(15) R. M. Fuoss and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York, N. Y.,1959. (16) G.Jones and M. Dole, J . Am. Chem. SOC.,51, 2950 (1929). (17) R.L. Kay, D. F. Evans, and T. Vituccio, to be published. (18) R. L. Kay, J . Am. Chem. Soc., 82, 2099 (1960). (19) L. G. Longsworth and D. A. MaoInnes, {bid., 59, 1666 (1937).
CONDUCTANCE O F TIZITRAALKYLAMMONIUM HALIDES
t
Table IV : Limiting Cation Conductances and Walden Products in DzO a t 25"
Me4N + Et4N+ PrdN +
BuaN +
Br -
I-
xo +vu
R
36.61 26.44 18.84 15.62
36.62
0.4009 0.2895 0.2063 0.1711
1.0124 1.008 0.9981 0.9948
(15.63)
4219
'
I
I
I
for aqueous solutioras.20 The final values obtained for To+ were 0.4943 and 0.3985 for KC1 and NaC1, respectively, and resulted in Xo(Br-) = 64.67 for D2O a t 25". A value of Xo(I-) = 63.79 was obtained from our data for Bu4NI, which leads to an excellent check since ho(Me4N+) = 36.62 from the iodide and 36.61 from the bromide.
Discussion Limiting Conductances. Since this is the first determination of limjting conductances for the tetraalkylammonium ions in D 2 0 with one possible exception,5 no independent check for our data is available. However, the excellent agreement in Xo(MekN+) obtained from the bromide and iodide and the fact that the same salts were used here as gave excellent agreement with the data of other workers for H2O solutions a t 25" adds considerably to the confidence to be placed on the reliability of our data. The lower limiting conductances in D2O as compared to H20, of course, reflect the 23% increase in the viscosity. However, the ratio of the conductanceviscosity product for D 2 0 relative to H20, as given by R in Table IV, shows systematic and significant deviations from unity. Although the deviations of R from unity are small, the differences between H2O and DzO in the properties which classical electrolyte theory takes into account are also very small. The dielectric constant and molar volume of H20 at 25" are 78.38 and 18.05 iml./mole while for DtO a t the same temperature the vahes are 78.06 and 18.11 ml./mole. I n Figure 1, the R values reported are compared, on a size basis, to those reported by Swain and Evans5 for a number of alkali and halide ions. Considerable data from thermodynamic, transport, and dielectric relaxation measurements involving these smaller ions have been interpreted satisfactorily21*22 by assuming that these ions are structure breakers in that they tend to break down the three-dimensional water structure in their vicinity. The resulting decrease in the local viscosity would be greater in the case of DzO than H2O owing to the greater structural order known to be present in liquid DzO at the same temperature.l I t is
.5
.7 .9 I /rx Figure 1. The limiting conductance-viscosity product in DzO divided by that, in HzO a t the same temperature as a function of ion size. .3
this greater decrease in the local viscosity around the alkali and halide ions in DzO over that of H20that produces the increase in the conductance-solvent viscosity product for these ions in D2O over that in H20. The value of R less than unity for the fluoride ion indicates that ion to be more highly hydrated in DzO than in HZO so that the increased viscous drag of the larger moving particle in D2O outweighs any structurebreaking properties of the ion. For PrdN+ and Bu4N+, R is significantly less than unity owing to greater enforcement of water structure about the hydrogen chains of these ions in D2O as compared to the effect in HzO. This hydration of the second kind has been detected by other independent m e a s ~ r e m e n t son ~ ~the ~ ~quaternary ammonium ions and is verified here by their transport properties. The values of R slightly above unity for the Et4N+and Me4N+ ions indicate that the structure-breaking tendencies of these large ions more than match the structure-making tendencies due to hydration of the second kind. The conclusions reached here concerning the relative effect of solvent structure in DzO and HzO on the ~ ~ _ (20) R. L. Kay and J. L. Dye, PTOC. Natl. Acad. Sei., 49, 5 (1963). (21) H. S. Frank and W. Y . Wen, Discussions Faraday SOC.,24, 133 (1957). (22) R. W. Gurney, "Ionic Processes in Solution," McGraw-Hill Book Co., Inc., New York, N. Y., 1953. ~~
~~
Volume 69, Number 12 December 1966
_
_
4220
ROBERT L. KAYAND D. FENNELL EVANS
I
'
I
'
I
'
I
'
I
i
F-
Bu,Nt
,
'
p/
Pr4N'
I
I
I
30
I
I
I
45
I
60
I
I
I
75
90
A0
Figure 2. The conductance-viscosity product ratio R as a function of the limiting ion conductance in H20.
mobilities of ions are in agreement with the explanation used by Greysonz3to account for the negative excess entropies and heats of transfer of ions from DzO to HzO a t infinite dilution. The results in Figure 1 demonstrate also that the interaction of a cation with water is different from that of an anion. This i s brought out more clearly in Figure 2 where the R values are plotted against the limiting equivalent conductance of the ions in HSO. I n conclusion, it should be said that the explanation for the solvent isotope effect given here is in complete agreement with the model of Frank and Wen21for ionic solutions. I n this model, they postulate three regions around an ion in aqueous solution. The first contains water molecules immobilized owing to hydration of the first kind (electrostriction) or hydration of the second kind (clathratelike cages). A second region separates the third region containing the bulk or normal water from the water of hydration around the ion. In this second region, the amount of hydrogen bonding or structure is less than in bulk water, owing to competition between the orienting influence of the central ion and that of the neighboring water molecules. The relative size or effect of these three regions depends on the charge, size, composition, and shape of the central ion. In general, high charge and small size or low The Journal of Physical Chemistry
charge and large hydrophobic surface area increase the size of the first region, whereas low charge and large size increase the effect of the second area on ionic properties. From the data represented in Figure 1, it would be possible to order the ions according to their structure-breaking ability. Strictly speaking, this order would apply only to the effect of solvent structure on ionic mobilities, and a somewhat different order could be obtained from thermodynamic data or other measurements. However, the order found here for the cations and the anions is in good agreement with that obtained from other measurements although the structure-breaking effect of cations relative to anions seems to depend more on the particular property investigated. Concentration Dependence. The d values shown in Table 111 for the tetraalkylammonium bromides and iodides are relatively low and of about the same magnitude as those found for the same salts in HzO solut i ~ n . An ~ analysis of the data for Bu4NI showed much better precision when eq. 2 was used, indicating a significant amount of association was present for that salt relative to the other salts studied. The magnitude of the association constant is the same as that found for this salt in HzO at two temperatures4 within experimental error. Thus, the concentration dependence for these salts in DzO appears to be identical wjth that obtained in H20solutions. This apparent association of Bu4NI and the lack of association for Bu4NBr is the most notable feature of the concentration dependence of conductance. Lindenbaum and Boydz4 required the same assumption of greater association of the quaternary ammonium iodides, as compared to the bromides, to explain their activity coefficient data in aqueous solution. It is difficult to explain association constants that increase with increasing anion size. Errors in the theory used for th6 evaluation of the conductance of the free ions should be present for both salts, and the excellent agreement obtained in the limiting conductance for our salts in HzO and D20 rules out salt impurities. Likewise, Diamond's suggestionz6that large ions will tend to form ion pairs more readily in aqueous solution owing to the fact that ion pairs will cause the least interference with the solvent structure is ruled out since a much larger KA would be expected for DzO solutions owing to the more extensive structure in DzO as compared to HzO. It is possible that the fact that large ions come into contact more often may not be properly (23) J. Greyson, J . Phys. Chem., 66, 2218 (1962). (24) S. Lindenbaum and G. E. Boyd, ibid., 68, 911 (1964). (25) R. N. Diamond, ibid., 67, 2513 (1963).
SPECTROSCOPIC AND PHOTOCONDUCTIVITY EFFECTS IN PERMSELECTIVE MEMBRANES
taken care of in the theory. This will explain the higher association of the iodides in aqueous and alcoholic SQlUtions but leaves the lack of association of Bu4NI in acetonitrile' to be explained. It would appear that the effect is characteristic of alcoholic14and aqueous solut i o n ~but ~ not solvents like acetonitrile,' nitromethane,26 and nitrobenzene.' It would appear to be insensitive to the degree of hydrogen bonding in the solvent, as seen here for DzO solutions and previously for aqueous solutions where a decrease in temperature had
422 1
little effect. It would appear that some as yet unidentified specific solvent effect must be present in alcoholic and aqueous solutions to account for the abnormally high association of the tetraalkylammonium iodides.
Acknowledgment. This work was supported by Contract 14-01-0001-359 with the Office of Saline Water, U. S. Department of the Interior. (26) R. L. Kay, 8. C. Blum, and S. 1223 (1963).
I. Schiff, J. Phys. Chem., 67,
Some Observations of Spectroscopic and Photoconductivity Effects in Permselective Membranes
by Chaim Forgacs and Gabriel Stein Neoeteu Institute for Arid Zone Research, Beersheba, and Department of Physical Chemistry, Hebrew University, Jerusalem, Israel (Received June BB, 1966)
Some permselective membranes colored through the existence of conjugated double bonds in the polymer backbone were investigated. Changes in the absorption spectrum due to different gegenions were found. The current through the membranes increases on illumination. The additional current depends on the magnitude of the dark current, light intensity, existence of dou.ble-bond structure, and the spectrum employed. Saturation of the double bond occurs during prolonged current transfer in the dark or light. The characteristics of such membranes as mixed ionic and electronic conductors and the role of internal polarization within the body of the membrane are discussed,
We have reported briefly1 observations w'hich showed that the absorption spectrum of some charged permselective membranes immersed in different electrolyte solutions is affected by the nature of the gegenion, opposite in sign to the charge of the membrane. It was also shown there that the conductivity of such membranes is increased by illumination. Both observations appeared to be related to the electric field existing within the membrane. They offered a method of obtaining further information on this component of membrane processes, which hitherto could not be investigated. I n the present paper, we report more
detailed results and attempt to give a largely qualitative interpretation of the phenomena.
Experimental Section Permselective Membranes. Most experiments were carried out using positively charged membranes synthesized at the Negev Institute for Arid Zone Research according to the process described by Korosy and Shorr.2 (1) G. Stein and Ch. Forgacs, Science, 142, 953 (1963). (2) F. de Korosy and J. Shorr, Dechema Monograph., 47,477 (1963).
Volume 69,Number 1B December 1966
