G. P. CUNN~NGHAM, D. F. EVANS, AND R. L. KAY
3998
Willis for their interest in the project and to Mr. B. D. Guerin for performing the analyses of the Mg(NO& solutions. The author is also indebted to
Dr. D. M. Gruen of the Chemistry Division, Argonne National Laboratory, in whose laboratory the necessary measurements were completed.
Transport Properties of the Tetraethanolammonium Ion in Nonaqueous Solvents at 10 and 25"
by G.P. Cunningham, D. F. Evans, and Robert L. Kay1 Mellon Institute, Pittsburgh, Pennsylvania
16916
(Received J u l y 18, 1966)
Precise conductance measurements are presented for (EtOH)4NI in methanol and acetonitrile a t 10 and 25", (Et0H)dNBr in methanol at 25", and tetraethanolammonium tetraphenylboride [(EtOH)4NBPh4] in acetonitrile a t 25". (EtOH)4NI in aqueous solutions at 10" was also measured in order to extend the known temperature coefficient over a larger temperature range. The halides are only slightly associated in methanol but considerably associated in acetonitrile, owing to the difference in the acid-base properties of these two solvents and the possibility of hydrogen bonding of the (EtOH)4N+ion to a halide anion. The small interaction of this large cation with acetonitrile as compared to that with methanol is reflected in the limiting ionic mobilities. A comparison of the limiting conductanceviscosity products for the (EtOH)4N+ ion in nonaqueous solvents with those for aqueous solution at different temperatures verifies the conclusion arrived a t from previous measurements that this ion does not enforce water structure in aqueous solutions as is the case with its alkyl analog, the Pr4N+ ion. I n contrast, there is some evidence that the (EtOH)4N+ion has considerable structure-breaking powers in aqueous solution.
Introduction Considerable interest has been generated in the properties of the tetraethanolammonium ion2 owing to its normal behavior in aqueous solution in contrast to what has been considered abnormal behavior exhibited by its alkyl analog, the tetrapropylammonium i ~ n . ~ The concentration dependence of partial molar heats of d i l ~ t i o n ,and ~ viscositye for the tetraalkylammonium ions can only be explained by appealing to the effect of these large hydrophobic ions on water structure. The available evidence indicates that water structure enforcement occurs around the inert hydrocarbon side chains of these ions. However, the effects attributable to such water-structure The Journal of Physical Chemistry
enforcement are not observed in similar data for the (EtOH)4N+ion, so that it would appear that the introduction of a terminal polar group into the otherwise inert side chains is sufficient to disrupt the increased degree of hydrogen bonding normally found the alkyl analog of this ion. - around ~ (1) To whom all correspondence is t o be addressed. (2) (a) W. Y. Wen and S . Saito, J . Phys. Chem., 69, 3569 (1965): (b) D. F. Evans, G. P . Cunningham, and R . L. Kay, ibid., 70, 2974 (1966). (3) W. Y.Wen and 5. Saito, ibid., 68, 2631 (1964). (4) S. Lindenbaum, ibid., 70, 814 (1966). (5) R. L. Kay and D. F. Evans, ibid., 70, 2325 (1966). (6) R. L. Kay, T. Vituccio, C . Zawoyski, and D. F. Evans, ibid., 70, 2336 (1966).
TRANSPORT PROPERTIES OF THE TETRAETHANOLAMMONIUM ION
This result is particularly evident when the limiting mobilities and their temperature dependence for aqueous solutions of the tetraalkylammonium ions are compared to those for the (EtOH),N+ ion.2b A further criterion that has been found useful in elucidating water-structural effects in the case of the tetraalkylammonium salts was the comparison of limiting mobilities in aqueous and nonaqueous solvent^.^ Here we report limiting ionic mobilities for the tetraethanolammonium halides in methanol and acetonitrile with the temperature dependence included for methanol solutions. The results support the conclusion already arrived at from a consideration of the data for aqueous solutions alone. Furthermore, the polyfunctional nature of the (EtOH),N+ ion gives rise to a variety of possibilities as far as solutesolvent interactions are concerned, depending on the particular properties of the solvent. I n solvents that can act as good acceptors and donors (water and methanol), this ion is strongly solvated, whereas in solvents with poor acid-base properties (acetonitrile) this ion is poorly solvated and is stabilized most readily by hydrogen bonding to the anion to form an ion pair.
Experimental Section All electrical equipment, cells, salt cup dispensing device, and general techniques for the conductance measurements mere the same as those previously described.'~~The method used to overcome the problems encountered in handling a salt as hygroscopic as (EtOH),XBr has been outlined in detail.2 The conductance baths were set at 10 and 25' within 0.003' with a calibrated platinum resistance thermometer. The change of cell constant with temperature was calculated to be less than 0.01% and therefore negligible. The viscosity measurements were carried out using a suspended-level Ubbelhode-type viscometers with a flow time of approximately 500 see for HzO at 25'. KO kinetic energy correction was found necessary at either temperature. The experimental techniques were the same as previously described.s The preparation and purification of (EtOH)4NBr and (EtOH)4NI have been described.2b The measurements described here were carried out at the same time as those for aqueous solutionzb using the same salt samples, thereby introducing no new variable. (EtOH)&BPh, was prepared by mixing equimolar aqueous solutions of (EtOH),NBr and NaBPL. The resulting precipitate was recrystallized only once with difficulty from acetone-water and finally dried in a vacuum oven at 50' for 12 hr. From previous experience, we have found this is not the best method
3999 _.
for the preparation of a pure salt, owing to the possibility of coprecipitation. However, for the purpose for which this salt was used, the salt purity proved adequate. The details describing the purification of the water,lO methanol," and acetonitrile8 have been adequately covered in previous papers. Briefly, conductivity water was obtained by passing distilled water through a mixed-bed ion exchanger. Reagent grade methanol was passed through a water-free mixed-bed ion exchanger and fractionally distilled under nitrogen. Reagent grade acetonitrile was prepared by the Coetaee method.l2
Results The density increments for the volume concentrations and viscosity measurements were obtained by direct measurements on 0.1 M solutions of (EtOH)4NBr in methanol at 25 and 45'. The 8 value in the density equation, d = do Orit, where rit is the concentration in moles per kilogram of solution, was equal to 0.12 at both temperatures, as was the case with aqueous solution^.^ The 8 value for the iodide in methanol solution was assumed to be 0.13 in keeping with the I - Br difference found previously for the quaternary salts." The e values for acetonitrile solutions were assumed to be approximately the same as those for methanol in keeping with previous experience.8i11 Cunningham has shown that the tetraphenylboride ion has about the same density increment as the iodide ion.la The viscosity data for (EtOH)4NBr in methanol are plotted in Figure 1 and can be seen to conform to the Jones-Dole equation14
+
$/C"z = A
+ BC'"
(1)
+
where = q/qo - 1. A straight line through the points gives an intercept A in good agreement with the Falkenhagen theoretical value15 of 0.02. It should be noted that B = 0.98 h 0.05 is the same at (7) J. L. Hawes and R. L. Kay, J. Phys. Chem., 69, 2787 (1965). (8) D. F. Evans, C. Zawoyski, and R. L. Kay, ibid., 69,3878 (1965). (9) Cannon Instrument Co., State College, Pa. (10) D. F. Evans and R. L. Kay, J. Phus. Chem., 70, 366 (1966). (11) R. L. Kay, C. Zawoyski, and D. F. Evans, ibid., 69, 4208 (1966). (12) J. F. Coetzee, G. P. Cunningham, D. K. McGuire, and G. P. Padmanabhan, Anal. Chem., 34, 1139 (1962). (13) G. P. Cunningham, Ph.D. Thesis, University of Pittsburgh, 1964. (14) G. Jones and M. Dole, J. Am. Chem. Soc., 51, 2950 (1929). (15) H.Harned aftd B. B. Owens, "The Physical Chemistry of Electrolyte Solutions, 3rd ed, Reinhold Publishing Corp., New York, N. Y.,1958,p 240.
Volume 70, Number 18 December 1966
G. P. CUNNINGHAM, D. F. EVANS, AND R. L. KAY
4000
Table I: Equivalent Conductances lO4C
A
(EtOH)hNI, 10' l@Ko = 1.1 5.971 75.56 10.168 73.95 14.564 72.60 20.710 71.05 25.938 69.94 30.614 69.06 68.23 35.462 lOSK,
4.205 8.336 13.696 19.258 22.911 28.519 32.575
= 1.0
76.58 74.71 72.95 71.49 70.67 69.54 68.79
10412
A
CH8OH (EtOH)&NI,25" 108~0 = 2.6 5,853 93.15 10.607 91.16 16.574 88.92 21.794 87.34 25.569 86.32 30.195 85.21 34.402 84.28 40.005 83.15 108Ko = 5.213 10.607 15.472 19.815 23.974 29.017 34.518 40.578
2.1 93.79 91.08 89.20 87.81 86.64 85.37 84.15 82.94
CHICN (EtOH)*NI, 10' (EtOH),NI, 25" 108Ko = 3.4 108Ko 1.7 7.091 124.18 5,110 148.75 9.581 140.34 12.570 116.66 14,319 133.42 18,080 110.80 19.691 127.13 23.391 106.15 23.937 122.91 28.809 102.11 29.709 117.95 34.196 98.66 34.620 114.30 40.708 94.99 39.478 111.06 (EtOH)hNI, 25' 108Ko = 1 . 9 4.879 149.65 9.115 141.46 14.282 133.86 19.430 127.91 24.385 123.03 28,620 119.38 33.754 115.47 39.236 111.75
10'C
A
(EtOH)&NBr,25' 10%0 = 2.7 6.769 87.21 13.775 84.16 22.013 81.61 29.243 79.82 34.394 78.72 41.074 77.43
loS,, = 2.6 7.544 86.78 14.457 83.95 21.844 81.67 28.830 79.92 35.865 78.45 45.256 76.71 52.369 75.56 61.692 74.20
A0
- S(C7)"'
(EtOH),NI, 10' 1 0 7 ~=~ 0.80 5.005 72.40 11.833 71.62 17.941 71.12 23,687 70.70 29.749 70.31
(EtOH)aNBPh*, 25' 108Ko = 1 . 7 4.632 115.52 12.948 111.54 17.461 110.04 21.806 108.82 26,528 107.66 30.410 106.81 34.932 105.91
+ E C y log Cy + ( J - BAo)C7 - K A C Y ~(2)~
The Journal of Physical Chemistry
I
I
I 0.2
I
0.1
I
1
I 0.3
Cl/2
Figure 1. Plot of the Jones-Dole viscosity equation (eq 1) for (EtOH)rNBr in methanol at 25 and 45".
-HzO-
both temperatures, as was the case with aqueous solutions of this salt.2b The measured equivalent conductances and corresponding concentrations in moles per liter of solution are given in Table I along with K O , the solvent specific conductance. The conductance parameters given in Table I11 were obtained from the Fuoss-Onsager conductance equationI6for associated electrolytes A =
1
0
using a least-squares computer analysis.' The dielectric constants, viscosities (poise) and densities (g ml-l) of the solvents at the two temperatures are collected in Table I1 from the various sources cited. Only the data for acetonitrile a t 10' were measured in this research using methods already described.8 Of the three parameters in eq 2, the value of the viscosity B coefficient used affects only the ion-size parameter CE, and then only slightly. A 10% change in B changes d by only 0.03 for both methanol and acetonitrile solutions. Consequently, B = 0.98 was Table I1 : Solvent Properties Temp,
Solvent
OC
f
10nIIo
do
Hz06 CHsOH" CHaOH" CHaCN CHsCN'
10 10 25 10 25
83.96 35.70 32.62 38.34 36.02
1.306 0.672 0.5445 0.397 0.341"
0.99973 0.80073 0.78658 0.7927 0.7766
This value of the viscosity was used in the calculation of limiting ionic conductanceviscosity products. However, in order to be consistent with previous calculations,8 the somewhat higher value of 0.3448 cp was used in eq 2 for the evaluation of the conductance parameters. Separate calculations have shown that changes of this magnitude in the solvent viscosity result in negligible changes in the conductance parameters.
(16) R. M. FUOSS and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York, N. Y., 1959.
4001
TRANSPORT PROPERTIES OF THE TETRAETHANOLAMMONIUM ION
Table 111: Conductance Parameters for Tetraethanolammonium Salts Salt
10
(EtOH)&I
25
( EtOH)4NBr
25
(Et0H)dNI
10 25
(Et0H)J (EtOH)4I
25
(Et0H)aSBPha
10
(EtOH),NI
An
KA
UA
80.64=k0.01 80.88 f 0 . 0 2 94.08 1.0.04 93.931.0.04 100.00 1.0 . 0 2 99.911.0.01
CHaOH 3.76 f 0.09 4 . 9 f0 . 3 4 . 5 i0 . 3 3.7 f0.1 4.1 f O . 1 4 . 0 f 0.07
9 . 9 f0 . 5 16.4 f 1 . 3 16 -12 10 f 1 1 2 . 4 f0 . 8 12.3 2~ 0 . 4
0.004 0.01 0.02 0.02 0.01 0.006
29.7 30.0 37.6 37.5 37.2 37.1
142.20f0.04 166.2 f 0 . 1 165.91-10.03 122.33 1 . 0 . 0 5
CH&N 1 . 6 1 f 0.09 1 . 9 f0 . 4 1 . 6 f0 . 8 5 . 2 6 -1 0 . 0 7
136.2 f 0.9 142 -13 143 f 0 . 7 0.0
0.01 0.07 0.01 0.05
63.5 63.2 64.2
0.006
18.35
73.74=kO0.008
H20 (0.30f0.04)
Discussion used for both salts in methanol and the somewhat lower value B = 0.8, for acetonitrile solutions.a The ion-size parameters, 8, given in Table I1 for the Since eq 2 gave small negative association constants tetraethanolammonium salts in methanol are in good for (EtOH),XBPh,, this salt was considered completely agreement with the value 3.8 obtained for the tetradissociated. The conductance parameters were obalkylammonium halides in methanol" a t 10 and 2 5 O , tained by setting y = 1 and K A = 0 in eq 2. acetonitrile* and nitromethaneZ0 at 25'. The small Included in Table I11 are data for (Et0H)rNI in departures from this value could be attributed to the aqueous solution at 10' that complement the previous difficulty of splitting the last two terms of eq 2 in results2for this salt in HzO at 25 and 45'. obtaining a and KA. We do not consider the magniThe anion limiting conductances required for the tude of the association constants for (EtOH)2J halides cation values given in the last column of Table I11 were in methanol to be significantly different from those obtained in various mays. The values for methanol obtained for the tetrapropylammonium halides" in at 25' were taken from a compilation by Kay and the same solvent ( K A = 5-17). Association constants Evans5 that is based on precise transference data." of the magnitude obtained here correspond to about The value Ao(I-) = 50.9 for methanol solution at 10' 3% association into pairs at 5 X M . Large is based on transference and conductance data reuncertainties are to be expected in the absolute value cently obtained in this laboratory.ia For acetonitrile of association constants of this magnitude since these at 25', the value of ho(I-) = 102.7 is based5 on the salts do not have the stability of the quaternary amassumption that both ions of i-Am3BuNBPh4have the monium or alkali metal halides, and furthermore they same limiting conductance. l 9 Keither transference are hygroscopic. From these results we have connumbers nor conductance data on a salt of two large cluded that the tetraethanolammonium salts in methions are available at the present time for acetonitrile anol at 10 and 25' have the same concentration deat 10'. pendence to a first approximation as the quaternary The value of Ao[(EtOH),S+] = 37.3 f 0.2 for methammonium salts. I n contrast to methanol, these halides are conanol at 25' as obtained from the bromides and iodides siderably associated in acetonitrile. The iodide has shows the typically lower precision to be expected if one salt is highly hygroscopic. For acetonitrile solutions, the agreement in the value of Xo[(EtOH)4- (17) J. A. Davies, R. L. Kay, and A. R. Gordon, J . Chem. Phys., 19, N + ] = 63.3 as obtained from the iodide is in poor 749 (1951). (18) G. A. Vidulich, G. P. Cunningham, and R. L. Kay, to be pubagreement with the value 64.2 obtained from the lished. tetraphenylboride, a result we attribute to the dif(19) M. A. Coplan and R. M. Fuoss, J . Phys. Chem., 68, 1181 ficulty encountered in the preparation and purifica(1964). tion of the tetraphenylboride salt. (20) R. L. Kay, S. C. Blum, and H. I. Schiff, ibid., 67, 1223 (1963). Volume YO,Number 18 December 1986
G. P. CUNNINGHAM, D. F. EVANS, AND R. L. KAY
4002
t
an association constant of about 140 with little dependence on temperature. Measurements were carried o.26 out on (EtOH),NBr in acetonitrile a t 2 5 O , but the -+ CH,OH rate of solution of this salt was so slow that data a t only three concentrations could be obtained, and those were of limited precision. Although it was not ~ C H ~ C N H20 possible to obtain an accurate value of Ao,a good estimate of K A = 1 X IOa 10% was obtained from a 6 0.23 Shedlovsky plot. On a size basis, the association of the bromide should be no greater than 10% above that of 0.22 the iodide rather than almost seven times greater as is I 0 CH&N the case in acetonitrile. I n contrast, the alkyl analogs of these salts, namely, Pr4NI and PsNBr, are essentially unassociated in acetonitrile (KA = 3-5).* Since ions of the same size as the tetraethanolammonium I I I 1 I I I halides are not significantly associated, the large 10 25 45 TnC association of the tetraethanolammonium halides is Figure 2. Limiting conductance-viscosity products for the not entirely coulombic in origin. Furthermore, if at (EtOH)aN+ ion ( 0 ) and its alkyl analog, the Pr4N+ion ( 0 )in all, solvation would tend to stabilize the ( E t O H ) S + , and methanol, - - - -, solutions as a function of aqueous. ion rather than the tetraalkylammonium ions. It is temperature. Data for acetonitrile are also included a t 25”. clear that a different effect is stabilizing the tetraethanolammonium ion pairs in acetonitrile solution. The most likely explanation is a stabilization of the The hove product for the (EtOH),K+ ion in these ion pairs by an acid-base interaction between the prononaqueous solvents substantiates these conclusions tons of the hydroxyl groups on the cation and the unas well as those that have been made concerning the solvated anion to form a hydrogen bond. This type properties of this ion in aqueous solutions.2b The data of ion pairing was used by Taylor and Kraus2I to for this ion as well as those for its alkyl analog, the explain the high degree of association of the picrates PrlN+ ion, are collected from various sources in Table of hydroxyl-substituted quaternary ammonium cations, I V and are shown in Figure 2 as a function of temperaKA[(CH3)3(OH)KPi] = 6 X lo4 and KA[(CHQ)S- ture. The point reported here for aqueous solutions (EtOH)NPi] = 140, compared to the symmetrical at 10’ is in good agreement with the near-zero temtetraalkylammonium cation, KA[(CH3),NPi] = 24, perature coeEcient found previously for this ion.2b in nitrobenzene, a solvent comparable to acetonitrile The much lower values of hove for the Pr4N+ ion in dielectric constant and acid-base properties. Acein water compared to those for that ion in methanol tonitrile is an exceedingly weak acid and baseIz2 and consequently only weakly solvates the anions and the terminal hydroxyl groups of the cation. Ionic Table IV : Limiting Ionic Conductance-Solvent charge-solvent dipole interaction is also weak because Viscosity Products of the relatively large sizes of these ions. On the other Pr4N Temp, (EtOH14N hand, methanol has much stronger acid and base A070 Solvent OC Xano properties and solvates both the anions and the ter0,2005 10 0,240 minal hydroxyl groups of the tetraethanolammo0.2067 Hz0 25 0.240 nium ion readily. Consequently, this stabilization of 0,2134 45 0,239 the free ions by solvation in methanol solution accounts 0.2500 10 0.201 for the low degree of association found here for the tetraCHaOH 0.2509 25 0.203 ethanolammonium halides in that solvent. The high 0.240 CHICN 25 0.216 degree of solvation of (Et0H)JBr by methanol is also reflected in the high-viscosity B coefficient of 0.98 compared to 0.67 for its alkyl analog, Pr4NBr, in methanol.6 Furthermore, it is likely that the poor (21) E. G. Taylor and C. A. Kraus, J . Am. Chem. Soe., 69, 1731 (1947). base properties of the tetraphenylboride ion as well as (22) J. Coetaee in “Progress in Physical Organic Chemistry,” A. its large size contribute to the complete dissociation Streitwieser and R. Taft, Ed., Interscience Publishers, Inc., New of (EtOH)&BPh, in acetonitrile. York, N. Y., 1966.
*
.,D
1 c
+
~~
The Journal of Physical Chemistry
+
TRANSPORT PROPERTIES OF THE TETRAETHANOLAMMONIUM ION
have been interpreted as indicating a considerable amount of water-structure enforcement around that ion in aqueous s ~ l u t i o n . ~Furthermore, the relatively large positive t,emperature coefficient of Xovo for the Pr4N+ ion in aqueous solutions as compared to that for methanol solutions has been shown to be added evidence for water-structure enforcement about the hydrocarbon side chains of this hydrophobic ion5 By inserting a dipole moment into the side chains by the replacement of a terminal methyl by a hydroxyl group to form the (EtOH)4N+ ion, the mobility in aqueous solution increases and becomes comparable to that for its analog, the Pr4N+ ion, in both methanol and acetonitrile, solvents in which three-dimensional structures are not possible. I n other words, the inclusion of the hydroxyl group in the side chain has sufficient orienting influence on the water dipoles so as to interfere with the enforcement of water structure around this ion. I n methanol, however, the (EtOH),N+ ion appears to be considerably solvated, and slower as seen by the low value of Xovo as compared to that for aqueous solution and by its slightly positive temperature coefficient. This is in agreement with the predictions made above from the association behavior of this ion in methanol. Also, in acetonitrile, the mobility is higher, indicating less solvation, a conclusion also in agreement with the association behavior. It would be interesting to verify this conclusion by the temperature coefficient of the Xovo product for this ion in acetonitrile, but lack of transference data does not permit this. However, the Aovo for the iodide salt gives 0.565 and 0.566 at 10 and 2 5 O , respectively, indicating very little temperature dependence. It would appear that one inconsistency remains.
4003
The Xovo product for the (EtOH)*N+ ion in aqueous solutions is higher than in acetonitrile although this ion must be solvated to a greater extent in water than in acetonitrile. This problem can be resolved by assuming this large ion to be a good structure-breaker in aqueous solution and therefore exhibits some excess mobility in aqueous solution owing to its ability to break water structure in its cosphere. The lack of a pronounced temperature dependence for the (EtOH)4N + ion in aqueous solution could then be the result of the negative temperature coefficient characteristic of structure breakers and a positive component due to the tendency of the hydroxyl groups to be solvated more extensively with water molecules a t lower temperatures. The results obtained here add considerably to the reliance to be placed on the criteria we have developed5 for the detection of the effect of solvent structure on the transport properties of electrolytes. These criteria are based on a comparison of both the magnitude of the transport properties and their temperature coefficients in aqueous and nonaqueous solvents. I n particular, the use of the limiting ionic conductance-viscosity product has in many cases been quite conclusive and certainly refutes the claims made in a recent discussion23concerning the usefulness of Stokes’ law.
Acknowledgment. This work was supported by Contract 14-01-0001-359 with the Office of Saline Water, U. S. Department of the Interior. (23) B. E. Conway and R. G. Barrados, Ed., “Chemical Physics of Ionic Solutions,” John Wiley and Sons,Inc., New York, N. Y., 1966 p 576.
Volume 70, Number 1.9 December 1966
