2639
PARTIAL MOLALVOLUMES OF TETRAALKYLAMMONIUM BROMIDE SOLUTIONS
Apparent and Partial Molal Volumes of Five Symmetrical Tetraalkylammonium Bromides in Aqueous Solutions
by Wen-Yang Wen and Shuji Saito Chemistry Department, Clark University, Worcester, Massachusetts
(Received April 97, 1964)
The apparent molal volumes, pz,and partial molal volumes, 7 2 , in aqueous solutions were obtained for (CH3)4SBr,(C2H5)4XBr,(n-C3H7)SBr,and (n-C6H1J4NBrat 25’ and for ( ~ L - C ~ H ~ ) ~a T t \15, J B 25, ~ and 35’. In dilute solutions cpz decreases with the increase of molar concentration, c, for all of the salts except (CHJ4NBr. At higher concentrations goes through a minimum and the plot of V2us. 4;for (n-C4H9)4r\iBrand (YZ-C~H~)~N\’B~ then turns upward. These observations are discussed in terms of the solute-water interaction in general, and of a clathrate-like structure for (n-C4Hg)4NBrsolution in particular. Extrapolated values of cpz a t infinite dilution are apportioned to Br- and cations, and cpz(+) ‘’ values so obtained are compared with cpZo values of simple aliphatic hydrocarbons in water.
Introduction The tetraalkglammonium salts constitute a class of electrolytes showing peculiar properties in their relation with water. The aqueous solutions of these salts have been found to give high viscosities with large temperature coefficients,l long dielectric relaxation ~ times, high apparent molal heat c a ~ a c i t i e s ,large Soret coefficients,4 and peculiar activity coefficient^.^'^ Some of the large tetraalkylammonium salts are known to form polyhedral clathrate hydrates’ with high water of crystallization. The structures of several of these crystalline hydrates have been carefully analyzed by X-ray and their close relation with the gas hydrates verified.a The low heats and entropies of fusion of two of these hydrates have also been ~ o n f i r m e d . ~ These studies seem to indicate that, in aqueous solutions, the peculiarities of the tetraalkylammonium ions are due not only to their large ionic sizes but also to the significant modification of water structure around the cations. It will be, therefore, of special interest to see how this structural interaction affects the volume of solutions. In this paper we are reporting measurements on the apparent and partial molal volumes of five symmetrical tetraalkylammonium bromides in aqueous solutions a t 25’. Temperature effect on the apparent molal volume has been determined for one of the salts, tetra-n-butylammonium bromide.
Experimental
Materials. Five compounds, (CH3)4NBr, (C2Hs)dNBr , (n-C3H7)4NBr, (n-C4Hg) .,SBr, and (n-CSHI1) 4NBr, were obtained from the Eastman Organic Cheniicals Department of Distillation Products Industries. They were purified by several recrystallizations from the following solvents: an ethanol-ether niixture for (CH3)4SBr,and chloroform-ether mixtures for (1) E. R. Nightingale, Jr., J . Phys. Chem., 66, 894 (1962); E. Hiickel and H. Schaaf, 2. physik. Chem., 21, 326 (1959). (2) G. H. Haggis, J. B. Hasted, and T. J. Buchanan, J . Chem. Phys., 20, 1452 (1952). (3) H. S. Frank and W. Y. Wen, Discussions Faraday SOC.,24, 133 (1957); Th. Ackerman and F. Schreiner, Z. Elektrochem., 62, 1143 (1958). (4) J. N. Agar and J. C. R. Turner, Proc. Roy. SOC.(London), 255, 307 (1960); P. N. Snowdon and J. C. R. Turner, Trans. Faraday Soc., 56, 1409 (1960). (5) L. Ebert and J. Lange, 2. physik. Chem., A139, 584 (1928); J. Lange, ibid., A168, 147 (1934). (6) H. S. Frank! J. Phys. Chem., 67, 1554 (1963); S. Lindenbaum and G. E. Boyd, Abstracts of Papers, Division of Colloid and Surface Chemistry, 145th National Meeting of the American Chemical Society, New York, N. Y., September, 1963, p. 71. (7) D. L. Fowler, W. V. Loebenstein, D. B. Pall, and C. A. Kraus, J . Am. Chem. Soc., 62, 1140 (1940); R. McMullan and G. A. Jeffrey, J . Chem. Phys., 31, 1231 (1959). (8) D. Feil and G. A. Jeffrey, ibid., 35, 1863 (1961); M. Bonamico, R. K. McMullan, and G. A. Jeffrey, ibid., 37, 2219 (1962); R. K. McMullan, M. Bonamico, and G. A. Jeffrey, ibid., 39, 3295 (1963). (9) W. Y. Wen. Ph.D. Thesis, University of Pittsburgh, 1957 (microfilm no. 24753).
Volume 68. Number 9 September, 1964
2640
WEN-YANGWEN AND SHUJISAITO
the other four compounds. Each salt was dried a t 60-70° in vacuo for a t least 1 week before use. Apparatus and Measurements. Density measurements were made in Weld-type pycnometers of 5and 25-ml. capacity standardized with doubly distilled water, 25-ml. pycnometers for dilute solutions (with concentrations below 0.3 m) and 5-ml. ones for concentrated solutions. The procedure of Weissbergerlo was followed in the measurements, all weights being reduced in vacuo. Density measurements are believed to be precise to within =k0.00005 and the precision for the apparent molal volume is k O . 1 ml. Pycnometers of different capacity (5 and 25 ml.) have been checked against each other, all yielding results in agreement with one another to within the above experimental error. Temperature controls. for the water baths used were 15 & O.0lo, 25 f 0.005', and 35 h 0.01O, respectively. Density measurements for each salt were carried out over a concentration range of about 0.05 m to near saturation.
Results and Discussion The apparent molal volumes of four tetraalkylammonium bromides obtained a t 25' are listed in Table I. The data for tetra-n-butylammonium bromide in aqueous solutions at 15, 25, and 35" are shown in Table 11. These apparent molal volumes, cpz, were calculated from the density data by the equation 1000
1 cpz = m -
(
+ mMe - 1%) d
do
/ /'
'\
290.
0
'\
0.5
where do is the density of pure water; Mz, the molecular weight of the salt; d, the density of the solution; and m, its molality. In Fig. 1 the values of cpz for (n-C4H9)&Brare plotted against a t 15, 25, and 35O, where c is the molar concentration. The values of cp2 for other salts a t 25' are similarly plotted against diin Fig. 2-5. In all cases 92 seems to vary linearly with at least in the dilute solution range following the equation
,/
1
/,
1.0
1.5
n
i
Figure 1. Apparent and partial molal volumes of ( ~ - C ~ H Q ) in ~N aqueous B ~ solutions plotted against Jc- where c is the molar concentration: -0-0-, p2; - - - - - - -, i i 2 .
1
t
e
1
IlOb
0.5
(1)
,'
2 .o
15
1.0
Figure 2. Apparent and partial molal volumes of ( CHs)4NBr in aqueous solutions a t 25": -0-0-, (p2; - - - - - -, v2.
-
.\/c
di
9 2
=
(P2O
+ svdc
(2)
where cpzo (= l72') is the cpz a t infinite dilution and S , is the limiting slope. Values of p 2 O and S, obtained from these plots are given in Table 111. The linear relation ( 2 ) has been known to be followed by many small strong electrolytes in dilute solutions and, for some salts, up to medium concentrations as well." The experimentally found values of S , for small electrolytes do not agree exactly with that predicted by the Debye-Hiickel theory; nevertheless, the agreement is good with regard to the sign and order of magnitude.12 The Journal of Physical Chemistru
L 0
0.5
1.0 4-F
1.5
20
Figure 3. Apparent and partial molal volumes of (CgH6)aNBr in aqueous solutions a t 25": -0-0-, (02; - - - - - -, v2.
-
(10) A. Weissberger, "Physical Methods of Organic Chemistry," Vol. I, 2nd Ed., Part I, Interscience Publishers, Inc., New York, N. Y., 1949. (11) D. 0. Masson, Phil. Mag., [ 7 ] 8 , 218 (1929); A. F. Sooth, J . Phys. Chem., 35, 2315 (1931); W. Geffeken, 2. physik. Chem.,
A155, 1 (1931).
264:1
PARTIAL MOLAL VOLUMES OF TETRAALKYLAMMONIUM BROMIDE SOLUTIONS
Table I : Apparent and Partial Molal Volumes of Four Tetraalkylammonium Bromides in Aqueous Solutions a t 25’ (unit: ml./mole)
115.1 115.2 115.3 115.3 115.4 115.4 115.4 115.3 115.3 115.3 115.1 115.1 115.0 114.9 114.8 114.6 114.4 114.2 113.9
115.0 115.1 115.1 115.1 115,2 115.2 115.2 1.15 2 115.2 115.3 115.2 115 2 115.2 115.1 115.1 115 0 115.0 114.9 114.7 114.6
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.5 3.0 3.5 4.0 5.0 7.0 10.0
I
173.7 173.1 172.6 172.2 171.9 171.5 171.1 170.7 170.4 170.1 169.4 168.9 168.3 167.9 167.5 166.7 166.2 165.7 165.3
174.3 173.8 173.5 173.2 173.0 172.8 172.6 172.4 172.2 172.0 171.6 171.3 171.0 170.7 170.4 169.8 169.2 168.7 168.3 167.7 166.6 165.9
...
238.9 238.2 237.6 237.1 236.7 236.3 235.8 235.4 234.9 234.5 233.8 233.2 232.6 232.2 231.8 231.2 230.7 230.5 230.4 230.5 231.2 231.8
... 164.2 .
.
I
237.9 236.8 236.0 235.3 234.6 233.8 232.9 231.8 231.0 230.5 229.7 229.2 228.9 228.7 228.6 228.7 229.0 229.5 230.1
362.9 362.0
361.8 360.3
233.2
...
Table I1 : Apparent and Partial Molal Volumes of Tetra-n-Butylammonium Bromide in Aqueous Solutions a t 15, 25, and 35’ (unit: ml./mole) Conon., m
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.2 1.4 1.6 1.8 2.0 2.5 3.0 3.5 4.0 7.0 10.0 17.0 24.0
9%
297.1 296.2 295.4 294. 294.0 293.5 293.1 292.7 292.3 292.0 291.4 291.3 291.3 291.4 291.5 292.3 293.1 293.8 294.3 296.8 298.6
r
---35*---.
---25°----
7--15°----
Bt 295.9 294.3 293.7 292.2 291.5 290.9 290.3 289.9 289.5 289,2 299.3 290.1 291.3 292.4 293.6 295.8 297.2 298.3 299.1 301 6 302.4 I
92
300.4 299.3 298.6 297.9 297.3 296.8 296.3 296.1 295.9 295.8 295.6 295.4 295.4 295.5 295.5 295 9 296.7 297.2 297.7 299.8 301.2 302.5 303.0 I
P2
299.1 297.6 296.5 295.5 294.9 294.3 294.1 293.9 293.9 294.0 294.4 294.9 298.5 296.1 296.9 298.6 300.0 301.0 301.6 303.5 304.1
,.. ...
92
Pa
302.9 302.2 301.5 301 1 300.6 300.3 300.0 299.8 299.6 299.5 299.3 299.2 299.1 299.2 299.3 299.7 300.2 300.6 301.0 302.8 303.7
301.9 300.8 300.0 299.4 298.8 298.4 298.2 298.0 297.9 298,O 298.4 298.9 299.6 300.1 300.7 301.8 302.7 303.4 303.9 305.6
I
~~~
0
Figure 4.
0.5
I .o
G
1.5
20
Apparent and partial molal volumes of
(n-CaH.i)aNBrin aqueous solutions a t 25”: -0-0-, ....--_-_, E.
rp~;
As can be seen from the figures, the ranges of applicability of the linear relation (2) for the tetraalkylammonium salts are m a l l , certainly siiialler than those of sinal1 electrolytes. More significantly, the sign of S, is found to be negative for all of the salts except
.... (12) See, for example, H. 8.Harned and B. B. Owen, “The Physical Chemistry of Electrolytic Solutions,” 3rd Ed., Reinhold Publishing Gorp., New York, N. Y . , 1958, pp. 360-364.
Volume 68, Number 9 September, 1064
2642
WHN-YANG WEK AND SHUJISAITO
Table 111: Apparent Molal Volumes a t Infinite Dilution and the Limiting Slopes for the Tetraalkylamrnonium Bromides Temp,,
OC. lDzO
15
S”
25 35 15 25 35
(CHdrNBr
(CzHdrNBr
(Ca HdrNBr
114.8
175.3
240.8
-3.3
-6.0
0.6
Figure 5 . Apparent and partial molal volumes of (n-C5H1&NBr in aqueous solutions a t 25‘: &O-,
---_-_ , 7-2 .
pz;
(CHJ4NBr. This negative and widely different X, is in contradiction to the Debye-Huckel limiting law. The discrepancy can be traced to the fact that the Debye-Huckel theory does not take into account the solute-water interaction which appears to be quite important in determining the apparent molal volumes of tetraalkylammonium salts even in dilute solutions. The partial molal volumes, of the five tetraalkylammonium bromides have been computed from 9 2 by the equation
v2,
L
v2
The values of so obtained are tabulated a t various molal concentrations, m, in Tables I and 11and plotted against dl in Fig. 1-5. The previous studies cited ab0vel-6’~all seem to indicate a presence of strong structural influence of large tetraalkylainnionium salts upon water. The structural influence has been described by words such as iceberg eff ect,13increase of ice-likenessJ3increase of icepatchesJg stabilizing of and tightening of The Journal of Physical Chemistry
(C4Hg)rNBr
300.0 302.9 304.8 -9.0 -8.4 -6.3
(CsHdrNBr
365.6
-8.3
hydrogen bonds of water molecules.16 To describe the structure of (C4Hg)rSBrsolutions under discussion we shall use the expression “formation of clathratelike structure.” As shown in Fig. 1, V2of (C4H9)4N3r gives a minimum at around 1.0 m (when the molar concentration c in the figure is converted to the molal concentration, m), The stoichiometric composition of the solution at the minimum V2 corresponds to (C4Hg)JJBr.(60 f 10)HzOwhich is somewhat different from the composition of a polyhedral clathrate hydrate, (C4HJ4NBr~32.8H20studied by Weng and by Jeffrey, et aL7 In spite of this stoichiometric difference, the existence of a clear minimum in V2 for the solution in this concentration range is considered as an indication that the structure is “clathrate-like.”16 The melting point of the solid hydrate is 12.5’, and the rigid structure cannot exist above this temperature. Yet the presence of a minimum v2 at 15, 25, and even at 35’ indicates that, in the Ijquid solution, the clathratelike interaction among the ions and water molecules still persists to a certain degree. It diminishes with the increase of temperature as shown by the shallowness of minimum a t 35’. This salt has been suspected to associate in aqueous solutions, but the nature of the association has not been clarified so far. In view of the above observation, the association of (C4H9)4S B r in water may be due to a clathrate-like arrangement and not necessarily due to the micelle formation as postulated by Lindenbaum and Boyd” to explain the low activity coefficients observed by them. At infinite dilution the cation and anion are far apart from each other, each affecting surrounding water in (13) H. 8. Frank and M. W. Evans, J . Chem. Phys., 13, 507 (1945). (14)
H. 8.Frank, Proc. Roy. SOC.(London), A247, 481 (1958).
(15) R. M. Diamond, J . Phys. Chem., 67, 2513 (1963). (16) T h e stoichiometric composition of the solution a t the minimum (see Fig. 1 and Table 11) corresponds to (ClH~)aNBr.(35f 2)HzO
‘PZ
which is rather close t o t h e solid hydrate composition. However, since t h e meaning of p 2 a t the relatively high solute concentration is somewhat complicated, the agreement of stoichiometry in this case may be accidental.
(17) S. Lindenbaum and G. E. Boyd, .J. Phys. Chem., 68, 911 (1964).
PARTIAL 1\/IOLAL
2643
VOLIJMES O F TETRAALKYLAMMONIUM BROMIDE SOLUTIONS
its unique way, (C4H9)4N+enhances water clusters around it while Br- slightly breaks the water structure. With the increase of the salt concentration, many cations and anions will come closer together. Water clusters surrounding the cations will begin to join with their neighbors and form flickering cages forcing the large cations as well as smaller anions to get inside these cages. This will cause the decrease of v z . With the concentration increase, pz continues to decrease until the solution reaches a stage at which clusters around cations and snions arrive at a maximum cooperation in constructing a clathrate-like structure. When the salt concentration is increased further, the number of water molecules will no longer be enough to form an adequate number of flickering cages to trap all the ions. With the gradual collapse of the clathratelike structure, i.'2begins to increase. Concerning the discrepancy between the stoichiometry of the solution having the minimum and that of the solid hydrate, we suspect it to be due to an inherent structural difference between the liquid and the solid. The liquid solution with the minimum v Z contains, in contrast to the solid hydrate, a considerable number of water molecules which are momentarily not participating in any hydrogen bonding. The inclusion of these extra nonhydrogen-bonded water molecules will necessarily make the solution more dilute than the melted hydrate. Conversely, when the solid hydrate is melted, a, certain fraction of water molecules will always be in a broken state and there will not be enough bonded water molecules to achieve the maximum cooperation with all the ions in the solution. This would result in a slightly higher Vz for the melted hydrate than the minimum value. As shown in Fig. 4 , Vz of (C3H7)4NBrgives a broad minimum a t around 2.2 rn concentration corresponding to the salt to water mole ratio of about 1:25. So far no crystalline hydrate of this salt has been reported in the literature. This may be taken to indicate that either a hydrate will be found in the future orl more likely, a minimum Vz a t a certain concentration In solution is not a sufficient condition for the existence of a crystalline hydrate. For the solutions of (C2Hs)bNBr and (C6H11)4NBrl as can be seen in Fig. 3 and 5, Yzalso decreases with the increase of salt concentration. Vz does not, however, show any minimum and never turns upward in these solutions. This observation is consistent with the fact that these salts do not form any polyhedral clathrate with high water of crystallization. l8 I n these solutions water clusters surrounding the cations will join and enhance each other, but they cannot form a clathrate-like structure because the cation is
vz
either too large or too small to support the cages. A rather low solubility of (C5HII)4NBrin water has severely limited the observable range of At 25O, S , is the largest negative for (C4H9)4NBr,a very close second for (C6H~1)4T\rTBr, third for (C3H,)4NBr, and the smallest negative for (CzH6)4NBr. These negative values of S , can be considered as a manifestation of the hydrophobic nature of these tetraalkylammonium ions and will be referred to hereafter as a hydrophobic effect. l9 For the (CHs)*SBr solution (see Fig. 2) S , is nearly zero (actually very slightly positive) and cpz is almost constant over the entire concentration range. This phenomenon is probably arrived a t by the balancing of two opposite effects the hydrophobic effect and the charge effect. The hydrophobic effect will tend to lower cpz with while the charge effect will tend to increase cpz with .\/E. For smaller electrolytes (such as RbBr or CsBr) cpz increases with -&because the overlapping of already electrostricted regions of water around ions will diminish the constrictive effect per ion. This consideration will lead us to conclude that, in the very vicinity of (CH3)4N+,water structures are under some influence of the charge. The influence of the charge becomes quite weak in case of (C2H6)4Xt and it may be entirely negligible for (C4Hd41\'+ and (C&)4Nf when compared with the dominating hydrophobic effect. If this is the case, we may proceed to make a coniparison of p0 values of tetraalkylammonium cations with those of hydrocarbons in water. cp+ O values of cations listed in Table IV are obtained by subtracting p - O of Br- (given by PadovaZ0as 25.638 ml./mole) from the cpzo of the corresponding salts. cp+ O values obtained from the cpzo data of Gilkerson and Stewart2] are included in Table I V for comparison. The agreement of the two sets of data is, in general, satisfactory. MastertonZ2has measured the cpzo of methane, ethane, and propane in water a t several temperatures with the results summarized in Table V. At 23O, methane occupies 36.3 ml./mole in water while ethane occupies 50.5 ml./mole corresponding to 25.3 ml./inole of methyl group. Propane occupies 66.6 ml./mole which corresponds roughly to 22.2 inl./mole of methyl
vz.
(18) G. Beurskens, G. A. Jeffrey, and R. K. McMullan, J . Chem. Phgs., 39, 3311 (1963). (19) In contrast to (CaH7)rNBr, the somewhat hydrophilic tetra-
ethanolammonium bromide, (HOCH&HdrNBr, has been found by us to give a small positive Sv in aqueous solutions. (20) J. Padova, J. Chem. Phgs., 39, 1552 (1963). (21) W. R. Gilkerson and J. L. Stewart, J . Phus. Chem., 65, 1465 (1961). (22) W. L. Masterton, J . Chem. Phgs., 22, 1830 (1954).
Volume 68, Number 9
September, 1964
WEN-YANGWEN AND SHUJISAITO
2644
Table IV : Apparent Molal Volumes of Tetraalkylammonium Cations at Infinite Dilution" Temp.,
........................
"C.
15 25 35 a 'pa
(CHdrN+
(CpHs),N+
149.6( 149.3)b
8 9 . 1 (88.4)b
for Br- taken as 25.7 ml./mole."
(p+o,
ml./mole
........................
(CsH j4N +
215.1 (214)b
(C4HshN
+
274.3 277.3 (279)b 279.1
(CaHiijrN
+
339.9
* Data in parentheses are obtained from Gilkerson and Stewart.21
group. For tetraalkylammoniuni ions i t 25O, po per methyl group is 15.1 ml./mole in going from (CHa)4N+ to (Cz&),N+, 16.4 ml./mole in going from (C2H5)4N+ to (CzH7)4N+, 15.6 ml./mole in going from (C3H7)4N+ to (C4Hg),N+, and 15.7 ml./mole in going from ((34H9)4N+to (C5Hl1)4N+. The temperature coefficient of (020 for the three hydrocarbons is in the range of 0.2-0.5 ml./mole deg., which happens to be the approximate value of dq+ O/dT for (C4H9)&+a t around room temperatures. Since the size of (C4H9)4N+ is much greater than the small hydrocarbons, this finding may be interpreted to indicate that water
clusters surrounding the large cation are stronger and better formed than those surrounding the hydrocarbon molecule. The thermal expansion of (C4Hg)4N+located inside a cluster cage would not increase the volume of solution significantly. Masterton's measurements could not supply information concerning the concentration dependence of p2 owing to the very low solubilities of hydrocarbons in water. From the similarities of water structure effect of tetraalkylammonium salts with that of aliphatic hydrocarbons, one can expect q2 of hydrocarbons to decrease with the increase of concentration. This would be plausible from the consideration of hydrophobic bonding of hydrocarbon molecules by the entropy effect of water.2a
Table V : Apparent Molal Volumes of Three Hydrocarbons a t Infinite Dilution'
Acknowledgments. We wish to thank Professor Henry S. Frank and Dr. Felix Franks for their reading of our manuscript and offering valuable comments and suggestions. This research is supported by the U. S. Department of the Interior, Office of Saline Water, through Grant No. 14-01-0001-306.
a
Temp.,
---____-
OC.
C H4
CaHs
CaHs
16.9 23.0 29.1
33.2 36.3 38.0
48.2 50.5 52.1
63.6 66.6 67.5
Data of Masterton.*2
The Journal of Physical Chemistry
czo, ml ./m ole--------
(23) W. Kauzmann, Advan. Protein Chem., 14, 1 (1959); G. NBmethy and H . A. Scheraga, J . Chem. Phys., 36,3401 (1962).
