164
T. L. BROADWATER AND D. FENNELL EVANS
Partial Molal Volume of [Bu,N-( CH,) ,-NBu,]Br,, a Large Bolaform Electrolyte, in Water at 10 and 25" by T. L. Broadwater and D. Fennel1 Evans1 Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 441 06
(Received August 19, 1968)
The apparent, bz, and partial, pg, molal volumes were determined for octane-l,&bis(tri-n-butylamn~onium) dibromide (DiBuBr2) in water at 10 and 25". Over the conc_entrationrange studied (0.3-1.2 M ) , the volume behavior was similar to that observed for BuANBr, with cp2 and V 2decreasing to a minimum, cp2(miii)= DiBuBrz. 100H20, and then increasing as the Concentration increased. The partial molal expansibilities,c p ~ of , ($E - 4 2 ) were positive, went through a maximum at the concentration where 42is at a minimum, and then decreased. The observed effects,including clathrate formation, all indicate that this large divalent salt affects water structure in the same manner as Bu4XBr. Introduction The partial molal volume behavior of symmetrical tetraalkylammonium halides has been investigated in aqueous2-s as well as nonaqueous solvents.g~10 It is firmly established from these studies as well as numerous others"J2 that the unusual concentration dependence of salts having ions with large hydrophobic surfaces arises from water structural effects. However, there is no general agreement on the exact nature of the ion-solvent interactions. Ion pairing,3r4,6micelle formation1l3 hydrophobic bonding,14 salting-inlsJ6 and cation-cation pairing2j7have been suggested to explain the anomalous concentration behavior. It has been pointed out that the first three alternatives listed above do not explain the observed volume behavior.' The compound studied here allows the implications of the last suggestion to be explored more thoroughly since DiBuBrz should serve as a good model for the Bu4N+ cation-cation pair. A second paper in this series will report the conductance behavior of cations with the general formula [(CnH~n+~)~N+-(CH2)~n-l?Tf(C,H~,+1)31, where n = 2, 3, or 4.'6 Experimental Section Density measurements were carried out below 0.5 il~? with two 25-ml Weld and three 30-ml capillary pycnometers. Above this concentration, three 5-ml Weld pycnometers were used. All calibrations were carried out using conductivity grade mater, and the procedure of Weissbergerl7 was used for measurements with the Weld pycnometers. Capillary pycnometers were filled with syringes equipped with long needles, and the unfilled portions of the capillaries were wiped clean with lens paper. The constant temperature baths were maintained at 25.000 f. 0.005 and 10.00 f. 0.01". The salt, was made by refluxing an excess of tributylamine with 1,8-dibromooctane in' ethanol. The excess amine was removed by extraction with ether and reThe Journal of Physical Chemistry
crystallizat,ions were carried out in methanol-ether mixtures (mp 123-125'). Anal. Calcd for C32H70' Br2N2: 59.80; H, 10.98; Br, 24.87. Found: 59-57'; H, 10.68; Br, 24.63.
c,
c,
Results The densities and molal concentrations at 25 and 10" are given in Table I, along with the corresponding values of 42. The latter were calculated from the density data using the equation 1 1000
42
-(
= m2
+ 1741 d
1000) do
where mz is the molal concentration of the solute, M is its molecular weight, cl is the density of the solution, and do is the density of the solvent. (I) T o whom all correspondence should be directed. (2) W. Y. Wen and S.Saito, J . Phys. Chem., 68, 2639 (1964). (3) L. G. Hepler, J. M. Stokes, and R. H. Stokes, Trans. Faraday Soc., 61, 20 (1965). (4) B. J. Levien, Aust. J . Chem., 18, 1161 (1965). (5) (a) B. E. Conway, R. E. Verrall, and J. E. Desnoyers, Trans. Faraday Soc., 62, 2738 (1966); (b) R. E. Verrall, unpublished Ph.D. dissertation, University of Ottawa, 1966. (6) H. E. Wirth, J . Phys. Chem., 71, 2922 (1967). (7) F. Franks and H. T. Smith, Trans. Faraday SOC., 63, 2686 (1967). (8) R. Gopal and M.A. Siddiqi, J . Phys. Chem., 72, 1814 (1968). (9) J. Padova and I. Abrahamer, ibid., 71, 2112 (1967). (10) W. Y . Wen, "Saline Water Conversion Report," Office of Saline Water, U. S. Department of the Interior, 1966, p 13. (11) T. L. Broadwater, Ph.D. Dissertation, Case Western Reserve University, 1968. (12) All of the pertinent references are too numerous to be conveniently listed here, but are given in ref 11. (13) S. Lindenbaum and G. E. Boyd, J . Phys. Chem., 68,911 (1964). (14) F. Franks and H. T. Smith, ibid., 68, 3581 (1964). (15) J. E. Desnoyers and M. Arel, Can. J . Chem., 45, 359 (1967). (16) D. F. Evans and T. L. Broadwater, to be published. (17) A. Weissberger, "Physical Methods of Organic Chemistry," 2nd ed, Vol. I, Part 1, Interscience Publishers, New York, N. Y., 1949.
165
PARTIAL MOLALVOLUMEOF [ B U ~ N - ( C H ~ ) ~ - N B U ~ ] B ~ ~ I
Table I : Densities, Molalities, and Apparent
1
I
I
I
0.6
0.8
I
Molal Volumes for DiBuBr?, m
Density
7.232 5.037 3.755 2.615 2.342 1,740 1.543 I.512 I.303 1.174 0.9842 0.7289 0.6499 0.6471 0.5420 0.4887 0.4241 0.3989 0,3494 0.3096 0.2932 0.2476 0.2400 0.2212 0.2092 0.2026 0,1900 0.1115 0.0932 0.0883 0.0799 0.0756 0.0631 0.0427 0.0317
1.08592 1.08061 1.07589 1.06911 1.06695 1.05911 1.05622 1.05543 1.05132 1.04862 1.04356 1.03488 1,03181 I.03184 1.02728 1.02485 1.02167 1.02047 1,01793 1.01568 1.01484 1.01216 1.01185 1.01076 1.01011 1.00976 1,00895 1.00423 1.00310 1 00278 1.00224 1,00195 1.00115 0 I99985 0.99914
9%
580.6 579.4 577 * 9 575.2 574.4 573.1 572.2 572.3 571.7 571 .O 570.5 570.8 571.0 570.7 571.3 571.6 572.2 572.2 572.6 573.4 573.5 574.6 574.2 574.5 574.4 574.4 574,9 575.9 576.1 576.5 576.6 576 9 577.3 577.6 577.8
Density
42
1.09620 1.09119 1.08680 1.07993 1.07807 1.06967 1.06700 1.06602 1.06156 1,05860 1.05265 1.04299
574.2 572.4 570.1 566.8 565.2 563.3 561.6 561.8 560.8 559.8 559.5 559 3 I
...
...
1.03925 1.03391
559.7 560.7
1.02744 1.02606 1.02314 1.02076 1.01960 1.01676 1.01622 1.01497 1.01425 1.01382 1.01289 1.00762 1.00636 1,00599
562.0 562.1 562.7 563.1 563.9 564,s 564.9 565.4 565.3 565.4 566.2 567.4 568.0 568.5
...
...
0.2
0.4
1.0
c1/2
Figure 1. Apparent molal volume (AMV) and partial molal volume (PMV) for DiBuBrn a t 10 and 25'.
Values of b+/dd/C were determined graphically from curves drawn through the 42 data. Plotted in Figure 1is the concentration dependence of +2 and 7 2 a t 25 and 10". Partial molal expansibilities, $E, were calculated a t different concentrations from the 42data a t 25 and 10". The resulting curve is plotted in Figure 2, along with data for Bu4NBr, Ka2S04,and KC1.
these salts decrease, go through a minimum, and then begin to increase. The value of '520 becomes smaller and the minima become more pronounced as the temperature decreases.2 I n contrast, for salts which are small and electrostrictively hydrated, the increase of ' 5 2 with concentration is reasonably linear.' Shown in Figure 1 are the apparent and partial molal volumes of DiBuBr2 a t 10 and 25". The curves display a dependence upon concentration similar to that observed for the large univalent tetraalkylammonium salts. The aqueous solution behavior has been explained for R&+ halides in terms of cation-cation ~ a i r i n g . ~ , ~ It was proposed that the increasing concentration of these large ions eventually led to cooperative networks in which the cations were forced to share sheaths of water. Time-averaged solution structures similar to the crystalline clathrates studied by Jeffrey and his associates1* were assumed to make a contribution. The sharing of water layers would place the cations in close proximity and presumably the anions would participate in the structure t o reduce the coulombic repulsion. The decrease in 42 was attributed to the association of more and more ions until the minimum, corresponding to a maximum solution structure, was reached. A further increase in concentration would break down the open hydrogen-bonded structure. Further evidence for cation-cation pairing has recently been offered by Wen and XaralDfrom data on the
Discussion The partial molal volume of tetraalkylammonium compounds has been studied over a large concentration range and as a function of temperature.2-s Above a concentration of -0.01 M , 4z and V2 for the larger of
(18) D. Feil and G. A. Jeffrey, J . Chem. Phys., 35, 1863 (1961); M. Bonamioo, R. K. McMullan, and G. A. Jeffrey, ibid., 37, 2219 (1962); 39, 3295 (1963). (19) W.Y . Wen and K. Nara, J . Phys. Chem., 71, 3907 (1967); 72, 1137 (1968); W. Y. Wen, K. Nara, and R. H. Wood, ibid., 72, 3048 (1968).
I
I
... ...
... ... ...
... ...
...
... 9 . .
Partial molal volumes were calculated from the 42 data using the relationship
Volume 78, Number 1 January 1969
166
T. L. BROADWATER AND D. FENNELL EVANS I
I
I
I
0
-5
-
O U
I>
I N
2 -10
KCI
I
0.5
1.0
1.5
2.0
c 1/2 Figure 2. Apparent molal expansibilities of DiBuBrt, BudNBr, NazSOd,and KCl.
-i5 I
I
I
I
I
I
0.2
0.4
0.6
0.8
1.0
1.2
cl/2
Figure 3. Concentration dependence of the partial molal volume for Bu4NBr and DiBuBm.
volume increases observed when R& halides are mixed with alkali metal halides. The data were analyzed in terms of Friedman’s ionic solution theory,20 which, when applied to the mixing of salts AX and BX, describes the volume change in terms of structural parameters due to an ion i in the vicinity of an ion j. The term involving i = j = R4N+ was found to be predominant where P q N + and Bu&+ were studied. In Figure 3, the concentration dependences of BudKBr and DiBuBrz are compared and the similarities in behavior are evident. The curve for the larger salt has the more negative slope and undergoes a larger decrease in $2. This is probably a reflection of the fact that more water per ion is being affected. The minima in the $2 curves correspond to BudNBr’55H20 and DiBuBrz 100HzO. The partial molal volumes a t infinite dilution, obtained by extrapolating the negative portion of the ~$2 curve to C = 0, were ( T‘.zo)2j = 580.9 It: 0.5 and ( Vzo)1o = 575.4 0.5. The large uncertainties are due to scatter in the density data a t low concentrations. In addition, the values obtained are too large because the extrapolation to zero concentration ignores the contribution from the limiting Debye-Huckel term which produces a positive slope for the region below 0.01 A I . The magnitude of this error can be estimated by constructing the limiting slope of 9.G62* so that it intersects the qi2 curve at C’’z = 0.16, this being the approximate concentration where the theoretical and the extrapolated experimental curves intersect for PrdNBr and BudNBr.’ At 25’, this procedure gives VZo= 577.9, a value which is 3.0 cmE/mol lower than the linear extrapolation. The value of Vzo for +
The Journal of Physical Chemistry
PrdNBr and BudNBr, obtained by Wen and Saito2by linear extrapolation, is too large by a value of 2.6 cm3/ mol, as can be determined from the data of Franks and Smith.’ Subtracting the value of VzO(Br-) = 30.2 gives a ratio Vzo(DiBu2+)/Vz0(Bu4N+)= 517.5/270.2 = 1.91. The volume decrease on the formation of a cation-cation pair for Bu4N+ ions has been evaluated by Wen, Nara, and Woodl9 to be 8.7-7.3, 5.4-9.2, and 7.2-11.5 cm3/mol a t ionic strengths of 1, 0.5, and 0.2, respectively. A value a t infinite dilution of 11 i2 cm3/ mol can be estimated by extrapolating to zero concentration the values at 0.5 and 0.2 ,u. An independent estimate of this volume decrease can be gained from the following simple calculation: [(2) (270.2) 517.51 = 22.9 cm3/mol. The magnitude of the value obtained for DiBu2+gives credence to our contention that this ion is a good model for a Bu4N+cation-cation pair. The similarities in behavior of the two cations are further emphasized by the fact that a crystalline clathrate was found for DiBuFz which melts at 5” and has approximately 40 waters as determined by the Karl Fischer titration method, The exact number of waters is uncertain because of the difficulties of obtaining a sample for titration free of adsorbed water. The exact composition can probably be resolved by X-ray analysis. The apparent molal expansibilities for DiBuBrz are
-
(20) H. L. Friedman, J. Chem. Phys., 32, 1134 (1960) ; H. L. Friedman, “Ionic Solution Theory,” Interscience Publishers, New York, N. Y . , 1962. (21) 0. Redlioh and D. M. Meyer, Chem. Rev., 64, 221 (1964).
A
DIiiIENSIONLESS CONSTANT CHARACTERISTIC O F
167
GASES
plotted in Figure 2 along with data for BurNBr, il‘a2S04, and KC1.2t22 The curves for both BurnTBr and DiBuBr, differ from the others in that they are positive, go through a maximum, and then decrease. The maximum for both curves occurs at the concentration where the & data pass through a minimum. The behavior shown for KC1 and Na2S04 is typical of small electrolytes.22 The curve for KCl does become positive at low concentrations (
