Journal o f Solution Chemistry, Vol. 6, No. 9, 1977

Apparent Molal Volumes of Aqueous Solutions of Bis-tetraalkylammonium Salts at 25~ Methylene-Group Contribution to the Limiting Molal Volume

Patrick Carpenter 1 and Siegfried Lindenbaum 1'2 Received April 20, 1977

Densities o f aqueous solutions of a series of polymethonium chloride and bromide salts (CH3)3-N-(CHz),-N-(CH3),~Xz have been measured at 25~ Apparent molal volumes have been calculated, and methylene-group contribu- tions to the limiting apparent molal volumes ~b v have been estimated. Constant values o f the methylene-group contribution of 16.5 and 17.0 cma-mole -1 were obtained for the bromide and chloride salts, respectively. These values are consistent with methylene-group contributions reported for other series o f organic electrolytes.

KEY WORDS: Density; apparent molal volume; bolaform electrolytes; bis-quaternary ammonium salts; methylene-group contribution.

1. I N T R O D U C T I O N

In an earlier study, (1) enthalpies of dilution were measured for aqueous solutions of a series of polymethylene a,o~-bis(trimethylammonium) salts, (CH3)3N(CH2),N(CHa)3X2 (X = Br or C1). The heat of dilution AHD for the chloride and bromide salts was found to increase with increasing n (the number o f - C H 2 - groups separating the positively charged nitrogen atoms) until a maximum value is reached at n = 5. For values of n greater than 5, the heat of dilution decreased with increasing n.

It was speculated at that time that the maximum was due to a competi- tion between water-structure-making and water-structure-breaking effects.

1 Pharmaceutical Chemistry Department, University of Kansas, Lawrence, Kansas 66045.

2 To whom correspondence should be addressed.

581

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582 Carpenter and Lindenbaum

The purpose of the present investigation is to determine the apparent molal volumes of this series of salts in order to observe whether a constant incremental increase in r would be obtained as n is increased, or whether a change in slope of r vs. n would be observed at n = 5 corresponding to the maximum in the AnD values.

2. E X P E R I M E N T A L

Pentamethonium dibromide (n = 5) was obtained from K & K Lab- oratories, Inc.; hexamethonium dibromide (n = 6) and decamethonium dibromide (n = 10) were obtained from Fluka. The compounds were re- crystallized twice from absolute ethanol and ether, and dried to constant weight under vacuum at room temperature.

The remaining dibromide salts were synthesized using the appropriate ~,oJ-dibromoalkane and trimethylamine. A fivefold excess of trimethylamine (25~o in methanol) was refluxed at 40~ for 48 h with the dibromoalkane. For the last 3 to 4 h, the temperature was increased to 60~ The salt was collected by evaporating the excess methanol and trimethylamine, recrystallizing from ethanol/ether, and drying to constant weight at room temperature.

The dichloride salts were prepared from the dibromide salts as previously described (2~ by converting the bromide salt to the hydroxide salt with freshly precipitated silver hydroxide, and neutralizing with hydrochloric acid to obtain the chloride salt.

The purity of all salts was determined by argentometric titration of halide ions. The purity in most cases, by this criterion, was better than 99.0~o. Even after repeated recrystallization, the trimethonium salts yielded purities of only 97.1 and 98.670 for the bromide and chloride salts, respectively.

Aqueous solutions of the bis-quaternary ammonium salts were prepared by weight using deionized water.

Densities were measured using a SODEV-model 01D densimeter38~ In this instrument, the solutions are circulated through an oscillating tube, the natural vibration period of which is read on a digital frequency meter. The density of the solution circulating in the tube is obtained from

d = d o + B ( r 2 - r~) (1)

where do and ~0 are the density and period of oscillation for pure solvent, and d and ~- are the corresponding quantities for the solution. The quantity B is a constant determined by calibration using nitrogen gas and distilled water. The densimeter was thermostated at 25.00~ by pumping water through the thermostating coil of the instrument from a constant-temperature

Aqueous Solutions of Bis-tetraalkylammonium Salts at 25~ 583

bath maintained constant during the course of an experiment to within 0.001 ~

3. RESULTS AND DISCUSSION

Apparent molal volumes 4v were calculated from the following equation:

1 [z00o + raM2 1000] 4v = m a ao J (2)

where m is the molality of the solution, and M2 the molecular weight of the solute. The density of pure water do is taken as 0.997047 g-cm-*. <*>

The data were fitted by least squares to

4v -- S , ,C zl= = 4 ~ + a C + b C 2 (3)

where 4 ~ = po is the limiting partial molar volume, S~ is the Debye-Hiickel limiting slope (9.706 cm3-mole-a/2-1iter 1/2 for a 2:1 electrolyte), <m and C is the molarity (moles-din-3). The data are shown as graphs of (4~ - S~ CI/~) vs. C in Figs. 1-4. The circles in each case represent the data as calculated

255

254

25~ 252

239 2 3 8

257

z~

222

~ 221

2r9

O

~ P en~u Br 2

�9

Tetro Br 2

o

Tri Br 2

o; o12 o'3 c

Fig. 1. Debye--Htickel limiting law corrected apparent molal volumes of trimethonium, tetramethonium, and pentamethonium dibromides at 25~

584 Carpenter and Lindenbaum

� 9 ~ O o t a Br 2 503 3O2 � 9

289 ~ 0

287 O -~ 286 Hapto Br 2

-e- 273 0 272 271 270 0

Hexe Br=

i i L

O. I 0.2 0.3 r

Fig. 2. Debye-Hiickel limiting law corrected apparent molal volumes of hexamethonium, heptamethonium, octamethonium, and decamethonium dibromides at 25~

222

2 2 1 ' ~ C 220 t 2

204

203 O0 ~ o~ 202 O0

201 Tti CI2

0.1 0.2 0.3 c Fig. 3. Debye-Hfickel limiting law corrected apparent molal volumes of trimethonium

and tetramethonium dichlorides at 25~

Aqueous Solutions of Bis-tetraalkylammonium Salts at 25~ 585

321 ~ c a CI 2

288

287 Cla

258 �9

257 �9

; . ?.55

%

.3 z4f 240

�9

258

O.I 0.2 0.3 c

Fig. 4. Debye--H~ickel limiting law corrected apparent molaI volumes of pentametho- nium, hexamethonium, octamethonium, and decamethonium dichlorides at 25~

from Eq. (2), and the curves are calculated from the least-squares fits to Eq. (3). Values of r and the coefficients a and b obtained from the least-squares fits are listed in Table I. The incremental differences in r and Ar are also listed. Values of Ar are the methylene-group contributions to the limiting partial molal volumes.

Apparent molal volumes were reported recently for hexamethonium and decamethonium bromides. (6) The results reported here are in fairly good agreement with the published Cv values, although the r values of this investigation are approximately 1% higher.

The excellent linearity of the graphs of 4~ vs. n (Fig. 5) indicates that there is a virtually constant incremental contribution to the 4'~ value for each additional methylene group. The slopes of these graphs yield values of the methylene-group contribution of 16.50 and 17.00 cm3-mole-1 for the bromide and chloride salts, respectively. These results are in good agreement with measurements of apparent molal volumes of other series of compounds for which incremental -CH2- group contributions have been estimatedF -19)

586 Carpenter and Lindenbaum

Table I. Apparen t Molal Volumes of Bis-quaternary A m m o n i u m Halides

~ a b A~.

Bromide salts

Deea 336.25 - 56.47 161.49 15.78 (average)

Octa 304.69 - 60. 31 174.18 16.33

Hepta 288.36 - 56.94 170.37 16.22

Hexa 272.14 - 58,62 229.79 17.60

Penta 254.54 - 53,32 170.64 16.24

Tetra 238.30 - 63.40 213.84 17.40

Tri 220.90 - 68.22 241.79

Average methylene group contribution: 16.50

Chloride salts

Deca 323.15 - 166.85 1531.2 16.72 (average)

Octa 289.72 - 53.04 138.97 16.56

Hexa 256.61 - 30.61 35.67 16.71

Penta 239.90 - 32.59 46.01 17.90

Tetra 220.00 - 27.72 32.01 18.63

Tri 203.37 - 56.39 185.30

Average methylene group contribution: 17.00

Most of the values reported vary between 15 to 17 cma-mole-1 per methylene

group. Since the slopes of Fig. 5 are almost identical, it is possible to calculate an average difference between bromide and chloride ~ contr ibut ion. The average value of 4v(Br) - ~(C1) was found to be 7.7 _+ 0.8 cm3-mole -1 This value is slightly larger than the chlor ide-bromide difference reported for other bolaform electrolytes for which an average value of 6.9 was reported.(l~, 20) The difference between limiting apparent molal volumes of

inorganic bromide and chloride salts yields values for the chlor ide-bromide difference ranging from 6.8 to 7.3 cm3-mole - 1. The values reported here are consistent with these differences within the precision of the incremental values.

Aqueous Solutions of Bis-tetraalkylammonium Salts at 25~ 587

Fig. 5. Limiting apparent molal volumes (ff~ = V~) vs. n, the number of methylene groups separating the N atoms of ~,~o-bis(tri- methylammonium) chlorides and bromides. Least-squares fits to these data yielded the equation ~ = 153.79 + 17.00n for chloride salts and ff~ = 172.21 + 16.50n for bromide salts.

340

320

3O0

280

260

240

220

200

Bromides

Chlorides

2 4 6 8 1O

n

R E F E R E N C E S

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R. A. Home, ed. (Interscience, New York, 1970), Chap. 15. 6. J. R. Johnson and R. Fleming, J. Phys. Chem. 79, 2327 (1975). 7. M. Sakurai, Bull. Chem. Soc. Jpn. 46, 1596 (1973). 8. M. Sakurai, J. Solution Chem. 4, 511 (1975). 9. J. E. Desnoyers and M. Arel, Can. J. Chem. 45, 359 (1967).

10. S. Cabani, G. Conti, and L. Lepori, J. Phys. Chem. 76, 1338 (1972). I1. S. Cabani, G. Conti, L. Lepori, and G. Leva, J. Phys. Chem. 76, 1343 (1972). 12. R. E. Verrall and B. E. Conway, J. Phys. Chem. 70, 3961 (1966). 13. L. H. Lalibert6 and B. E. Conway, J. Phys. Chem. 74, 4116 (1970). 14. M. Manabe and M. Koda, Bull. Chem. Soc. Jpn. 48, 2367 (1975). 15. J. A. Burns and R. E. Verrall, J. Solution Chem. 3, 289 (1974). 16. T. Nakajima, T. Komatsu, and T. Nakagawa, Bull. Chem. Soc. Jpn. 48, 783 (1975). 17. H. H~iland, J. Chem. Soc. Faraday Trans. 1 71,797 (1975). 18. W.-Y. Wen, A. LoSurdo, C. Jolicoeur, and J. Boileau, J. Phys. Chem. 80,466 (1976). 19. C. Jolicoeur and J. Boileau, J. Solution Chem. 3, 889 (1974). 20. G. Perron and J. E. Desnoyers, J. Chem. Eng. Data. 17, 136 (1972).