Dee., 1963
EXTRACTION OF TETRAALKYLAMMONIUM HYDROXIDES
free-radical chains might tend to concentrate crosslinks locally and give a broader distribution of strand lengths than a random chemical reaction. Loss Mechanisms at Long Times and Low Frequencies.--Accord ing to the theories quoted above, 21-24 tan 6 should vanish a t low frequencies and H and L should vanish a t long times, approaching ideal elastic behavior. The norrsulfur vulcanizates deviate the most from this prediction, and further study of the differences may provide clues concerning the origin of these puzzling low-frequency losses. The heterogeneity of strand lengths alluded to above may be a partial sourco of the higher losses of the nonsulfur vulcanizates, but it seems unlikely that the entire effect can be attributed to it. Another source may be in the length, and consequently the mobility, of the crosslinkage itself. In vulcanizates with radiation30 or dicumyl peroxide,a1the backbone chain carbon atoms of the rubber strands are believed to be bonded directly, forming a rather bulky junction. In vulcanizates with tetramethylthiuram disulfide, there is evidence that the chain backbones are joined with a single interstitial sulfur t t t ~ m . In ~ ~conventional sulfur vulcanizates, on the other hand,3334 the chain backbones are believed to be joined through bridges of three or more sulfur atoms, in some cases considerably more. Such a junction would be expected to have considerably less frictional resistance to motion in its environment, and to dissipate less energy in coordinated motions involving groups of linked strandsbZ2 However, it (33) hf. L. Studebaker and L. G. Xabols, Rubber Chem. Technol., 32, 941 (1959). ( 3 4 ) C. G. 17Ioore and B. R. Trego, J . A p p l . Polymer Sci., 5, 299 (1961).
2785
does not seem possible to relate the observed differences to any quantitative theory thus far available. A third possible source of the low-frequency losses is the effect of loose ends (ie,,network strands attached a t one end only) on the motion of network junctions as treated by Bueche.3.28 This would predict an increase in tan 6 with an increase in the ratio M c / M n , where Mn is the number-average molecular weight before vulcanization, in agreement with the results for the three sulfur vulcanizates for which Mn is presumably the same. From the functional form of Bueche's equations, however, it does not appear that they can describe quantitatively the persistence of loss mechanisms to such very low frequencies in the nonsulfur vulcanizates. In accordance with the principles of linear viscoelasticity, the losses observed a t very low frequencies should be related to the very slow relaxation mechanisms observed in stress relaxation experiment^.^,^^ Gent6 has reported measurements of both tan 6 arid rate of stress relaxation on several vulcanizates, among which higher values mere obtained for one cross-linked with tetramethylthiuram disulfide than for most of the sulfur cures cited. It would be of interest to correlate these two types of measurement in further detail. Acknowledgments.-This work was supported in part by a grant from the National Science Foundation, and in part by the Research Committee of the Graduate School of the University of Wisconsin from funds supplied by the Wisconsin Alumni Research Foundation. (35) Reference 3, p. 151. (36) P. Thirion, Rev. gen. Caoutchouc, 39, 611 (1962).
THE EXTRACTION OF TETRAALKYLAMMONIUM HYDROXIDES AND THE SOLVATION OF THE HYDROXIDE ION1& BY
BHARAT R. A G A R W AAKD L ~ ~a.11.DI.4MOND
Lawrence Radiation Laboratory, University of California, Berkeley, Calijornia Received July 18, 1965 The extraction of tetrahexyl- and tetrapentylammonium hydroxides into solutions of alcoho!s in benzene and of tetrabutyl- and tetrapropylammonium hydroxides into dilute solutions of alcohols in nitrobenzene has been studied. The variation of the extraction into benzene as a function of the aqueous base Concentration a t a fixed alcohol concentration indicated that the extracting ions associate as ion pairs. The dependence of the extracting species on alcohol concentration at a fixed aqueous base concentration indicated that three molecules of alcohol are complexed by each extracted ion pair of the base. A similar study with a solution of benzyl alcohol in nitrobenzene of fixed concentration indicated that the extracted species in Buch a high dielectric constant solvent is in the form of two dissociated ions. The extractions of the smaller substituted ammonium bases into varying concentrations of alcohols in nitrobenzene also showed that three alcohol molecules are bound to the extracted hydroxide as observed with the associated systems in benzene. I t is suggested that the OHion alone provides the three sites for hydrogen bonding to the alcohol molecules, and a structure for the extracted species is proposed. The tricoordination of the OH- ion is considered similar to that of the hydronium ion which has been established in earlier studies.
Introduction The nature of the solvation of the hydroxide ion has received relatively little attention2 compared to the discussions in the litera,ture3-10on the hydration of the (1) (a) This work was done under the auspices of the U.
S.Atonlfc Energy
Commission; ( b ) appointment supuorted by the International Coopetation Administration under the Visiting Research Scientists Program administered by the National Academy of Sciences of the United States of America. (2) (a) T. Ackermann, Discussions Faraday Soc., 24, 133 (1957); (b) 1-1.6. Frank, Abstract8 of Papers, IT, Division of Physical Chemistry, 140th liational Meetihg of the American Chemical Society, Chicago, Ill., Sept., 1961.
(3) E. Wicke, M. Eigen, and T . hckermann,
Z.physik. Chem. (Frankfurt),
1, 340 (1954). (4) E. Glueckauf and G. P. Kitt, Proc. Roy. Soc. (London), A228, 32% (1955). ( 5 ) K. N. Bascombe and R. P. Bell, Discussions Payadag Soe., 24, 168
(19.57). (6) AI. Eigen arid L. UeMaeyer, in "The Structure of Electrolyte Solu-. tions," W. J. Hamer, Ed., Jobn Wiley and sohs, Inc., New York, N. Y.,1959. (7) H. D. Beckey, Z. N a t ~ ~ r f o ~ s c14% h . , 712 (1959). ( 8 ) (a) R . M. Diamond, d . rhus. Chem., 63, 659 (1959); (b) H . M. T h rnond and D. G. Tuck, in "Progress in Inorganic Chemistry," Vol. 11, F. A. Cotton, Ed., Interscience Publishers, Inc., New York, N. Y . , 1960. (9) E. Hogfeldt, Acta Chem. Scand., 14, 1597 (1960). (10) D. C. Whitney and R.. M.Diamond, J . Phys, Chern., 6'7, 209 (19ci3).
BHARAT R. AGARWAL AND R. M. DIAMOND
2786
hydronium ion, HaO+. For the latter ion, the existence of three sites of partial positive charge (the protons) leads quite naturally to a model in which three water molecules are hydrogen-bonded tightly in the first hydration shell. Additional water molecules are bound in further shells, less and less tightly as the distance from the ion increases, until the structure far enough out merges into that of ordinary water. The hydronium ion (and its trihydrate) is probably pyramidal, but an indication of the strength of the hydrogen-bonding of the three waters in the first shell is given by the calculations of Grahn'l for a planar hydronium ion model. He calculates that the hydrogen bond holding each of these three water molecules to the H 3 0 +ion has an energy of -45 kcal. far larger than the ordinary water-water bond of -6 kcal. Similar considerations suggest that the hydroxide ion in aqueous solution also might hold a first shell of three water molecules by unusually strong hydrogen bonds and so be trisolvated too. Ackermaiinlzafrom a comparison of the specific heats of dilute solutions of strong acids and bases has suggested such a model, and Frank2bhas made a similar proposal on the basis of the similarity of the broad Raman bands of HC1 and KOH solutions, the relation to each other of the single ion partial molal entropies of H+ and of OH-, and the rates of proton transfer in acidic and basic solutions. The present authors hoped that experimental verification of the trisolvation of the OH- ion could be obtained from solvent extraction studies by a method similar to that used recently to investigate the solvation of the hydronium ion.l"I12 In this latter work, the extraction of strong acids from aqueous solutions into dilute solutions of tributyl phosphate (TBP) or trioctyl phosphine oxide (TOPO) in octane, CCl,, or xylene, etc., was measured as a function of the (small) TBP or TOPO conceiitration in the organic phase. All other components were either held constant or corrected to a constant value. From the power of the extractant dependence, it was shown that three T B P molecules are involved in the extracted complexes of the strong acids HC1O4, HRe04, HBr, HAuC14, and HAuBr4, and that three TOPO molecules are bound to extracted HC104, HRe04, and HAuCL. With the HClOl and HBr systems, it was possible to show further that a minimum of one water molecule (to form the HaOf ion) is involved in the complex. To apply the same sort of technique to study the solvation of the OH- ion, conditioiis for the successEuI extraction of a strong base into a dilute solution of an extractant in an "inert7' diluent had to be found. A necessary requirement for the extractant molecule is that it be weakly acidic so that it contains a hydrogen atom capable of (hydrogen-) bonding to the hydroxide ion, but not so strongly acidic so as to react to form a salt and water. The large water-insoluble alcohols seemed likely candidates, and were, in fact, successfully used. Two important conditions or1 the bases used are that the catioii of the base be as hydrophobic as possible, in order to promote the distribution into the dilute organic extractant solution from the aqueous phase, and that the cation not be capable of bonding to the (11)
R. Grahn, Arlciv Pysilc, 22, 13 (1962).
(12) D. C. Whitney and R. M. Diamond, J . Phys. Chhem., 67,2583 (1963).
Vol. 67
extractant molecules. The latter requirement is necessary so that the observed dependence of the extraction on the alcohol concentration would be due only to the solvation of the OH-, and not to solvation of both the cation and OH- ion. The tetraalkylanimonium ions seemed the best choice, and were used in the present study. Experimental Reagents.-Tetrapropylammonium hydroxide, TPrAH, was obtained from Eastman Organic Chemicals and standardized against hydrochloric acid using neutral red as indicator. Tetrabutylammonium hydroxide, TBAH, tvaa prepared from the bromide salt (Eastman Organic Chemicals) by passing an aqueous solution of the Iatter through a column of strongly basic anion exchange resin (Dower: AGI-XX, 100-200 mesh, Bio-Rad Laboratories, Richmond, California) in the hydroxide form. It was standardized in the same way as the TPrAH. Tetrapentylammonium hydroxide, TPAH, and tetrahexylammonium hydroxide, THAH, were prepared by shaking suspensions of their iodides (Eastman Organic Chemicals) in water with freshly prepared silver oxide. These were standardized using phenol red as indicator. Benzyl alcohol (Cl free) and 4-methyl cyclohexanol (both from Eastman Organic Chemicals) were dissolved in nitrobenzene (Eastman Organic Chemicals) to give stock solutions of 0.100 M and 0.400 M concentration, respectively. Other solutions were prepared by dilution. Similarly, stock solutions of 0.10 iM decyl alcohol (Matheson Coleman and Bell) and of 0.15 iM benzyl alcohol were prepared in benzene (Allied Chemicals, New York) and less concentrated solutions obtained by volumetric dilution. Procedure.-The base (2 ml.), when using TPrAH and TBAH solutions, waswdded to 20 ml. of the dilute alcohol solutions in the organic solvent. When TPAH and THBH solutions and benzene solutions of the alcohol were used, the ratio of the volumes of the base to the organic phase was 5 to 15. The mixtures were shaken for 1 hr. on a mechanical shaker; shaking for a longer period (up to 12 hr.) did not change the results, indicating that equilibrium had already been achieved. Then the mixture was centrifuged and the two phases were separated. Two 5-ml. aliquots of the organic phase were shaken with 2 nil. of water for about 15 min. and titrated against 0.0100 A T HCI. Similar ext'ractions were performed with no alcohol present in the organic phase in order to determine the correction for the extraction into the solvent alone. All experiments were done a t room t,emperature, 22 =k 2".
Results The data for the extraction of the OH- ion into the organic phase when varying concentrations of THAH were added to 0.100 ilf benzyl alcohol in benzene are given in Table I. Explanations for the corrections made in order to arrive a t the results listed in columns 3 and 4 of Table I are given in the Discussion section. Similarly, the data for the extraction of varying concentrations of THAH into 0.100 flil decyl alcohol in benzene are given in Table 11,for THAH into 0.075 M benzyl alcohol in benzene in Table 111, and for TPAH into 0.100 144 benzyl alcohol in benzene in Table IV. There was negligible extraction of TPAH and of THAH into pure benzene. TABLE I EXTRACTION OF THAH INTO 0.100 M BENZYL ALCOHOL IN BENZENE Equil. base concn. (as.)
1.36 X 10-l 9.50 x 1 0 - 2 6.00 X 4.50 x 3.10 X IO-e 1.70 x 10-2 1.39 X 10-2 9.20 X
Equil. base concn. (org.)
8.60 X 8.80 x 6.96 X 5.28 x 3.16 X 1.10 x 7.60 X 3.80 X
10-3 10-3 10-3 10-3
10-3
Equil. alcohol conan.
7.42 7.36 7.91 8.42 9.05 9.67 9.77 9.89
X
x
10-2
X
X X IOW2 x 10-2 X X
Corrected equil. base concn. (org.)
2.11 X IOTa 2.21 x 10-2 1.41 X 8.85 X 4.26 X 1.22 x 10-3 8.06 X 3.93 X
EXTRACTION OF TETRAALKYLAMMONIUM HYDROXIDES
Dec., 1963
TABLE I1 EXTRACTION OF THAH INTO 0,100 M DECYL ALCOHOL IN BENZENE Equil. base concn. (aq.)
Equil. base concn. (org.)
1.53 X 10-1 1.10 X 10-1 7.45 X 10-2 3.87 X 1.58 x 10-2
3.18 X 3.48 X 2.18 X 6.00 >< lo-* 1.20 x 10-4
Equil. alcohol concn.
9.05 8.96 9.35 9.82 9.96
X X X X
x
10-2
Corrected equil. base concn. (ore.)
4.29 X 4.83 X 2.67 X 6.32 X 1.21 x 10-4
TABLE I11 EXTRACTION OF THAH INTO 0.075 M BENZYL ALCOHOL IN BENZENE Equil. base concn. (aq.)
1.49 X 1.08 X 6.92 X 5.24 X 3.57 x 1.86 x 1.52 x
lo-' 10-1 10-2 10-2
Equil. base concn. (org.)
4.40 )( 10-3 4.64 )( 3.94 >( 10-3 2.88 X 10-8 1.60 X 4.80 X lod4 3.20 >: lom4
Equil. alcohol concn.
6.18 6.11 6.32 6.64 7.02 7.36 7.40
X X X lob2 X X IOm2 X X
Corrected equil. base concn. (org.)
7.87 X 8.57 X 6.59 X 4.16 X 1.95 X 5.08 X 3.53 X
5.64 X 3.00 X 2.24 X 1.51 X 1.13 X 7.53 X 5.07 X
lo-' lo-' lo-'
lo-' lo-' 10-2
Equil. base concn. (org.)
1.90 x 1.82 >I: 1.38 X 7.80 x 5.00 x 2.40
x
10-3 10-3 10-3 10-4 10-4 10-4
1.00 x 10-4
Equil. alcohol concn.
9.43 x 1 0 - 2 9.45 x 1 0 - 2 9 59 X 9.77 x 1 0 - 2 9.85 x 1 0 - 2 9.93 x 1 0 - 2 9.97 x 10-2
Corrected equil. base concn. (org.)
2.26 x 2.15 x 1.57 X 8.38 x 5.23 x 2.45 x 1.01 x
10-3 10-3 10-4 10-4
10-4 10-4
The extractions of OH- into the organic phase when a given volume of a fixed initial concentration of THAH or TPAH was added to varying concentrations of benzyl alcohol in benzene are given in Table V (initial concentration of TECAH equals 0.162 M ) , Table VI (initial concentration of TPAH equals 0.305 M ) , and Table VI1 (initial concentration of THAH equals 0.061 M ) . TABLE V EXTRACTION OF 0.162 M THAH INTO BENZYL ALCOHOL IN BENZENE Alcohol concn. 1.50 X 10-1 1 . 0 0 x 10-1 7 . 5 0 X 10-2 5.00 X 10-2 3.50 X 10-2 2.50 X 10-2
Equil. base concn. (org.) 2.04 X 10-2 S . B O x 10-3 4,bO X 10-8 1.76 X 10-3 7.650 X 10-4 3.60 X 10-4
Equil. base concn. (as.) 1.01 X 10-1 1 . 3 6 x 10-1 1.48 X 10-1 1 . 5 7 X 10-1 1.60 X 10-1 1 . 6 1 X 10-1
Corrected equil. base concn. (org.) 1 . 8 x 10-2 7 . 9 x 10-3 4.35 x 10-3 1 . 7 x 10-3 7 . 5 x 10-4 3 . 6 x 10-4
Corrected alcohol concn. 8.90 x 10-2 7 . 4 2 x 10-2 6 . 1 5 X 10-2 4.47 x 10-2 3.27 X 10-2 2.39 X 10-'2
TABLE VI EXTRACTION OF 0.305 i W TPAH INTO BENZYL ALCOHOLI N BENZEXE lllcohol concn. 1.60 X 10-1
1 . 0 0 x 10-1 7 . 5 0 X 10-2 5 . 0 0 X 10-2
Equil. base concn. (org.) 6.40 X 10-3 1 . 8 8 x 10-3 1 . 0 0 X 10-8 3 . 6 0 X 10-4
'Equil. base concn. (aq.) 2 . 8 9 X 10-1 2.99 x 10-1 3.02 X 10-1 3.04 X 10-1
TABLE VI1 EXTRACTION OF 0.061 M THAH INTO BENZYL ALCOHOL IX BEXZEKE Alcohol concn. 1.50 X 10-1 1 . 0 0 X 10-1 7.50 X 10-2 5.00 X 10-2 3.00 x 10-2
Equil. base concn. (org.) 9 . 3 4 X 10-3 5 . 0 4 X 10-3 2.90 X 10-3 1.26 X 10-8 3.80 x 10-4
Equil. base concn. (aq.) 3.30 X 10-2 4.60 X 10-2 5 . 2 3 X 10-2 5.72 X 10-2 5.99 x 10-2
Corrected equil. base concn. (org.) 3.19 X 10-2 8 . 9 2 X 10-8 3 . 9 7 X 10-8 1.44 X 10-3 3.94 x 10-4
Corrected alcohol concn. 1.22 X 10.-1 8.50 X 10'-2 6 . 6 3 X lo'-* 4.62 X 10-2 2.89 x 10-2
into pure nitrobenzene and the concentrations listed in column 2 are those obtained after subtracting the blanks. Explanations for the corrections made in order to arrive a t the results given in columns 3, 4, a:nd 5 are given later. TABLE VI11 EXTRACTION OF TBAH INTO 0.075 i M BESZYLALCOHOLIN NITROBENZENE
TABLE IV OF TPAH INTO 0.100 M BEXZYL ALCOHOL IN EXTRACTION BENZENE Equil. base concn. (as.)
2787
Corrected Corrected equil. base alcohol concn. (org.) concn. 5 . 8 8 X 10-3 1.34 X 10-1 1.93 x 10-3 9 . 4 4 x 10-2 1 . 0 1 X 10-3 7 . 2 0 X 10-2 3 . 6 0 X 10-1 4.89 X 10-2
Table VI11 gives the extraction of the OH- ion into the organic phase when varying concentrations of TBAH are added to 0.075 M benzyl alcohol in nitrobenzene. Blank determinations were made of the extraction of the various concentrations of the base
Equil. base concn. (aq.) 3.70 X 10-1 2.76 X 10-1 2.09 X 10-1 9.24 X 10-2 3.69 X 10-2 1 . 8 5 X 10-2
Equil. base concn. (org.) 6.70 X 1.0-3 5.02 X 10-8 3.60 X 1.0-3 1.66 X 10-8 6.80 X 10-4 3.40 X 10-4
Equil. alcohol concn. 5.49 X 10-2 5.99 X 10-2 6.64 X 10-2 7.01 X 10-2 7.36 X 10-2 7.40 X 10-2
Corrected equil. base concn. (org.) 1.08 X 10-2 7.02 X 10-3 4.32 X 10-3 1 . 8 4 X 10-3 7 . 0 0 X 10-4 3.49 X 10-4
Corrected equil. mean base activity (org.) 8 . 4 2 X 10-8 5 . 6 1 X 10-3 3 . 5 4 X 10-8 1.64 X 10-8 6.51 X 10-4 3 . 2 8 X 10-4
The data for the extraction of OH- into the organic phase when a fixed volume of a given initial concentration of TBAH or TPrAH is added to varying concentrations of benzyl alcohol in nitrobenzene are given in Tables IX and X, respectively. Determinations were made of the extraction of TBAH and TPrAH into pure nitrobenzene and the equilibrium concentrations given in column 2 are those obtained after subtracting these blanks. The corrections made to arrive a t the values given in columns 3, 4, 5 and 6 are discussed later. Similarly, the data for the extraction of a solution of TBAH, initially 0.437 M , into varying concentrations of 4-methyl cyclohexanol in nitrobenzene is given .in Table XI.
Discussion The equation for the extraction of a tetraalkylanimonium hydroxide by a dilute solution of an alcohol in an "inert" so1ven.t can be written as nROH(0,
+ R4X+ + OH-
=
where eq. 1 represents creation of an ion-pair in the organic phase, while eq. 1' shows the formation of a pair of dissociated ions. Which expression is correct in a given case depends upon the conditions, particularly upon the dielectric constant of the solvent. The possible influence of water on the extraction has not been shown as it was not determined in the preserit study, but all aqueous solutions used were dilute (
