ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 607

Highly Hydrated Associate of Tributyl Phosphate in

Hirochika NAGANAWA and Shoichi TACHIMORI

Department of Fuel Cycle Safety Research, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-11, Japan

The hydration and association of tributyl phosphate (TBP) in dodecane were investigated on the basis of the water distribution between aqueous solutions and TBP-dodecane solutions at 298 K. The water distribution data were analyzed as functions of the TBP concentration in the organic phase and the water activity in the aqueous phase. A highly hydrated TBP trimer was found in the organic phase at high TBP concentrations by the data analysis. At [TBP]org,totat~ 0.02 mol dm-3, no TBP association occurred and only TBP •H2O formed as a unique hydrate. In the range of 0.02 mol dm-3 < [TBP]org,totaic 0.3 mol dm-3, a part of the monohydrate dimerized to form (TBP)2(H20)2. At [TBP]org,total>0.3 mol dm-3, trimerization also occurred at high water activities. The trimer was found to be hydrated with six water molecules, (TBP)3(H20)6.

Keywords Tributyl phosphate, hydration, highly hydrated associate

The distribution of water between aqueous solutions and tributyl phosphate (TBP)-dodecane solutions has Experimental been the subject of numerous studies in the - reprocessing industry. The subject has been discussed Preparation from the viewpoint of complex-formation between TBP All of the reagents were of reagent grade. TBP and water, and many complexes have been suggested solutions diluted with dodecane were washed several previously.)-14 Monomeric TBP monohydrate, TBP • times with a 1 mol dm 3 sodium hydroxide solution, then H2O, was supposed in most such studies.)-1o However, with a 0.1 mol dm 3 solution, and finally with there is no definite answer regarding other complex- deionized-distilled water. Lithium chloride (purity> formations. 99.0%) and dodecane (purity 99.3%) were used without In the present study, hydration and the association of further purification. TBP were investigated stoichiometrically. The data concerning the water distribution were analyzed as Distribution of water functions of the TBP concentration in the organic phase A portion of water, or an aqueous lithium chloride and the water activity in the aqueous phase. In the solution, and the same volume of a TBP-dodecane calculations, the association number of TBP and the solution were placed in a stoppered glass tube. The two hydration number for the associates were treated as phases were vigorously agitated for 15 min and then unknown values. centrifuged in a chamber thermostated at 298 K. The In the range of TBP concentrations up to 0.3 mol dm3 concentration of water extracted into the organic phase in dodecane, the hydration and dimerization of TBP was measured by Karl-Fischer titration. had been shown in a previous study in our laboratory.15 However, in the reprocessing industry, a TBP solution Co-extraction of water with LiCI--.TBPcomplexes that is much more concentrated, i.e., 30vol% TBP Lithium chloride was added to the liquid-liquid system (1.1 mol dm 3), has been used. At the higher TBP in order to decrease the water activity in the aqueous concentration, successive complicated TBP-water com- phase (a'). Although this inorganic salt distributes low plexation occurs. In the present study, the water in the organic phase, the concentration of water co- distribution was examined over an extended TBP extracted with LiCI-TBP complexes is not negligible at concentration range up to 1.5 mol dm 3 in dodecane (40 extremely high concentrations of the aqueous salt. For vol% TBP). By analyzing the data, a highly hydrated instance, when [LiCI]aq,total>6.2mol dm3, that is, a'<0.6, TBP trimer, (TBP)3(H20)6, was found in addition to the water co-extraction has to be taken into account at TBP •H2O and (TBP)2(H20)2, in the higher TBP concen- [TBP]org,totalhigher than 0.3 mol dm 3. tration range. The highly hydrated trimer was sup- The concentration of the salt actually extracted was posed to be a kind of ring polymer. also checked by means of ion chromatography using 608 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10

K22 [(TBP)2(H20)2]0 a 2.5X103 mol dm-3 aqueous potassium hydrogen- (2) Kl l phthalate solution as an eluent; a portion of the organic [TBP •H2O]o ' phase was shaken with water, and the concentration of salt back-extracted into the aqueous phase was measured where by ion chromatography. While taking note of the relation between the amount of the salt and of water K 22 _ [(TBP)2(H20)2]0TBP extracted into the organic phase, the conditions under ah 2•aW (3) which the water co-extraction with salt complexes can be neglected were chosen in the experiments. Formation of highly hydrated TBP associates When the TBP concentration is higher than 0.3 mol dm 3, other TBP hydrates can form in addition to Theoretical TBP•H20 and (TBP)2(H20)2. The equilibrium can be written as: Hydration and dimerization of TBP The formation of the TBP monomer monohydrate1S mTBP(ah)(0) + nH2O (TBP)m(H20)n(0), can be written as: [(TBP)m(H20)n]0 K mn- TBP TBP(ah)(0) + H2O TBP•H20(0), ah o •aW

K [TBP•H20]0 11_ [TBP( (m-1,2,3,..., n=1,2,3,...). (4) ah)]o•a ' (1) The following mass balances can be written as: w where the subscript "o" denotes the species in the organic phase, while the lack of subscript denotes those in the [TBP]0,t = [TBP(ah)]0 + [TBP • H20]0 aqueous phase, TBP(ah) is the anhydrous TBP monomer + 2[(TBP)2(H20)2]0 and aw is the water activity in the aqueous phase; aw=1 + (m[(TBP)m(H20)n]0) for pure water. m=] n=l The monomer monohydrate can dimerize in the = (1+ K1i • aW)[TBP(ah)]0 organic phase15, which can be written as: + 2K22• aW• [TBP(ah)]l 2TBP•H20(0) (TBP)2(H20)2(0), + (m • Kmn• aW• [TBP(ah)J ), m=1 n=1 (5)

Fig. 1 The concentration of water extracted by TBP as a function of the total TBP concentration in the organic phase: (a) 0.005 mol dm-3 < [TBP]0,t <_0.02 mol dm-3, (b) 0.02 mol dm-3 < [TBP]0,t < 0.3 mol dm-3, (c) 0.3 mol dm-3 <_[TBP]0,t <_ 1.5 mol dm-3. Calculated lines: (...... ) only TBP • H2O was taken into account; (-----) TBP •H2O and (TBP)2(H20)2 were taken into account; ( ) TBP•H2O, (TBP)2(H20)2 and (TBP)3(H20)6 were taken into account. These lines were calculated by using equilibrium constants shown in Table 2. ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 609

[H2O]0,t= [H20(free)]0 + [H20]0,E obtained precisely to be 3.06(±0.55), 6.02(±0.88) and = [H20(free)]0+ [TBP•H20]0 0.037(±0.007), respectively. The analysis assuming two additional complexes was also made. No combination + 2[(TBP)2(H20)2]0 of two different additional complexes gave a standard + A deviation better than that obtained in the analysis n= 1(n [(TBP)m(H20)n]0) assuming the sole additional complex of (TBP)3(H20)6. = (KfW+ K11• [TBP(ah)] 0)aW Then, the resulting standard deviation from assuming two additional complexes was rather worse. From + 2K22• aW• [TBP(ah)]o these, no additional complexes other than (TBP)3(H20)6 + (n • Kmn• aW • [TBP(ah)]o ), (6) were found practically in the analyses. m=1 n=1 where [H20]0,E is the concentration of water extracted Stoichiometry for TBP water complexation along with the extractant of TBP. KfWis the distribution By the addition of TBP at high concentration to constant of free water: KfW=[H2O(free)]0/aW. dodecane, the activity coefficient of TBP could be On the basis of Eqs. (5) and (6), the data of [H2O]0,t changed from unity, and, thus, the equilibria for the free can be analyzed as functions of [TBP]0,t and aW by a water distribution and TBP-water complexation could successive-approximation method using a least-squares be apparently changed. In order to appraise this, both computer program. the equilibrium constants and their errors were deter- mined at individual TBP concentrations by analyzing the water-distribution data as a function of aW. When the Results TBP concentration in dodecane is constant, the medium of the organic phase can be uniform. In such a uniform Formation of highly hydrated TBP associates medium, the values of KfW,K11, K22 and K36 can be Figures 1(a), 1(b) and 1(c) give the water distribution as regarded as being constants. Figures 2(a), 2(b) and 2(c) a function of the TBP concentration in dodecane: (a) give the distribution of water as a function of aWat (a) 0.005 mol dm 3 < [TBP]0,t <_0.02 mol dm 3, (b) 0.02 mol [TBP]0,t=0.005, 0.01 and 0.02 mol dm 3, (b) [TBP]0,t= dm-3 < [TBP]0,t <_ 0.3 mol dm-3, (c) 0.3 mol dm-3 0.02, 0.05, 0.1 and 0.3 mol dm 3, (c) [TBP]0,t=0.5, 0.7, [TBP]0,t<_ 1.5 mol dm 3. The values of aWwere calcu- 0.9,1.1,1.3 and 1.5 mol dm-3, respectively. The data in lated on the basis of data concerning the vapor pressure these figures were analyzed on the basis of Eqs. (5) and above aqueous lithium chloride solutions from the (6). In the calculations, the equilibrium constants of literature.16 In a previous study conducted at our KfW,K11, K22 and K36 at the respective TBP concen- laboratory15, TBP•H20 and (TBP)2(H20)2 were postu- trations were treated as unknown values. These con- lated in the range of TBP concentrations up to 0.3 mol stants obtained by the calculations are given in Table 1 dm 3 in dodecane. The broken lines in Figs. 1(b) and along with 3Q errors. Although the value of KfWwas 1(c) were calculated by taking account of TBP•H20 and changed only slightly by a change in the TBP concen- (TBP)2(H20)2, whereas the dotted lines in Figs. 1(a) and tration, the error in the value was very large at high TBP 1(b) were calculated by assuming only TBP• H2O. concentrations; the actual value of KfW cannot be However, in the range of 0.3 mol dm 3 < [TBP]0,t <_ 1.5 determined at a TBP concentration range higher than mol dm-3, the experimental data deviate positively 0.3 mol dm-3. The value of K11was precisely obtained from the broken lines, as can be seen in Fig. 1(c). The over the entire TBP concentration range, whereas precise deviation at low aWis much smaller than that at high aW. values of K22 and K36were obtained at high TBP con- At aW=0.609, which is the lowest water activity in Fig. centrations. No systematical change in the values of 1(c), the broken line approximately fits the experimental these equilibrium constants due to a change in the TBP data. Therefore, the values of K11and Ku remain con- concentration can be seen in the results given in Table 1. stant up to [TBP]0,t=1.5 mol dm 3 under this condition. Based on this fact, the effect of a change in the TBP From this, it appears that stoichiometric complex- concentration on the equilibrium constants does not formation occurs even at high TBP concentrations, and, appear to be very important, though the physical pro- thus, the data can be treated without any correction in perties of the organic phase should be changed by the TBP activity at low aW. If the stoichiometry also applies addition of TBP to dodecane. to the case of aW higher than 0.609, the discrepancies The total water concentration in the organic phase can between the experimental and calculated values can be be broken down into four species: free water, TBP•H2O, due to other complex-formation than TBP•H20 and (TBP)2(H20)2 and (TBP)3(H20)6. Figures 3(a), 3(b) (TBP)2(H20)2. For the data analysis, at least a complex and 3(c) give the ratios at aW=0.609, 0.762 and 1.000, in addition to TBP•H20 and (TBP)2(H20)2 must be taken which were calculated by using the equilibrium con- into account. An additional complex, (TBP)m(H20)n stants listed in Table 2. A considerable amount of (m ? 1, n >_1), was first assumed and the data of[H20]0,E (TBP)3(H20)6 exists in the high TBP concentration range as a function of [TBP]0,t at aW=0.609, 0.762 and 1.000 at high aW,as can be seen in these figures. The con- were analyzed at the same time on the basis of Eqs. (5) centration of water extracted as (TBP)3(H20)6 is higher and (6). By the analysis, the values of m, n and Kmnwere than that extracted as TBP•H20 in the range of 610 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10

Fig. 2 The total water concentration in the organic phase as a function of aWat (a) [TBP]o,t=0.005, 0.01 and 0.02 mol dm-3, (b) [TBP]o,t=0.02, 0.05, 0.1 and 0.3 mol dm-3, (c) [TBP]o,t=0.5, 0.7, 0.9, 1.1, 1.3 and 1.5 mol dm-3. Calculated lines: (.....) only TBP•H2O was taken into account; (-----) TBP•H2O and (TBP)2(H20)2 were taken into account; ( ) TBP•H2O, (TBP)2(H20)2 and (TBP)3(H20)6 were taken into account. These lines were calculated by using equilibrium constants shown in Table 2.

Table 1 Equilibrium constants at various TBP concentrations with 3Q errors

Table 2 Equilibrium constants for TBP-water complexes [TBP]o,t > 1.0 mol dm3 at a1.000. with 3Q errors obtained by a simultaneous analysis of the data in Fig. 1 Discussion

Highly hydrated TBP trimer Although the self-association of TBP has been pointed out in numerous previous reports3,7-14,17-21, most of them did not refer to higher associates than the dimer. The higher association is still confusing. A few investigators have supposed the TBP associates ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 611

Fig. 3 Ratios of water extracted as free water, TBP• H2O, (TBP)2(H20)2 and (TBP)3(H20)6 as a function of [TBP]o,t at (i) aw=0.609, (ii) aW=0.762, (iii) aW=1.000.

higher than the dimer. Roland and Duyckaerts have Only by an analysis of data obtained at various TBP suggested the formation of (TBP•H20), (1 ? 3) in carbon concentrations, the TBP association number was tetrachloride and carbon disulfide.3 Roddy has sug- determined precisely to be three. gested (TBP•H20)4 in octane.' However, in the present study, such a higher associate, whose hydration number Reliabilityfor hydration numbers of TBP dimer and trimer is the same as the association number of TBP, was not The anhydrous (TBP)211,17,18and (TBP)2•H2Og-12have found in dodecane; the TBP trimer was not (TBP•H20)3, been supposed in many studies. However, in a previous but (TBP)3(H20)6. The trimer of (TBP•H20)3 may study in our laboratory15, these dimers were not found, at be less stable, and might immediately change into least in an analysis of the water-distribution data. The (TBP)3(H20)6 due to an addition of three water mole- optimum hydration number for the TBP dimer (=y) and cules. On the other hand, Bullock and Tuck12 have the equilibrium constant (=K2y) obtained in this study suggested the presence of ring polymer complexes in the were 2.04(±0.16) and 0.22(±0.03), respectively. Here, TBP-water binary system based on PMR and viscosity the effect of the variation in the hydration number on the measurements; however, the complexes were not iden- calculation is shown in Fig. 4(a). The experimental data tified in this study. The complex of (TBP)3(H20)6 is of [H2O]o,t at [TBP]o,t=0.3 mol dm-3 as a function of aW considered to be a kind of ring polymer. A six-member are given together with the calculated lines in this figure; ring of water may be the most stable structure in the at this TBP concentration, the formation of the TBP trimer hexahydrate, as is also seen in the water network in trimer is negligible. The solid and dotted lines in Fig. aqueous solution; three TBP molecules may surround the 4(a) were calculated by usingy=l.5, 2 and 2.5(varied) and water ring with hydrogen bonds in dodecane. Tetramer K2y=0.220(fixed). The slope of the lines is very much and hexamer, as other good symmetrical hexahydrates, affected by the value of y. In Fig. 4(b), the calculated were not major, perhaps because of the stereological lines by using y=2(fixed) and K2yfrom 0.1 to 0.3(varied) strain of the alkyl chains among TBP molecules. are given. From these figures, there is no value of K2y Further information is necessary for the complete which can be applied to the fitting at y=1.5 and 2.5. understanding of the structural problem. Thus, the hydration number of the TBP dimer and its Tedder also pointed out a sixth-order dependence of equilibrium constant can be precisely determined by the [H2O]o,t on aW in a liquid-liquid system of aqueous analysis. solutions and 100% TBP.11 He suggested the TBP Furthermore, the experimental data were analyzed monomer hexahydrate, TBP(H20)6, in the system. again by assuming anhydrous (TBP)2 and (TBP)2•H20 However, any one of m=1, 2, 3, • • • in (TBP)m(H20)6 can in addition to (TBP)2(H20)2. The equilibrium con- explain the water distribution data very well in the 100% stants for the dimers, K20 and K21, were obtained to be TBP system; the water distribution ratio calculated 0.001(±0.02) and 0.015(±0.02), respectively. Thus, any by assuming TBP(H20)6 to be a function of aw was TBP dimers other than (TBP)2(H20)2 were concluded to nearly the same as that calculated by assuming be practically negligible in the analysis. (TBP)2(H20)6 or (TBP)3(H20)6. Thus, in fact, the asso- A similar examination of the hydration number of the ciation number of TBP in the hydrate cannot be deter- TBP trimer was also made, which is shown in Figs. 5(a) mined only from the data at a fixed TBP concentration. and 5(b). Although the value of six was quite good, it 612 ANALYTICAL SCIENCES AUGUST 1994, VOL. 10

Fig. 4 Calculated values of [H2O]o,t as a function of aWat [TBP]o,t=0.3 mol dm-3: (a) variation of hydration number for TBP dimer (=y) at a fixed Key,(b) variation of Key at a fixed y. K11(=0.11) was introduced as an already-known value in the calculations.

Fig. 5 Calculated values of [H2O]o,t as a function of aW at [TBP]o,t=1.5 mol dm-3; (a) variation of hydration number for TBP trimer (=n) at a fixed Kin, (b) variation of Kin at a fixed n. K,1(=0.11) and K22(=0.22) were introduced as already-known values in the calculations.

was not obtained as precisely as the dimer hydration important. This is because the set of equilibrium con- number. stants shown in Table 2 interprets all of the experimental data up [TBP]o,t=1.5 mol dm 3 very well without any Activity coefficient of TBP in dodecane consideration for the correction in TBP activity; the When [TBP]o,t is not sufficiently low, the change in the change in the medium of the organic phase by an addition activity coefficient of TBP in the organic phase must be of TBP to dodecane affects the equilibrium constants taken into account in the data analysis. However, in the only slightly, which is shown in Table 1. Possibly, the case of this study, the change does not seem to be very change in the activity coefficient of TBP may be chiefly ANALYTICAL SCIENCES AUGUST 1994, VOL. 10 613 due to TBP self-association and hydration of the "Proc. Int. Extraction Conf,1971 ", Vol. 2, p.1236, associates in the organic phase. Similarly, in a previous Society of Chemical Industry, London, 1971. study in the TBP-hexane binary systems', the change in 6. S. J. Lyle and D. B. Smith, J. Inorg. Nucl. Chem., 39, 865 the TBP activity coefficient calculated on the basis of the (1977). 7. W. Roddy, J. Inorg. Nucl. Chem., 40, 1787 (1978). deviations from Raoult's law can be simply explained 8. C. J. Hardy, "The Extraction of Acids by Tributyl by assuming a TBP association in hexane with the Phosphate (TBP)", Technical Report AERE-R-3124, equilibrium constant obtained spectroscopically. U. K. Atomic Energy Authority, Harwell, Berkshire, England, 1959. In conclusion, three complexes, TBP•H2O, (TBP)2- 9. C. J. Hardy, D. Fairhurst, H. A. C. McKay and A. M. (H20)2 and (TBP)3(H20)6, were postulated as being the Willson, Trans. Faraday Soc., 58,1626 (1964). most probable species. The following equilibrium 10. S. Nishimura, C. H. Ke and N. C. Li, J. Am. Chem. Soc., stages in the organic phase can be written: 90, 234 (1968). 11. D. W. Tedder, "Water Extraction", in "Science and Monomer hydration: Technology of Tributyl Phosphate", Vol. IV, ed. W. W. Schulz, I. D. Navratil and A. S. Kertes, Chap. 3, CRC TBP + H2O TBP • H2O, Press, Boca Raton,1991. 12. E. Bullock and D. G. Tuck, Trans. Faraday Soc., 59,1293 Dimerization of TBP•H2O: (1963). 2TBP•H20 (TBP)2(H20)2, 13. C. R. Blaylock and D. W. Tedder, Solvent Extr. Ion Exch., 7, 249 (1989). 14. D. W. Tedder and W. Davis, Jr., Solvent Extr. Ion Exch., Trimerization and successive hydration: 1, 43 (1983). 3TBP•H20 [(TBP•H20)3] 3H2O~(TBP)3(H20)6.+ 15. H. Naganawa and S. Tachimori, Anal. Sci.,10, 309 (1994). 16. W. Kangro and A. Groeneveld, Z. Physik. Chem. [Frankfurt], 32, 1/2, 110 (1962). References 17. D. Dyrssen and Dj. M. Petkovic, J. Inorg. Nucl. Chem., 27, 1381 (1965). 18. Dj. M. Petkovic, J. Inorg. Nucl. Chem., 30, 603 (1968). 1. E. Buarer and E. Hogfeldt, J. Inorg. Nucl. Chem., 23,115 19. Dj. M. Petkovic, "Solvent Extraction Chemistry", p. 305, (1961). North-Holland, Amsterdam, 1967. 2. D. C. Whitney, "The Effects of Ion Hydration in Solvent 20. Dj. M. Petkovic and Z. B. Maksimovic, J. Inorg. Nucl. Extraction and Ion Exchange", U. S. Atomic Energy Chem., 38, 297 (1976). Commission Report UCRL-10505, University of Cali- 21. Dj. M. Petkovic, "Progress in Coordination Chemistry", ed. fornia, Berkeley, 1962. M. Cais, p. 665, Elsevier, Amsterdam, 1968. 3. G. Roland and G. Duyckaerts, Spectrochim. Acta, 24A, 529 (1968). 4. J. J. Bucher and R. M. Diamond, J. Phys. Chem., 73, 675 (Received March 25, 1994) (Accepted June 16, 1994) (1969). 5. Yu. G. Frolov, A. A. Pushkov and V. V. Sergrensky,