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WATERPROTON IN AQUEOUS TETRAALKYLAMMONIUM HALIDES tion into two distinct lines. This broadening may be due to a slower rate of exchange but also to broadening of the resonance line due to bound molecules,22
Acknowledgment. We are grateful to Professor Gabriel Stein for helpful discussions. (22) Y. Haas and G. Navon, to be published.
The Nuclear Magnetic Resonance Chemical Shift of the Water Proton in Aqueous Tetraalkylammonium Halide Solutions at Various Temperatures by Marie-Madeleine Marciacq-Rousselot, Laboratoire de Physique Ezperimentale MolBculaire, Facultb des Sciences, 9, Quai Saint-Bernard, 76 Paris 66me,Tour SB, 26me dtage, France
Anne de Trobriand, and Michel Lucas* Department de G h i e Radioactif, C E N F A R , B.P.No6, 92-Fontenay-auz-Roses, France
(Received August 20, 1971)
Publication costs assisted by Commissariat a I'Energie Atomique
In the present article we report the results of nmr measurements of the chemical shift of the water proton in solutions of tetrabutyl-, tetrapropyl- and tetramethylammonium bromides, chlorides, and fluorides at various temperatures and concentrations. Changes in the molal heat capacity of chloroform and of solid tetrabutylammonium bromide in aqueous solutions of tetrabutylammonium fluoride, chloride, and bromide are included as well. At low temperatures the downfield shift of the water proton increases in the order Br- < C1- < Fand Me4N+ < Pr4Nf < Bu4N+. This is consistent with an increase in water structure promotion when the alkyl chains of the quaternary ammonium salts are lengthened. At higher temperatures the order is the same for the anions but is reversed for the cations. Heat capacity changes have been measured at 20 and 25". According to the currently accepted interpretations of such measurements, Bu4NFshould be a strong structure promoter (which is also consistent with the nmr measurements). The structure of water apparently collapses or changes abruptly when the molality of BurNBr is 1.2 m. There are thus conflicting conclusions regarding the effect of dissolved BudNBr on the structure of water. The change in heat capacity and the nmr spectra indicate that it may be a structure breaker, whereas a number of other measurements suggest that it is a structure maker.
I. Introduction Since Frank and Wen' introduced the concept of water structure promotion by the salt tetrabutylammonium bromide, aqueous solutions of this salt have been extensively studied, and evidence has accumulated which suggests that this salt is a strong water structure former. Among the properties of the solution which are taken as showing the effect, we may include the high heat capacity, the near-infrared spectra of HzOD20 solutions,2 and the increase of the reorientation time of water molecules upon the addition of the quaternary ammonium bromide. However not all properties of solutions of this compound in water lead directly to the conclusion that it is a structure former a t various temperatures and concentrations. Thus the lowering of the temperature of maximum density of the solution (TMD) by the addition of structure-breaking salts would require that, if tetra-n-butylammonium bromide were a structure former, the T M D would be raised. One would expect
this from the behavior of tert-butyl alcohol which does raise the T M D of its aqueous solution^,^ and which in other respects appears to be a water structure f ~ r m e r . ~ However the bromide lowers the TMD of its aqueous solution comparatively to pure water.6 In the same vein, nmr measurements of the chemical shift of the water proton in solutions of the bromide are also p u ~ z l i n g . ~The shift is in the same direction as that caused by such structure breakers as CsI, or by higher temperatures. The decision is a difficult one be(1) H. S. Frank and W.-Y. Wen, Discuss. Faraday SOC., 24, 133 (1957). (2) J. D. Worley and I. M. Klotz, J . Chem. Phys., 45, 2868 (1964). (3) H. G. Hertz and M. D. Zeidler, Ber. Bunsenges. Phys. Chem., 68, 821 (1964). (4) F. Franks and D. J. G. Ives, Quart. Rev., 20, 1 (1966). (5) R. G. Anderson and M. C. R. Symons, Trans. Faraday Soc., 65, 2550 (1969). (6) A, J. Darnel1 and J. Greyson, J . Phys. Chem., 72, 3021 (1968). (7) H. G . Hertz and W. Spalthoff, J . Elektrochem., 63, 1096 (1959).
The Journal of Physical Chemistry, Vol. 76, No. 10, 1972
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M.-M. MARCIACQ-ROUSSELOT, A. DE TROBRIAND, AND M. LUCAS
cause of the questions which have been raised regarding the nature of the hydrogen bond. If one assumes that the H bond has some covalent character, then the direction of the shift (upfield with increasing salt concentration) can be explained by an increase in the covalent character in the presence of the organic salt with an accompanying increase in the proportion of organized water in the solution.8 On the other hand, if an electrostatic model of the H bond is assumed, the direction of the shift caused by this salt must be interpreted as a decrease in the H bonding in the water, that is, the salt is a structure breaker. In addition, recent measurements of the change in heat capacity (AC,) of chloroform in the solutions of the bromide showed that at a salt molality of 1.2 AC, decreased a b r ~ p t l y . ~This result can be interpreted as a sign of the collapse of the water structure in solutions more concentrated than 1.2 m at room temperature. Finally an additional complication in the interpretation of the nmr spectra of water in aqueous solutions of quaternary ammonium chlorides is found in the recent ' They reported that communication by Davies, et aZ.O they had confirmed the results of Hertz and Spalthoff at 25'; increasing the length of the alkyl chains of quaternary ammonium salts causes the water proton resonance to shift upfield (in the structure-breaking direction). However lowering the temperature to 0" reverses the trend. That is at the lower temperature the larger the cation, the more it acts as a structure promoter. In an attempt to make clear the effect of the tetraalkylammonium salts on the water structure, we report here the results of nmr measurements of the chemical shift of the water proton in solutions of tetrabutyl-, tetrapropyl-, and tetramethylammonium bromides, chlorides, and fluorides when the temperature and salt concentration are varied. In addition some molal heat capacity changes for chloroform and tetrabutylammonium bromide in aqueous solutions of tetrabutylammonium fluoride, chloride, and bromide are also reported.
11. Experimental Section Chemicals. Bu4NBr,Pr4NBr,Me4NBr, Pr4NC1, and Me4NC1 (Eastman Kodak), were purified according ref 11. Bu4NC1, Bu4NF, Pr4NF, and Me4NF were prepared by titration of the corresponding hydroxides (Fluka). The Bu4NC1 and Bu4NF solutions were in addition purified by several recrystallizations of the hydrates. The solutions of the tetraalkylammonium fluorides were analyzed for the cation and anion content by gravimetric analysis with Na(CeH5)4Band CaC12. The analyses agreed with each other to within 1%. Nmr Measurements. "r spectra were obtained with a Varian A 60 spectrometer operating at 60 MHz and equipped with the V 6040 temperature controller. The Journal of Phvsical Chemistry, Vol. 76, No. 10, 197.2
.
0
' , . ' 6 4 0
1
1 0
m 1
L
t
m 2
m Z
Figure 1. Plots of the chemical shift between the peak due to the water proton and a peak due to protons in the CH4 groups against the salt molality for tetraalkylammonium fluoride and chloride solutions.
The chemical shift between the peak due to the water protons and a peak due to protons in the CH1 groups of the tetraalkylammonium ions was measured. The CHI protons were used as an internal standard. This eliminates the need for corrections due to differences in the bulk diamagnetic susceptibilities of the various solutions. It was shown5 that similar results are obtained from an external standard like chloroform or an internal standard like tetramethylammonium chloride. Heat Capacity Measurements. The calorimeter and the procedure have already been described in detail. l 2 The heat of solution AH of small quantities of liquid chloroform or solid tetrabutylammonium bromide in the various salt solutions investigated was measured at two temperatures: 15 and 25" for chloroform and 20 and 30" for tetrabutylammonium bromide. The heat capacity change for the solution process from the pure liquid or pure solid state is AC, = A ( A H ) / A T . (8) J. Clifford and B. A. Pethica, Trans. Faraday Soc., 60, 1483 (1964). (9) M.Lucas and A. Feillolay, J . Phys. Chem., 75, 2330 (1971). (10) J. Davies, S. Ormondroyd, and M. C. R. Symons, Chem. Commun., 1426 (1970). (11) B. E. Conway, R. E. Verrall, and J. E. Desnoyers, Trans. Faraday Soc., 62, 2738 (1966). (12) M.Lucas, Bull. SOC.Chim. France, 2902 (1970).
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WATERPROTON IN AQUEOUS TETRAALKYLAMMONIUM HALIDES
0
1
m z 0
0
1
m
a
Figure 2. Plots of the chemical shift between the peak due to the water proton and a peak due to protons in the CHs groups against the salt molality for tetraalkylammonium bromide solutions.
Results I n Figures 1 and 2 the chemical shift (as ordinate) is plotted against salt molality (as abscissa) for different salts and different temperatures. The temperature in general was controlled within 1 2 ' . Figure 3a shows the plots of the heat capacity change at 25" against salt molality for the process of dissolving solid Bu4NBr in various aqueous salt solutions. Figure 3b shows similar plots for the heat capacity change at 20" for liquid chloroform in the same salt solutions. Each measurement of the enthalpy of solution represents the mean of four determinations. The range of the salt concentration investigated was limited by the crystallization of the clathrate. The estimated errors are shown by vertical bars in the figures. 111. Discussion 1. AC,, Measurements. Figures 3a and 3b show some interesting features. The first is that the heat capacity change in tetrabutylammonium bromide solution is almost constant until a concentration of 1.2 m is reached. At this point it decreases abruptly. I n tetrabutylammonium chloride or fluoride solutions this abrupt decrease does not appear. I n addition the AC, in dilute fluoride solutions at first increases with fluoride molality and then falls slightly, but is always higher than in bromide solutions. According to currently accepted interpretations of heat capacity measurements,l 3 this should indicate a breaking of the water structure in the Bu4iVBr aqueous solutions when the salt molality is higher than 1.2 m. At lower molalities the AC,, is rather constant, and one cannot decide the effect of the changing salt concentration on the structure of the water. I n aqueous Bu4NC1 solutions the sudden decrease in AC, is not evident, and in BudNF solutions the measurements appear to show an
0
1
m
2
0
1
m
2
Figure 3. (a) Heat capacity change a t 25' for the process of solution of pure solid Bu4NBr in aqueous tetrabutylammonium halide solutions: 1, BuaNCl solutions; 2, BuaNBr solutions; 3, Bu4NF solutions. (b) Heat capacity change a t 20" for pure liquid CHC18 in aqueous tetrabutylammonium halide solutions: 1, Bu4NCl solutions; 2, BunNBr solutions. Abscissa: salt molality.
increase in the extent of hydrogen bonding by the salt. A similar conclusion was reached by Narten and Lindenbaum'4 from their X-ray studies. They showed that the near-neighbor distance between water molecule: at 25" decreased from 2.85 in pure water to 2.80 A in aqueous 1.4 m Bu4NF solution, indicating stronger H bonding. The Bu4NF salt thus appears to be a structure promoter. BuaNBr may be a structure breaker in its effects on the structure of pure water, because the structure-breaking effect of Br- competes with the structure-forming effect of Bu4N +, as postulated by Lindenbaum, et to explain the variation of the osmotic coefficients of tetraalkylammonium fluorides and iodides with temperature. From the picture of the aqueous Bu4NF solution given by Karten and Lindenbaum,I4 an explanation for the sudden break a t 1.2 m in Bu4NBr aqueous solutions may be proposed. In Bu4NF solutions the water is pictured as an extended ice-I latticein which all water molecules, F- ions, and N atoms arc found to occupy network positions. The butyl chains of the cations are visualized to be located in the cavities. There is one-half cavity for one network position, and a butyl chain occupies two cavities. A crude calculation shows that at a salt molality of about 3, all cavities should be occupied so that a t higher molalities this structure would no longer be (13) R. K. Mohanty, T. S. Sarma, S. Subramanian, and J. C . Ahluwalia, Trans. Faraday SOC.,67, 305 (1971). (14) A. H. Narten and S. Lindenbaum, J . Chem. Phys., 51, 1108 (1969). (15) S. Lindenbaum, I. Leifer, G. E. Boyd, and J. W. Chase, J . Phys. C h a . , 74, 761 (1970). The Journal of Physical Chemistry, Vot. 76, N o . 10, 1972
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Ma-M.MARCIACQ-ROUSSELOT, A. DE TROBRIAND, AND M. LUCAS
stable. If in the solution the F- ions are replaced by the structure-breaking Br-, then it may be supposed that the collapse of the water structure should appear at a lower molality. Now if the quaternary ammonium bromide is indeed a water structure breaker, there is an apparent contrttdiction between this property and the high heat capacity of its aqueous solution. The proposal has been made by Millero,16 from apparent molal volumes of NaB(CaH6)4,by Kalfoglou, l7 from osmotic measurements on tetraphenylphosphonium halide solutions, and by Jolicoeur, et aZ.,18 from near-infrared measurements, that ions like B(C6H614or (CeH6)4P+ should be water structure breakers- The reason for this effect is not clear at present but is evidently related to the interactions of phenyl groups with liquid water. The AC, for solid NaB(CBH6)4in water at 25" is, however, rather large and very similar to that of solid Bu4NBr (170 and 180 cal/deg mol, respectively's). Thus a high heat capacity could be associated with a structure-breaking character. In addition the partial molal heat capacities CP02of KF, KC1, KBr, and K I at infinite dilution in water a t 25" are, respectively, -28, -29, -29.5, and -30.2 cal/deg mo1.'?20 That is they are very similar although the water structure forming character of the anions is very different. The structure-breaking character certainly increases much from C1- to I- and the C,"2 should have been much more negative for the largest anion. Clearly there is some other effect than the structure-modifying ability of the ion which must be taken into account to explain the experimental value. It would be associated with a positive contribution increasing with the size of the anion, and the absolute value should depend on the balance between the size effect and the structure-breaking character of the anion. Thus a large heat capacity of a solution would not in itself be sufficient to conclude that the water is more strongly organized, Rather one should take the increase or decrease of the heat capacity change associated with the solution of a given molecule or salt on the addition of another compound as an indication of the water structure modifying ability of this compound. 2 . Nmr Measurements. The results given in Figures 1 and 2 are summarized in Figure 4, which shows plots of the chemical shift against salt molality at 2, 25, and 43". It is apparent that the effect of the salt depends on the nature of the salt, the concentration of the solution, and its temperature. The effect of the salts at low molalities (
