K. W. BUNZL
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Near-Infrared Spectra of Aqueous Solutions of Some Tetra-n-alkylammonium Bromides’
by K. W.Bunzl Oak Ridge National Labwatwy, Oak Ridge, Tennessee 57850 (Received October 5, 1966)
The change in the position of the 0.97-p near-infrared band of water has been measured in aqueous solutions of (C4Hs)4NBr,(CaH&NBr, (C2Ha)4NBr, and (CH&NBr as a function of concentration in the temperature range from 10 to 70”. The observed band shifts are explained as arising from structural changes of the water present in these solutions. These effects were strongly temperature dependent for solutions of (C4H&NBr, (C3H7)4NBr,and (CHJ4NBr, while in the case of (GH&NBr no appreciable dependence was observed. The results are compared with other physicochemical measurements available for these solutions. The diffculties in obtaining information about the water structure from nearinfrared measurements in electrolyte solutions are discussed.
Introduction Since Suhrmann and Breyer2showed in 1933 that the near-infrared (NIR) bands of water are characteristically changed by the presence of dissolved electrolytes, this technique has been used repeatedly3-10 to obtain information about the structure of water in electrolyte solutions. The purpose of the present investigation has been to show to what extent the NIR absorption of water is changed if the tetra-n-alkylammonium bromides are dissolved in it. A variety of other measurements on these solutions (e.g., nmr and dielectric relaxation times, viscosity, density, thermodynamic measurements)”-” have shown that the effect of these solutes is to increase the water structure around them. These ideas concerning the formation of a more ordered water structure around the hydrophobic surfaces of these ions have been mainly developed by Frank and his coworkers. 1 2 3 3 The NIR method has been used very little to study quaternary ammonium salt solutions. In 1964, Yamatera and co-workers9 concluded from observations on the 1.15-1.25-p band of water (assigned as v1 v2 v3)18 that a 2 m solution of (CHa)4NCIshowed a “structure-breaking” effect, while (C4Hg)4NBrseemed to indicate a “structure promotion” of the water. However, the choice of the 1.15-1.25-p band to observe these effects seems to be somewhat unfortunate, since,
+
The Joumcrl of Physical Chemisttu
+
owing to the organic nature of these cations, absorption bands of the C-H overtones are also present in this wavelength region. This can be shown by measuring the NIR difference spectrum of (C4H9)4NBrdissolved (1) Research sponsored by the U. 8. Atomic Energy Commission under contract with Union Carbide Corp. (2) R. Suhrmann and F. Breyer, 2.Physik. Chem. (Leipzig), 20, 17, 23, 193 (1933). (3) E. Ganz, aid.,33, 163 (1936); 35, 1 (1937). (4) W. Luck, Fwtschr. Chem. Fomch., 4, 653 (1964). (5) G. R. Choppin and K. Buijs, J . Chem. Phys., 39, 2042 (1963). (6) D.F. Hornig, ibid., 40, 3119 (1964). (7) K.Buijs and G. R. Choppin, ibid., 40,3120 (1964). (8) I. M.Klotz, Federation PTOC.,24, 5-24 (1965). (9) H.Yamatera, B. Fitzpatrick, and G. Gordon, J. Mol. Spectry., 14, 268 (1964). (10) E. R. Nightingale, Jr., “Chemical Physics of Ionic Solutions,” B. E. Conway and R. G. Barradas, Ed., John Wiley and Sons, Inc., New York, N. Y., 1966. (11) H.G.Hertz and M. D. Zeidler, Ber. Bunsenges. Physik. Chem., 68, 821,907 (1964). (12) H. S.Frank and W. Y. Wen, Discussions Faraday Soc., 24, 133 (1957). (13) H. S. Frank, PTOC. Roy. SOC.(London), A247, 481 (1958). (14) R. L. Kay, T. Vituccio, C. Zawayski, and D. F. Evans, J . Phya. Chem., 70, 2325, 2336 (1966). (15) W. Y.Wen and S. Saito, ibid., 68, 2639 (1964). (16) H.S.Frank and M. W. Evans, J . Chem. Phys., 13, 507 (1945). (17) J. B. Haggis, J. B. Hasted, and T. J. Buchanan, ibid., 20, 1452 (1952). (18) Where vi is the symmetric stretching, Y Z the bending, and YII the asymmetric stretching mode of the water molecule.
SPECTRA OF TETRA-WALKYLAMMONIUM BROMIDES
1359
O.96Ot m, rn 3(7 m2.P42 m,. 1.96
-
0.990
c ($H5l4NBr
0.900
0.970
m,sl46 m20-8 rn6=047t H20
Figure 1. Temperature dependence of the wavelength of maximum absorbance of the 0.97-4 band of water in aqueous solutions of tetra-n-alkylammonium bromides at different concentrations (m = molality).
in acetonitrile (-2 m in a 2-cm cell) us. pure acetonitrile in the reference cell. The thus obtained absorption bands of (C4H9)4NBrare: 1.145 p (m), 1.167 p (m), 1.186 p (s), 1.360 p (m), 1 . 3 7 6 (m), ~ 1.395 p (b, s), 1.430 p (b, s). Absorption bands from C-H overtones are, of course, also present in all of the tetra-n-alkylammonium salts and will therefore be superimposed on adjacent water bands. Thus the 1.15-1.25-p band of water in a (C4Hs)4NBr solution will always be altered by the 1.186-p band of the solute and so complicate the interpretation. This possibility of an interference of solute and solvent absorption is also present to some extent in the measurements of Klotz,* who aelected the 1.45-p band of water to study the solid hydrate (C4Hg)4NBr.32H20. Apart from these two measurements at one concentration and at one temperature, no systematic study of these solutions in the NIR has been reported. Therefore, we have measured the NIR absorption of aqueous solutions of (CH3)4NBr,(C2H&NBr, (CaH&NBr, and (C4H9)4NBras a function of concentration and temperature. The 0.97-p band was selected to avoid the above mentioned difficulties in interpreting the expected changes of the water bands in these solutions, as this band appears in a wavelength region where the solutes do not absorb.
Experimental Section All spectra were taken with a Cary 14-PM recording spectrophotometer. Quartz cells, 5 cm in length, were
used, which could be thermostated by a coaxial glass jacket. The reference cell was always filled with CCL. The temperature was controlled by two thermostats to within AO.1'. If the cell was cooled below room temperature, argon was flushed through the cell compartment to prevent water condensation from the air on the cell windows. All solutions were made with doubly distilled water, using purified and dried salts, and then filtered through a glass frit to remove dust particles.
Results and Discussion As a first-order approximation, the changes of the 0.97-p water band in these solutions will be characterized by the change in its wavelength of maximum absorbance, A., (Changes in the intensity and the band width will be discussed later.) Following Bernal and Fowler,l9 we then describe the solution by its structural temperature, tSt,; the structural temperature of a solution is defined as the temperature at which pure water has the same degree of association. Figure 1 shows Amax of the 0.97-p water band for the four solutions at different concentrations and temperatures. The A,,&, of pure water is also plotted in Figure 1 as a function of the temperature. The latter values agree very well with those obtained recently by Luck4
(19) J. D. Bernal and R. H.Fowler, J .
Chem. Phya., 1, 516 (1933).
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and Waggener,ZO who measured the NIR spectrum of water up to the critical point. From these graphs we can obtain tstr by selecting a point on one of the solution curves and finding the corresponding temperature a t which pure water has the same The quantity At = (tsoln - tetr) wavelength,. , , ,A taken at a given temperature and concentration is then a measure of the change in the association of the water molecules in the solution compared with pure water. The difference, At, is plotted for the various solutions as a function of the solution temperature, tsoln, keeping the concentration constant (Figure 2), and as a function of the concentration, keeping the solution temperature constant (Figure 3). As may be seen from these curves, At was positive for all the solutes examined, indicating that the structural temperature of these solutions is always smaller than the solution temperature. Since it is well known that the structure of pure HzO increases with decreasing temperatures, one is tempted to interpret the above result as a corresponding increase in the water structure in these solutions. However, before arriving at this conclusion, it is necessary to discuss the quantity measured in a little more detail. The 0.97-p band of water is usually assigned to the combination band ( 2 v 1 ~ 3 or) by ~ Luck4 to (314. In any case, the behavior of this band is mainly determined by the O-H stretching frequencies. If the temperature of water is decreased and more hydrogen bonds between water molecules are formed to give O-H---O (b) the water will essentially be attracted through the hydrogen atoms of the water molecules. This will result in a loosening of the O-H bond and its stretching (20) W. C. Waggener, A. J. Weinberger, and R. W. Stoughton, 149th National Meeting of the American Chemical Society, Detroit, Mich., April 1966.
SPECTRA OF TETRA-WALKYLAMMONIUM BROMIDES
40
la
’
I
1361
I
I D
MOLALITY (nil
Figure 3. Concentration dependence of At = (tsoletion tatructural) for water in aqueous solutions of tetra-n-alkylammoniumbromides at different temperatures.
frequency will shift to a longer wavelength. If a cab ion is adjacent to the water molecule (structure b), the oxygen atom of the water molecule will be attracted and the hydrogen atoms will be somewhat repelled by the cation, which also causes a loosening of the O-H bond. However, the ion-water interaction is of a different type, and therefore the function relating the magnitude of this interaction to the stretching frequency of the O-H bond will no longer be the same as in the case of the water-water interaction. It is obvious, then, that one cannot obtain a meaningful structural temperature of an electrolyte solution if one obtains it from the total observed shift of the O-H stretching frequency of the water molecules present. No interaction exists at any temperature in pure water, which influences the O-H stretching frequency in the same way as does the interaction between an ion and an adjacent water molecule. We will call the shifts of the O-H stretching frequencies which arise from water molecules adjacent to an ion “charge-induced shifts.” As an example, we use the observed displacement of the 0.97-p band to longer wavelengths in a solution of CsCl compared with water, which appears as if the water structure around the ions had increased. However, since one knows from other measurements (e.g., n m r reorientation times)21that CsCl acts as a “structurebreaker” on water, one must assume that the chargeinduced shift more than compensates the structureinduced shift (which would be to lower wavelengths according to the disordered water structure).
For ordinary electrolyte solutions, one can therefore certainly not assign the observed shift in the 0.97-p band entirely to a change in the structural temperature. However, the situation becomes less complicated if one measures aqueous solutions of the larger quaternary ammonium ions. As has been shown by nmr measurements,l 1 the water molecules on the hydrophobic surface of these ions are very little affected by the ionic charge or by the hydrocarbon chains. Consequently, we can expect then the “charge-induced shift” in the 0.97-p water band arising from the presence of these ions to be small. The nevertheless large observed shifts should then be due to changes in the water structure around these ions arising from their hydrophobic nature. The effect of the bromide ion should be approximately the same in all four solutions. As long as the shift in the 0.97-p band arising only from the Br- ions is not known, we can only compare the four solutions with each other. To obtain at least an estimate of the influence of Br- ions on the shift of the 0.97-p band, we measured the shift of an aqueous 3 m NHdBr solution. Since the ammonium ion is unique in its similarity to water,12 we expect such a solution to reflect mainly the behavior of the Br- ions and the ammonium ions only to substitute for water molecules. The shift of the 0.97-p band, which such a solution shows, is always somewhat to longer wavelengths, but always much smaller than the one observed in the tetra-n-alkylammonium bromide solutions at the same concentration and temperature. It can be seen from examination of Figure 3 that there exists no great difference in At for the four salts at the same temperature and concentration. The reason for this behavior is probably due to the “charge-induced shift” which should become already somewhat important for water around (C1H&N+ ions and will certainly be present in the case of (CHa)aN+ ions. This effect will simulate a too large At as explained above. Conclusions about the water structure using the magnitude of At only are thus more difficult to obtain for the smaller quaternary ammonium ions. On the other hand, a very decided difference in the temperature dependence of At for the four salts can be seen from Figure 2, Since changes in the water structure should indeed be very sensitive to temperature changes, the slope of these curves, i.e., bAt/dt, should be a reliable and sensitive criterion for the detection of structural effects. Positive At values which decrease (21) H.G.Hertc, Bet. Bumengee. Phyeik. Chem., 67, 311 (1863). (22) For a review of the different measurements leading to this oonclusion, see P. M. Vollmar, J . Chem. PhyS., 39, 2286 (1063).
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K. W. BUNZL
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with increasing temperature @e., b A t / d t = negative) as observed for dilute solutions of (C4H9)4NBrcan thus be interpreted as arising from a water-structure enhancement. The smaller a possible structure enhancement around an ion would be, the less sensitive would this entity be to temperature changes, and consequently b A t / b t should be smaller at a given temperature and concentration. Figure 2 reveals then that (C3H&NBr can be regarded as a structure maker, although less efficient than (C4Hg)4NBrbecause of the smaller observed b A t / b t at a given temperature and concentration. The positive temperature coefficient of At for solutions of (CH3)4NBrwould then classify this ion as a structure breaker; however, in this caae At should be negative, contrary to the observation. Nevertheless, it is conceivable that the influence of the "charge-induced shift" for this ion is already large enough to determine the magnitude of At to a great extent. Without the charge effect the curves for (CH-J4NBrin Figure 2 would be shifted parallel to the At axis to smaller At values and possibly even to negative values. The positive temperature coefficient of At together with the possibility that the At values, which arise from structural changes only, could be negative would suggest then that (CH3)4NBrindeed acts as a structure breaker when dissolved in water. With increasing temperature, the (CH3)4N+and Br- ions would become less efficient as structure breakers since there is less structure available to be broken and, consequently, the negative At values should decrease (positive b A t / b t ) . For solutions of (C2H5)4NBrno appreciable temperature coefficient of At could be detected (Figure 2), suggesting that the structure-breaking and structuremaking effects of this salt cancel each other in this case. The above outlined conceptions are essentially the same as already obtained from recent ionic mobility measurements by Kay and co-w~rkers'~ and from heat of dilution measurements of Lindenbaum.2a The NIR measurements thus certainly confirm the ideas of a water-structure enhancement around ((34+ (as obtained by many preH9)4N+and ( G H T ) ~ Nions vious workers), since in this case At as well as b A t / b t have the correct sign. The NIR measurements also can lead to the same conclusions about the structural changes in solutions of (C2H5)4NBrand (CH&NBr aa they have been developed by Kay114if one makes an additional assumption regarding the magnitude of At as shown above. On the other hand, measurements on the smaller quaternary ammonium ions, especially on solutions of (CH3)4NBr,seem in many cases to lead to seemingly contradicting results, like nmrll and dielectricI2 reorientation times which would classify this ion The J O U Tof ~Physical Chemistry
as a structure maker (contrary to thermodynamic and ionic mobility measurements). It is very likely, however, that these methods (especially nmr) also measure effects in the immediate vicinity of the ion which are not directly related to the degree of hydrogen bonding between the water molecules but, nevertheless, occur with the presence of electric charges. It seems, therefore, that in all these cases the temperature dependence of the measured quantity rather than its absolute value could give more reliable information about structural changes in electrolyte solutions, since these changes are probably more temperature dependent than possible charge effects. Wen and SaitoI5 measured apparent molal volumes, &, of some quaternary ammonium salt solutions and concluded from an observed minimum in a 42 vs. Concentration plot for (C4H9)4NBr that a clathratelike arrangement of the water molecules around this (C4H9)4N+ion is likely. The observed minimum in this case occurs as a concentration which corresponds to a stoichiometric composition of (C4He)4NBr.(35f 2)H20 and is somewhat different from the composition of the solid hydrate (C4Hg)4NBr.32.8H20.24 Since our At values for this solution as obtained from shifts in the 0.97-p water band should represent mostly structural changes of the water only (see above), we can compare their observation with the corresponding curves of Figure 3. There we find, indeed, a change in this slope of the curve which is fairly pronounced at lower temperatures around 2 m,which is slightly higher than the concentration corresponding to the solid hydrate (see arrow in Figure 3.)26 If one uses b A t / b t rather than At as a measure for the water-structure enhancement around (C4H&NBr and plots this quantity vs. m,curves very similar to the ones in Figure 3 are obtained. However, at 30" the break occurs at a stoichiometric concentration of (C4HB)43)H20 and at higher concentrations the NBre(32 curve is almost horizontal. The large concentration range over which this break occurs can be considered as an indication that the clathrate structure of water in this solution is by far not as well defined as in the solid hydrate. At concentrations above 1.9 m, the structure-enhancing ability seems to be reduced con-
*
(23) s. Lindenbaum, J . Phya. Chem., 70, 814 (1966). (24) D. L. Fowler, W. V. Loebenstein, D. B. Pall, and C. A. Kraus, J. A m . Chem. Soc., 6 2 , 1140 (1940); R. McMullan and G. A. Jeffrey, J . Chem. Phys., 31, 1231 (1959). (25) Wen and Saito measurements show that the molar quantity, &, decreases by some per cent already in dilute solutions, while At/m as obtained from Figure 3 would be roughly constant in this concentration range. However, if 4 could be measured with the same accuracy as &. it would be doubtful if At/m would still be constant.
SPECTRA O F
1363
TETRA-n-ALKYLAMMONIUM BROMIDES
siderably. In a plot of bAt/bt vs. m for (CsH&NBr a t 30" (also obtained from the corresponding slopes of the curves in Figure 2), an even broader break occurs around the stoichiometric composition (C3H&NBr-(27 f 4Hz0). This makes it doubtful to speak of a defined clathratelike arrangement of the water molecules around this ion in solution. However, this break seems to become more pronounced at lower temperatures. The observed minimum in occurs in this solution a t a salt to water ratio of about 1 :25.15 For solutions of (C2H5)4NBrand ( C H S ) ~ N Bone ~ , is no longer justified to interpret bAt/bt vs. m curves in terms of possible clathratelike water structures, since the bAt/bt us. t curves, as discussed above, already show a behavior that excludes this possibility. We mentioned a t the beginning of our discussion that the change in A, of the 0.97-p band can only approximately describe changes in the water structure. For a refinement one would also have to consider changes in the intensity and the band width. To obtain the extinction coefficients of these solutions, one would have to know the density of all solutions for the whole concentration and temperature range. Beside the fact that these data are not yet available, one faces the difficulty of finding the correct baseline of the 0.97-p band,
+ +
since the neighboring band (Y, v2 v3) also extends somewhat into the 0.97-p band.26 For these reasons, we did not use the intensity changes of this band in our discussion. However, we noticed that especially a t higher concentrations the band width of the 0.97-p band is somewhat unsymmetrically increased toward longer wavelengths for the ( C ~ H S J ~ Nand B ~ the ( G H T ) ~ N B ~ solutions. If one assumes with Wall and HornigZ7 an almost continuous distribution of bond energies between the water molecules, then this result leads also to the conclusion that the distribution of bond energies in these solutions has shifted more to the side of higher water-water interaction energies. Acknowledgments. It is a pleasure to express my appreciation to Dr. G. E. Boyd for his continued interest and sponsorship of this work. I wish to thank him as well as Dr. S. Lindenbaum for all their helpful suggestions and discussions during the course of this work. (26) Nevertheless, the two bands are separate enough that no appreciable change in the position of ,A,, of the 0.97-r band should arise from this slight over1ap.m (27) T. T. Wall and D. F. Hornig, J. Chem. Phys., 43, 2079 (1965).
Volume 71,Number 6 April 1067
