84
S. SUBRAMANIAN AND HARVEY F. FISHER
Near-Infrared Spectral Studies on the Effects of Perchlorate and Tetrafluoroborate Ions on Water Structure by S. Subramanian and Harvey F. Fisher *I Veterans Administration Hospital, K a n s a s Citv, M i s s o u r i 64168, and the Department of Biochemistry, Uniuersitv of K a n s a s Medical Center, K a n s a s City, K a n s a s
(Received August 5 , 1971)
Publication costs assisted by the V e t e r a n s A d m i n i s t r a t i o n Hospital
The effect of perchlorates of sodium, magnesium, zinc, and aluminum, and sodium tetrafluoroborate on the structure of water has been studied with respect to the anion effect,cation effect, concentration,and temperature effects. All of these salts split the 1.45-p band of water, an effect which is ascribed to their special waterstructure-breakingeffect. The anions play the predominant role and the effect of cations is found to be only of secondary importance. The effects of the anions and increase of temperature seem to be complementary. It is reasoned that the perchlorate and tetrafluoroborateions are not significantly hydrated in aqueous solutions.
I. Introduction The structure of water and the effect of electrolytes on the structure of water have been widely studied by several physico-chemical techniques. I n view of the prevailing uncertainty over the structure of water, the molecular nature of the electrolyte-water interactions is also not completely understood. The importance of solute-water interactions in determining biopolymer conformations has been realized lately. The effects of neutral salts on the structure of water and protein conformation have been reviewed2 recently. Except for the perchlorate ion, no direct quantitative correlation has been found3 between the effect of ions on water structure and their protein denaturing potential. The effect of perchlorate ion on water structure has been studied recently4z6by ir and raman spectroscopy. I n continuation of our studiessJ on the structure of water and hydration of alkali halides in aqueous solutions in the near-infrared region, we investigated the effect of perchlorate and tetrafluoroborate ions on water structure, their hydration, if any, and the influence of cations and temperature on the tendency of perchlorate ion in disrupting water structure, by studying the 1.45 p , ( v l v3) combination band of water,
+
prior to recording the spectra in order to minimize the effect of hydrolysis on the spectra recorded. Sodium trifluoroacctate was made by titrating trifluoroacetic acid (Eastman Organic Chemicals) with X'aOH t o neutral p H and crystallizing the salt from the solution. All spectra were recorded using a Cary Model 14R double-beam recording spectrophotometer. The spectra recorded were the average of three or four scans; the averaging was done with the help of a Varian Spectro System 100 computer linked to the Cary 14 spectrophotometer. The absorption cells were of the insert type with a pathlength of 0.05 & 0.0001 em. The temperature of the contents in the cells was controlled to zkO.02" using a Lauda thermostat. The direct spectra were recorded by using water (or solution) in the sample cell against a matched empty cell in the reference compartment. The difference spectra were obtained by placing water in the reference cell and solution in the sample cell and maintaining appropriate temperatures. The pH's of the solutions were nieasured using a Radiometer PHM26 p H meter.
111. Results arid Discussion A . The Absorption Spectra of Water and Solutions in the 1.45-p Region. The spectra of HzO and 2 M
11. Experimental Section The water used in this study was deionized tap water which was found in this laboratory to be equivalent to glass-distilled water. Sac104 (as Pl'aC104. HzO, purified) and Mg(ClO& were obtained from Fisher Scientific Company. Zn(C1O4),.6Hz0 and Al(C104)3. 9Hz0 were obtained from I< & (.I Laboratories, Inc. These were used as such. KaBF, was purchased from Alfa Inorganics and was recrystallized from aqueous solution; the insoluble residue present was filtered off. The sodium tetrafluoroborate solution mas made just T h e Journal of Physical Chemistrv, V o l . 7 6 , No. 1, 1976
(1) Address correspondence to this author at the Veterans Administration Hospital. (2) P. H. von Hippel and T. Schleich, "Structure and Stability of Biological Macromolecules," S. K , Tiinasheff and G. D. Fasman, Ed., Marcel Dekker, New- York, N. Y . , 1969, Chapter 6. (3) T. Schleich and P. H . von Hippel, Biochemistry, 9, 1059 (1970). (4) (a) P. Dryjanski and Z. Keeki, Rocz. Chem., 43, 1053 (1969); (b) G. E. Walrafen, J . Chem. Phys., 5 2 , 4176 (1970). ( 5 ) G. Brinclc and M. Fallc, C a n . J . Chem., 48, 3019 (1970). (6) W. C. McCabe and H . F. Fisher, J . Phvs. Chem., 74, 2990 (1970) (7) IT. C. McCabe, S. Subramanian, and H . F. Fisher, ibid., 74, 4360 (1970).
EFFECT OF C104- AND BF4- IONS ON WATERSTRUCTURE
1
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Figure 1. The near-infrared absorption spectra of water and aqueous solutions (salt concentration = 2 M ) of sodium, magnesium, zinc, and aluminum perchlorates a t 20" measured us. the empty cell. The cell pathlengths were 0.05 cm.
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Figure 2. The spectra of water and aqueous perchlorate solutions (concentration = 2 M ) a t 60". The cell pathlengths were 0.05 cm.
solutions of Na, Mg, Zn, and AI perchlorates a t 20" are shown in Figure 1. The electrolytes do not have any absorption of their own in the spectral region studied. The spectrum of water in the various aqueous perchlorate solutions is shifted toward higher frequencies (shorter wavelength) relative to that of pure water, the maxima being almost the same for the different perchlorates. The spectrum of water itself shifts to higher frequencies with increase of temperature as shown in Figure 2, suggesting a similarity in the effects of temperature and perchlorate salts in disrupting water structure. At 20°, the spectra of water in aqueous perchlorate solutions show a splitting into (1) a high-frequency peak (-1.42 p ) and (2) a lowfrequency shoulder (1.46 p ) . The spectra of water in the same solutions a t 60" (Figure 2) are not significantly different from those a t 20". In Figure 3 are shown the spectra of water in aqueous sodium tetrafluoroborate a t 20". Here again the changes are similar to those in aqueous perchlorate solutions. A cursory examination of Figures 1-3 reveals certain character-
i
I .3
1.4
Figure 3. The spectra of aqueous sodium tetrafluoroborate solutions a t 20". The cell path lengths were 0.05 cm. The vertical dotted line in Figures 1-3 refers to the position of the shoulder a t ~ 1 . 4 p6.
istic features. The high-frequency peak of water in aqueous perchlorates and tetrafluoroborate indicates an extensive breakdown of the water structure. The shoulder in the solutions spectra a t 1.46 p is seen only with perchlorates and tetrafluoroborate and not in the presence of other common anions.8 The shoulder a t 1.46 p is also seen in the spectra of water a t high temperatures.' The position of the shoulder in the solutions spectra does not appear t o shift measurably when the temperature is increased from 20" to 60". Furthermore, the position of the shoulder has very little dependence on the species of cation. These features demonstrate that the major effect in the spectra is due to the anions and that their effects are similar to that of a temperature increase. We had also noticed earlier' that the intensity, but not the frequency, of the highfrequency peak is dependent on the concentration of perchlorate. B. Digerence Spectra of Aqueous c104-, BF4- us. Water at $0". The difference spectra of 2 M solutions of Na, Mg, Zn, and AI perchlorates (Figure 4) and 2 M , 4M, and 5 M solutions of NaBF4 (Figure 5 ) measured vs. water a t 20" consist, in general, of a negative component arising from excluded volume6 for water in the sample cell, with a positive peak in the 1.42-p region superimposed. The difference spectra for aqueous solutions of alkali halides consisted6 of similar curves with the positive peak centered a t 1.43 p . The difference spectra for A1 and Zn perchlorates also show positive absorbances a t wavelengths greater than 1.57 and 1.65 p, respectively. In these two cases and possibly for n!lg(C104),, the positive absorbance could be due to partial hydrolysis of the solutions resulting in acidic or basic solutions. It is known9-11 that hy(8) Unpublished results of this laboratory. (9) G. E. Walrefan, J . Chem. Phys., 36, 1035 (1962). (10) T. T. Wall and D. F. Hornig, ibid., 47, 784 (1967). (11) W. R. Busing and D. F. Hornig, J . P h y s . Chem., 65, 284 (1961). T h e Journal of Physical Chemistry, Vol. Y 6 , No. 1, 1372
86
8. SUBRASIAlVIAN A K D HARVEY E'. FISHER W
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Figure 4. Difference spectra of the 2 Af aqueous perchlorate solutions measured 21s.water a t 20'. The cell path lengths were 0.05 cm.
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Figure 6. Effect of change in concentration on the difference spectra of aqueous sodium perchIorate at 20". The reference cell contained a 2 M solution and the sample cell, 3, 4, and 6 iM. The cell path lengths were 0.05 cm.
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Figure 5. Difference spectra of aqueous sodium tetrafluoroborate solutions measured vs. water a t 20'. The cell path lengths were 0.05 em.
dronium and hydroxide ions have broad absorption bands in the long tvavelength region of the water spectrum. We measured the pH's of the 2 M perchlorate solutions and they were: All 1.0; Zn, 4.0; NIg, 8.1; while NaC1O4 solution was nearly neutral. Hence, instead of the negative component, being proportional to the excluded voldme, increasing in the order A1 > Zn > R'lg > Na throughout the spectral region the reverse sequence is obtained in the longer wavelength region. The high-frequency positive peak is only slightly affected by the cation change. The position of the peak is 1.416 p (7063 cm-l) for Na and hIg perchlorates and 1.419 p (7049 cm-l) for Zn and AI perchlorates. The change in frequency (-15 cm-l) for a change in the cationic charge (+1 to $3) is negligible enough to be nothing more than a secondary effect due to the cations, The difference spectra for the tetrafluoroborate solutions are similar to n'aC104 difference spectrum except for the fact that the high-frequency positive peak is at 1.412 p (7081 em-l) instead of the 1.416 p. C. Temperature and Concentration Digerence Spectra for Aqueous NaC104. In Figure 6 are shown the difference spectra obtained by varying the concentration of n'aC104 in the sample cell while keeping the reference solution a t a constant concentration. The effect of concentration is not to change the features but The Journal of Physical Chemistry, Vol. 76, X o . 1, 1978
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WAVELENGTH ( p )
Figure 7 . Effect of temperature on the difference spectrum of 2 M sodium perchlorate solution us. water: a, 20"; b, 30"; c, 39.6"; d, 49.2'; e, 59.8". The cell pathlengths were 0.05 cin. The sample and reference cells were maintained at the same temperature in each case.
only to intensify or diminish the changes. The positive peak a t 1.416 p is invariant with concentration. Figures 7 and 8 show the simultaneous effect of temperature and perchlorate addition. With the same or different temperatures in reference and sample cells, addition of perchlorate is seen to complement the effect of temperature. While there are intensity changes, no frequency change is noticed. This suggests that the perchlorate anion disrupts water structure in the same manner as temperature does. This would also suggest that bonding interactions between C104- and water are very weak. D. Analysis and Assignments. The most important question we address ourselves to is the nature of the interaction between the C104- and BF4- ions and water. To determine this we need a parameter which represents such interactions directly. In an earlier study16we made use of the concept of "excluded volume" in determining the hydration of alkali halides in aqueous solutions. Using the same approach, we attempted to derive "hydration spectra" for the c10,and BF4- salts. The hydration of halide anions and several cations is well established. However, it
87
EFFECT OF Clod- AND BF4- IONS ON WATERSTRUCTURE
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Figure 8. Difference spectra of 2 hi’sodium perchlorate solutions at several temperatures vs. water at 20. The cell path lengths were 0.05 cm. Key: a, 20”; b, 30.1”; c, 40.1”; d, 51.2’; e, 60”.
is still not clear whether or not the C104- and BFdare h ~ d r a t e d .1 ~ 2 , 1’3 ~ An examination of the difference spectra and pH measurement of all the solutions indicate that except for XaClOd, all other solutions studied were either acidic or basic, thereby contributing somc absorption in the long wavelength tail of the difference spectra. It is difficult to evaluate such absorption quantitatively and hence we have derived “hydration spectrum” only for NaC104. We will use the hydration spectrum of XaC1O4 to draw infermces with regard t o other solutions. The positive peak in the high-frequency region of the difference spectra of perchlorates (Figure 4) could arise either from hydration of perchlorate or from an excess production of singly bonded mater.7 Figure 9 describes the resolution of the difference spectrum for 2 M KaClOe curve a, into two arbitrary component spectra, one (curve b) representing the net contribution of the absorption by water in the refercnce cell. This was effected with the help of the Varian Spectro System 100 Computer and a duPont Model 310 Curve Resolver. Curve b has the same shape as the spectrum of normal water and by definition it is the excluded volume spectrum. Curve c is the hydration ~ p e c t r u m . ’ ~ We shall define curve c as the “apparent hydration spectrum” pending the settlement of the question whether the perchlorate ion is hydrated or not. In Figure 10 we have shown the resolution of the ‘Lapparenthydration spectrum” for 2 M NaC104 into component Gaussian curves B and C. The appropriate Gaussian distribution parameters are as follows : curve B (1) absorption maximum, vOJequal t o 7060 (i) 5 cm-I (1.419 p ) , ( 2 ) absorbance at voJ AVO,equal to 0.27;and (3) half-band width AvliZJ 225 ==! 10 em-‘; curve C (1) v o equal to 6816 i 6 em-’ (1.467 p ) ; (2)
Figure 9. Determination of the excluded volume and “apparent” hydration spectrum for 2 M NaC104 Curve a is the difference spectrum for 2 ‘74XaC104 measured US. water; the optical path length was 0.05 cm and the cells were thermostated a t 20’. Curve b is the normalized water spectrum determined by normalizing the spectrum for water measured us. empty cell (curve HzO in Figure 1) to curve a a t 1.68 p. Curve c is the “apparent” hydration spectrum for 2 M NaC104. Curve b is negative but plotted in the positive sense. The algebraic sum of curves c and b produces curve a.
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Figure 10. The resolution of the “apparent” hydration spectrum for 2 M XaC104 into two components. Curve Ais the hydration spectrum for 2 M XaC104; it is the same as curve c in Figure 9. The appropriate Gaussian distributions of absorption parameters are listed in the text.
Avo equal to 0.116, and (3) half-band width Avl,, = 214 f 10 em-I. These curves have been compared with the absorption parameters for anion and cation hydratesJ6 singly bonded water in acetone and the high frequency peak of water in the temperaturc difference spectra for water? in Table I . The “C” curve in the present study is in agreement with sodium ion hydrate in the previous study6 and hence we assign it to sodium ion hydrate. The “B” curve (12) K. A. Hartman, Jr., J . Phys. Chem., 70, 270 (1966). (13) P. Dryjanski ahd Z . Kecki, Rocz. Chem., 44, 1141 (1970). (14) For more details regarding the procedure, see ref 6.
T h e Journal of Physical Chemistry, Vol. 76, NO.1, 1972
88
S. SUBRAMANIAN AND HARVEY E’. FISHER
has absorption parameters closely similar to the singly bonded mater in acctoric with regard to frequency and bandwidth and resembles the halide anion hydrate in bandwidth while differingin frequency.
Table I : Abtsorption Parameters for Some Spectral Component Curves
____-
,----Crn-1 Absorption maximum Spectral component
Water in acetone peak High-frequency peak in temperature differelice spectra for water (“B” curves) Halide anion hydrate Sodiuiii ion hydrate Sodium perchlorate hydration spectruiii “B” curve “C” curve
Half-band width
Refer-
yo
A”l/‘Z
ence
7075 7081 i.5
200 216 It 10
7 7
6997 6800
212 200
6 6
7050 f.5 6818 f.5
225 i 10 214 i.10
This work This work
On the basis of this comparison, one could assign the “E” curvc in Figure 10 to singly bonded water produced by the perchlorate addition or to mater weakly H-bonded6 to the perchlorate anion. We have assigned’ the following structures for the singly bonded water (a) absorbing a t 7080 cm-1 (in comparison witth singly bonded water in acetone absorbing a t 7075 cm-l) and halide anion hydrate (b) absorbing a t 6997 cm-’. The lower frequency of absorption of b relative
0
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to a is easy to reconcile since it is known from the Raman spectral studies of I Br- = CCl3C0O- > C1> NO3-- > F- = HzO > C104-. It is clear from the above series that water-perchlorate H bonds are weaker than water-water H bonds and hence the “B” curve in Figure 10 with absorption maximum a t 7050 em-I cannot arise from waterperchlorate H bonds since such bonds would result in the shifting of the frequency of absorption maximum of the “B” curve to greater than 7075 cm-l. On the other hand, if one considers that the effect of perchlorate is to convert doubly bonded water into singly bonded water, the “B” curve would account for this. The “B” curve differs in frequency from that of The Journal of Physical Chemistry, Vol. 76, No. 1, 1978
singly bonded water by 25 cin-’ toward the lowcr frrquency sidc. This can be explained as a medium effect arising from thc cIcctrostatic fidd of the hTa+ and c104- ions. It must bc emphasized homver, that thc delimitation brtwccn ion-dipole interaction and hydrogen bond is not sharp. Brinck and Falk6 have favored assigning the highfrequency peak in their studies of the effect of perchlorate on HDO in liquid water to a weak C104-. . . . . HzO H-bond in analogy with studies of NaC1O4.H20 in the solid state, wherein the water-perchlorate interactions seem to satisfy the geometric criteria of hydrogen bonding.16 There are othcr evidences which indicate that the pcrchloratc ion is not H-bonded t o the water in aqueous solution. Walrafen’s Raman spectral studies4 indicate that the Clod- ion is a strong structure-breaker and that it is not hydrated appreciably. Walrafen concludes4 that “nondirected ionic interactions between C104- and water would thus be consistent with the failure to observe Raman bonds characteristic of hydrated C104- ions.” Samoilov reports17 that the C1-0 stretching frequency is 1085 cm-I in NaC1O4.H2O (where the water is H bonded to the clod- ion) whereas in both anhydrous NaC104 and in aqueous solution of NaC104 the frequency is 1115 cm-l, suggesting that the C10,- ion in aqueous solution is not H-bonded to water. It is interesting to note that so far no complex of a transition metal with perchlorate as the ligand has been reported.ls Inasmuch as the tendencies to function as a ligand in complex formation and as an acceptor in an H-bond are interrelated we conclude that C104does not form an H-bond with water in solution. It was suggested by Icecki and Dr~rjanski‘~ that the high-frequency peak in the infrared spectrum of HOD in liquid water in the presence of NaC104 could be due to water molecules trapped between anion and cation without H bonding to the anion. From our studies it is evident that the high-frequency peak in the difference spectra (Figure 4) shifts only by 15 em-’ as the charge on the cation is increased from 1 to 3. It is known19 that highly charged cations form partially covalent bonds with water molecules which are most likely to produce significant effects on the OH and OD stretching frequencies. The reasoning of Mecki and Dryjanslri would be valid only if the water molecule is trapped between the cation and perchlorate physically (16) Z. Kecki, J. Witanowski, K. Akst-Lipszyc, and S. Minc, Rocz. Chem., 40, 919 (1966). (16) G. Brink and M. Falk, C a n . J . Chem., 48, 2096 (1970). (17) 0. Ya Samoilov, “Water in Biological Systems,” L. P. Kayushin, Ed., Consultants Bureau, New York, N. Y., 1969, p 25. (18) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry: A Comprehensive Text,” Interscience, New York, N. Y., 1962, p 450. (19) (a) D. E. Irish, B. Mecarroll, and T. F. Young, J . Chem. Phys.1 39, 3436 (1963); (b) R. E. Hester, R. A. Plane, and G. E. Walrafen, ibid., 38, 249 (1963).
EFFECT OF C104- A N D BF,- IONS ON WATERSTRUCTURE without getting involved in a chemical interaction. It is difficult to see how that can be so.2o We have also studied CF,COONa solutions. The difference spectra laclied the feature of a high-frequency positive peak, found in the perchlorate and tetrafluoroborate solutions. Ihclti and Dryjanski reported13 that the C104-, BF4-, and CC13COO- ions behave in a similar manner in affecting water structure, since these three ions give rise to very strong acids. Our studies agree with this conclusion except for the fact that with CF,COO- ions, the effect is not very predominant as is the case with (3104- and BF,-. The size of the ions may be a factor insofar as the sizes of C10,- and BF4- are almost the same, and they would be expected to behave similarly; also both the ions have a low surface charge density. In the absence of knowledge about the hydronium and hydroxyl absorption contribution in the longwavelength region of the spectra, we are not reporting the hydration spectra for NaBF, and CF3COONa. However, we made qualitative resolutions and the two component curves, as in the case of NaC104 (Figure 10) were obtained. The sodium ion hydrate curve is common in all the three cases a t the same frequency and with the same bandwidth while the “B” curve not differing in bandwidth from that for NaC104, changes in frequency. The Y O for CF3COONa “B” curve is 7042 cm-l and for NaBF4 it is 7075 cm-l. These differences could arise from small differences in the ionic fields. Mention must be made about the causes of the appearance of the shoulder in the direct spectra of the perchlorate and tetrafluoroborate solutions. Since the shoulder does not shift observably in frequency with increase of temperature and change of cation
89
and it does appear in a high-temperature pure water spectrum,’ it must be a property of water structure influenced but not caused by the Clod- and RP4anions. It is not due to a cation hydrate since highly charged cations shiftz1 the OH stretch frequencies of the hydrated mater molecules in addition to the effect due to H bonding of the hydrated water with anions. This effect of the cations is also observed in the nmr study of the aqueous solutions of highly charged cations where separate resonances for bulk and hydrated water are obtained,22the hydrated water resonance appearing at a lower field than the bulk water resonance. In our studies’ on the effect of temperature on the structure of water, x e could not assign a spectral component centered at 1.46 p to any specific species. It was a temperature-invariant component and in accordance with the observation of Luckz3we assign the shoulder component at 1.46 p to some energetically unfavorable H bonded species of water. The shoulder appears only in solutions containing non-H-bonding anions since, if the anions H bond with water, the absorption maxima are shifted towards longer wavelength with concomitant disappearance of the shoulder.
Acknowledgment. This work was supported in part by a Public Health Service research grant (GM15188) from the National Institutes of Health and a National Science Foundation grant (GB29023). (20) (a) S. Subramanian and H. F . Fisher, Rocz. Chem., 45, 933 (1971); (b) 2 . Kecki and P . Dryjanski, ibid., 45, 937 (1971). (21) I. Gamo, Bull. Chem. Soc. J a p . , 34, 760 (1961). (22) A. Fratiello, R. E. Lee, 17. M. Kishida, and R. E. Schuster, J. Chem. P h y s . , 48, 3705 (1968). (23) W. A. P. Luck, “Physico-Chemical Processes in Mixed Aqueous Solvents,” F . Franks, Ed., Heinemann, London, 1968, Chapter 2.
T h e Journal of Physical Chemistry, Vol. 76, X o . 1 , I972
