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Determination of the total hydration number of a licl cation-anion pair via collective proton motions
Volume 134, number 4
CHEMICAL PHYSICS LETTERS
6 March 1987
DETERMINATION OF THE TOTAL HYDRATION NUMBER OF A LiCl CATION-ANION PAIR VIA COLLECTIVE PROTON MOTIONS I.L GREEN, A.R. LACEY and M.G. KEATS ~e~rtment of Physicaf Chemists, university of Sydney, Sydwy, N.S. W. 2006, A~ir~ia Received 12 September 1986; in final form 5 January 1987
A study of the in-phase collective OH stretching band in the Raman spectrum of LiCl aqueous solutions ranging from 5 to 15 mol% LiCl and at temperatures from 70 to - 30°C has been carried out. The value ofthe total hydration number obtained namely 5.3 is in good agreement with thermodynamic and diffraction studies. This investigation provides insight into the mechanism by which the anomalous properties of bulk liquid water are quenched in LiCl solutions.
We have recently shown that analysis of the OH stretching Raman spectrum in water [ l-31 and various aqueous solutions [4,5] in terms of collective proton motions can yield important information regarding the structure of the water network in these systems. The low-frequency band in the OH Raman stretching spectrum is strongly polarized and is assigned to small amplitude in-phase collective stretching motions. Theoretical calculations [ 6-91 of the spectra of the tetrahedral networks of ice I and amorphous solid water, Hz0 ( as), have clearly established the collective character, and the premise of our previous work [ l-5 ] is that the collective character is attenuated in the defective networks of liquid water and aqueous solutions. In this note we extend the study to solutions of electrolytes, in particular LiCl, to investigate the effect of electrolytes on the network structure of water. A wide range of the~~yn~ic, spectroscopic and transport properties of electrolyte solutions including LiCl have been investigated. Infrared studies at room temperature carried out by Draegert and coworkers [ lo] concentrated on the librational and translational regions of 4 M LiCl in DzO. They concluded that there is very little change in the frequency of these spectra on comparison with pure D20. Draegert was also able to catergorise the Lif cation as a “structure maker” and the Cl- anion as a “structure breaker”, a concept initiated by Frank et
al. [ 11,12]. Other infrared studies on the OH stretching fundamental of HDO (uncoupled OH) with LiCl (2-16 M) [13,14] and NaCl down to - 30°C [ 151 also show no new features when compared with the bulk spectrum as well as no significant frequency shifts. However, Verrall [ 161 separates the cation and anion pairs when discussing their effect on the frequency of the OH stretch. He concludes that Li+ cation-water pairs do not perturb the water stretching spectrum whereas Cl- anionwater pairs show a shift to higher frequency. Nonetheless, the spectrum of decoupled OH oscillators is so weakly perturbed that to a good approximation we can conclude that, on the basis of the spectral frequency distribution, the ability of OH oscillators to couple is unaffected in these solutions relative to bulk water at the same temperature. There are, however, structural reasons which may inhibit coupling and decrease the collective character in Hz0 solutions simply because the pairwise coupling depends on hydrogen bond-hydrogen bond correlations. Raman spectral studies of aqueous LiCl solutions [ 17,181 have been reported. While these studies concentrated on the translational and librational regions of the water spectrum above O”C, OH stretching spectra were recorded [ 171 and these show a decrease in intensity, with added LiCl, of the band centred at 3225 cm-’ which we have interpreted [ l-5 ] as arising from small-amplitude in-phase col-
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lective OH stretching motions. Walrafen had assigned this band to Fermi resonance of v1 and 2vZ, and concluded that the resultant decrease in intensity of this band upon the addition of electrolytes arises from the breaking of hydrogen bonds. The small change in the decoupled oscillator spectrum is not consistent with this interpretation. We will show in this paper that the decrease is due to change in the network structure, and that this can be quantified. Kint and Scherer [ 181 have studied the influence of Kl on the Raman spectrum of HZ0 and interpreted their results in terms of disruption of the intermolecular coupling. Their study however did not quantify the extent of this disruption. Neutron and X-ray studies have been reported on dilute and concentrated aqueous LiCl solutions. Narten and co-workers [ 191 concluded from their X-ray and neutron studies that the average coordination of water molecules around Li+ ions and Cl- ions is four and six respectively. Neutron diffraction studies with isotopic substitution [ 20,2 1] have shown that bridging waters between the LiCl cation-anion pair and HZ0 shells are important at high salt concentrations. It has also been deduced from these studies that the overall coordination number is 5.5. Numerical simulations [ 22,231 have been performed on electrolyte solutions, and of particular relevance to this work has been a study of the HZ0 infrared spectrum [23] in these solutions. Unfortunately the Raman spectrum was not calculated, but it is noted that the use of a frozen field approximation precludes the dynamical coupling between oscillators on different molecules but does incorporate that between OH oscillators on the same molecule. Both contribute to the collective band studied in this paper. An important feature of LiCl solutions above 10 molI is that they quench both the anomalous properties of water in the supercooled region and the homogeneous nucleation to ice I, thereby allowing the formation of a glass [ 22-291. Diffraction studies [ 241 on LiCl samples have measured the kinetics of formation of ice in such glassy samples. These show that 8 mol% solutions homogeneously nucleate at a temperature TH,the homogeneous nucleation temperature, forming ice I,, during a liquid nitrogen quench. On the other hand, 10 mol% solutions form glass samples which precipitate ice I, (metastable) at temperatures a few degrees above the glass transition 386
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temperature ( Tgx 142 K) for concentrations > 9 molW LiCl. At 14 mol% however samples remain vitreous when warmed from Tgto 193 K. Thermodynamic measurements also relating to TH and Tghave given important structural insights into aqueous LiCl solutions. Angel1 and Sare [25] have shown that LiCl compared with other salts of the same molarity raises Tgby the smallest amount. Two processes were identified as being important, namely phase separation (i.e. liquid-liquid immiscibility) and nucleation. NMR studies [ 261 using ‘Li, *H, and ‘H and 18 mol% solutions show that the water in the system is still quite free at the glass transition temperature. Angel1 and co-workers [27] have also looked at T, and Tgof Li salts in DzO and H,O and found doubly unstable glass regions. MacFarlane et al. [ 281 investigated the cooling rate dependence of TH for several LiCl emulsion systems. Shear viscosity and shear impedance measurements [29] have also led to an estimate of Tg as well as some information on the mechanism of ionic conduction. Further structural information has also been gleaned through dielectric loss measurements [ 301 on LiCl solutions with R = 4.66-6.72 (where R = 55.5/M) as well as Rayleigh and Brillouin scattering [ 3 I] over a wider range of concentrations. The studies discussed above do not directly shed light on the central question of how the LiCl at 10 mol% quenches [32] the anomalous properties of pure water. These anomalies arise from fluctuations between configurations of water molecules having similar energies but different densities [ 33,341. In H20/H20 mixtures [ 51, where the anomalous properties are also quenched [35], the intensity of the collective band was shown [ 51 to arise from destruction of the tetrahedral water network with one H202 molecule introducing between 2 and 3 network defects. A study of the collective character in LiCl electrolyte solutions is carried out in this paper with the goal of extracting similar information. As previously stated, we have shown [ 1,2] that the band centred around 3200 cm-’ in the Raman spectrum of H,O can be attributed to small amplitude inphase collective OH stretching motions. The first mechanism whereby defects give rise to a reduction in the collective band intensity has been postulated [ 361 in terms of a decoupling of a defect OH oscillator when its stretching frequency lies outside the
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span of the vibron density of states determined by the nearest neighbour coupling. The large vibron bandwidth is primarily a property of the tetrahedral coordination of the network, as shown by analysis of available data on the ice polymorphs [ 361. The second mechanism arises when an OH oscillator is found in an environment of reduced coordination number in which there are fewer nearest neighbour oscillators with which it can couple. This will lead to an overall reduction of the vibron bandwidth when the number of such environments become sufficiently large, and a decrease of the intensity of the in-phase collective band. Both mechanisms can play a role in aqueous solutions, but when there are only small changes in the decoupled OH stretching spectrum but large changes in the in-phase collective band the latter mechanism is dominant. This is the case in both LiCl and H202 solutions. The OH stretching I,, and I,_ Raman spectra of 5, 10 and 15 mol% aqueous LiCl solutions were recorded as previously described [ 1,2] over a temperature range of - 30 to 70°C. The samples were contained in sealed Pyrex capillaries (with a vapour bubble) in order to enhance the extent of supercooling by minimising the sample volume. Raman spectra in the metastable supercooled region as well as polarized spectra in the stable liquid region have not been previously reported. In figs. 1A, lB, 2A and 2B, the I,, and II spectra are shown at 30°C and - 5 “C respectively. These show, as for H202, that the major effect of the addition of LiCl is to reduce the intensity of the collective band. Qualitatively, it appears that the collective band intensity is reduced to a large extent with the addition of only 5 mol% LiCI, and a roughly linear dependence is observed upon further addition of LiCl. The relative intensity C,(T) of the band assigned to the in-phase collective motions in the I,, spectrum was extracted using the spectral stripping procedure introduced in ref. [ 11. The spectra of the resulting collective bands are shown for 30°C in fig. 3A and - 5 ‘C in fig. 3B. The value of C for ice I and H20( as) is 0.54 + 0.02 [ 1 ] and any decrease relative to this is interpreted in terms of a variation of the tetrahedral nature of the network, by either of the mechanisms discussed above. Values of C,( T) obtained from each set of spectra are plotted in fig. 4.
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Firstly, it is interesting to note that the values of C for 15 mol*/oat 30 and 70°C are very small. Thus the structural disorder in LiCl solutions of greater than 15 mol% together with the intrinsic network defects in bulk water above 30°C are sufficient to uncouple every OH oscillator in solution. On the basis of the small shifts in the uncoupled oscillator spectrum [ 13- 16 1, we attribute this decoupling of OH oscillators to the Lit and Cl- ions changing the network coordination with respect to OH oscillators, and not the presence of a greater number of distorted or broken hydrogen bonds as previously argued [ 171. It has been argued [ 1,2,36] that defect-free tetrahedral networks have a Cvalue equal to C,, viz. 0.54 f 0.02 [ 11. We have shown [ 21 that in ice I the collective motions can be destroyed by mass defects (OD oscillators) and that the intensity of the collective band is reduced as (I -poD)2, where poD is the mole fraction of OD. Liquid water is characte~zed by fluctuating hydrogen bond defects [ 371, and a selfconsistent analysis showed [2] that the collective band intensity is reduced by these intrinsic defects as (1 -pi)’ where pr is the probability that an OH oscillator is a vibrational defect, which can be broadly interpreted as the probability of a broken hydrogen bond. Consequently C(~)=Gzef1-Pl~2
9
(1)
where C(T) refers to the pure liquid water data obtained previously [ 1 ] . In order to extract structural information from the results of this study, we consider a model of a hypothetical fully hydrogen bonded reference network, containing the Li+ and Cl- ions. In this case all the hydrogen bonds which can form would have OH stretching frequencies within the vibron bandwidth, but the in-phase collective band intensity would be reduced because water molecules in the first solvation shell will have oxygen atoms which exhibit a lower coordination number with respect to OH oscillators (as donors or acceptors) than that of a perfect tetrahedral network such as ice I. The intensity of the collective band C, would be independent of temperature assuming no underlying change of “structure” of the reference network, and C, < Ci,, which is the second mechanism mentioned above. In the real liquid, the network structure exhibits hydrogen bond defects and if the extent and dist~bution of these is 387
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B
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Fii. 1. The OH stretching I,, (A) and I, (B) spectra at 30°C for a range of mole fractions of LiCi.
the same as in bulk water at the same temperature, then C, ( r) will be reduced because of a reduction in the number of resonant nearest neighbour couplings by the same extent as in liquid water. This is the first mechanism, hence we postulate that
A crude model of C, can be developed on the basis that one Li+ and one Cl- effectively remove 2n OH oscillators from the hypothetical fully hydrogenbonded tetrahedral network. For small mole fractions (p-J of LiCl, it follows that
G(n=cx(l-Pd2
Cx=CiceC1-nPx)
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-
(2)
-
(3)
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Fig. 2. The OH stretching I,, (A) and II (B) spectra at - 5°C for a range of mole fractions of LiCI.
Combining (1 ), (2) and (3) gives the simple result that GA~f=CtW1--WX)
+
(41
It is this relationship which is used to evaluate the only parameter of the model namely n. The same
argument was applied to H202/H20 mixtures at low mole fractions of Hz02 with the result that n x 2-3. Values of n extracted using the C,( T) data of fig. 4 are shown in table 1. They exhibit little variation with mole fraction of LiCl and a slight drift towards lower values with temperature. However, it is likely 389
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Table 1 Estimate of hydration number in concentrated LiCl solutions
A 30%
Mole fraction LiCl px 0.05 0.10 0.15
n=~l-G(WC(~)l@X 70°C
30°C
-5°C
- 30°C
6.3 6.1 5.6
5.0 5.2 5.4
5.6 4.8 5.3
a) 4.1 4.4
a) Nucleation was obtained before - 30°C was reached.
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Fig. 3. The collective band I, of the LiCl solutions is shown at 30°C (A) and - 5°C (B) for a range of mole fractions in LiCl.
4
200
300 TEMPERATURE
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Fig. 4. The relative in-phase collective band intensity C,(r) is plotted as a function of temperature for moIe fractions of LiCI: (.)0.00,(,.)0.05,(~)0.10,(*)0.15.
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that the drift with temperature is attributed to systematic errors in the spectral stripping procedure used to evaluate C,( 7). From the data at 30°C and - 5”C, it is judged that the temperature dependence of n is small. The values of C,( T) at 70°C are so small that the estimates of n at 70°C will be susceptible to systematic error. The fall in n at - 30 * C could be meaningfull, but more experiments in the supercooled regime would be required to verify this. The mean value of n is 5.3 k 0.6. This means that for every LiCl molecule introduced approximately 5.3 water molecules are disrupted in the water network. The value of n= 5.3 2 0.6 agrees well with the thermodynamic studies of Angel1 and Sare [ 25 1. Angel1 and Sare have calculated the total hydration number (i.e. the number of Hz0 molecules that a cation-anion pair can remove from the water network) of LiCl to be between 6 and 8. Even closer agreement is obtained with diffraction studies [ 20,211 where a coordination number of 5.5 was obtained. The constant value of n is indicative of a single mechanism for the reduction of the collective band intensity C, of the reference network. As previously postulated, this occurs when an OH oscillator is found in an environment of reduced coordination number. The temperature independence of n in the equilibrium liquid regime confirms that the dominant contribution to the temperature dependence of C,( T) in LiCl solutions arises not only from the same mechanism as in bulk water, namely the creation of defective hydrogen bonds, i.e. O-H oscillators associated with “broken bonds” which vibrate at a frequency above the high-frequency edge of the vibron density of states, but also to the same extent as in bulk water. This is consistent with the small changes in the HDO spectrum. The concentration independence of n at
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all temperatures suggests that from 5 to 15 mol% LiCl, the structural characteristics of the hydration shells and the remaining bulk water are not significantly changed. This has interesting ramifications because it is precisely in this concentration regime that the anomalous properties of the water are progressively suppressed. Insofar as the anomalies arise from density fluctuations of a modest spatial extent, it is possible that the suppression is attributable to breakup of patches of water molecules in configurations of different density by the LiCl without significantly altering the hydrogen bond energy distribution. Any significant alteration of the hydrogen bond energy distribution would be reflected by a change in the HDO spectrum, which is small, i.e. the suppression arises primarily from geometric, rather than “energetic”, perturbations introduced by the LiCl. In summary, we have shown that the OH stretching Raman spectrum of collective proton motions can be used to quantitatively probe the number of water molecules that a LiCl cation-anion pair can effectively remove from the tetrahedral water network. This work was supported by the Australian Research Grants Scheme. JLG acknowledges the award of a Commonwealth Postgraduate Scholarship.
References [ 1] J.L. Green, A.R. Lacey and M.G. Sceats, J. Phys. Chem. 90 (1986) 3958. [2] J.L. Green, A.R. Lacey and M.G. Sceats, Chem. Phys. Letters 130 (1986) 67. [ 31 J.L. Green, A.R. Lacey M.G. Sceats, S.J. Henderson and R.J. Speedy, J. Phys. Chem., submitted for publication. [ 41 J.L. Green, A.R. Lacey and M.G. Sceats, J. Phys., to be published. [5] J.L. Green, A.R. Lacey and M.G. Sceats, J. Chem. Phys., to be published. [ 61 R. McGraw, W.G. Madden, M.S. Bergren, S.A. Rice and M.G. Sceats, J. Chem. Phys. 69 (1978) 3483. [ 71 W.G. Madden, M.S. Bergren, S.A. Rice and M.G. Sceats, J. Chem. Phys. 69 (1978) 3497.
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[ 81 M.S. Bergren and S.A. Rice, J. Chem. Phys. 77 (1982) 583. [ 91 G. Neilson and S.A. Rice, J. Chem. Phys. 78 (1983) 4824. [ lo] D.A. Draegert and D. Williams, J. Chem. Phys. 48 (1968) 401. [ 111 H.S. Frank and M.W. Evans, J. Chem. Phys. 13 (1945) 507. [ 121 H.S. Frank and W.Y. Wen, Discussions Faraday Sot. 24 (1957) 133. [ 131 K.A. Hartman Jr., J. Phys. Chem. 70 (1966) 270. [ 141 R.D. Waldron, J. Chem. Phys. 26 (1957) 809. [ 15 ] H.R. Wyss and M. Falk, Can. J. Chem. 48 (1970) 607. [ 161 R.E. Verall, in: Water, a comprehensive treatise, Vol. 3, ed. F.Franks (Plenum Press, New York, 1973) ch. 5. [ 171 G.E. Walrafen, J. Chem. Phys. 36 (1962) 1035. [ 181 S. Kint and J.R. Scherer, J. Chem. Phys. 69 (1978) 1429. [ 191 A.H. Narten, F. Vaslow and H.A. Levy, J. Chem. Phys. 58 (1973) 5017. [ 201 J.R. Newsome, G.W. Neilson and J.E. Enderby, J. Phys. C 13 (1980) L923. [21] AK. Soper, G.W. Neilson, J.E. Enderby and R.A. Howe, J. Phys. Cl0 (1977) 1793. [22] R.O. Watts, Mol. Phys. 32 (1976) 659; K. Heinxinger, in: Ions and molecules in solution, ed. R. Tamamushi (1982) p.61; M. Mezei and D.L. Beveridge, J. Chem. Phys. 74 (1981) 6902. [23] M.F. Mills, J.R. Reimers and R.O. Watts, Mol. Phys. 57 (1985) 777. [24] A. Elarby-Aoulzerat, J.F. Jal, C. Ferradou, J. Dupuy, P. Chieux and A. Wright, J. Phys. Chem. 87 (1983) 4170. [25] C.A. Angel1 and E.J. Sare, J. Chem. Phys. 52 (1970) 1058. [26] E.J. SutterandJ.F.Harmon, J. Phys. Chem. 79 (1975) 1958. [ 271 C.A. Angell, E.J. Sare, J. Donnella and D.R. MacFarlane, J. Phys. Chem. 85 (1981) 1461. [ 28 ] D.R. MacFarlane, R.K. Kadlyala and C.A. Angell, J. Phys. Chem. 87 (1983) 235. [29] C.T. Moynihan, N. Balitactac, L. Boone and T.A. Lotovitz, J. Chem. Phys. 55 (1971) 3013. [30] C.T. Moynihan, R.D. Bressel and C.A. Angell, J. Chem. Phys. 55 (1971) 4414. [31] S.-Y. Hsich, R.W. Gammon, P.B. Macedo and C.J. Montrose, J. Chem. Phys. 56 (1972) 1663. [ 321 C.A. Angell, Nature 296 (1982) 138. [33] R.J.SpeedyandC.A.Angell, J. Chem.Phys. 65 (1976) 851. [34] C.A. Angell, Ann. Rev. Phys. Chem. 34 (1983) 593. [ 351 M. Oguni and C.A. Angell, J. Chem. Phys. 73 (1980) 1948. [ 361 J.L. Green, A.R. Lacey and M.G. Sceats, in preparation. [ 371 A. Geiger, A. Rahman and F.H. Stillinger, J. Chem. Phys. 70 (1979) 263.
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DETERMINATION OF THE TOTAL HYDRATION NUMBER OF A LiCl CATION-ANION PAIR VIA COLLECTIVE PROTON MOTIONS I.L GREEN, A.R. LACEY and M.G. KEATS ~e~rtment of Physicaf Chemists, university of Sydney, Sydwy, N.S. W. 2006, A~ir~ia Received 12 September 1986; in final form 5 January 1987
A study of the in-phase collective OH stretching band in the Raman spectrum of LiCl aqueous solutions ranging from 5 to 15 mol% LiCl and at temperatures from 70 to - 30°C has been carried out. The value ofthe total hydration number obtained namely 5.3 is in good agreement with thermodynamic and diffraction studies. This investigation provides insight into the mechanism by which the anomalous properties of bulk liquid water are quenched in LiCl solutions.
We have recently shown that analysis of the OH stretching Raman spectrum in water [ l-31 and various aqueous solutions [4,5] in terms of collective proton motions can yield important information regarding the structure of the water network in these systems. The low-frequency band in the OH Raman stretching spectrum is strongly polarized and is assigned to small amplitude in-phase collective stretching motions. Theoretical calculations [ 6-91 of the spectra of the tetrahedral networks of ice I and amorphous solid water, Hz0 ( as), have clearly established the collective character, and the premise of our previous work [ l-5 ] is that the collective character is attenuated in the defective networks of liquid water and aqueous solutions. In this note we extend the study to solutions of electrolytes, in particular LiCl, to investigate the effect of electrolytes on the network structure of water. A wide range of the~~yn~ic, spectroscopic and transport properties of electrolyte solutions including LiCl have been investigated. Infrared studies at room temperature carried out by Draegert and coworkers [ lo] concentrated on the librational and translational regions of 4 M LiCl in DzO. They concluded that there is very little change in the frequency of these spectra on comparison with pure D20. Draegert was also able to catergorise the Lif cation as a “structure maker” and the Cl- anion as a “structure breaker”, a concept initiated by Frank et
al. [ 11,12]. Other infrared studies on the OH stretching fundamental of HDO (uncoupled OH) with LiCl (2-16 M) [13,14] and NaCl down to - 30°C [ 151 also show no new features when compared with the bulk spectrum as well as no significant frequency shifts. However, Verrall [ 161 separates the cation and anion pairs when discussing their effect on the frequency of the OH stretch. He concludes that Li+ cation-water pairs do not perturb the water stretching spectrum whereas Cl- anionwater pairs show a shift to higher frequency. Nonetheless, the spectrum of decoupled OH oscillators is so weakly perturbed that to a good approximation we can conclude that, on the basis of the spectral frequency distribution, the ability of OH oscillators to couple is unaffected in these solutions relative to bulk water at the same temperature. There are, however, structural reasons which may inhibit coupling and decrease the collective character in Hz0 solutions simply because the pairwise coupling depends on hydrogen bond-hydrogen bond correlations. Raman spectral studies of aqueous LiCl solutions [ 17,181 have been reported. While these studies concentrated on the translational and librational regions of the water spectrum above O”C, OH stretching spectra were recorded [ 171 and these show a decrease in intensity, with added LiCl, of the band centred at 3225 cm-’ which we have interpreted [ l-5 ] as arising from small-amplitude in-phase col-
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lective OH stretching motions. Walrafen had assigned this band to Fermi resonance of v1 and 2vZ, and concluded that the resultant decrease in intensity of this band upon the addition of electrolytes arises from the breaking of hydrogen bonds. The small change in the decoupled oscillator spectrum is not consistent with this interpretation. We will show in this paper that the decrease is due to change in the network structure, and that this can be quantified. Kint and Scherer [ 181 have studied the influence of Kl on the Raman spectrum of HZ0 and interpreted their results in terms of disruption of the intermolecular coupling. Their study however did not quantify the extent of this disruption. Neutron and X-ray studies have been reported on dilute and concentrated aqueous LiCl solutions. Narten and co-workers [ 191 concluded from their X-ray and neutron studies that the average coordination of water molecules around Li+ ions and Cl- ions is four and six respectively. Neutron diffraction studies with isotopic substitution [ 20,2 1] have shown that bridging waters between the LiCl cation-anion pair and HZ0 shells are important at high salt concentrations. It has also been deduced from these studies that the overall coordination number is 5.5. Numerical simulations [ 22,231 have been performed on electrolyte solutions, and of particular relevance to this work has been a study of the HZ0 infrared spectrum [23] in these solutions. Unfortunately the Raman spectrum was not calculated, but it is noted that the use of a frozen field approximation precludes the dynamical coupling between oscillators on different molecules but does incorporate that between OH oscillators on the same molecule. Both contribute to the collective band studied in this paper. An important feature of LiCl solutions above 10 molI is that they quench both the anomalous properties of water in the supercooled region and the homogeneous nucleation to ice I, thereby allowing the formation of a glass [ 22-291. Diffraction studies [ 241 on LiCl samples have measured the kinetics of formation of ice in such glassy samples. These show that 8 mol% solutions homogeneously nucleate at a temperature TH,the homogeneous nucleation temperature, forming ice I,, during a liquid nitrogen quench. On the other hand, 10 mol% solutions form glass samples which precipitate ice I, (metastable) at temperatures a few degrees above the glass transition 386
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temperature ( Tgx 142 K) for concentrations > 9 molW LiCl. At 14 mol% however samples remain vitreous when warmed from Tgto 193 K. Thermodynamic measurements also relating to TH and Tghave given important structural insights into aqueous LiCl solutions. Angel1 and Sare [25] have shown that LiCl compared with other salts of the same molarity raises Tgby the smallest amount. Two processes were identified as being important, namely phase separation (i.e. liquid-liquid immiscibility) and nucleation. NMR studies [ 261 using ‘Li, *H, and ‘H and 18 mol% solutions show that the water in the system is still quite free at the glass transition temperature. Angel1 and co-workers [27] have also looked at T, and Tgof Li salts in DzO and H,O and found doubly unstable glass regions. MacFarlane et al. [ 281 investigated the cooling rate dependence of TH for several LiCl emulsion systems. Shear viscosity and shear impedance measurements [29] have also led to an estimate of Tg as well as some information on the mechanism of ionic conduction. Further structural information has also been gleaned through dielectric loss measurements [ 301 on LiCl solutions with R = 4.66-6.72 (where R = 55.5/M) as well as Rayleigh and Brillouin scattering [ 3 I] over a wider range of concentrations. The studies discussed above do not directly shed light on the central question of how the LiCl at 10 mol% quenches [32] the anomalous properties of pure water. These anomalies arise from fluctuations between configurations of water molecules having similar energies but different densities [ 33,341. In H20/H20 mixtures [ 51, where the anomalous properties are also quenched [35], the intensity of the collective band was shown [ 51 to arise from destruction of the tetrahedral water network with one H202 molecule introducing between 2 and 3 network defects. A study of the collective character in LiCl electrolyte solutions is carried out in this paper with the goal of extracting similar information. As previously stated, we have shown [ 1,2] that the band centred around 3200 cm-’ in the Raman spectrum of H,O can be attributed to small amplitude inphase collective OH stretching motions. The first mechanism whereby defects give rise to a reduction in the collective band intensity has been postulated [ 361 in terms of a decoupling of a defect OH oscillator when its stretching frequency lies outside the
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span of the vibron density of states determined by the nearest neighbour coupling. The large vibron bandwidth is primarily a property of the tetrahedral coordination of the network, as shown by analysis of available data on the ice polymorphs [ 361. The second mechanism arises when an OH oscillator is found in an environment of reduced coordination number in which there are fewer nearest neighbour oscillators with which it can couple. This will lead to an overall reduction of the vibron bandwidth when the number of such environments become sufficiently large, and a decrease of the intensity of the in-phase collective band. Both mechanisms can play a role in aqueous solutions, but when there are only small changes in the decoupled OH stretching spectrum but large changes in the in-phase collective band the latter mechanism is dominant. This is the case in both LiCl and H202 solutions. The OH stretching I,, and I,_ Raman spectra of 5, 10 and 15 mol% aqueous LiCl solutions were recorded as previously described [ 1,2] over a temperature range of - 30 to 70°C. The samples were contained in sealed Pyrex capillaries (with a vapour bubble) in order to enhance the extent of supercooling by minimising the sample volume. Raman spectra in the metastable supercooled region as well as polarized spectra in the stable liquid region have not been previously reported. In figs. 1A, lB, 2A and 2B, the I,, and II spectra are shown at 30°C and - 5 “C respectively. These show, as for H202, that the major effect of the addition of LiCl is to reduce the intensity of the collective band. Qualitatively, it appears that the collective band intensity is reduced to a large extent with the addition of only 5 mol% LiCI, and a roughly linear dependence is observed upon further addition of LiCl. The relative intensity C,(T) of the band assigned to the in-phase collective motions in the I,, spectrum was extracted using the spectral stripping procedure introduced in ref. [ 11. The spectra of the resulting collective bands are shown for 30°C in fig. 3A and - 5 ‘C in fig. 3B. The value of C for ice I and H20( as) is 0.54 + 0.02 [ 1 ] and any decrease relative to this is interpreted in terms of a variation of the tetrahedral nature of the network, by either of the mechanisms discussed above. Values of C,( T) obtained from each set of spectra are plotted in fig. 4.
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Firstly, it is interesting to note that the values of C for 15 mol*/oat 30 and 70°C are very small. Thus the structural disorder in LiCl solutions of greater than 15 mol% together with the intrinsic network defects in bulk water above 30°C are sufficient to uncouple every OH oscillator in solution. On the basis of the small shifts in the uncoupled oscillator spectrum [ 13- 16 1, we attribute this decoupling of OH oscillators to the Lit and Cl- ions changing the network coordination with respect to OH oscillators, and not the presence of a greater number of distorted or broken hydrogen bonds as previously argued [ 171. It has been argued [ 1,2,36] that defect-free tetrahedral networks have a Cvalue equal to C,, viz. 0.54 f 0.02 [ 11. We have shown [ 21 that in ice I the collective motions can be destroyed by mass defects (OD oscillators) and that the intensity of the collective band is reduced as (I -poD)2, where poD is the mole fraction of OD. Liquid water is characte~zed by fluctuating hydrogen bond defects [ 371, and a selfconsistent analysis showed [2] that the collective band intensity is reduced by these intrinsic defects as (1 -pi)’ where pr is the probability that an OH oscillator is a vibrational defect, which can be broadly interpreted as the probability of a broken hydrogen bond. Consequently C(~)=Gzef1-Pl~2
9
(1)
where C(T) refers to the pure liquid water data obtained previously [ 1 ] . In order to extract structural information from the results of this study, we consider a model of a hypothetical fully hydrogen bonded reference network, containing the Li+ and Cl- ions. In this case all the hydrogen bonds which can form would have OH stretching frequencies within the vibron bandwidth, but the in-phase collective band intensity would be reduced because water molecules in the first solvation shell will have oxygen atoms which exhibit a lower coordination number with respect to OH oscillators (as donors or acceptors) than that of a perfect tetrahedral network such as ice I. The intensity of the collective band C, would be independent of temperature assuming no underlying change of “structure” of the reference network, and C, < Ci,, which is the second mechanism mentioned above. In the real liquid, the network structure exhibits hydrogen bond defects and if the extent and dist~bution of these is 387
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B
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Fii. 1. The OH stretching I,, (A) and I, (B) spectra at 30°C for a range of mole fractions of LiCi.
the same as in bulk water at the same temperature, then C, ( r) will be reduced because of a reduction in the number of resonant nearest neighbour couplings by the same extent as in liquid water. This is the first mechanism, hence we postulate that
A crude model of C, can be developed on the basis that one Li+ and one Cl- effectively remove 2n OH oscillators from the hypothetical fully hydrogenbonded tetrahedral network. For small mole fractions (p-J of LiCl, it follows that
G(n=cx(l-Pd2
Cx=CiceC1-nPx)
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-
(2)
-
(3)
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Fig. 2. The OH stretching I,, (A) and II (B) spectra at - 5°C for a range of mole fractions of LiCI.
Combining (1 ), (2) and (3) gives the simple result that GA~f=CtW1--WX)
+
(41
It is this relationship which is used to evaluate the only parameter of the model namely n. The same
argument was applied to H202/H20 mixtures at low mole fractions of Hz02 with the result that n x 2-3. Values of n extracted using the C,( T) data of fig. 4 are shown in table 1. They exhibit little variation with mole fraction of LiCl and a slight drift towards lower values with temperature. However, it is likely 389
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Table 1 Estimate of hydration number in concentrated LiCl solutions
A 30%
Mole fraction LiCl px 0.05 0.10 0.15
n=~l-G(WC(~)l@X 70°C
30°C
-5°C
- 30°C
6.3 6.1 5.6
5.0 5.2 5.4
5.6 4.8 5.3
a) 4.1 4.4
a) Nucleation was obtained before - 30°C was reached.
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Fig. 3. The collective band I, of the LiCl solutions is shown at 30°C (A) and - 5°C (B) for a range of mole fractions in LiCl.
4
200
300 TEMPERATURE
(K)
Fig. 4. The relative in-phase collective band intensity C,(r) is plotted as a function of temperature for moIe fractions of LiCI: (.)0.00,(,.)0.05,(~)0.10,(*)0.15.
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that the drift with temperature is attributed to systematic errors in the spectral stripping procedure used to evaluate C,( 7). From the data at 30°C and - 5”C, it is judged that the temperature dependence of n is small. The values of C,( T) at 70°C are so small that the estimates of n at 70°C will be susceptible to systematic error. The fall in n at - 30 * C could be meaningfull, but more experiments in the supercooled regime would be required to verify this. The mean value of n is 5.3 k 0.6. This means that for every LiCl molecule introduced approximately 5.3 water molecules are disrupted in the water network. The value of n= 5.3 2 0.6 agrees well with the thermodynamic studies of Angel1 and Sare [ 25 1. Angel1 and Sare have calculated the total hydration number (i.e. the number of Hz0 molecules that a cation-anion pair can remove from the water network) of LiCl to be between 6 and 8. Even closer agreement is obtained with diffraction studies [ 20,211 where a coordination number of 5.5 was obtained. The constant value of n is indicative of a single mechanism for the reduction of the collective band intensity C, of the reference network. As previously postulated, this occurs when an OH oscillator is found in an environment of reduced coordination number. The temperature independence of n in the equilibrium liquid regime confirms that the dominant contribution to the temperature dependence of C,( T) in LiCl solutions arises not only from the same mechanism as in bulk water, namely the creation of defective hydrogen bonds, i.e. O-H oscillators associated with “broken bonds” which vibrate at a frequency above the high-frequency edge of the vibron density of states, but also to the same extent as in bulk water. This is consistent with the small changes in the HDO spectrum. The concentration independence of n at
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all temperatures suggests that from 5 to 15 mol% LiCl, the structural characteristics of the hydration shells and the remaining bulk water are not significantly changed. This has interesting ramifications because it is precisely in this concentration regime that the anomalous properties of the water are progressively suppressed. Insofar as the anomalies arise from density fluctuations of a modest spatial extent, it is possible that the suppression is attributable to breakup of patches of water molecules in configurations of different density by the LiCl without significantly altering the hydrogen bond energy distribution. Any significant alteration of the hydrogen bond energy distribution would be reflected by a change in the HDO spectrum, which is small, i.e. the suppression arises primarily from geometric, rather than “energetic”, perturbations introduced by the LiCl. In summary, we have shown that the OH stretching Raman spectrum of collective proton motions can be used to quantitatively probe the number of water molecules that a LiCl cation-anion pair can effectively remove from the tetrahedral water network. This work was supported by the Australian Research Grants Scheme. JLG acknowledges the award of a Commonwealth Postgraduate Scholarship.
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[ 81 M.S. Bergren and S.A. Rice, J. Chem. Phys. 77 (1982) 583. [ 91 G. Neilson and S.A. Rice, J. Chem. Phys. 78 (1983) 4824. [ lo] D.A. Draegert and D. Williams, J. Chem. Phys. 48 (1968) 401. [ 111 H.S. Frank and M.W. Evans, J. Chem. Phys. 13 (1945) 507. [ 121 H.S. Frank and W.Y. Wen, Discussions Faraday Sot. 24 (1957) 133. [ 131 K.A. Hartman Jr., J. Phys. Chem. 70 (1966) 270. [ 141 R.D. Waldron, J. Chem. Phys. 26 (1957) 809. [ 15 ] H.R. Wyss and M. Falk, Can. J. Chem. 48 (1970) 607. [ 161 R.E. Verall, in: Water, a comprehensive treatise, Vol. 3, ed. F.Franks (Plenum Press, New York, 1973) ch. 5. [ 171 G.E. Walrafen, J. Chem. Phys. 36 (1962) 1035. [ 181 S. Kint and J.R. Scherer, J. Chem. Phys. 69 (1978) 1429. [ 191 A.H. Narten, F. Vaslow and H.A. Levy, J. Chem. Phys. 58 (1973) 5017. [ 201 J.R. Newsome, G.W. Neilson and J.E. Enderby, J. Phys. C 13 (1980) L923. [21] AK. Soper, G.W. Neilson, J.E. Enderby and R.A. Howe, J. Phys. Cl0 (1977) 1793. [22] R.O. Watts, Mol. Phys. 32 (1976) 659; K. Heinxinger, in: Ions and molecules in solution, ed. R. Tamamushi (1982) p.61; M. Mezei and D.L. Beveridge, J. Chem. Phys. 74 (1981) 6902. [23] M.F. Mills, J.R. Reimers and R.O. Watts, Mol. Phys. 57 (1985) 777. [24] A. Elarby-Aoulzerat, J.F. Jal, C. Ferradou, J. Dupuy, P. Chieux and A. Wright, J. Phys. Chem. 87 (1983) 4170. [25] C.A. Angel1 and E.J. Sare, J. Chem. Phys. 52 (1970) 1058. [26] E.J. SutterandJ.F.Harmon, J. Phys. Chem. 79 (1975) 1958. [ 271 C.A. Angell, E.J. Sare, J. Donnella and D.R. MacFarlane, J. Phys. Chem. 85 (1981) 1461. [ 28 ] D.R. MacFarlane, R.K. Kadlyala and C.A. Angell, J. Phys. Chem. 87 (1983) 235. [29] C.T. Moynihan, N. Balitactac, L. Boone and T.A. Lotovitz, J. Chem. Phys. 55 (1971) 3013. [30] C.T. Moynihan, R.D. Bressel and C.A. Angell, J. Chem. Phys. 55 (1971) 4414. [31] S.-Y. Hsich, R.W. Gammon, P.B. Macedo and C.J. Montrose, J. Chem. Phys. 56 (1972) 1663. [ 321 C.A. Angell, Nature 296 (1982) 138. [33] R.J.SpeedyandC.A.Angell, J. Chem.Phys. 65 (1976) 851. [34] C.A. Angell, Ann. Rev. Phys. Chem. 34 (1983) 593. [ 351 M. Oguni and C.A. Angell, J. Chem. Phys. 73 (1980) 1948. [ 361 J.L. Green, A.R. Lacey and M.G. Sceats, in preparation. [ 371 A. Geiger, A. Rahman and F.H. Stillinger, J. Chem. Phys. 70 (1979) 263.
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