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UV spectral shift of benzene in sub- and supercritical water
Chemical Physics Letters 394 (2004) 85–89 www.elsevier.com/locate/cplett
UV spectral shift of benzene in sub- and supercritical water Noritsugu Kometani a,*, Koji Takemiya a, Yoshiro Yonezawa a, Fujitsugu Amita b, Okitsugu Kajimoto b a
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan b Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Received 21 May 2004; in final form 21 June 2004 Available online 17 July 2004
Abstract UV absorption spectra of benzene have been measured over the wide range of temperature and pressure from the ambient state to the supercritical state (T = 400 C and P = 40 MPa). The analysis of the spectral shift of benzene in water relative to that in the gas indicates that at T = 380 and 390 C the local solvent density around benzene is likely to be depressed below the bulk density for densities near the critical density. It is found that p-hydrogen bond between benzene and water becomes evident with lowering temperature below T = 340 C. 2004 Elsevier B.V. All rights reserved.
1. Introduction Supercritical water (SCW), which refers to water above the critical temperature (TC = 374 C) in this study, possesses interesting properties which are quite different from those of ambient water [1]. Especially, SCW is completely miscible with nonpolar organic compounds such as benzene because of the low dielectric constant and the weak hydrogen-bonding network [2,3]. As water is an environmentally benign solvent, SCW can be an alternative medium for various organic chemical processes [4,5]. In order to control organic chemical reactions in SCW, it is important to understand the density dependence of the hydration structure around the solute. Considering the drastic change in the solvent properties of SCW with pressure or density, the remarkable variation in the hydration structure would be also expected. To examine the solvation in supercritical fluids, it is often *
Corresponding author. Fax: +81-66690-2743. E-mail address: [email protected] (N. Kometani). 0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.115
useful to measure the spectral shift of proper probe molecules. In the earliest work, Bennett and Johnston [6] reported UV–Vis absorption spectral shift of acetone and benzophenone in SCW and observed that the local density around these solutes are enhanced above the bulk density for densities near the critical density. Oka and Kajimoto [7] also reported the similar results for 4-nitroaniline in SCW. Osada et al. [8] have studied the hydrogen bonding between quinoline and water in SCW by UV–Vis spectroscopy and observed that the degree of the solute–solvent hydrogen bonding decreased for densities near the critical density due to the local density depression around the solute. In all of these preceding works, polar solutes have been employed as probe molecules. In this study, we have focused our attention on the hydration of the nonpolar aromatic compound in SCW. Benzene is employed as a probe because it is typical nonpolar aromatic molecule as well as enough stable in SCW. UV absorption spectra of benzene have been measured in SCW over the wide range of temperature and pressure. The hydration of benzene in SCW is discussed by analyzing the spectral shift on the basis of dielectric theory.
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2. Experimental Benzene (spectral grade, KISHIDA Chemical Co.) was used as received. Benzene aqueous solution was prepared with distilled water. The concentration was adjusted among 2–6 · 103 M so that the optical density of benzene at k = 257 nm might be less than 0.15. In this concentration range, the spectral shift does not depend on the concentration. The detail of the high-pressure optical cell has been already described elsewhere [9]. In brief, the cell was made of Hastelloy and equipped with two optical windows of synthetic sapphire. The optical path length is 10 mm. A small electric furnace heats the high-pressure optical cell up to T = 400 C while a water-cooled thermal shielding jacket protects the spectrometer. Temperature was detected and controlled by a thermocouple inserted into the cell within ±1 C accuracy. The solution was introduced to the high-pressure cell by intelligent HPLC pump (PU-1580, Jasco Co.) after deaeration with nitrogen. Pressure was monitored by a strain gauge (PG-1TH, Kyowa) and controlled by a back-up regulator (SCF-Bpg, Jasco Co.) within ±0.1 MPa accuracy. UV spectra were recorded on a UV– Vis spectrophotometer (V-560, Jasco Co.). The water density was calculated from the temperature and pressure by the 58-coefficient equation of state reported by Saul and Wagner [10].
3. Results Absorption spectra of benzene in sub- and supercritical water have been measured as a function of pressure up to P = 40 MPa at various temperatures. Fig. 1 shows the typical absorption spectra of benzene in SCW (T = 400 C). A broad vibronic band belonging to A1B2u ‹ X1A1g electronic transition of benzene is observed at around k = 257 nm. The peak position of this band slightly shifts toward longer wavelength with in-
creasing pressure. Absorption spectra at other temperatures are nearly the same as those in Fig. 1. To analyze the spectral shift, the peak frequency, mmax was evaluated by fitting a Gauss function to the spectral curves near the highest peak of this band. The obtained mmax values for different temperatures are plotted against pressure in Fig. 2. It is found that mmax is approximately independent of pressure for temperatures below T = 340 C, because water is in the liquid state and the water density does not change significantly with pressure. At T = 360 C, the peak position suddenly changes at about P = 10 MPa due to the gas–liquid phase transformation. When the temperature exceeds TC, mmax continuously shifts toward lower frequency with increasing pressure because of continuous and wide variation of water density with pressure. Generally, the spectral shift with temperature and pressure consists of the contributions of the effect of solute–solvent interaction as well as the temperature effect (thermochromic effect). As we are interested only in the contribution of the solute–solvent interaction, the thermochromic effect must be excluded from the observed spectral shifts. We have then measured the absorption spectra of benzene in gaseous nitrogen at P = 0.1 MPa for various temperatures to normalize the thermochromic effect on the spectral shift. Since the interaction between benzene and nitrogen is negligible at low pressure, the peak frequency in gaseous nitrogen includes only the thermochromic effect. The peak frequency observed in gaseous nitrogen decreases approximately linearly with elevating temperature (see Fig. 3), reflecting that the molecular rotational distribution of benzene shifts toward higher energy with temperature. Assuming that the temperature effect in aqueous solution is similar to that in gaseous nitrogen, the spectral shift arising from the solute–solvent interaction, Dm, can be estimated by sub-
[× 104]
100˚C 150˚C 200˚C 250˚C 300˚C 340˚C 360˚C 380˚C 390˚C 400˚C
35.0 MPa 30.0 MPa 28.0 MPa 27.0 MPa
0.1
ν max / cm
Absorbance
0.15
-1
3.92
3.91
3.9
0.05 3.89 0
240
260 Wavelength / nm
280
Fig. 1. UV absorption spectra of benzene in water at T = 400 C for different pressures.
20
30 Pressure / MPa
40
Fig. 2. The pressure dependence of the peak frequency of benzene absorption, mmax, for different temperatures.
N. Kometani et al. / Chemical Physics Letters 394 (2004) 85–89
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kg/m3, the Dm values decrease with lowering temperature.
[× 104] 3.94
3.93
ν max / cm
-1
4. Discussion 3.92
3.91
3.9
3.89
100
200
300
400
Temperature / ˚C Fig. 3. Thermochromic effect on mmax of benzene in gaseous nitrogen at P = 0.1 MPa.
tracting the peak frequency in gaseous nitrogen at a given temperature from that in water at the same temperature. Fig. 4 shows the Dm values obtained in this manner as a function of water density, q, for different temperatures. The critical density, qC = 322 kg/m3, is shown by the dashed line in Fig. 4 for reference. As seen in this figure, the Dm values increase with density up to about q = 600 kg/m3 and reach the maximum at a density near q = 650 kg/m3. For densities higher than q = 700 kg/m3, Dm decreases with increasing q. It is pointed out here that the increase in the density above q = 700 kg/m3 results from lowering temperature rather than increasing pressure, i.e., for densities higher than q = 700
ρr
0 200
∆ν / cm
-1
150
100
1
2
3
25˚C 100˚C 150˚C 200˚C 250˚C 300˚C 340˚C 360˚C 380˚C 390˚C 400˚C
50
0 0
200
400
600
ρ / kg/m
800
1000
3
Fig. 4. The density dependence of Dm for different temperatures. The solid line represents Dmcal calculated by Eq. (1). The dashed line represents the critical density of water.
It has been demonstrated that the solvatochromic shift in liquid solutions where there is no specific solute–solvent interaction could be quantitatively predicted by the dielectric theory [11]. Amos and Burrows have derived the following expression to calculate the solvatochromic shift based on the dielectric theory [12]: lug ðlue lug Þ 2ðe 1Þ 2ðn2 1Þ 2 hcDmcal ¼ a3 eþ2 n þ2 2 h i 1 n 1 4pq 3 ðlue Þ2 ðlug Þ2 3 ðlvg Þ2 ðaue aug Þ a n2 þ 2 3a 2 u u u v 3 ae ag I I n 1 ; ð1Þ 2 a3 I u þ I v n2 þ 2 where Dmcal denotes the calculated solvatochromic shift; l, a and I are the dipole moment, polarizability and ionization energy, respectively; e and n are the dielectric constant and refractive index of the solvent, respectively; a is the cavity radius; superscripts, u and v, refer the solute and the solvent, respectively; subscripts, g and e, refer ground and excited state, respectively. In Eq. (1), the first term represents the interaction between solute and solvent dipoles, the second term the interaction between solute and solvent induced dipoles, the third term the interaction between solvent dipole and solute induced dipole, and the forth term the interaction between solute and solvent induced dipoles. As benzene possesses no permanent dipole moment, both first and second terms in Eq. (1) vanish and only third and forth terms survive. In order to calculate Dmcal for benzene in water by Eq. (1), we have taken the following data from the literatures: lvg ¼ 1:85 D [13], Iu = 9.246 eV, Iv = 12.61 eV [14], aue aug ¼ 1:2 1024 cm3 [15]. The refractive index of water at high temperature and high pressure was calculated by the formula reported by Harvey et al. [16]. The cavity radius of benzene in water, a = 0.496 nm, was calculated by [17] 1=3 4 vdu 1þ M M udv 3M u a3 ¼ ; ð2Þ 1=3 4pN A d u vdu p M M udv where M, d and NA are molecular weight, liquid density and AvogadroÕs number, respectively; subscripts, u and v, refer to the solute and solvent, respectively. The solid line in Fig. 4 represents Dmcal calculated by Eq. (1). It is found that Dmcal increases almost linearly with q but shows no appreciable dependence on temperature. This means that, within the framework of the di-
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electric theory, the Dm values for different temperatures should converge on the single line when they are plotted against the solvent density. We first compare Dmcal with the experimental Dm for densities higher than q = 700 kg/m3, i.e., the temperature range of T < 340 C. In this region, Dm decreases while Dmcal increases with increasing q, resulting in increasing deviation between Dm and Dmcal. As mentioned in the previous chapter, the density variation in this range mainly arises from the temperature change rather than the pressure change. This observation therefore implies that there is some specific solute–solvent interaction, which becomes evident and gives rise to the decrease in Dm with lowering temperature. It has been well known that benzene forms p-hydrogen bond with water in an ambient state [18]. The experiments of jet-cooled benzene–water clusters revealed that UV absorption spectrum of benzene is blue-shifted due to p-hydrogen bond with water [19]. The blue shift of benzene absorption leads to the decrease in Dm. Furthermore, the magnitude of the deviation between Dm and Dmcal amounts to about 100–150 cm1 as indicated in Fig. 4, which is almost comparable to those observed in jet-cooled benzene–water clusters. These facts strongly suggest that benzene would form p-hydrogen bond with water below T = 340 C, causing the blue shift of benzene absorption and the decrease in Dm. As p-hydrogen bond becomes stronger with lowering temperature, the deviation between Dm and Dmcal would increase with lowering temperature. According to the results of ab initio calculations, the binding energy of such p-hydrogen bond has been found to be 3.9 ± 0.2 kcal/mol, which is only 20% weaker than that of water–water hydrogen bond [20]. It is therefore reasonable that p-hydrogen bond of benzene with water may persist even at high temperatures up to T = 340 C. In fact, the infrared absorption spectroscopy of benzene–water mixture has demonstrated the existence of p-hydrogen bond at T = 250 C [21]. We next focus our attention on the data for densities lower than q = 600 kg/m3, which corresponds to the temperature region higher than T = 380 C (supercritical state). As seen in Fig. 4, the Dm values at T = 400 C increase almost linearly with q, which is approximately consistent with the density dependence of Dmcal. On the other hand, the Dm values at T = 380 and 390 C exhibit the convex density dependence. The Dm values at T = 380 and 390 C are close to those at T = 400 C for low densities near q = 100 kg/m3 and for high densities near q = 600 kg/m3, but they deviate from each other for medium densities around qC. Such tendency is more evident at T = 380 C as temperature is closer to TC. We have tried to ascribe these observations to the difference between the local density around the solute and the bulk density, as often discussed in other systems.
Firstly, we have assumed that at T = 400 C the solvent density is homogeneous and there is no p-hydrogen bond between benzene and water, resulting in the linear density dependence of the Dm values at T = 400 C as predicted by Eq. (1). Under this assumption, one can estimate the local solvent density around the solute from the deviation of the observed Dm from those at T = 400 C. As the Dm values at T = 380 and 390 C are larger than those at T = 400 C for medium densities around q = 200–400 kg/m3, it might be expected that the local solvent density is enhanced above the bulk density. Similar results have been reported for 4-nitroaniline in SCW [7]. In contrast, the completely different conclusion may be drawn from the present observations. Here, we have assumed that p-hydrogen bond exists even at high temperatures of T = 380–400 C. This assumption is not unreasonable considering the large bonding energy of p-hydrogen bond [20]. In this case, absorption spectra of benzene are subject to the blue shift due to the p-hydrogen bond, resulting in smaller Dm than Dmcal. This agrees with the observation in Fig. 4. If the local density in the vicinity of the solute is depressed below the bulk density, the average number of p-hydrogen bond would also decrease. As a result, the observed Dm would be increased due to the depressed local density, since the spectral shift is sensitive to the p-hydrogen bond. We can therefore conclude that the convex density dependence of the Dm values around q = 200– 400 kg/m3 at T = 380 and 390 C is due to the local density depression around the solute. This conclusion is similar to that of Osada et al. [8]. The hydration of benzene in SCW has been studied by computer simulations. Gao [22] has examined the potential of mean force for benzene dimer in SCW at T = 400 C and P = 350 atm using the Monte Carlo method and found that solvent molecules do not form stable clusters near the solute. Cummings and et al. [23] have carried out molecular dynamics simulations for aqueous solutions of various solutes including benzene at q = qC and at Tr = T/Tc = 1.05. They observed that the nearest-neighbor water density around benzene is depressed below the bulk average. These simulation studies obviously support the latter conclusion which we have made in this study: the depression of the local solvent density in the vicinity of the solute and the decrease in p-hydrogen bond in SCW at T = 380 and 390 C.
References [1] E.U. Franck, in: E. Kiran, P.G. Debenedetti, C.J. Peters (Eds.), Supercritical Fluids – Fundamentals and Applications, Nato Science Series E, vol. 366, Kluwer Publishers, Dordrecht, 2000, p. 307.
N. Kometani et al. / Chemical Physics Letters 394 (2004) 85–89 [2] J.F. Connolly, J. Chem. Eng. Data 11 (1966) 13. [3] A. Alwani, G. Schneider, Ber. Bunsenges. Phys. Chem. 71 (1967) 633. [4] R.W. Shaw, T.B. Brill, A.A. Clifford, C.A. Eckert, E.U. Franck, Chem. Eng. News 69 (1991) 26. [5] J.W. Tester, H.R. Holgate, F.J. Armellini, P.A. Webley, W.R. Killilea, G.T. Hong, H.E. Barnes, ACS Symp. Ser. 518 (1993) 35. [6] G.E. Bennett, K.P. Johnston, J. Phys. Chem. 98 (1994) 441. [7] H. Oka, O. Kajimoto, Phys. Chem. Chem. Phys. 5 (2003) 2535. [8] M. Osada, K. Toyoshima, T. Mizutani, K. Minami, M. Watanabe, T. Adschiri, K. Arai, J. Chem. Phys. 118 (2003) 4573. [9] F. Amita, K. Okada, H. Oka, O. Kajimoto, Rev. Scientific Instrum. 72 (2001) 3605. [10] A. Saul, W. Wagner, J. Phys. Chem. Ref. Data 18 (1989) 1537. [11] P. Suppan, N. Ghoneim, Solvatochromism, MPG Books, Cambridge, 1997.
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[12] A.T. Amos, B.L. Burrows, Adv. Quantum Chem. 7 (1973) 289. [13] S.A. Clough, Y. Beers, G.P. Klein, L.S. Rathman, J. Chem. Phys. 59 (1973) 2254. [14] S.G. Lias, J.E. Bartmess, J.L. Holmes, R.D. Levin, J.F. Liebman, W.G. Mallard, J. Phys. Chem. Ref. Data 17 (Suppl. 1) (1988). [15] R. Mathies, A.C. Albrecht, J. Chem. Phys. 60 (1974) 2500. [16] A.H. Harvey, J.S. Gallagher, J.M.H. Levelt Sengers, J. Phys. Chem. Ref. Data 27 (1998) 761. [17] J. Otomo, S. Koda, Chem. Phys. 242 (1999) 241. [18] O.D. Bonner, Y.S. Choi, J. Phys. Chem. 78 (1974) 1723. [19] A.W. Garrett, T.S. Zwier, J. Chem. Phys. 96 (1992) 3402. [20] D. Feller, J. Phys. Chem. A 103 (1999) 7558. [21] S. Furutaka, S. Ikawa, J. Chem. Phys. 117 (2002) 751. [22] J. Gao, J. Am. Chem. Soc. 115 (1993) 6893. [23] P.T. Cummings, A.A. Chialvo, H.D. Cochran, Chem. Eng. Sci. 49 (1994) 2735.
UV spectral shift of benzene in sub- and supercritical water Noritsugu Kometani a,*, Koji Takemiya a, Yoshiro Yonezawa a, Fujitsugu Amita b, Okitsugu Kajimoto b a
Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan b Division of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan Received 21 May 2004; in final form 21 June 2004 Available online 17 July 2004
Abstract UV absorption spectra of benzene have been measured over the wide range of temperature and pressure from the ambient state to the supercritical state (T = 400 C and P = 40 MPa). The analysis of the spectral shift of benzene in water relative to that in the gas indicates that at T = 380 and 390 C the local solvent density around benzene is likely to be depressed below the bulk density for densities near the critical density. It is found that p-hydrogen bond between benzene and water becomes evident with lowering temperature below T = 340 C. 2004 Elsevier B.V. All rights reserved.
1. Introduction Supercritical water (SCW), which refers to water above the critical temperature (TC = 374 C) in this study, possesses interesting properties which are quite different from those of ambient water [1]. Especially, SCW is completely miscible with nonpolar organic compounds such as benzene because of the low dielectric constant and the weak hydrogen-bonding network [2,3]. As water is an environmentally benign solvent, SCW can be an alternative medium for various organic chemical processes [4,5]. In order to control organic chemical reactions in SCW, it is important to understand the density dependence of the hydration structure around the solute. Considering the drastic change in the solvent properties of SCW with pressure or density, the remarkable variation in the hydration structure would be also expected. To examine the solvation in supercritical fluids, it is often *
Corresponding author. Fax: +81-66690-2743. E-mail address: [email protected] (N. Kometani). 0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.06.115
useful to measure the spectral shift of proper probe molecules. In the earliest work, Bennett and Johnston [6] reported UV–Vis absorption spectral shift of acetone and benzophenone in SCW and observed that the local density around these solutes are enhanced above the bulk density for densities near the critical density. Oka and Kajimoto [7] also reported the similar results for 4-nitroaniline in SCW. Osada et al. [8] have studied the hydrogen bonding between quinoline and water in SCW by UV–Vis spectroscopy and observed that the degree of the solute–solvent hydrogen bonding decreased for densities near the critical density due to the local density depression around the solute. In all of these preceding works, polar solutes have been employed as probe molecules. In this study, we have focused our attention on the hydration of the nonpolar aromatic compound in SCW. Benzene is employed as a probe because it is typical nonpolar aromatic molecule as well as enough stable in SCW. UV absorption spectra of benzene have been measured in SCW over the wide range of temperature and pressure. The hydration of benzene in SCW is discussed by analyzing the spectral shift on the basis of dielectric theory.
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2. Experimental Benzene (spectral grade, KISHIDA Chemical Co.) was used as received. Benzene aqueous solution was prepared with distilled water. The concentration was adjusted among 2–6 · 103 M so that the optical density of benzene at k = 257 nm might be less than 0.15. In this concentration range, the spectral shift does not depend on the concentration. The detail of the high-pressure optical cell has been already described elsewhere [9]. In brief, the cell was made of Hastelloy and equipped with two optical windows of synthetic sapphire. The optical path length is 10 mm. A small electric furnace heats the high-pressure optical cell up to T = 400 C while a water-cooled thermal shielding jacket protects the spectrometer. Temperature was detected and controlled by a thermocouple inserted into the cell within ±1 C accuracy. The solution was introduced to the high-pressure cell by intelligent HPLC pump (PU-1580, Jasco Co.) after deaeration with nitrogen. Pressure was monitored by a strain gauge (PG-1TH, Kyowa) and controlled by a back-up regulator (SCF-Bpg, Jasco Co.) within ±0.1 MPa accuracy. UV spectra were recorded on a UV– Vis spectrophotometer (V-560, Jasco Co.). The water density was calculated from the temperature and pressure by the 58-coefficient equation of state reported by Saul and Wagner [10].
3. Results Absorption spectra of benzene in sub- and supercritical water have been measured as a function of pressure up to P = 40 MPa at various temperatures. Fig. 1 shows the typical absorption spectra of benzene in SCW (T = 400 C). A broad vibronic band belonging to A1B2u ‹ X1A1g electronic transition of benzene is observed at around k = 257 nm. The peak position of this band slightly shifts toward longer wavelength with in-
creasing pressure. Absorption spectra at other temperatures are nearly the same as those in Fig. 1. To analyze the spectral shift, the peak frequency, mmax was evaluated by fitting a Gauss function to the spectral curves near the highest peak of this band. The obtained mmax values for different temperatures are plotted against pressure in Fig. 2. It is found that mmax is approximately independent of pressure for temperatures below T = 340 C, because water is in the liquid state and the water density does not change significantly with pressure. At T = 360 C, the peak position suddenly changes at about P = 10 MPa due to the gas–liquid phase transformation. When the temperature exceeds TC, mmax continuously shifts toward lower frequency with increasing pressure because of continuous and wide variation of water density with pressure. Generally, the spectral shift with temperature and pressure consists of the contributions of the effect of solute–solvent interaction as well as the temperature effect (thermochromic effect). As we are interested only in the contribution of the solute–solvent interaction, the thermochromic effect must be excluded from the observed spectral shifts. We have then measured the absorption spectra of benzene in gaseous nitrogen at P = 0.1 MPa for various temperatures to normalize the thermochromic effect on the spectral shift. Since the interaction between benzene and nitrogen is negligible at low pressure, the peak frequency in gaseous nitrogen includes only the thermochromic effect. The peak frequency observed in gaseous nitrogen decreases approximately linearly with elevating temperature (see Fig. 3), reflecting that the molecular rotational distribution of benzene shifts toward higher energy with temperature. Assuming that the temperature effect in aqueous solution is similar to that in gaseous nitrogen, the spectral shift arising from the solute–solvent interaction, Dm, can be estimated by sub-
[× 104]
100˚C 150˚C 200˚C 250˚C 300˚C 340˚C 360˚C 380˚C 390˚C 400˚C
35.0 MPa 30.0 MPa 28.0 MPa 27.0 MPa
0.1
ν max / cm
Absorbance
0.15
-1
3.92
3.91
3.9
0.05 3.89 0
240
260 Wavelength / nm
280
Fig. 1. UV absorption spectra of benzene in water at T = 400 C for different pressures.
20
30 Pressure / MPa
40
Fig. 2. The pressure dependence of the peak frequency of benzene absorption, mmax, for different temperatures.
N. Kometani et al. / Chemical Physics Letters 394 (2004) 85–89
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kg/m3, the Dm values decrease with lowering temperature.
[× 104] 3.94
3.93
ν max / cm
-1
4. Discussion 3.92
3.91
3.9
3.89
100
200
300
400
Temperature / ˚C Fig. 3. Thermochromic effect on mmax of benzene in gaseous nitrogen at P = 0.1 MPa.
tracting the peak frequency in gaseous nitrogen at a given temperature from that in water at the same temperature. Fig. 4 shows the Dm values obtained in this manner as a function of water density, q, for different temperatures. The critical density, qC = 322 kg/m3, is shown by the dashed line in Fig. 4 for reference. As seen in this figure, the Dm values increase with density up to about q = 600 kg/m3 and reach the maximum at a density near q = 650 kg/m3. For densities higher than q = 700 kg/m3, Dm decreases with increasing q. It is pointed out here that the increase in the density above q = 700 kg/m3 results from lowering temperature rather than increasing pressure, i.e., for densities higher than q = 700
ρr
0 200
∆ν / cm
-1
150
100
1
2
3
25˚C 100˚C 150˚C 200˚C 250˚C 300˚C 340˚C 360˚C 380˚C 390˚C 400˚C
50
0 0
200
400
600
ρ / kg/m
800
1000
3
Fig. 4. The density dependence of Dm for different temperatures. The solid line represents Dmcal calculated by Eq. (1). The dashed line represents the critical density of water.
It has been demonstrated that the solvatochromic shift in liquid solutions where there is no specific solute–solvent interaction could be quantitatively predicted by the dielectric theory [11]. Amos and Burrows have derived the following expression to calculate the solvatochromic shift based on the dielectric theory [12]: lug ðlue lug Þ 2ðe 1Þ 2ðn2 1Þ 2 hcDmcal ¼ a3 eþ2 n þ2 2 h i 1 n 1 4pq 3 ðlue Þ2 ðlug Þ2 3 ðlvg Þ2 ðaue aug Þ a n2 þ 2 3a 2 u u u v 3 ae ag I I n 1 ; ð1Þ 2 a3 I u þ I v n2 þ 2 where Dmcal denotes the calculated solvatochromic shift; l, a and I are the dipole moment, polarizability and ionization energy, respectively; e and n are the dielectric constant and refractive index of the solvent, respectively; a is the cavity radius; superscripts, u and v, refer the solute and the solvent, respectively; subscripts, g and e, refer ground and excited state, respectively. In Eq. (1), the first term represents the interaction between solute and solvent dipoles, the second term the interaction between solute and solvent induced dipoles, the third term the interaction between solvent dipole and solute induced dipole, and the forth term the interaction between solute and solvent induced dipoles. As benzene possesses no permanent dipole moment, both first and second terms in Eq. (1) vanish and only third and forth terms survive. In order to calculate Dmcal for benzene in water by Eq. (1), we have taken the following data from the literatures: lvg ¼ 1:85 D [13], Iu = 9.246 eV, Iv = 12.61 eV [14], aue aug ¼ 1:2 1024 cm3 [15]. The refractive index of water at high temperature and high pressure was calculated by the formula reported by Harvey et al. [16]. The cavity radius of benzene in water, a = 0.496 nm, was calculated by [17] 1=3 4 vdu 1þ M M udv 3M u a3 ¼ ; ð2Þ 1=3 4pN A d u vdu p M M udv where M, d and NA are molecular weight, liquid density and AvogadroÕs number, respectively; subscripts, u and v, refer to the solute and solvent, respectively. The solid line in Fig. 4 represents Dmcal calculated by Eq. (1). It is found that Dmcal increases almost linearly with q but shows no appreciable dependence on temperature. This means that, within the framework of the di-
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electric theory, the Dm values for different temperatures should converge on the single line when they are plotted against the solvent density. We first compare Dmcal with the experimental Dm for densities higher than q = 700 kg/m3, i.e., the temperature range of T < 340 C. In this region, Dm decreases while Dmcal increases with increasing q, resulting in increasing deviation between Dm and Dmcal. As mentioned in the previous chapter, the density variation in this range mainly arises from the temperature change rather than the pressure change. This observation therefore implies that there is some specific solute–solvent interaction, which becomes evident and gives rise to the decrease in Dm with lowering temperature. It has been well known that benzene forms p-hydrogen bond with water in an ambient state [18]. The experiments of jet-cooled benzene–water clusters revealed that UV absorption spectrum of benzene is blue-shifted due to p-hydrogen bond with water [19]. The blue shift of benzene absorption leads to the decrease in Dm. Furthermore, the magnitude of the deviation between Dm and Dmcal amounts to about 100–150 cm1 as indicated in Fig. 4, which is almost comparable to those observed in jet-cooled benzene–water clusters. These facts strongly suggest that benzene would form p-hydrogen bond with water below T = 340 C, causing the blue shift of benzene absorption and the decrease in Dm. As p-hydrogen bond becomes stronger with lowering temperature, the deviation between Dm and Dmcal would increase with lowering temperature. According to the results of ab initio calculations, the binding energy of such p-hydrogen bond has been found to be 3.9 ± 0.2 kcal/mol, which is only 20% weaker than that of water–water hydrogen bond [20]. It is therefore reasonable that p-hydrogen bond of benzene with water may persist even at high temperatures up to T = 340 C. In fact, the infrared absorption spectroscopy of benzene–water mixture has demonstrated the existence of p-hydrogen bond at T = 250 C [21]. We next focus our attention on the data for densities lower than q = 600 kg/m3, which corresponds to the temperature region higher than T = 380 C (supercritical state). As seen in Fig. 4, the Dm values at T = 400 C increase almost linearly with q, which is approximately consistent with the density dependence of Dmcal. On the other hand, the Dm values at T = 380 and 390 C exhibit the convex density dependence. The Dm values at T = 380 and 390 C are close to those at T = 400 C for low densities near q = 100 kg/m3 and for high densities near q = 600 kg/m3, but they deviate from each other for medium densities around qC. Such tendency is more evident at T = 380 C as temperature is closer to TC. We have tried to ascribe these observations to the difference between the local density around the solute and the bulk density, as often discussed in other systems.
Firstly, we have assumed that at T = 400 C the solvent density is homogeneous and there is no p-hydrogen bond between benzene and water, resulting in the linear density dependence of the Dm values at T = 400 C as predicted by Eq. (1). Under this assumption, one can estimate the local solvent density around the solute from the deviation of the observed Dm from those at T = 400 C. As the Dm values at T = 380 and 390 C are larger than those at T = 400 C for medium densities around q = 200–400 kg/m3, it might be expected that the local solvent density is enhanced above the bulk density. Similar results have been reported for 4-nitroaniline in SCW [7]. In contrast, the completely different conclusion may be drawn from the present observations. Here, we have assumed that p-hydrogen bond exists even at high temperatures of T = 380–400 C. This assumption is not unreasonable considering the large bonding energy of p-hydrogen bond [20]. In this case, absorption spectra of benzene are subject to the blue shift due to the p-hydrogen bond, resulting in smaller Dm than Dmcal. This agrees with the observation in Fig. 4. If the local density in the vicinity of the solute is depressed below the bulk density, the average number of p-hydrogen bond would also decrease. As a result, the observed Dm would be increased due to the depressed local density, since the spectral shift is sensitive to the p-hydrogen bond. We can therefore conclude that the convex density dependence of the Dm values around q = 200– 400 kg/m3 at T = 380 and 390 C is due to the local density depression around the solute. This conclusion is similar to that of Osada et al. [8]. The hydration of benzene in SCW has been studied by computer simulations. Gao [22] has examined the potential of mean force for benzene dimer in SCW at T = 400 C and P = 350 atm using the Monte Carlo method and found that solvent molecules do not form stable clusters near the solute. Cummings and et al. [23] have carried out molecular dynamics simulations for aqueous solutions of various solutes including benzene at q = qC and at Tr = T/Tc = 1.05. They observed that the nearest-neighbor water density around benzene is depressed below the bulk average. These simulation studies obviously support the latter conclusion which we have made in this study: the depression of the local solvent density in the vicinity of the solute and the decrease in p-hydrogen bond in SCW at T = 380 and 390 C.
References [1] E.U. Franck, in: E. Kiran, P.G. Debenedetti, C.J. Peters (Eds.), Supercritical Fluids – Fundamentals and Applications, Nato Science Series E, vol. 366, Kluwer Publishers, Dordrecht, 2000, p. 307.
N. Kometani et al. / Chemical Physics Letters 394 (2004) 85–89 [2] J.F. Connolly, J. Chem. Eng. Data 11 (1966) 13. [3] A. Alwani, G. Schneider, Ber. Bunsenges. Phys. Chem. 71 (1967) 633. [4] R.W. Shaw, T.B. Brill, A.A. Clifford, C.A. Eckert, E.U. Franck, Chem. Eng. News 69 (1991) 26. [5] J.W. Tester, H.R. Holgate, F.J. Armellini, P.A. Webley, W.R. Killilea, G.T. Hong, H.E. Barnes, ACS Symp. Ser. 518 (1993) 35. [6] G.E. Bennett, K.P. Johnston, J. Phys. Chem. 98 (1994) 441. [7] H. Oka, O. Kajimoto, Phys. Chem. Chem. Phys. 5 (2003) 2535. [8] M. Osada, K. Toyoshima, T. Mizutani, K. Minami, M. Watanabe, T. Adschiri, K. Arai, J. Chem. Phys. 118 (2003) 4573. [9] F. Amita, K. Okada, H. Oka, O. Kajimoto, Rev. Scientific Instrum. 72 (2001) 3605. [10] A. Saul, W. Wagner, J. Phys. Chem. Ref. Data 18 (1989) 1537. [11] P. Suppan, N. Ghoneim, Solvatochromism, MPG Books, Cambridge, 1997.
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[12] A.T. Amos, B.L. Burrows, Adv. Quantum Chem. 7 (1973) 289. [13] S.A. Clough, Y. Beers, G.P. Klein, L.S. Rathman, J. Chem. Phys. 59 (1973) 2254. [14] S.G. Lias, J.E. Bartmess, J.L. Holmes, R.D. Levin, J.F. Liebman, W.G. Mallard, J. Phys. Chem. Ref. Data 17 (Suppl. 1) (1988). [15] R. Mathies, A.C. Albrecht, J. Chem. Phys. 60 (1974) 2500. [16] A.H. Harvey, J.S. Gallagher, J.M.H. Levelt Sengers, J. Phys. Chem. Ref. Data 27 (1998) 761. [17] J. Otomo, S. Koda, Chem. Phys. 242 (1999) 241. [18] O.D. Bonner, Y.S. Choi, J. Phys. Chem. 78 (1974) 1723. [19] A.W. Garrett, T.S. Zwier, J. Chem. Phys. 96 (1992) 3402. [20] D. Feller, J. Phys. Chem. A 103 (1999) 7558. [21] S. Furutaka, S. Ikawa, J. Chem. Phys. 117 (2002) 751. [22] J. Gao, J. Am. Chem. Soc. 115 (1993) 6893. [23] P.T. Cummings, A.A. Chialvo, H.D. Cochran, Chem. Eng. Sci. 49 (1994) 2735.