Local density augmentation of supercritical water probed by 4,4′-bpyH radical: A pulse radiolysis study

Chemical Physics Letters 657 (2016) 78–82

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Research paper

Local density augmentation of supercritical water probed by 4,40 -bpyH radical: A pulse radiolysis study Zhe Liu a, Zhong Fang a, Yusa Muroya b, Haiying Fu c, Yu Yan d, Yosuke Katsumura b,d, Mingzhang Lin a,e,⇑ a

School of Nuclear Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, China Nuclear Professional School, School of Engineering, The University of Tokyo, 2-22 Shirakata-shirane, Tokai-mura, Naka-gun, Ibaraki 319-1188, Japan c Shanghai Institute of Applied Physics, CAS, 2019 Jialuo Road, Jiading district, Shanghai 201800, China d Department of Nuclear Engineering and Management, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan e Institute of Nuclear Energy Safety Technology, CAS, No. 350 Shushanhu Road, Hefei, Anhui 230031, China b

a r t i c l e

i n f o

Article history: Received 17 April 2016 In final form 25 May 2016 Available online 26 May 2016 Keywords: Supercritical water Pulse radiolysis Radical Local density Solute–solvent interaction

a b s t r a c t Solvatochromic shift of 4,40 -bpyH in aqueous solutions at elevated temperatures up to supercritical conditions and in various organic solvents with different dielectric constants, is investigated by pulse-radiolysis technique. 4,40 -bpyH shows a stronger solvent–solute interaction in water than in organic solvents, perhaps due to the hydrogen bond between 4,40 -bpyH and water. At 380 °C, local density augmentation, namely qlocal–qbulk, in supercritical water becomes 280 kg m3 (qbulk = 208 kg m3), and the density enhancement factor is 8.9. Density fluctuation maximizes when qbulk is around 120 kg m3. Density inhomogeneity decreases as temperature rises, but is still remarkable at 400 °C. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. SCFs have many special properties, for example, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties of a supercritical fluid to be ‘‘fine-tuned”. Since SCFs are good solvents for both nonpolar organic solutes and polar components, SCFs are considered to be environmentally benign solvents for many important applications, such as the synthesis of functional materials, nuclear reactor coolant, waste destruction, and biomass processing [1]. Density inhomogeneity is a characteristic phenomenon in supercritical fluids (SCFs) [2–6]. In pure SCFs the density fluctuation reaches its maximum when the bulk density is around critical density. In supercritical solutions, local density around attractive/ repulsive solute is higher/lower than the bulk density [7–9]. Usually this phenomena is termed as local density augmentation. The local density augmentation has been investigated widely, since the local density variation affects the changes in chemical potential of the reactants, intermediates and products, which should have ⇑ Corresponding author at: School of Nuclear Science and Technology, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, China. E-mail address: [email protected] (M. Lin). http://dx.doi.org/10.1016/j.cplett.2016.05.059 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

remarkable effect on the chemical reactions, especially highspeed or high-selectivity reactions in supercritical solutions [8,10–13]. The local density augmentation of a SCF is reflected on some changes in some properties of solute dissolved in it, such as the spectral shift of the solute. This is because the variation of solvent environment will influence the energies of ground and excited state of solute molecule. Several groups have studied the local density augmentation around some solutes in supercritical water (SCW, critical temperature tc = 374 °C, critical pressure Pc = 22.1 MPa) by UV–Vis and Raman spectroscopies [14–19]. Aizawa et al. investigated the solvent clustering around certain short-lived species (acetophenone N,N,N0 ,N0 -tetramethylbenzidine exciplex and excited 1-(dimethylamino)naphthalene) in SCW via transient absorption technique and fluorescence, where strong solute–solvent interactions were observed [20–22]. As is well known, radicals are the intermediate species for many reactions in radiation chemistry. The study on local density augmentation of water molecules around radical is essential to understand the kinetic and mechanism of chemical reactions in SCF. In our previous works, the spectral shifts of several charged/neutral radicals in sub- and supercritical water were observed [23–28]. Generally speaking, the radical anions and cations are red-shifted or invariant as the temperature increases, while the neutral radicals of aromatic compounds show a blue-shift. Since the spectral shift in supercritical conditions reflects the clustering of solvent

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molecules, the radical can be used as a probe for the local density of SCW. The density effects on the absorption spectra of solvated electron in sub- and supercritical water and alcohols have been reported and the clustering of solvent molecules has been demonstrated [29,30]. In the present work, instead of solvated electrons as mentioned above, 4,40 -bpyH radical produced by pulse radiolysis of 4,40 bipyridyl aqueous solution in sub- and supercritical water, is used to probe the local density augmentation by observing its solvatochromic shift of optical absorption spectra. Since the spectral shift is considered to be largely relevant to the polarity of solvents, the absorption spectra for 4,40 -bpyH in various organic solvents with different dielectric constants (hexane, cyclohexane, ethyl acetate, tetrahydrofuran, ethylenediamine, methanol, and acetonitrile) at room temperature are also measured as comparison. 4,40 -bpyH is selected due to its good thermal stability, high absorption coefficient, large solubility in neutral medium, and relative long lifetime (ls at even supercritical conditions) [31,32]. In addition, it is noted that 4,40 -bpyH can be easily detected while the solvated electrons in organic solvents are rather difficult or even impossible to be recorded by pulse radiolysis with nanosecond time-resolution. 2. Experimental 4,40 -bipyridyl (4,40 -bpy) and the organic solvents (hexane, cyclohexane, ethyl acetate, tetrahydrofuran, ethylenediamine, methanol, and acetonitrile), purchased from Wako Pure Chemical Industries, Ltd., were used as received. The ultra-pure water (resistivity >18.25 MX cm, purified by a Millipore Milli-Q system) was used. pH value was adjusted by adding HClO4 or NaOH. The sample solutions were freshly prepared, deaerated by Ar gas for about 15 min, and then continuously purged with Ar gas during the measurements. The nanosecond pulse (energy = 28 MeV and pulse width = 10 ns) was produced by the linear electron accelerator in the University of Tokyo. An absorption spectroscopic detection system was used in the radiolysis experiments. The optical flow cell for the high temperature, high pressure water was produced in Taiatu Techno(R). The highest temperature/pressure was 400 °C/40 MPa. This apparatus was made of Hastelloy HC22 and the windows were of sapphire. The temperature was monitored by 4 thermocouples and controlled by heater controllers. The pressure was adjusted by a back-pressure regulator. More details are presented in previous works [33,34]. A blocking filter at 340 nm was used to cut the scattered and multiple-orders light within the wavelength range of 340– 520 nm, while a filter at 520 nm was used for wavelength from 520 to 900 nm. The optical path length of the quartz cell was 15 mm. N2O-saturated KSCN aqueous solution (10 mM), with Ge 4 [(SCN) m2/J at 475 nm, was used for dosimetry. 2 ] = 5.2  10 The dose was not measured pulse by pulse, due to the choice of full-metal high-temperature cell. However, during a daylong experiment, the fluctuation of dose was less than 5%.

3. Results and discussions



þ

4; 40  bpyH þ Hþ $ 4; 40  bpyH2

ð3Þ

The temperature dependent absorption spectra of the radicals produced by pulse radiolysis of 4,40 -bipyridyl aqueous solutions were reported in our previous works [32,36]. It has been demonstrated that at temperatures >350 °C, the monoprotonated form, the N-hydro radical 4,40 -bpyH, becomes predominant. The lifetime of 4,40 -bpyH is in ls scale. Such long lifetime benefits the optical detection. Fig. 1(a) shows the absorption band for the radiolysis of 4,40 -bpy in water at 0.5 ls after radiolysis.1 Since the absorption peak for 4,40 -bpyH in aqueous solution at ambient condition locates at 540 nm [37], the bands around 540 nm in Fig. 1(a) refers to 4,40 -bpyH. The absorption peak was blue-shifted with increase in temperature at a fixed pressure, and red-shifted with elevation of pressure at a fixed temperature. The absorption peak as a function of the bulk water density is shown in Fig. 1(b), with t from 25 to 400 °C and P from 0.1 to 40 MPa. Generally, the spectral peak is red-shifted with increasing density, and the behaviors for SCW and water with temperatures
mmax ¼ A

  n2  1 e  1 n2  1 þ m0 þ B  2n2 þ 1 e þ 2 n2 þ 2

ð4Þ

The first term describes the polarization of solvent, and the second one accounts for the influence of the solvent permanent dipole. The second term is dominant when the permanent dipoles of both solute and solvent are large. m0 stands for the spectral peak with no solute–solvent interaction. There are few reports on the dielectric property of 4,40 -bpyH. The spectral shifts as a function of ee1  nn2 1 in several polar organic þ2 þ2 solvents (ethyl acetate, tetrahydrofuran, ethylenediamine, methanol, and acetonitrile) and non-polar organic solvents (hexane and cyclohexane) at ambient condition are presented in Fig. 3. The 2

3.1. Absorption spectra of 4,40 -bpyH 4,40 -bpyH was formed by pulse radiolysis of 0.5 mM 4,40 bipyridyl aqueous solution. The related reactions are expressed as [32,35]:

H2 O ! eaq ; H ;  OH;H2 ; H2 O2 ;  HO2

ð1Þ 

eaq þ 4; 40  bpy þ H2 O ! 4; 40  bpyH þ OH

ð2Þ

absorption peak is red-shifted as ee1  nn2 1 increases. The data for þ2 þ2 those polar organic solvents have an approximately linear relation 2

1 The data at ambient condition (t = 25 °C, P = 0.1 MPa) was measured in strong alkaline solution, and other values were obtained in neutral solution.

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Z. Liu et al. / Chemical Physics Letters 657 (2016) 78–82 Table 1 The dielectric constants and refractive indexes of several solvents at ambient condition (25 °C, 0.1 MPa), and the corresponding peak position for 4,40 -bpyH. Solvent

kmax (nm)

e

n

e1  n2 1 eþ2 n2 þ2

Hexane Cyclohexane Ethyl acetate Tetrahydrofuran Ethylenediamine Methanol Acetonitrile

505 505 517 523 530 535 535

1.8 2.0 6.0 7.6 16.0 32.7 37.5

1.38 1.43 1.37 1.40 1.45 1.33 1.34

0.0183 0.0026 0.3998 0.4423 0.5625 0.7107 0.7122

Fig. 3. The dependence of mmax on the polarity parameter ee1  nn2 1 . mmax is the þ2 þ2 frequency for the absorption peak of 4,40 -bpyH. Additionally, the absorption  0 maxima of 4,4 -bpyH in some non-polar (hexane, cyclohexane) and polar (ethyl acetate, tetrahydrofuran, ethylenediamine, methanol, and acetonitrile) organic solvents at ambient condition are represented by ► and r. The polarity parameters e1  n2 1 of these organic solvents are listed in the last column of Table 1. The eþ2 n2 þ2 2 dashed and solid lines show the least-squares fittings to ee1  nn2 1 of 4,40 -bpyH in þ2 þ2 water with temperature below tc and several polar organic solvents, respectively. 2

Fig. 1. (a) The optical absorption spectra of 4,40 -bpyH in aqueous solution. The values with different temperatures (380–400 °C) and pressures (23–30 MPa) are denoted by dots with different shapes. (b) Density dependence of peak position for 4,40 -bpyH in supercritical water and water below tc. Dose = 49 Gy.

in Fig. 3, where the dielectric constants and refractive indexes are calculated from temperature and pressure in the experiment [41,42]. The values for 4,40 -bpyH in water with temperatures 500 kg m3) are in good linearity with ee1  nn2 1 , which furþ2 þ2 ther confirms that the dipole of 4,40 -bpyH should be large. A leastsquares fitting to these data, which represents the values in homogenous solution [16,22], is denoted by a dashed line in Fig. 3. The slope B is 7743 cm1, which is much larger than that in polar organic solvents. It is reported that the N atom in pyridyl can form hydrogen bond with the H atom in water molecule [43,44]. Therefore, the enhancement of solute–solvent interaction can be attributed to the hydrogen bond between the N-hydro radical 4,40 -bpyH and water molecule. It is seen that, the |B| for 4,40 -bpyH is larger than that (1575 cm1) of excited 1- (dimethylamino)naphthalene in water [20]. Such strong solute–solvent interaction for 4,40 -bpyH comes from both dipolar effect and hydrogen bond. 2

Fig. 2. Time-dependent absorption Dose = 55 Gy, t = 25 °C, P = 0.1 MPa.

spectra

for

4,40 -bpyH

in

acetonitrile.

3.3. Local density augmentation

with ee1  nn2 1 , which indicates the dominance of dipolar effect for þ2 þ2 0 4,4 -bpyH. A least-square fitting to those data is denoted by a solid line in Fig. 3. The slope B is 1891 cm1. As for the nonpolar organic solvents (hexane and cyclohexane) where the permanent dipole moment is absent, the spectral shift deviates from the linear 2

function of ee1  nn2 1 obviously. þ2 þ2 Moreover, the spectral shifts for 4,40 -bpyH in water with tem2

peratures up to 400 °C as a function of

e1  n2 1 eþ2 n2 þ2

are also shown

In Fig. 3, the experimental data for SCW, with temperature = 380, 390, or 400 °C, do not match the linear behavior. Such deviation indicates that the local density around solute molecule is different from the bulk density in SCW. One model for the relation between spectral shift and bulk density in homogenous solution is expressed as: [22]

kmax ¼ C q þ D

ð5Þ

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The bulk density dependence of the measured spectral peak in 4,40 -bpyH aqueous solution is shown in Fig. 4(a). The values of kmax are in good linearity with bulk density when the bulk density is over 500 kg m3. The local density augmentation in SCW, expressed by Dq = qlocal–qbulk, can be derived via Eq. (5). It should be noted that, the thermochromic effect is not included in our analysis, as the spectral shift due to thermochromic effect is usually quite small (leV in benzene case [17]). In Fig. 4(b), the Dq for 4,40 -bpyH in SCW at different temperatures are plotted as functions of bulk density. The Dq for 4,40 -bpyH reaches 280 kg m3 when t = 380 °C and P = 23 MPa (qbulk = 208 kg m3). This value is higher than the maximal local density augmentation for the short-lived species reported by Aizawa et al. [19,20,22]. At 390 °C and 400 °C, Dq in SCW is still considerable, but the local density fluctuation already vanishes at 400 °C in solutions of some longlived species [17]. These results show that, since the solute–solvent interaction between 4,40 -bpyH and water molecule is strong regardless of its short lifetime, there is significant local density effect for supercritical aqueous solution of 4,40 -bpyH. To further discuss the local density augmentation, an analytical expression for Dq is needed. The relation between local density and bulk density is commonly described by an exponential function reported in the literature [20,21,45]

Dq=qbulk ¼ d1 expðd2 qbulk Þ

ð6Þ

Here d1 and d2 are fitting parameters. 1/d2 refers to the position of maximal local density augmentation. (1 + d1), namely, the density enhancement factor, indicates the relation between solute– solvent interaction and solvent–solvent effect. The fitting based on Eq. (6) is denoted by the curves in Fig. 4(b), and the fitting parameters d1 and d2 at different tempertures are summarized in Table 2. At 380 °C, the density enhancement factor for 4,40 -bpyH is 8.9. This value is slightly larger than but comparable with that for other solutes in SCW, for examples, (1 + d1) is 6.1 for excited 1-(dimethylamino)naphthalene, and is 3.3 for acetophenone N,N, N0 ,N0 -tetramethylbenzidine exciplex [19,21]. This large density enhancement factor can be expressed by the intense solute– solvent interaction for 4,40 -bpyH, as shown in Fig. 3. The maximum of local density augmentation locates around 120 kg m3, which is much lower than the critical density (323 kg m3). The enhancement factor decreases obviously as the temperature increases, while it is still significant at 400 °C. The d2 shows a slow descent with temperature, i.e., the position of maximal local density fluctuation varies mildly with temperature. 4. Conclusions In summary, the solvatochromic shift was measured via pulseradiolysis technique for 4,40 -bpyH radical in aqueous solutions up to supercritical conditions and some organic solvents at ambient condition. The solvatochromic shift for 4,40 -bpyH has nearly linear e1  n2 1 eþ2 n2 þ2

relation the polarity parameter both in water with temperature below tc and in polar organic solvents at ambient condition. This phenomenon indicates that 4,40 -bpyH should have a remarkable dipole moment. Perhaps due to the hydrogen bonding between 4,40 -bpyH and water molecule, 4,40 -bpyH shows a stronger solute–solvent interaction in aqueous solutions than in organic solvents. The local densities of SCW with different temperatures are estimated from the solvatochromic shift. At 380 °C, the local density fluctuation reaches 280 kg m3 when P = 23 MPa, and the density enhancement factor is 8.9. These values are higher but still in the same order with those for the other short-lived species in SCW. The maximal local density augmentation occurs when bulk density is around 120 kg m3. Density inhomogeneity declines as the temperature increases, and keeps obvious with temperature

Fig. 4. (a) Optical absorption maximum as a function of bulk density for 4,40 -bpyH in supercritical water and water with temperature below critical temperature. The experimental data with temperature below tc are represented by ., and the values with temperature = 380, 390, and 400 °C are denoted by ▲, d, and j, respectively. The solid straight line represents the fitting to measured data for water below the critical temperature. The dashed straight line marks the critical density (323 kg m3) for pure SCW. (b) The local density fluctuation as a function of bulk density of the supercritical 4,40 -bpyH aqueous solution with temperature = 380 (▲), 390 (d), and 400 (j) °C. The solid, dashed, and chain curves are the fitting to data with temperature = 380, 390, and 400 °C, separately. The critical density of pure SCW is represented by the dashed straight line. Table 2 The fitting parameters 1 + d1 and d2 for the bulk density dependence of local density augmentation of SCW at different temperatures. The bulk density dependence is expressed as Dq=qbulk ¼ d1 expðd2 qbulk Þ. The parameter 1 + d1 is known as density enhancement factor in previous works [19,21]. d2 (103 m3 kg1)

1 + d1 380 °C 8.9

390 °C 7.0

400 °C 5.0

380 °C 8.5

390 °C 8.0

400 °C 7.5

up to 400 °C. The position of peak for local density fluctuation varies slowly as temperature increases. These results indicate a quite significant solute–solvent effect and local density fluctuation for 4,40 -bpyH in SCW. In the study of radiolysis or reaction mechanism in SCW, the local density property of water should play an important role. Acknowledgments This work was partly supported by National Natural Science Foundation of China (No. 21377122). We are thankful to Mr. T. Ueda and Professor M. Uesaka for the technical assistance in the

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