ATR FT-IR studies of supercritical methanol

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J. of Supercritical Fluids 46 (2008) 206–210

ATR FT-IR studies of supercritical methanol Dmitry S. Bulgarevich a , Yoshiteru Horikawa b , Takeshi Sako c,∗ a

Organic Nanomaterials Center, National Institute for Materials Science, 1-1, Namiki, Tsukuba 305-0044, Japan b LC & SFC Application Laboratory, JASCO Co., 2967-5 Ishikawa-cho, Hachioji, Tokyo 192-8537, Japan c Department of Materials Science, Shizuoka University, Johoku 3-5-1, Hamamatsu, Shizuoka 432-8561, Japan Received 29 November 2007; received in revised form 28 January 2008; accepted 28 January 2008

Abstract To measure the Fourier transform infrared spectra of supercritical methanol at 523 K and pressures up to 30 MPa, the application of attenuated total reflectance optical cell is reported. Together with previously published transmittance infrared spectra of pure supercritical methanol at low pressures (up to 5 MPa), present studies covered the entire region from gas-like to liquid-like densities for this simplest organic compound capable of exhibiting hydrogen bonding. At 8 MPa, the position of methanol O H stretching vibration corresponded to the hydrogen-bonded dimer formation. At higher pressures, the methanol O H peak maxima were located between dimer and trimer or tetramer infrared absorption peaks. The shift of the hydrogen-bonding equilibrium with pressure toward higher aggregates was also indicated by the absorptivity changes in the region of O H stretching vibration. © 2008 Elsevier B.V. All rights reserved. Keywords: Supercritical methanol; Infrared spectroscopy; Attenuated total reflectance

1. Introduction Methanol is the simplest organic compound capable of hydrogen-bond formation. Therefore, it is no surprise that for decades its hydrogen-bonding attracted the great experimental and theoretical interests [1]. Such studies were also extended to the supercritical (SC) region (critical temperature, Tc = 512.6 K; critical pressure, Pc = 8.1 MPa; critical density, ρc = 8.6 mol/l [2]). As a result, the hydrogen-bond formation can be studied continuously from gas-like to liquid-like densities. In particular, the infrared (IR) [3,4], nuclear magnetic resonance (NMR) [5–8], X-ray and neutron diffraction [9,10] as well as Raman [11] spectroscopic methods were used. Molecular dynamics (MD) [12] and ab initio molecular orbital (MO) [13] simulations were also applied to estimate the extent of the methanol hydrogen-bonding in SC state. Among such methods, infrared spectroscopy has an advantage of direct observation at least two methanol species: methanol monomer and hydrogen-bonded dimer. Our previous report on Fourier transform infrared (FT-IR) spectroscopic studies of the

∗

Corresponding author. Fax: +81 53 478 1165. E-mail address: [email protected] (T. Sako).

0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.01.013

hydrogen-bonding interactions in SC methanol demonstrated that at 523 K and below ∼0.3 mol/l methanol chiefly existed in a monomeric form [3]. Between ∼0.3 and 1 mol/l there were clear indications of methanol monomer–dimer equilibrium: eight isosbestic points through the mid-IR region. At densities higher than ∼15 mol/l, the hydrogen-bonding equilibrium started to shift toward larger aggregates. Even though a short pathlength optical cell (∼0.4 mm) was used, the SC methanol/methanol-d1 mixtures had to be measured at densities higher than ∼2 mol/l due to the large molar absorptivity of methanol in mid-IR region. For direct studies of highly absorbing substances in mid-infrared spectral region, we developed a high temperature/pressure optical cell for attenuated total reflectance (ATR) FT-IR spectroscopy [14]. Due to small penetration depth of the evanescent IR radiation into the sample and durability of the optical cell, now we are able to record ATR FT-IR spectra of highly concentrated solutions and heterogeneous mixtures at various SC conditions. In this work, we report on the application of the technique to measure ATR FT-IR spectra of supercritical methanol from gas-like to liquid-like densities and compare these results with our published transmittance FT-IR spectra of SC methanol as well as with reported ATR spectrum of liquid methanol. As a result, the extent of hydrogen-bonding in

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SC methanol at 523 K and at pressures up to 30 MPa can be understood. 2. Experimental Fig. 1A shows the high-temperature/pressure apparatus used for ATR FT-IR spectroscopic measurements [14]. The optical cell (inconel body), which used a fiber-optic REMSPEC ATR probe, was custom built by JEOL and can withstand conditions up to 30 MPa at 693 K. The key element of the cell is a diamond ATR crystal (like hexadecagonal pyramid, high quality, natural Type IIa) used as a single window (see Fig. 1B and C). To avoid damage to the optical fiber at high temperature operations, a ZnSe rod was used to separate it from the heated diamond crystal. Cold airflow cooled the optical fiber and ZnSe rod to increase the optical throughput. The cell design sets 45◦ angle of incidence, θ, of the infrared radiation with usable wavenumbers in the ranges of ∼4000–2600 and ∼2000–900 cm−1 , which are determined by the transmittance of the optical elements and sensitivity of the MCT detector. The penetration depth, dp , of IR radiation with wavelength λ = 11–2.5 ␮m from diamond surface (refractive index n1 = 2.4) into the sample (n2 ≈ 1–1.6) can be calculated as

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follows [15]: I(z) = I0 e−z/dp λ  dp = ≈ 0.3–3 ␮m, 2 2π n1 sin2 θ − n22

(1)

where I0 and I(z) are the evanescence field intensity at zero and at perpendicular distance z from the interface. The critical angles, θ c , were between 25◦ and 42◦ for listed n1 and n2 . Due to such small penetration (exponential decay) of evanescence IR radiation into the sample (z ≈ 2–3dp ), the ATR FT-IR spectra of highly concentrated solutions and heterogeneous mixtures can be measured at conditions that correspond to many industrial applications. To measure ATR FT-IR spectra of methanol, the cell was sealed and after temperature adjustment the baseline was measured. The digital temperature controller was connected to the cartridge heaters and thermocouple inserted directly into the holes in the cell body. Note that thermocouple measured the actual temperatures of sample inside the optical cell. Then, methanol was loaded into the cell with a pump (Jasco PU1586). To avoid temperature stress on the diamond, methanol was pumped through the preheater. After closing of the shutoff valves (Sno-Trik® SS-410-FP), the pressure of the methanol was monitored using a pressure sensor (Tsukasa Sokken PLCJ3318) connected to the digital pressure gauge (Tsukasa Sokken PE-33-A digital manometer) and hybrid recorder (Yokogawa HR-1300). The accuracy of the pressure and temperature measurement were ±10 kPa and within ±0.2 K. Density was calculated analytically by solving the IUPAC equation of state for methanol [2]. Spectroscopic measurements were performed with 4 cm−1 resolution by co-adding 320 scans for each spectrum. 3. Results and discussion

Fig. 1. Apparatus used to measure ATR FT-IR spectra of subcritical and supercritical methanol: (A) general view; (B) side view of the ATR element; (C) top view of the diamond crystal.

Fig. 2 shows transmittance [3] and ATR FT-IR spectra of subcritical and supercritical methanol. They cover the regions of ν(O H), ν(C H), and ν(C O) stretching as well as δ(CH3 ) and δ(C O H) bending, and γCH3 rocking vibrations. The peak positions in ATR spectrum of liquid methanol measured with the apparatus agreed well with previously published ATR [16] and transmittance IR spectra of methanol [17]. This is important since the shape of the ATR spectrum generally depends on dp (see Eq. (1)), which is a function of wavelength and refractive indices of the ATR element and sample. Strictly speaking, for accurate comparison of the ATR and transmittance spectra, it is necessary to scale the ATR absorbance values by dp function. However, at present, accurate refractive indices of diamond and methanol at different wavelengths are not known for current experimental conditions. Nevertheless, the magnitude of dp effect in the region of ν(O H) vibrations should be small compared to the infrared intensity changes due to the hydrogenbond formation. For example, Fig. 3 shows a plot of dp under conditions of vacuum and in liquid methanol as a function of wavenumber. The values of dp were calculated by Eq. (1) using n1 = 2.4 and reported refractive indices of liquid methanol in mid-infrared region [18]. The fixed value of n1 was used since

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Fig. 4. Pressure dependences of the methanol density, ρ, in subcritical and supercritical regions and scaled absorbance of the O H stretching vibration (sum of absorbances between 3200 and 3800 cm−1 normalized using ρ). Arrows indicate the corresponding scales.

Fig. 2. ATR and transmittance FT-IR spectra of subcritical and supercritical methanol.

current temperature as well as pressure and spectral ranges have very little effect on diamond refractive index:   1 ∂n = −3.6 × 10−4 GPa−1 , (2) n ∂P   1 ∂n (3) = 7.17 × 10−6 K−1 , n ∂T n = A + BL + CL2 + Dλ2 + Eλ4 ,

(4)

Fig. 3. Wavenumber and refractive index dependences of the penetration depth of infrared radiation for current experiments.

where L = 1/(λ2 − 0.028), A = 2.37553, B = 3.36440 × 10−2 , C = −8.87524 × 10−2 , D = −2.40455 × 10−6 , E = 2.21390 × 10−9 , and λ is in micrometers [19–21]. For λ between 2.5 and 11 ␮m, P = 30 MPa, and T = 220 K, the maximum | n1 | is only ∼0.007. In addition, since n2 does not exceed the value of liquid methanol, the dp for the region of ν(O H) will be changed by 40% at most. In other words, it will be confined between dp in vacuum and dp in liquid methanol (Fig. 3) for P = 0.1–30 MPa at 523 K. As shown in Fig. 2, the ν(O H) vibration in SC methanol at highest measured pressure (∼3500 cm−1 ) was shifted from its position for hydrogen-bonded dimer in the gas phase (∼3590 cm−1 ). However, it was still far away from the liquid phase absorption band maximum at ∼3350 cm−1 . Such a difference is attributed to the higher density of liquid methanol compared to SC methanol even at highest measured pressure (Fig. 4). The assignments of the methanol IR bands in the gas, liquid [17], and SC states are listed in Table 1. Fig. 5 displays the evolution of the ATR spectra by application of pressure in the region of ν(O H) and ν(C H) vibrations. Interestingly, at 8 MPa, the peak maximum for ν(O H) was shifted bathochromically from methanol dimer absorption maximum in the gas phase and was close to that for dimer in liquid CCl4 [1]. Note that CCl4 has similar dipolarity/polarizability parameter [22] as SC methanol under these conditions [23]. Since the ν(O H) peak position for dimer also depends on the polarity of the medium [24,25], the location of the ν(O H) vibration at 8 MPa was attributed to the shifted (from gas phase) dimer absorption band. Between 8 and 30 MPa, the density of methanol was changed from 3.5 to 17.6 mol/l, i.e. increased by five times. However, the ν(O H) intensity was increased by only two times. This probably indicated the shift of the hydrogen-bonding equilibrium from dimer to trimer or tetramer. In other words, the concentration of dimer in SC methanol decreased due to the formation of other hydrogen-bonded aggregates having molar absorptiv-

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Table 1 Infrared vibrational frequencies of gaseous (g), liquid (l) and supercritical methanol MeOH

ν(OH) (cm−1 ) Monomer

Subcritical

ν(CH3 ) (cm−1 ) Dimer

3681 (g, M) 3590 (g)

Supercritical 3680 (M)

δ(CH3 ) (cm−1 )

δ(OH) (cm−1 )

γ(CH3 ) (cm−1 )

ν(CO) (cm−1 )

1480 (as, l, M) 1450 (s, l, M) 1477 (as, g, M) 1455 (s, g, M)

1418 (l, M) 1345 (g, S)

1115 (l, M) 1060 (g, W)

1030 (l, VS) 1033 (g, VS)

1450 (M)

1380 (S)

Not visible

1030-1020 (VS)

Polymer 3328 (l)

2980 (as, l, M) 2946 (as, l, S) 2834 (s, l, S) 3000 (as, g, M) 2960 (as, g, S) 2844 (s, g, S) 3600 (S) up to 3500 (S) 2950 (as, S) 2840 (s, S)

The approximate intensities of vibrational bands are also given for comparison: medium (M), strong (S), and very strong (VS). Abbreviations (s) and (as) correspond to “symmetric” and “anti-symmetric” vibrations, respectively.

ity similar to the dimer. Fig. 4 also shows the plot of scaled absorbance of ν(O H) band versus pressure. Initially, it rose due to the hydrogen-bonded dimer formation having higher molar absorptivity compared to the methanol monomer. Then, it dropped and leveled off at higher pressures due to the formation of larger hydrogen-bonded aggregates. The expected cyclic trimer or tetramer absorption should be centered at ∼3390 cm−1 [1]. However, there were little changes in the ν(O H) position and band shape for this pressure range. Fig. 6 displays the region of δ(C O H), δ(CH3 ), and γCH3 vibrations. Again, the peak maxima of δ(C O H) vibration showed no changes at pressures higher than 8 MPa, which was consistent with the behavior of the ν(O H) vibration. The shape of the ν(C O) stretching showed RQP vibrational–rotational branches for methanol [26] monomer/dimer mixture at gas-like densities and sharp strong band (Q branch) at liquid-like densi-

Fig. 6. Pressure dependence of the ATR FT-IR spectra of C O H bending and C O stretching vibrations of supercritical methanol.

ties. The central peak of the Q branch at ∼1033 cm−1 kept its position through the studied density range. 4. Conclusions

Fig. 5. Pressure dependence of the ATR FT-IR spectra of O H and C H stretching vibrations of supercritical methanol.

High temperature/pressure apparatus for ATR FT-IR spectroscopy opened the possibilities to measure the infrared spectra of SC fluids from gas-like to liquid-like densities. The studies of the hydrogen-bond formation in pure SC methanol with this method confirmed our previous results obtained with transmittance FT-IR spectroscopy for SC methanol/methanol-d1 mixtures. At 8 MPa, ATR FT-IR spectrum of SC methanol corresponded to the hydrogen-bonded dimer formation. At higher pressures, the shift of the hydrogen-bonding equilibrium toward higher aggregates (trimer or tetramer) was observed. In addition,

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