Article pubs.acs.org/JPCB
Blue-Colored tert-Butylamine Clathrate Hydrate Atsushi Tani,*,† Satoshi Koyama,† Yusuke Urabe,‡ Kenji Takato,‡ Takeshi Sugahara,‡ and Kazunari Ohgaki‡ †
Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan
‡
ABSTRACT: Clathrate hydrates preserve active species more stably than the other icy materials and investigation of the behavior of the active species elucidates the physicochemical properties of clathrate hydrates like guest−guest interaction. Color of the tert-butylamine clathrate hydrate changes to blue after gamma irradiation and is bleachable with visible light. The electron spin resonance (ESR) spectrum at 120 K mainly consists of a triplet signal of the C-centered radical NH2C(CH3)2CH2• together with a single signal at g = 2.0008. The latter signal disappears after light exposure. These results indicate that both the blue color and the single ESR signal are derived from trapped electrons in the hydrate. They thermally decay around 140−160 K by the firstorder reaction, and the activation energy is 27 kJ/mol. Since tert-butylamine molecules can capture protons due to the high proton affinity, electrons may remain in the hydrate without reacting with protons, making the hydrate blue after gamma irradiation. The long-lived trapped electrons in the tert-butylamine hydrate have an advantage to investigate those in icy materials because tertbutylamine hydrate is nonionic and has a tetra-coordinated host water network like crystalline ice without any substitution for water molecules.
1. INTRODUCTION Clathrate hydrates are crystalline inclusion compounds of guest molecules encaged with water-bonded cage structure1 and have been classified into three groups: gas hydrates, alkylamine hydrates, and peralkylammonium hydrates.2 Gas hydrates are genuine nonionic clathrate hydrates, whereas alkylamines except tert-butylamine (hereafter, tBA) form nonionic “semiclathrate” hydrates, where a nitrogen atom of amines replaces an oxygen atom of hydrogen-bond network.3,4 Peralkylammonium hydrates are ionic and anions are substituted for water molecules in a host structure.5,6 If a peralkylammonium ion is too large to be occupied in a cage, the guest ion molecule can be enclathrated over several cages where hydrogen bonds are partially broken by connection between water molecules and ionic guests (semiclathrate). Most clathrate hydrates are nearly colorless, though water molecules slightly absorb the red end of the visible spectrum.7 If guest molecules absorb the visible region of the spectrum, clathrate hydrates have intrinsic color. For example, ozone hydrate is bluish due to the optical absorption by ozone molecules.8,9 On the other hand, color can be acquired by an additional procedure like radiation exposure. Dark blue coloration was observed in X or gamma-irradiated tetramethylammonium hydroxide (TMAOH) hydrate,10,11 where a tetramethylammonium ion is enclathrated in a distorted truncated octahedron without any replacement of water molecules.4 The transient dark blue coloration by electron pulse irradiation has also been observed in alkylamine and peralkylammonium hydrates12,13 as well as crystalline ice.14,15 This blue coloration in icy materials is caused by trapped electrons and have been investigated intensively in both © 2014 American Chemical Society
experimental and theoretical studies because the behavior of the electrons is of fundamental importance not only in radiation physics and chemistry but also in biological systems and in degradation of materials.16−18 We have investigated the behavior of unstable radical species in the nearly spherical water cages and revealed the guest−guest interaction through radicals in nonionic clathrate hydrates.19−24 For example, a methyl radical is induced in methane hydrate after gamma-irradiation and is stably stored in the hydrate at relatively low temperature like 120 K.19 Once the hydrate is heated, the methyl radical may start to withdraw a hydrogen atom of methane in an adjacent cage by a hydrogen-picking reaction and finally dimerize to ethane.25 Almost all alkyl radicals in clathrate hydrates may enable hydrogen picking from guest molecules in adjacent cages. However, none of the previously studied nonionic “clathrate” hydrates has the blue coloration, even at 77 K after gamma irradiation. To investigate the behavior of trapped electrons in nonionic clathrate hydrates, we focus on tBA hydrate because radiationinduced protons may be captured by tBA due to higher proton affinity,26 suggesting that higher stability of the trapped electrons is expected in the tBA hydrate. The tBA hydrate is nonionic clathrate hydrate (structure-VI) at the composition of 16(CH3)3CNH2·156H2O.4 The tBA is not substituted for a water molecule. As we expected, the color of the tBA hydrate Special Issue: Physics and Chemistry of Ice 2014 Received: May 30, 2014 Revised: August 19, 2014 Published: August 19, 2014 13409
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sample was set to the holder, and the absorption spectrum was measured under short light illumination within 0.5 s. The irradiated sample taken from the cuvette was also measured by ESR (JEOL, JES-FA200). Light from the same xenon lamp was exposed to the sample through the front window of the microwave cavity in the ESR system. Measurement temperature was 120 K, controlled with the cold gas flow system (JEOL, ES-EVT4). Microwave power was 0.01 mW and 100 kHz modulation field was 0.1 mT.
becomes deep blue after gamma irradiation. In this paper, we report the optical absorption and the gamma-induced active species in the hydrate and discuss the characteristics of the trapped electrons in the alkylamine clathrate hydrate.
2. EXPERIMENTAL SECTION A plastic cuvette (1 mL) was filled with tBA aqueous solution (tBA:H2O = 1:10.11 in molar ratio) prepared using ultrapure water (Milli-Q) and reagent tBA (Sigma-Aldrich, 98%). Transparent solid sample in the cuvette was formed at ∼253 K in a freezer and confirmed as the structure-VI tBA hydrate by means of Raman spectroscopy27,28 and differential scanning calorimetry (DSC).29 It was immersed in liquid nitrogen and irradiated at 77 K by γ rays using a source of 60Co. Total dose was estimated to be ∼5 kGy by calibration with the electron spin resonance (ESR) signal of the alanine standard sample. Several samples were prepared in the same procedure and stored at 77 K until the following measurements. A cryo-measurement system of optical absorption was developed as shown in Figure 1. Light from the xenon lamp
3. RESULTS AND DISCUSSION The colorless sample of tBA hydrate turned blue after gammairradiation (Figure 2). The hydrate will be crucial for this color
Figure 2. Blue-colored tBA hydrate after gamma-irradiation. Figure 1. Schematic illustration of the sample holder in the cryomeasurement system of optical absorption.
change because no color appeared at 77 K in the gammairradiated solid tBA sample. This color was observed not only near the surface of the sample but also inside. The color does not change for at least a few months at 77 K. Nevertheless, it returns to colorless and transparent by exposure of visible light from xenon lamp at 77 K. These results indicate that a kind of defect with an optical absorption will be induced in the tBA hydrate by γ rays and be bleachable by visible light. Transmitted light of the sample was measured in the abovementioned system. With a number of light exposures (i.e., a number of optical absorption measurements), the intensity of transmitted light (I) gradually increases and finally reaches to constant due to optical bleach of the sample itself. Assuming the intensity of transmitted light in the completely bleached sample as the original intensity of transmitted light (I0) in the sample, absorbance (A) of the blue-colored hydrate was calculated in each light exposure by A = −log (I/I0). The absorption spectra at 135 K are shown in Figure 3. The absorption is observed in almost whole visible region with a maximum peak at ∼590 nm. Absorbance in the first light exposure is the most intense and becomes weaker with a number of measurements (n), though the spectral shapes do not change during the experiments. In spite of different measurement intervals, the decay of absorbance shows a simple exponential curve against n, as shown in Figure 4. This means that the blue color does not decay thermally at 135 K during the experiment (within an hour).
(Asahi Spectra, Max-301) was exposed to the sample holder through an optical fiber. Its spectrum covered a visible region from 400 to 750 nm. Transmitted light was detected by CCD spectrometer (B&W Tek, BTC112E) through another optical fiber attached to the opposite side of the sample holder. Intensity of the light was adjusted using a ND (neutral density) variable controller in the light source to avoid the signal saturation in the CCD detector. The sample holder and the cuvette were cooled by cold and dry gas evaporated from liquid nitrogen. The cold gas was flowed from the bottom through spaces in both sides of the lower part of the cuvette. To heat the sample, two ceramic heaters were attached to the other faces of the cuvette that were usually used for optical absorption measurements. As a result, optical path in the cuvette was perpendicular to the original direction and its optical length was 0.4 cm, shorter than 1 cm in ordinary measurements. Temperature between the cuvette and the heater was monitored using a type K thermocouple and controlled by a thermo-regulator (Chino, KP1000C). The inner temperature of the cuvette was measured separately and used for calibration. The sample holder was cooled to a setting temperature with a dummy cuvette at first. After the temperature became steady, the cuvette with the irradiated 13410
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Figure 5. ESR spectra of the irradiated tBA hydrate at 120 K. (a) Before light exposure and (b) after light exposure. (c) The difference spectrum by subtracting the triplet signal in (b) from the initial in (a).
from tBA. They are proposed by Smith and Swan who found products from both NH2C(CH3)2CH2• and (CH3)3CNH• in gamma-irradiated liquid tBA.33 The last one is an N-centered radical cation (CH3)3CNH2•+, which is proposed to explain quintet ESR signal observed in tBA-adsorbed silica gel after gamma-irradiation.34 Since only the C-centered radical among them shows a triplet signal due to hyperfine of two hydrogens, the mainly observed ESR signal in the tBA hydrate will be assigned to NH2C(CH3)2CH2•. An OH radical induced from a host water molecule is not observed, even at 77 K in the tBA hydrate, which is concordant with the previous studies on alkane hydrates.19,21−23 Since an OH radical is observed in CO2 hydrate and Xe hydrate after gamma-irradiation,24,35 it may react with tBA at 77 K by hydrogen abstraction to form the Ccentered radical. In comparison between two spectra in Figure 5 (panels a and b), peak height of the central signal decreases after light exposure, whereas one of the small peaks at the lower magnetic field slightly increases. After the triplet signal in Figure 5b is subtracted from the initial signal in Figure 5a by adjusting the signal intensity, an isotropic signal remains at g = 2.0008, as shown in Figure 5c. Similar spectrum are observed in hydrated medium as well as crystalline ice and assigned to trapped electrons.10,15,36−38 We conclude that our blue color is due to trapped electrons in tBA hydrate. For the thermal decay kinetic analysis, it is necessary to remove the effect of optical bleaching during measurements. As we mentioned, no thermal decay of the trapped electrons was observed at 135 K within an hour. Assuming that the opticalbleaching rate for all measurements from 135 to 159 K is constant, the difference between the observed decay and optical decay curves was regarded as the thermal decay curve of the trapped electron (Figure 6). This semilogarithmic plot makes sure that the decay of the blue color is followed by first-order kinetics. Arrhenius plot of the decay, shown in Figure 7, gives a straight line in this temperature region. Activation energy is estimated to be 27 kJ/mol. As binding energy of the trapped electron in water system seems to be around 300 kJ/mol,39−41 it is not easy to release thermally from the trap in the lower temperature region like 135−159 K. Nilsson et al. proposed that trapped electrons in electronpulse-irradiated ice was reacted with protons transferred by proton hopping through water molecules.32 However, tBA has high proton affinity26 and will easily capture protons induced during irradiation to form the tBA cation. In the case of tBA hydrate, both of tBA and trapped electrons may compete to capture protons. Protons may be consumed at 77 K, which causes higher stability of the trapped electrons in tBA hydrate.
Figure 3. Absorption spectra of tBA hydrate after gamma-irradiation at 135 K. n is the number of light exposures.
Figure 4. Decay of absorbance at 590 nm against a number of light exposures, n. Total time of light exposure is shown in upper axis together (each exposure time is 0.5 s). Measurement temperature is 135 K.
The similar absorption spectrum with a halflife of 0.38 ms was observed at 253−256 K in the electron-pulse-irradiated tBA hydrate.12 The absorption maximum is shifted by ∼30 nm: ∼590 nm at 135 K in the gamma-irradiated tBA hydrate and ∼620 nm at 253−256 K in the electron-pulse-irradiated tBA hydrate. The wavelength shift of absorption maximum will be caused by different measurement temperatures because the similar peak shift (∼30 nm/100 K) was observed in pulse radiolysis studies of crystalline ice.30−32 These previous papers suggested that the optical absorption is derived from trapped electrons in tBA hydrate or crystalline ice. The blue color in the gamma-irradiated tBA hydrate may also be caused by trapped electrons. To confirm the model of trapped electrons in the gammairradiated tBA hydrate, the sample was measured with ESR. ESR spectra of the irradiated tBA hydrate before and after light exposure are shown in Figure 5 (panels a and b). A triplet signal (g-factor is 2.0024 and hyperfine constant is 2.0 mT) is mainly observed in both cases. Three kinds of radicals will be induced in gamma-irradiated tBA hydrate. Two of those are a Ccentered radical NH2C(CH3)2CH2• and an N-centered radical (CH3)3CNH•, which may be formed by hydrogen abstraction 13411
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ACKNOWLEDGMENTS
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REFERENCES
Article
The authors appreciate Dr. T. Yamamoto at the Institute of Scientific and Industrial Research, Osaka University, and Dr. K. Kitano at the Graduate School of Engineering, Osaka University, for their support in 60Co gamma-ray irradiation and optical absorption measurements, respectively. This study was supported by Grant-in-Aid for Scientific Research (A) 21246117. We also acknowledge the scientific support from the “Gas-Hydrate Analyzing System (GHAS)” of the Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University.
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Figure 6. Thermal decay of the blue color in irradiated tBA hydrate at different temperatures.
Figure 7. Arrhenius plot of thermal decay of the blue color in irradiated tBA hydrate.
This will be the reason why only tBA hydrate becomes blue after gamma irradiation because none of the other nonionic clathrate hydrates has such a color, even at 77 K after gamma irradiation as far as we know.19,21−24
4. CONCLUSION Blue-colored gamma-irradiated tBA clathrate hydrate has been investigated by optical absorption and electron spin resonance (ESR) from 120 to 160 K. The absorption spectrum and ESR signal at g = 2.0008 in the irradiated tBA hydrate indicate that the blue color is derived from trapped electrons. Since tBA has a high affinity for proton capture, electrons may remain in the hydrate without reacting with protons and make it blue after gamma irradiation. The long-lived trapped electrons in tetracoordinated hydrogen-bonded water network suggest that tBA hydrate can be used as an analogous of crystalline ice for a further study of trapped electrons.
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