In this work, we measured the absorption spectra in primary alcohols, at elevated temperature with fixed pressure, by using nanosecond electron pulse radiolysis technique. Including above data, a compilation of currently available experimental data on the energy of absorption maximum (Emax) of solvated electrons changed with temperature in monohydric alcohols, diols and triol is presented. The molecular structure effect, including OH numbers, OH position and carbon chain length, are investigated. And then, the optical absorption spectra of the solvated electron (e-sol) in sub- and supercritical methanol and ethanol are measured by electron pulse radiolysis and laser photolysis techniques, at temperatures in the range of 220-270℃. The effect of addition of a small amount of water to the alcohol on the optical absorption energy of e-sol is also investigated. Then we measured the time dependent G(e-sol) in water and methanol at elevated temperatures by using both scavenging method (indirect method) and picosecond pulse radiolysis (direct method).

1. Introduction

In the last two decades, there has been an increased interest in the supercritical fluids (SCFs). Besides its important contribution to the fundamental chemistry, SCFs has drawn interest because of its innovative role in a variety of chemical processes and technological applications, such as the synthesis of new materials and the destruction of hazardous wastes. It is well known that supercritical fluids (SCF) possess many peculiar and desirable characters, and it is possible to control the reaction rate and the selectivity sensitively by changing temperature and pressure. In particular, the nature of SCF is that the density can be varied continuously at constant temperature over a wide range using only small changes in the applied pressure. However, the density is in fact microscopically inhomogeneous and there is clustering structure. The studies of the density and solvent structure effects are thus important.

And fundamental research on the solvated electron in alcohols is of importance in deep understanding of dynamics of salvation process and solvent structures, as well as subsequently developing applications in many industrial arenas.

Some behaviors of the solvated electrons in supercritical fluids of alcohols, including their absorption spectra, decay kinetics and yields after their production by use of pulse radiolysis and/or laser photolysis methods, are studied in this work.

The purpose of this study is to investigate the temperature and structure of the solvent effects on the energy of maximum absorption (Emax), the density effect on the Emax in sub- and supercritical solvent, and the yield of solvated electron at different times and temperatures.

2. Experimental

In this work, a method of nanosecond pulse radiolysis, picosecond pulse radiolysis and/or laser photolysis combined with absorption spectroscopy, have been used for measurement of the time-resolved absorption spectrum of the transient species which produced by irradiation of the sample with high energy electron beam or ultraviolet laser beam. And the transient absorption spectroscopic method has an advantage that, it enables us to measure signals deriving from the short-lived species in the solution, as like solvated electrons in this work. We combined the advantage of pulse radiolysis for producing the solvated electrons in alcohols with a high efficiency, and that of transient absorption spectrum for measuring the low concentration short-lived species in liquids with a high sensitivity.

Two different high-temperature and high-pressure (HTHP) cells (Taiatsu TechnoR) were used in pulse radiolysis and laser photolysis experiments, coupled with a linear electron accelerator (nanosecond or picosecond) or a KrF excimer laser (Lambda Physik, Compex 102; pulse duration: 20 ns; λ= 248nm), respectively, at the Nuclear Professional School of The University of Tokyo.

3. Solvated electrons at elevated temperatures in different alcohols

Jay-Gerin et al. compared available experimental data on some physicochemical properties of solvated electrons in 99 pure polar solvents at room temperature, and Krebs et al. discussed the correlation between mobility and the absorption spectra of solvated electron in polar solvents. However there is not simple correlation between the physicochemical properties of solvents and the position of the absorption spectrum of solvated electron, which indicate that it is governed by solvent molecular structure. Further investigation has shown that the absorption spectra of solvated electron are also dependent on the temperature and the pressure, which suggests a charge transfer to solvent (CTTS) state for their absorption band.

In this work, we measured the absorption spectra of solvated electrons in methanol, ethanol, butanol, pentanol, hexanol, octanol from room temperature to higher temperatures. We fixed pressure at 20 MPa in methanol and ethanol, and at 15 MPa in butanol, pentanol, hexanol and octanol. For methanol we changed temperature from room temperature (22 °C) to supercritical condition (270 °C). And the absorption spectra of solvated electrons in ethanol, butanol, pentanol, hexanol and octanol are measured from 22 to 230, 22 to 175, 22 to 200, 22 to 175, and 50 to 150 °C, respectively. For all the investigated alcohols, Emax will have a red-shift with increasing temperature.

Including above data, a compilation of currently available experimental data on the energy of absorption maximum of solvated electrons changed with temperature in monohydric alcohols, diols and triol is presented. The molecular structure effect, including OH numbers, OH position and carbon chain length, are investigated. For the primary alcohols with same OH group number and position (at the end of chain), the temperature coefficient increases with increasing chain length. For the alcohols with same chain length and OH numbers, temperature coefficient is larger for the symmetric alcohols than the asymmetric ones. Emax will be larger in the alcohol which has more OH groups at all the temperatures.

4. Temperature and density effects on the absorption maximum of solvated electrons in sub- and supercritical methanol and ethanol

Alcohols form an interesting class of liquids in which to investigate the factors influencing the formation and stabilization of electrons in fluids. Considering the structure of methanol is very like that of water (methanol, the simplest alcohol, may be considered as "methylated water"), one methyl group just replacing one H atom, it is reasonable to assume that density dependent behaviors similar to those observed for water also exist in sub- and supercritical methanol and ethanol .

(1) In methanol: The optical absorption spectra of e-sol in sub- and supercritical methanol are measured by both electron pulse radiolysis and laser photolysis techniques, at temperatures in the range 220-270℃. Over the density range studied (~0.45-0.59 g/cm3), the position of Emax of e-sol is found to shift only slightly to the red with decreasing density. In agreement with our previous work in water, at a fixed pressure, Emax decreases monotonically with increasing temperature in passing through the phase transition at critical temperature (Tc) (239.5℃). By contrast, at a fixed density, Emax exhibits a minimum as the solvent passes above the critical point into the supercritical state. These behaviors are discussed in terms of microscopic arguments based on the changes that occur in the methanol properties and methanol structure in the sub- and supercritical regimes. The effect of addition of a small amount of water to the alcohol on the optical absorption energy of esol- is also investigated.

(2) In ethanol: The optical absorption spectra of e-sol in sub- and supercritical ethanol are measured by electron pulse radiolysis technique, at temperatures in the range 220-260℃. Over the density range studied (~0.45-0.60 g/cm3), the position of Emax of e-sol is found also to shift slightly to the red with decreasing density. In agreement with in water and methanol, at a fixed pressure, Emax decreases monotonically with increasing temperature in passing through the phase transition at Tc (240.8℃). However, at a fixed density, Emax exhibits a minimum as the solvent passes above the critical point into the supercritical state.

5. Time dependent G(e-sol) in water and methanol at elevated temperatures

In order to investigate the spur reaction, we can use picosecond pulse radiolysis (direct method) or nanosecond pulse radiolysis (indirect method). It is possible to trace directly the decay of transients from a few picoseconds to several nanoseconds by using picosecond pulse radiolysis. But it needs sophisticated and complicated systems, which is very expensive, so called 'rich man' method. The indirect method is to use a chemical (scavenger) to react with e-sol to produce a relative stable intermediate radical which can be detected by nanosecond pulse radiolysis, also be called as scavenger method or 'poor man' method.

(1) In water: Methyl viologen (MV2+) was used as a scavenger of hydrated electron for estimation of its yield from room temperature to 300 °C with a fixed pressure 25 MPa. By changing the concentration of MV2+ from 0.01 to 10mM, the time dependent yield of hydrated electrons were estimated at different temperatures. The corresponding time were calculated by equation: t=(k[MV2+])-1, where k is the reaction rate of MV2+ and e-aq; [MV2+] is the concentration of MV2+. By using picosecond pulse radiolysis method, the kinetics of hydrated electrons were measured at fixed pressure of 25 MPa from room temperature to 300 °C. The wavelengths of analysis laser light are 700nm for room temperature, 860nm for 150 °C, and 940nm for above 250 °C. They are in good agreement between direct and indirect methods.

(2) In methanol: 4,4'-bipyridyl (Bpy) was used as a scavenger of hydrated electron for estimation of its yield from room temperature to 250 °C with a fixed pressure of 10 MPa. By changing the concentration of Bpy from 0.05 to 100mM, the time dependent yield of solvated electrons were estimated at different temperatures. The corresponding time were calculated by equation: t=(k[Bpy])-1, where k is the reaction rate of Bpy with and e-aq and [Bpy] is the concentration of Bpy. The rate constant k at different temperatures were measure by change of Bpy concentration. By using picosecond pulse radiolysis method, the kinetics of solvated electrons were measured at a fixed pressure of 10 MPa below supercritical temperature and 15 MPa in 250 °C.

6. Conclusion

For the investigated alcohols, the Emax will decrease with the increasing the temperature, and the decreasing rate has the relationship with the chain length, the number and position of the OH group. Temperature effect on Emax will increase with increasing chain length. For the isomers, the temperature effect on symmetric one will be smaller. Emax will be higher with more OH groups.

In both methanol and ethanol, the temperature dependence of Emax in sub- and supercritical fluids (SFC) reveals that, at a fixed pressure, Emax decreases monotonically with increasing temperature in passing through the liquid-SCF phase transition at tc, but exhibits a minimum at a fixed density as the fluids passes above Tc into SCF.

The yields of solvated electron in water decrease with increasing temperature up to 300 °C, monotonously, by both scavenging method and picosecond pulse radiolysis method. Below supercritical temperature, the yields of solvated electron in methanol decrease with increasing temperature up to 200 °C, monotonously, by both scavenging method and picosecond pulse radiolysis method. For scavenging method, the yield of solvated electron in methanol, at earlier time, is higher in supercritical state than that in lower temperature. And the decreasing rate of it is the fastest of all temperatures.

超臨界流体の特性は通常状態とは大きく異なり、化学反応の制御場として活用、機能性材料開発、新材料合成、環境に優しい利用法の可能性等の観点から、超臨界流体への興味やその応用についての興味が増大している。近年、超臨界水の放射線分解について、パルスラジオリシス法を用いた研究が行われるようになり、水和電子の吸収スペクトルや収量の温度依存性、密度効果についての知見が蓄えられるようになってきた。

本研究は亜臨界、超臨界状態のアルコールの放射線分解挙動の解明を目的に、溶媒和電子に着目し、その吸収スペクトル特性や収量の温度、圧力依存性について実験を行ったものである。

本論文は全6章で構成されており、第1章では超臨界流体の特徴とともに、水溶液の放射線照射による水和電子や溶媒和電子の生成、その特性についてこれまでの知見をまとめている。これらを踏まえ本研究の目的を述べている。

第2章は実験方法についてまとめている。本研究で採用したナノ秒時間分解能パルスラジオリシスとピコ秒の分解能を有するパルスラジオリシスのみならず、レーザーフォトリシスも相補的な測定法として採用している。さらに、亜臨界、超臨界状態のアルコール試料を供給するための流通システムについても述べている。

第3章はペンタノール、ヘキサノール、およびオクタノール中、15 MPaの圧力下、室温から各々200, 175, 150℃までの温度領域での溶媒和電子の吸収スペクトルの温度依存性を測定し、吸収ピークは温度増加に伴う長波長シフトの測定について述べている。その吸収ピークエネルギーは温度に比例して、単調に減少する。これらの観測は従来の他のアルコール中で観測されたものに対応する。さらに、現在手に入れることができる全てのデータを収集、整理し、その温度依存性を比較検討した。いずれのアルコール中でも溶媒和電子の吸収ピークエネルギーは温度上昇に伴い直線的に減少することから、その負の温度係数を比較した。それらを用いて室温での吸収ピークエネルギーと温度係数の関係をプロットした。温度係数の大小は分子中のOHの数、分子の対称性などと相関があることを見いだしている。

第4章はメタノール中一定密度下で温度を変化させたとき、溶媒和電子の吸収ピークエネルギーが臨界点で最小値を示すか否かの実施結果を報告している。水和電子では同様の挙動を示すことが最近報告されており、同じ現象がメタノールで観測できるかどうかを確認するためである。純度の高いメタノール中で実験したところ、最小値は観測できるものの臨界点よりも高温側に存在した。少量の水が存在するためとの推測のもと、含水量50ppm 以下のメタノールを用いた場合には、期待通り臨界点に極小値が見いだされた。さらに、共存する水が原因であることを確認するため、1 mol%の水を加えた場合には極小値は高温側に大きくシフトした。以上から、メタノール中一定密度下で温度を変化させたとき、溶媒和電子の吸収ピークエネルギーが臨界点で最小値を示すことを確認するとともに、極小値の温度は水の存在により大きく影響を受けることを明確にした。メタノールでは臨界点近傍での平均的クラスター数の推定は困難であった。

同様の実験をエタノールで実施し、エネルギー最小値を見いだしたが、エタノールの熱力学データ(温度、圧力、密度) の精度が十分でなく、詳細は今後の課題としている。

第5章はピコ秒パルスラジオリシス法によりメタノール中の水和電子の挙動を直接観測するとともに、従来から広く利用されてきた捕捉剤を用いた間接的な評価の比較を、室温から250℃の温度範囲で行った結果を述べている。メタノール実験に先立って、水中の水和電子を対象に、メチルビオローゲン(MV2+)を捕捉剤として用いた実験を室温から300℃の温度範囲で実施した。この結果と水和電子とMV2+の反応速度の温度依存性データから、間接法による水和電子の時間挙動を算出し、直接測定値と比較し、よい一致が得られた。これにより、直接法と間接法は同じ結果を得ることが実験的に確認された。同様に、メタノール中の溶媒和電子の直接測定と、4,4'-bipyridyl (Bpy)を捕捉剤とした間接測定を行った。Bpyと溶媒和電子の反応速度の温度依存性を決定し、温度を変えて捕捉収量の捕捉剤濃度依存性の測定し、両結果を用いて間接法による溶媒和電子の時間挙動を算出した。これを直接測定と比較したところ、10 MPaの一定圧力下の100, 150, 200℃ではほぼ一致する。250℃での直接測定は信号が弱く圧力 15 MPaのデータを取得した。そのため比較はできない。以上より、ピコ秒パルスラジオリシスにより高温メタノール中の溶媒和電子の挙動測定を初めて観測し、Bpyを捕捉剤とした間接測定とよく対応することを確認した。

第6章は本研究の結論であり、得られた成果をまとめている。

以上、要するに室温から高温、超臨界領域のアルコール中の溶媒和電子の吸収スペクトル、収量に対する温度と密度効果に関する実験を進め、高温、超臨界アルコールの放射線反応理解に関わる貴重な知見を得ており、放射線・量子ビーム科学分野への寄与は大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。