Radiolysis of sodium p-cumenesulfonate in aqueous solution

Radiation Physics and Chemistry 87 (2013) 71–81

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Radiolysis of sodium p-cumenesulfonate in aqueous solution Lidia Osiewa"a a,n, Adam Socha a, Marian Wolszczak b, Jacek Rynkowski a a b

Institute of General and Ecological Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland Institute of Applied Radiation Chemistry, Lodz University of Technology, Wroblewskiego 15, 93-590 Lodz, Poland

H I G H L I G H T S c c c

We determined the minimum hydrotrope concentration (MHC) for SCS. We analyzed the SCS reactions with oxidizing and reducing species. The rate constants for the oxidation and reduction of SCS were given.

a r t i c l e i n f o

abstract

Article history: Received 14 January 2013 Accepted 20 February 2013 Available online 28 February 2013

 The reactions of hydrated electron eaq , hydrogen atom (H) and CO2 (reducing species) as well as Cl2 , Br2 , N3 , OH, O  , SO4 radicals (oxidizing species) with sodium p-cumenesulfonate (SCS) in aqueous solution below minimum hydrotrope concentration have been studied by the method of steady-state and pulse radiolysis. The spectra of transient intermediates, leading to the final products, are presented. The rate constants for the reduction or oxidation reaction of the SCS are also given. The fate of the primary products of the SCS reaction produced during the pulse radiolysis under reductive or oxidative conditions is discussed. & 2013 Elsevier Ltd. All rights reserved.

Keywords: Pulse and steady-state radiolysis Minimum hydrotrope concentration Oxidation and reduction of sodium p-cumenesulfonate

1. Introduction Sodium p-cumenesulfonate (SCS) belongs to a class of water-soluble organic compounds, commonly referred to as hydrotropes (King Jr., 2004), substances which solubilize water-insoluble organic compounds. Common hydrotropes are urea, citric acid, sodium benzoate, sodium salicylate, aromatic sulfuric acids and their sodium salts. Hydrotropes generally contain small hydrophobic and hydrophilic moiety, thus do not form micelles, but facilitate solubility of organic compounds like drugs in water (Sharmad and Zeob, 2007). Hydrotropes are used in cleaning products, as inhibitors of corrosion, in electroplating baths, and in extraction processes (Stanton et al., 2009). The minimum hydrotropic concentration (MHC) is the concentration at which hydrotropes begin to aggregate, i.e., forming new microenvironments with features different from those in dilute solutions. MHC is determined by measuring the effect of hydrotrope concentration on the chemical properties of solutions, such as solubility enhancement or increase in hydrolysis reaction rates. Some physical properties of hydrotrope solutions also show discontinuities when the hydrotrope changes from its monomolecular form to aggregates (Neumann et al., 2007). The literature data concerning the degradation of SCS are not numerous. Kimura and Ogata (1983) studied the decomposition of SCS by the photochemical oxidation (UV light l 4290 nm) in an alkaline aqueous solution with an addition of sodium hypochlorite (NaClO). The SCS mineralization was 38% after 45 min reaction at room temperature and the following products were detected: cumene, 2-isopropylphenol, 4-isopropylphenol, 2-phenyl-2-propanol. In the work Behar (1991) the pulse radiolysis of alkylbenzenesulfonates was described. The reactions of p-methylbenzenesulfonate, m-methylbenzenesulfonate, m-xylene-4-sulfonate and isopropylobenzenesulfonate with the following short-living radicals: dOH, O  , Hd and SO4 , were studied.

n

Corresponding author. Tel.: þ48 42 631 31 30; fax: þ 48 42 631 31 03. E-mail address: [email protected] (L. Osiewa"a).

0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.02.031

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This paper presents the results of investigations concerning the pulse and steady-state radiolysis of SCS aqueous solutions with the  concentration below MHC with radicals: dOH, Nd3 , Br2 , Cl2 , O  , SO4 and Hd, eaq , CO2 .

2. Experimental 2.1. Reagents The solutions were prepared using purified water (double purification in Millipore Milli-Q Plus system). All chemicals were purchased from Aldrich-Sigma and were used without further purification.

2.2. Methods 2.2.1. Fluorescence method Fluorescence experiments were carried out at room temperature with an Aminco Bowman Series 2 spectrofluorimeter equipped with xenon arc lamp as a light source. The experiments were performed in 10-mm path-length quartz cells. The solutions were excited by irradiation with the wavelength of 240 nm.

2.2.2. Differential pulse voltammetry Differential pulse voltammetry measurements were carried out using an Autolab Potentiostats PGstat30 (Eco Chemie, Holland) in order to determine the half-wave potential of SCS oxidation and reduction. A three-electrode cell was applied in all experiments. Platinum electrode was used as a working electrode in the oxidation process, while the mercury electrode was a working electrode in the reduction process.

2.2.3. Pulse radiolysis Pulse radiolysis was performed using a 6 MeV linear accelerator. The equipment for pulse radiolysis with the optical detection was described elsewhere (Karolczak et al., 1991). Electron pulses of 17 ns duration delivered doses in the range 50–65 Gy. UV–vis absorption spectra were acquired using a Cary 5E (Varian) spectrophotometer. The flow system was used in pulse radiolysis of sodium p-cumenesulfonate solution with the volume of 250 ml. The solutions were saturated with N2 or N2O. The SCS concentrations in pulse and steady-state radiolysis experiments were 2  10  4 M and 1  10  3 M in oxidation and reduction processes, respectively. The water radiolysis results in formation of three well-characterized reactive radical species used to initiate radical reactions, and formation of molecular products with amounts (mM per absorbed dose of 1 Gy (J kg  1)) are given in parenthesis (Anderson et al., 2008). H2O-edaq  (0.28),dOH (0.28), H (0.055), H2 (0.04), H2O2 (0.07), H þ (0.28)

(1)

The hydroxyl radical is a powerful oxidant with a standard reduction potential of 2.7 and 1.8 V vs. SHE in acidic and neutral solutions, respectively. A system containing only dOH radicals can be obtained by purging the aqueous solution with N2O, while the hydrated electrons are converted into the dOH radicals. In alkaline solution, the above reaction causes deprotonation of dOH to Od  . The azide radicals, Nd3 , commonly used as a one-electron oxidants, can be formed in irradiated N2O-saturated aqueous solution containing NaN3. Similarly, the sulfate radical anions, SOd4  can be formed in the reaction of edaq or Hd with K2S2O8. The dihalide radical anions (Xd2  , X¼Cl, Br) can be generated by pulse radiolysis carried out in KBr or NaCl N2O-saturated aqueous solution, because the dOH radicals react with halide ions to form oxidizing species (reaction 2). The halide radicals are rapidly converted into Xd2  (reaction 3). OH þX  -Xd þ  OH

d

(2)

Xd þX  -Xd2 

(3)

One of the early discoveries employing pulse radiolysis was the detection of hydrated electron by Boag and Hart (1963). This reaction was studied in N2-saturated aqueous solution containing tert-butyl alcohol (t-BuOH) to effectively scavenge the dOH radicals. The hydrogen atoms (H) are generated in Ar or N2 saturated acidic solution containing t-BuOH in order to remove dissolved oxygen and hydroxyl radicals. In acidic solution the hydrated electrons are converted into H atoms (reaction 4). edaq þH-Hd

(4)

Hydrogen atoms absorb weakly in the UV region at lmax ¼200 nm. The Hd atom is a powerful reducing agent and its standard reduction potential is 2.31 V vs. SHE in acidic solution. However, the Hd atom is a slightly less powerful reducing agent than edaq  . The CO2 radical is a strong one-electron reducing radical and its standard potential is  1.9 V vs. SHE. It is obtained in the N2Osaturated aqueous solution containing a formate ion or formic acid (HCOO  ). Reaction leadings to CO2 radicals are as follows (Yadav et al., 2007). edaq þN2O-Od  þN2 OH/Hd þHCO2 -COd2  þ H2O/H2

d

(5) (6)

L. Osiewa!a et al. / Radiation Physics and Chemistry 87 (2013) 71–81

73

Scheme 1. Structure of sodium p-cumenesulfonate.

0.6

2.5

0.10 fluorescence

0.08

0.5

2.0

spectrum absorption spectrum

1.5

I357/I285

F

A

0.06

0.4

0.3

0.04

1.0

0.02

0.5

0.00

0.0 λ

0.2

0.1

0

2

4

6

8

10

CSCS, 10-3 M Fig. 1. Iaggr./Imonomol. ratio aqueous solution at various concentrations of SCS. Insert: absorption spectrum of SCS (2  10  4 M) and emission spectrum of SCS (1  10  3 M) in water, pH ¼6.5.

3. Results and discussion 3.1. Structure of SCS The molecular structure of SCS is shown in Scheme 1. It contains a benzene ring with a connected isopropyl group as a hydrophobic moiety and a sulfonic group as a hydrophilic moiety. The UV–vis absorption spectra of SCS aqueous solution show two absorption bands, one at lmax ¼221 nm and another with three peaks in the region from 240 to 275 nm as shown in Fig. 1 (insert). 3.2. Fluorescence studies Spectroscopic methods can be considered as one of the best ways of determining the aggregation features of hydrotropes and MHC. This method was widely described in the paper (Neumann et al., 2007). The measurement of the I(aggregate)/I(monomolecular) results in a good insight into the evolution of the aggregate formation in the solution when the hydrotropic concentration increases. The main advantage of the fluorescence method is that it does not require the use of probes or other additives, which might disrupt the process of aggregation. The change in the emission spectrum of the SCS aqueous solution was observed with the increase in the concentration of luminophore (Fig. 1). The analysis of the ratio of the emission bands at 357 to 285 nm as a function of SCS concentration showed that MHC for SCS is equal to 1  10  3 M. 3.3. Oxidative species The electrode reactions characterizing SCS electrooxidation at the platinum electrode were studied using the differential pulse voltammetry. In the aqueous solutions, SCS oxidation peak was not observed at potential lower than the potential at which oxygen evolution started. However in acetonitrile, SCS was oxidized in two steps and half-wave potentials were 1.97 V and 2.3 V vs. SCE (1.73 V and 2.06 V vs. SHE). 3.3.1. Oxidative species reactive towards SCS SCS oxidative processes induced by different radicals were investigated using pulse radiolysis. As described in the experimental section, it is possible to generate oxidizing radicals such as Nd3, SOd4  Cld2  , Brd2  , dOH and Od  by radiolysis. The azide radical is an oxidant which reacts exclusively by one-electron transfer (E0 ¼1.33 V vs. SHE). The lack of reactivity in the case of SCS arises from the higher oxidation potential of the hydrotrope as compared with the oxidation potential of the azide radical.

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0.20

2.7 μs 19 μs 77 μs

ΔA

0.15 0.10 0.05 0.00 250

300

350

400

λ, nm 0.20 330 nm

ΔA

0.15

315 nm

0.10 0.05

285 nm

0.00 0.00

0.05

0.10

0.15

t, ms Fig. 2. (A) Transient absorption spectra recorded at different times in N2O-satured aqueous solution (pH¼ 6.5) containing SCS (2  10  4 M) after 17 ns pulse irradiation with a dose of 65 Gy. (B) Time profiles (between  20 and 180 ms) of the absorbance recorded at 285, 315 and 330 nm. The kinetic patterns are normalized on the absorbance for the oscilloscope trace recorded at 285 nm.

The Brd2  (E0 ¼1.7 V vs. SHE) anion radical formed during pulse radiolysis do not react with SCS either. On the other hand, SOd4  (E0 ¼2.43 V vs. SHE) and dOH (E0 ¼2.7 V vs. SHE) radicals are stronger oxidants and therefore this radicals react with SCS. Cld2  (E0 ¼2.1 V vs. SHE) and Od  (E0 ¼1.77 V vs. SHE) anion radicals which have slightly lower potential, also react with SCS. These studies show that the standard potential value for the formation of the SCS oxidized form is in the range from 1.7 to 1.77 V vs. SHE.

3.3.2. The reaction of SCS with dOH radicals The absorption of the transient species formed after irradiation is shown in Fig. 2. The solution contained SCS at the concentration of 2  10  4 M and was N2O-saturated (pH¼6.5). The spectra show strong absorption at lmax ¼285 nm and in the region of 300–340 nm (Fig. 4A). The kinetic transient absorption signals at lmax ¼285, 315 and 330 nm are formed in the same time scale, with the secondorder rate constant k285,315,330 ¼6.5  1010 M  1 s  1. The transient absorption signals at lmax ¼285, 315 and 330 nm recorded in the time domain:  20 to 180 ms are shown in Fig. 2B. The first-order rate constant for band at lmax ¼285 nm is equal to k285 ¼3.7  104 s  1, while for bands at lmax ¼315 and 330 nm is equal to k315,330 ¼2.1  104 s  1. The traces at 315 and 330 nm are formed with the same kinetics, and it seems reasonable to conclude that the absorption bands with maxima at these wavelengths are due to the existence of the same moiety. The different kinetic pattern recorded at 285 nm may suggest the existence of the another product. Taking into consideration the obtained data, it can be concluded that at least two types of radical species are formed as the result of SCS reaction with the hydroxyl radicals. The first type of radicals are formed by addition of hydroxyl radical which we denoted as HO-adducts (reaction 7), and the second type—benzyl type radical (Cn) results from hydrogen separation (reaction 8).

(7)

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75

(8)

One can expect that the band at lmax ¼285 nm corresponds to benzyl type radical and wide band with two peaks at lmax ¼315 and 330 nm corresponds to OH-adducts. A steady-state radiolysis experiment of SCS reaction with OH radicals was carried out with the SCS solution as in the case of the pulse radiolysis experiments. The solution was irradiated with different doses. The absorption spectra of the products formed in the reaction of d OH radicals with SCS are shown Fig. 3. The progressive oxidation results in the decrease in absorbance within the spectral range characteristic for SCS (band at lmax ¼221 nm). Formation of a new wide band at lmax ¼280 nm was also observed. This band probably corresponds to the products of SCS reaction with the hydroxyl radicals. The one-step oxidation is reflected by the presence of isosbestic points at 232 and 210 nm. 3.3.3. The reaction of SCS with Cld2  radicals The absorption spectra of the transient species formed after irradiation of the N2O-saturated aqueous solution (pH¼6.5) containing SCS (2  10  4 M) and sodium chloride (0.1 M) are presented in Fig. 4. The spectra show strong absorption band at lmax ¼285 nm and the weaker one in the region of 300–350 nm (Fig. 4A). The band with maximum absorbance at 340 nm corresponds to the generation of Cld2  radicals (Shirdhonkar et al., 2008). The transient absorption signals detected at lmax ¼285, 315 and 330 nm in the time scale to 5 ms are presented in Fig. 4B. The transient spectra show that the bands are formed in the same time scale (similar to the reaction of SCS with hydroxyl radicals) with the second-order rate constant k285,315,330 equal to 9.4  109 M  1 s  1. The transient absorption patterns recorded at lmax ¼285, 315 and 330 nm are shown in Fig. 4C. The first-order rate constants for the traces detected at lmax ¼315 and 330 nm are the same and equal to k315,330 ¼1.7  104 s  1. An analogous constant obtained for the wavelength at lmax ¼285 nm is equal to k285 ¼3.5  104 s  1. The values of calculated rate constants are very close to those obtained in the reaction of SCS with hydroxyl radical. Thus it seems probable that the same types of radical species are obtained in the reactions of SCS both with hydroxyl and Cld2  radicals. The steady-state radiolysis experiments of the SCS reaction with Cld2  radicals were carried out in the same solution as in the case of the pulse radiolysis. The solution was irradiated with different doses. The absorption spectra of the products formed in the reaction of Cld2  radicals with SCS are shown Fig. 5. During the oxidation, a decrease in absorbance in the spectral range characteristic for SCS (the band at lmax ¼221 nm) is observed as well as formation of a new absorption band in the range from 232 to 350 nm. The irradiation also resulted in the formation of absorption band below 210 nm. The one-step oxidation was reflected by the presence of isosbestic points at 232 and 210 nm. The irradiation of the solution under such conditions leads to the spectral changes which are very similar to those observed for the reaction of SCS with hydroxyl radicals (Fig. 3). This indicates that the species formed in the reaction of SCS with Cld radicals are very 2 similar or the same as those formed in the reaction of SCS with hydroxyl radicals. HPIC analysis show that the increase in an irradiation dose results in a linear decrease of SCS concentration by about 75%. Simultaneously A/A0 ratio (band at lmax ¼221 nm) also decreases linearly by about 25% but with a different slope, while the concentration of SO2d increases linearly by about 12% (dose of 0.65 kGy) (Fig. 5, insert). 4

3.0 2.5

Dose [kGy] 0 0.195 0.39

A

2.0

0.65 0.95

1.5

1.95 3.25

1.0 0.5 0.0 200

220

240

260

280 λ, nm

300

320

340

Fig. 3. Absorption spectra recorded in N2O-saturated aqueous solution of SCS (2  10  4 M, at pH¼6.5) at different doses between 0 to 3.25 kGy (irradiation with 17 ns electron pulse of the dose of 65 Gy).

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0.15 2.7 μs 26 μs 55 μs

ΔA

0.10

0.05

0.00 250

300

350

400

λ, nm 0.15

ΔA

0.10 285 nm

0.05

315 nm 330 nm

0.00

0

2 t, μs

1

3

4

5

0.15

315 nm

0.10 ΔA

330 nm 0.05 285 nm 0.00 0.00

0.05

0.10

0.15

t, ms Fig. 4. (A) Transient absorption spectra recorded at different times in N2O-satured aqueous solution (pH¼6.5) containing SCS (2  10  4 M) and NaCl (0.1 M) after 17 ns pulse irradiation with a dose of 65 Gy. (B) Time profiles (between  0.5 and 5 ms) of the absorbance recorded at 285, 315 and 330 nm. (C) Time profiles (between  20 and 180 ms) of the absorbance recorded at 285, 315 and 330 nm. The kinetic patterns are normalized on the absorbance for the oscilloscope trace recorded at 285 nm.

The decrease in absorbance at lmax ¼221 nm is not directly coupled with SCS degradation, probably because in the same spectral region, light can be absorbed by the product of SCS oxidation. The above results show that desulfonation occurs only in a slight extent.

3.3.4. The reaction of SCS with SOd4  radicals The reaction of SCS with SOd radicals was also studied. The N2-satured aqueous solution (pH¼6.5) containing SCS (2  10  4 M), K2S2O8 4 (0.1 M) and t-BuOH (0.1 M) was irradiated and SO4 radicals were formed in the reaction of S2O2 with hydrated electrons ed 8 aq (reaction 9).  S2O28  þ edaq -SO2d þSOd4  4

(9)

To prevent the reaction of dOH with SCS, dOH radicals were scavenged with t-BuOH (reaction 10). OH þC(CH3)3OH-H2Oþ dCH2C(CH3)2OH

d

(10)

L. Osiewa!a et al. / Radiation Physics and Chemistry 87 (2013) 71–81

3.0 0 0.195 0.39 0.65 0.95

A

2.0 1.5

1.95

A/A0

0.6 0.10

0.4

0.05

0.2

SO

0.00 0.0

0.2

3.25

1.0

0.8

SCS

0.15 C, mM

2.5

1.0

0.20

0.4 Dose, kGy

A/A0

Dose [kGy]

77

0.6

0.0

0.5 0.0 200

220

240

260

280 λ, nm

300

320

340

Fig. 5. Absorption spectra recorded in N2O-saturated aqueous solution (pH¼6.5) of SCS (2  10  4 M) in NaCl (0.1 M) at different doses between 0 and 3.25 kGy, Insert: The dependence of SCS and SO24  (left axis) and A/A0 ratio at the wavelength of 221 nm (right axis) vs. irradiation dose.

The transient species formed after irradiation are shown in Fig. 6. The spectrum recorded at the early stage of radiolysis (240 ns after 17 ns pulse irradiation with the dose of 65 Gy) is dominated by the bleached absorption in the UV region. The bleaching of the absorption band around 260 nm is due to the depletion of the SCS ground state which absorbs in this spectral range. The spectra recorded at longer time scales show strong absorption at lmax ¼285 nm and two additional bands in the spectral region of 300–350 nm and 360–550 nm. The latter band corresponds to the generation of SOd4  anion radicals (Shirdhonkar et al., 2008). The absorption decay at 338 nm is accompanied by an increase in absorption at lmax ¼ 285 nm. It is evident that the species decaying at 338 nm (first-order rate constant is equal to k338 ¼9  104 s  1 for the OH-adducts) is the precursor of a benzyl type radicals, which strongly absorbs at lmax ¼285 nm (second-order rate constant is equal to k285 ¼2.75  109 M  1 s  1). Taking into consideration the obtained results, it can be concluded that in the reaction of SCS with SOd4  anion radical at least two types of radical species are produced (reaction 11–13). The first stage (reaction 11) is too fast to be recorded due to the long time of response of the apparatus.

(11)

Reaction 12 is followed by a rapid reaction with water resulting in formation of OH-adducts at different positions in the benzene ring.

(12)

L. Osiewa!a et al. / Radiation Physics and Chemistry 87 (2013) 71–81

ΔA

78

0.08

0.24 μs

0.06

0.7 μs 2.35 μs

0.04

8.7 μs

0.02 0.00 -0.02 -0.04 250

300

350

400 λ, nm

450

500

550

0.10 285 nm

0.08

ΔA

0.06 0.04 338 nm

0.02 440 nm

0.00 0

2

4 t, μs

6

8

Fig. 6. (A) Transient absorption spectra recorded at different times in N2-satured aqueous solution (pH¼ 6.5) containing SCS (2  10  4 M), K2S2O8 (0.1 M) and t-BuOH (0.1 M) after 17 ns pulse irradiation with a dose of 65 Gy. (B) Time profiles (between  0.5 and 9 ms) of the absorbance recorded at 285, 338 and 440 nm.

The OH-adducts are unstable and the transient species recorded at 338 nm are the precursors of the benzyl type radicals (lmax ¼285 nm) (reaction 13).

(13)

Similar results were presented in the Sehested and Holcman (1978) paper. The authors reported the OH-radicals induced formation of radical cations of methylated benzenes in aqueous acidic and neutral solutions. The precursor of the radical cation in acid media is the OH-adduct, whereas the cation is formed directly in the reaction with SO4 radical anions in the neutral solution.

3.3.5. The comparison of SCS and SBS reaction with Cl2 radicals In order to confirm that the band at lmax ¼285 nm corresponds to the benzyl type radical, we performed the reaction of Cld2  with sodium benzenesulfonate (SBS), because a compound not containing the alkyl group cannot form the benzyl type radical. The reaction was carried out in the same conditions as in a case of SCS reaction with Cld2  . Fig. 7 shows two transient absorption spectra recorded after 2.7 ms in the reaction of SBS and SCS with Cld2  anion radicals. In spectrum recorded for SBS, a high sharp band at lmax ¼285 was not observed in contrary to the reaction of SCS with Cld2  . Only the wide band in the range from 280 to 350 nm was found. The differences in the spectra indicate that the band within 280–350 nm corresponds to the formation of the OH-adducts with the benzene ring according to the reaction 14 and 15.

L. Osiewa!a et al. / Radiation Physics and Chemistry 87 (2013) 71–81

0.15

79

SCS

2.7 μs

SBS

ΔA

0.10

0.05

0.00 250

300

350 λ, nm

400

450

Fig. 7. Transient absorption spectra recorded at 2.7 ms in N2O-saturated aqueous solutions (pH¼ 6.5) containing SBS (2  10  4 M, (0.1 M) after 17 ns pulse irradiation with a dose of 65 Gy.

) or SCS (2  10  4 M, ’) in NaCl

(14)

(15)

This indicates that the band at lmax ¼ 285 nm, which was observed in reaction Cl2 with SCS, can be ascribed to the formation of benzyl radicals. 3.4. Reductive species Electrode reactions characterizing SCS electroreduction at mercury electrode were also studied using the differential pulse voltammetry. In the aqueous solution peak of SCS reduction was observed at a potential lower than the potential at which hydrogen evolution started. The half-wave potential was equal to  1.41 V vs. SCE ( 1.65 V vs. SHE). 3.4.1. Reductive species reactive towards SCS Reductive transformation of SCS by different radicals was investigated using pulse radiolysis. As it was described in the experimental section, it is possible to generate secondary reducing radicals such as edaq , COd2  and Hd. In reaction of SCS with CO2 radical (E0 ¼  1.9 V vs. SHE) a transient absorption spectra corresponding to the SCS reaction products were not observed, because UV region was dominated by absorption of CO2 radical. The hydrated electron edaq (E0 ¼  2.9 V vs. SHE) which is formed during pulse radiolysis of N2-saturated aqueous solution containing t-BuOH (0.1 M), is able to react with the substrate. Hydrogen atom Hd (E0 ¼  2.3 V vs. SHE) which is generated in acidic solution was also reactive towards SCS. 3.4.2. The reaction of SCS with edaq radicals Fig. 8A shows transient absorption spectra recorded for the reaction of hydrated electron with SCS (1  10  3 M) obtained in an aqueous solution deoxygenated by N2 in the presence of t-BuOH (0.1 M). During the reaction, it was found that a strong absorption spectrum with a very short life-time (0.48 ms) appeared in the wavelength range from 450 to 800 nm. This broad spectrum with maximum at 720 nm can be attributed the hydrated electrons. This band disappeared after 3 ms. The edaq decay is accompanied by the appearance of a new transient absorption band at lmax ¼331 nm in a longer time scale. The formation of a sharp absorption band in the spectral region of 310–320 nm was also observed immediately after the pulse. Probably this band corresponds to the formation of the reduction product of SCS in the reaction with a dry electron. The formation of the band in the region with lmax ¼331 nm (second-order rate constant k331 ¼3.6  109 M  1 s  1) is accompanied by the decay of the band with lmax ¼720 nm (first-order rate constant k720 ¼2.05  106 s  1) (Fig. 8B).

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0.4

ΔA

0.3 0.2

0.15 μs 0.35 μs 1.8 μs

0.1 0.0

300

400

500 λ, nm

600

700

800

0.30 0.25 331 nm

ΔA

0.20 0.15 0.10 0.05

720 nm

0.00 0

1

2

3

4

t, μs Fig. 8. (A) Transient absorption spectra recorded at different delay times in N2-saturated aqueous solution (pH ¼6.5) containing SCS (1  10  3 M) and t-BuOH (0.1 M) after 17 ns pulse irradiation with a dose of 65 Gy. (B) Time profiles (between  0.5 and 4 ms) of the absorbance recorded at 331 and 720 nm. The kinetic decay patterns are normalized on the absorbance for the oscilloscope trace recorded at 720 nm.

According to the results obtained in the reaction of the hydrated electrons with SCS, the following mechanism of the one-electron reduction of SCS can be suggested (reaction 16 and 17). The anion radicals formed as a result of the reaction of SCS with the hydrated electrons (reaction 16) can attach a proton (H þ ) from water molecule in order to neutralize a negative charge at SCS molecules according to the reaction 17. Protonated species are formed in this reaction.

(16)

(17)

L. Osiewa!a et al. / Radiation Physics and Chemistry 87 (2013) 71–81

A

2.0

0

1.0

1.0 A/A

SCS

0.15

1.065

1.5

0.8 0.6

0.10 0.4

2.6

0.05

4.55

0.00

SO

0

1

2

0.2

3

0.0

Dose, kGy

6.5 9.75

0.5 0.0 200

0.20

A/A

2.5

Dose [kGy]

C, mM

3.0

81

225

250

275 λ, nm

300

325

350

Fig. 9. Absorption spectra recorded in N2-saturated aqueous solution (pH¼ 6.5) of SCS (1  10  3 M) and t-BuOH (0.1 M) at different doses between 0 and 9.75 kGy. Insert: The dependence of SCS and SO24  concentration (left axis) and A/A0 ratio at the wavelength of 221 nm (right axis) vs. irradiation dose.

The steady-state radiolysis experiment of the SCS reaction with hydrated electrons was carried out in the same solution as in the case of the pulse radiolysis experiments. The solution was irradiated with different doses. The absorption spectra of the products formed in the reaction of edaq with SCS are shown in Fig. 9. A progressive reduction results in the decrease in absorbance in the spectral range corresponding to SCS (band at lmax ¼221 nm). Moreover, there was no change in the absorbance of the band 240–275 nm. The analysis of the HPIC data shows that the increase in an irradiation dose results in a linear decrease in the SCS concentration by about 50%. Simultaneously, A/A0 ratio (band at lmax ¼221 nm) also decreases linearly by about 40% with similar slope, while the concentration of SO24  ions increases by about 25% (at the dose of 3.25 kGy). Such results indicate that the decrease in absorbance at lmax ¼221 nm can be coupled with SCS degradation and 50% of the SCS reduction products do not contain the sulfonic groups.

4. Conclusions Sodium p-cumenesulfonate is very reactive towards dOH, Cld2 , SOd4, Od radicals. On the other hand, weaker oxidants such as Nd3 or Brd2 are not able to oxidize SCS. The lack of reactivity results from the lower oxidation potential of both radicals in comparison with the oxidation potential of SCS. One-electron oxidation of SCS in the reaction with OH and Cl2 radicals leads to the formation of the intermediate species in the same time scale: OH-adduct (band at lmax ¼315 and 330 nm) as well as to the benzyl type radicals (band at lmax ¼285 nm). In both cases, steady-state radiolysis experiments prove the formation of the new absorption band in the range from 232 to 350 nm. The oxidation with SOd4  anion radicals leads to the formation of OH-adducts (band at lmax ¼338 nm), which are precursors of benzyl type radicals (band at lmax ¼285 nm). SCS is very reactive towards edaq  and Hd. One-electron reduction of SCS in the reaction with edaq  leads to the formation of the intermediate species which absorb at lmax ¼331 nm. In this reaction the primary reduction product is unstable and undergoes protonation, the second-order rate constant is equal to 3.6  109 M  1 s  1. The reaction of SCS with edaq  leads to the desulfonation. Using the same dose of 0.65 kGy, the concentration of SCS decreased by 75% during oxidation, and by 20% during reduction. References Anderson, R.F., Shinde, S.S., Maroz, A., Boyd, M., Palmer, B.D., Denny, W.A., 2008. Intermediates in the reduction of the antituberculosis drug PA-824, (6S)-2-nitro-6-{[4(trifluoromethoxy)benzyl]oxy}-6,7-dihydro-5h-imidazo[2,1-b] [1,3] oxazine, in aqueous solution. Org. Biomol. Chem. 6, 1973–1980. Behar, D., 1991. Are positive ion radicals formed in pulse radiolysis of alkylbenzenesulfonates? J. Am. Chem. Soc. 95, 4342–4347. Boag, J.W., Hart, E.J., 1963. Absorption spectra in irradiated water and some solutions: absorption spectra of hydrated electron. Nature 197, 45–47. Karolczak, S., Hodyr, K., Polowinski, M., 1991. Pulse radiolysis system based on ELU-6E Linac-II. Development and upgrading the system. Radiat. Phys. Chem. 39, 1–5. Kimura, M., Ogata, Y., 1983. Photo-oxidation some aromatic sulfuric acids with alkaline hypochlorite. Bull. Chem. Soc. Jpn. 56, 471–473. King Jr., A.D., 2004. The solubility of ethane, propane, and carbon dioxide in aqueous solutions of sodium cumene sulfonate. J. Colloid Interface Sci. 273, 313–319. Neumann, M.G., Schmitt, C.C., Prieto, K.R., Goi, B.E., 2007. The photophysical determination of the minimum hydrotrope concentration of aromatic hydrotropes. J. Colloid Interface Sci. 315, 810–813. Sehested, K., Holcman, J., 1978. Reactions of the radical cations of methylated benzene derivatives in aqueous solution. J. Phys. Chem. A 82, 651–653. Sharmad, J.C., Zoeb, A.F., 2007. Syntheses of quinolines by Hriedlander’s heteroannulation method in aqueous hydrotropic medium. J. Dispersion Sci. Technol. 28, 279–283. Shirdhonkar, M., Mohan, H., Maity, D.K., Rao, B.S.M., 2008. Oxidation of phenyl trifluoromethyl sulphide: a pulse radiolysis and theoretical study. J. Photochem. Photobiol., A 195, 277–283. Stanton, K., Tibazarwa, C., Certa, H., Greggs, W., Hillebold, D., Jovanovich, L., Woltering, D., Sedlak, R., 2009. Environmental risk assessment of hydrotropes in the United States, Europe, and Australia. Integr. Environ. Assess. Manag. 6, 155–163. Yadav, P., Kulkarni, M.S., Shirdhonkar, M.B., Rao, B.S.M., 2007. Pulse radiolysis: Pune University LINAC facility. Curr. Sci. 92, 599–605.