Methylene blue sensitized oxidation of ethylene thiourea in cationic micellar medium

1 Introduction

Dye sensitized photooxidation reactions have been widely studied both in homogeneous and heterogeneous media (Murgia et al., 1998; Gutierrez et al., 2000; Memarian et al., 2006; Criado et al., 2008). The electron transfer mechanism has been proposed for a number of reactions and solvent reorganization plays an important role in such reactions (Tanielian and Mechin, 1997; Nad and Pal, 2000a,b, 2002; Singh et al., 2000; Alcantara et al., 2000). Solvent also affects the reactions involving production of ¹O₂. The surfactants have wide ranging effects on the photochemical reactions. There may be possible changes in the mechanism (Moore and Burt, 2006) and other effects (Zhang and Xu, 1994; Gupta and Dhamiwal, 1994). The two mechanisms operating in such systems are influenced by both substrates and solvent (Ohtani et al., 1986; Bonnett and Martinez, 2001; Schmidt, 2006; Cabrerizo et al., 2005; Bosca et al., 2006).

Thioureas have a lot of industrial applications and thus studies pertaining to their oxidation may provide some useful information. Ethylene thiourea is water soluble and mobile and is taken by the plant roots which are metabolized forming ethylene urea and other 2-imidazole derivatives. Photo sensitizers too oxidise ethylene thiourea readily. The present investigation has been carried out in order to look into the mechanistic aspects of methylene blue (MB) sensitized oxidation of ETU in cetyltrimethyl ammonium bromide (CTAB) aqueous micellar medium as photooxidation of sulfides and thioureas especially in heterogeneous medium is not so well studied.

2 Materials and methods

Methylene blue and CTAB were used as received from Aldrich. Conductivity water was prepared by two stage distillation. Other reagents were of high purity grade and were crystallized/distilled before use. Ethylene thiourea (ETU) was prepared by the method reported elsewhere (Johnson and Edens, 1942).

Halogen lamp (Philips, India; 1000 W) fed with a stabilized power supply was used for irradiations. A flat bottom Pyrex cell fitted with an outer jacket for circulating cold water as a heat sink was used for photolysis. The products were monitored conductometrically with a microprocessor based conductivity meter (EI Products) by inserting the conductivity cell directly into the reaction mixture. A wooden shutter was used to stop the irradiations during conductance measurements.

3 Results and discussion

The solution containing ETU (8.0 × 10⁻⁴ M), MB (5.0 × 10⁻⁵ M) and CTAB (2.0 × 10⁻³ M) in water was photolysed for 90 min in the presence of oxygen at constant temperature. The lone product formed in the reaction mixture was identified as 2-imidazoline-2-yl-sulphinic acid by the methods reported elsewhere (Rani et al., 1984). For kinetic runs the products were monitored conductometrically with unirradiated solution as reference. Acid is a product in the reaction and thus the conductance of the reaction mixture is expected to be enhanced with irradiations. The plot of conductance versus irradiation time (Fig. 1) shows regular increase in conductance with irradiation time in micro heterogeneous medium. The overall low conversion, in the presence of CTAB as compared to the homogeneous system, accounts for the enhanced quenching by the product in micro heterogeneous medium.

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Figure 1

Effect of irradiation time. [ETU] = 1 × 10⁻² M, [MB] = 2.68 × 10⁻⁵ M, [CTAB] = 1 mM and solvent = water.

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Sensitizer plays an important role in sensitized reactions as all the incident radiations are absorbed by it and thus the effect of varying concentrations of MB was studied for determining its optimum concentration for the complete absorption of light. The results of this study are given in Fig. 2. There is an initial increase in the quantum yield (Φ) of product formation which becomes practically constant before decreasing with the increase in [MB]. There is an incomplete absorption of incident radiations at lower [MB] and the entire incident light is absorbed by MB when its concentration reaches 5.32 × 10⁻⁵ M. Flattening is observed when concentration of MB is further increased because extra amount of MB does not absorb any additional light. A slight decrease in the quantum yield at high [MB] may be because of the deactivation of triplet dye by the dye itself or its dimeric species.

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Figure 2

Effect of MB concentration. [ETU] = 1 × 10⁻² M, [CTAB] = 1 mM, solvent = water.

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A comparison of the product formed in homogeneous and micellar media shows that the same products are formed in the two media which may account for the similar kinetic features except that the relative efficiency of different pathways is affected by the surfactant due to their effect on the energy transfer/ decay rate of triplet MB. Thus it becomes reasonable to study the effect of varying CTAB concentration and Fig. 3 gives the results. The quantum yield of product formation first decreases and then increases with increase in CTAB concentration. The concentration of triplet MB decreases due to the decrease in the singlet lifetime of MB in the premicellar region (Barnadas et al., 2009) which may account for the initial decrease in the quantum yield. The increase in the quantum yield beyond minimum could be due to the protection of excited state of MB encapsulated in micelles from bimolecular deactivation (Bilski et al., 1991). The enhanced electron transfer efficiency (Chen et al., 2001) in the post micellar region may result in the increase in quantum yield. The relative increase in the overall quantum yield is less because of lesser efficiency of the electron transfer than energy transfer.

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Figure 3

Effect of CTAB concentration. [ETU] = 1.0 × 10⁻² M, [MB] = 2.68 × 10⁻⁵ M, solvent = water.

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Both the pathways (electron transfer and singlet oxygen participation) (Sconfienza et al., 1979) are operative in photosensitized oxidations and their relative applicability depends upon the concentrations of ETU and oxygen. The reaction rate is expected to be strongly dependent on ETU concentration as it is one of the reactants. Effect of varying [ETU] was, therefore, examined at constant [CTAB] and the results are given in Fig. 4. The reaction extent first increases and then decreases after reaching a maximum value with the increase in [ETU]. In order to explain the observed results, the different possible processes involved in sensitized oxidations need consideration which are given in Scheme 1.

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Figure 4

Effect of varying ETU concentration. [MB] = 2.68 × 10⁻⁵ M, [CTAB] = 0.9 mM, solvent = water.

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Scheme 1

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A steady state treatment of Scheme 1 in the absence of steps k₇ and k₈ gives,

The initial increase in the quantum yield may be attributed to the reaction undergoing through step k₄ as there is a competition between the energy transfer from triplet MB to ground state oxygen and its interaction with ETU respectively. The relative efficiency of steps k₄ and k₇ determines the overall reaction extent. As the concentration of ETU increases, more and more reaction goes through k₇ and its efficiency is several times lower than that of process k₄. As the quantum yield of product formation is proportional to the substrate concentration if the reaction goes through step k₄ alone, the initial increase in the quantum yield with increase in [ETU] is expected. A study in CTAB free system shows that process k₇ starts operating at relatively lower [ETU] in the presence of CTAB than in its absence (Sconfienza et al., 1979) showing that the electron transfer-exciplex formation is more facilitated by CTAB possibly through the stabilization of ionic intermediates.

The reaction involves the ionic intermediates particularly at high substrate concentrations. The presence of salts may affect the reactions where ions are involved (Chesta et al., 1987). The effect of sodium chloride (salt) was studied and the results are given in Fig. 5. It is clear from the results that the quantum yield increases with the increase in [NaCl] which provides support to the ionic mechanism as the polar transition state is expected to be stabilized by the added salt.

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Figure 5

Effect of NaCl concentration. [ETU] = 1.0 × 10⁻² M, [MB] = 5.35 × 10⁻⁵ M, [CTAB] = 0.9 mM, solvent = water.

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It is well known that the photooxygenation reactions undergo through the participation of singlet oxygen at low substrate concentration because the excited sensitizer interacts with both oxygen and the substrate and at low substrate concentrations the former process dominates over the latter (Tanielian et al., 2003). The reaction is, therefore, affected by the presence of the singlet oxygen quenchers. The effect of 1,4-diazabicyclo [2:2:2] octane (DABCO), a well known singlet oxygen quencher was studied. Fig. 6 gives the Stern–Volmer plot with intercept which equals to unity (Wilkinson et al., 1993) supporting the participation of singlet oxygen at low [ETU].

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Figure 6

Effect of DABCO concentration. [ETU] = 4.0 × 10⁻³ M, [MB] = 2.68 × 10⁻⁵ M, [CTAB] = 0.9 mM. Solvent = water system (♦), acetonitrile:water, 1:9 (■), methanol:water, 1:9 (▴).

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On the basis of the results given above it is clear that the photooxidation of ETU sensitized by MB undergoes through both the mechanisms (singlet oxygen participation and electron transfer-exciplex formation) in micellar media and the extent of reaction is strongly dependent on [ETU] in addition to the other added substances. The switch over from singlet oxygen mechanism to ionic mechanism is not so distinct because there is a competition between the occurrence of the two mechanisms. The mechanism former occurs at a relatively faster rate than the latter which can account for the decrease in the quantum yield at higher ETU concentrations.