Oxidation of iminodiethanol by Ce (IV) in microheterogeneous system: a comprehensive kinetic analysis

Abstract A comprehensive kinetic analysis of catalyzed oxidative degradation of iminodiethanol by Ce(IV) in pure aqueous and microheterogeneous or surfactant media has been reported. The oxidation reactions were first order with respect to [Ce (IV)], the dependence of rate on iminodiethanol concentration is first order, the rate constant values increased with increasing dielectric constant of the medium and the entropy of activation was negative. Kinetic information suggested that the oxidation reactions proceed through intermediate complex formation. The profile of pseudo-first order rate constant, kψ against [SDS] (sodium dodecyl sulfate) shows a maximum at about the concentration near which the randomized surfactant monomers started aggregating for the formation of micelle. To demonstrate the kinetic pattern and to evaluate the micelle-reactant binding constant values, Berezin’s model has been advanced. The transfer free energy per mole of the solute from water to the micellar phase has also been computed. Activation parameters for the oxidation reaction are obtained from the temperature dependence studies. Basing on all analytical results plausible mechanisms are proposed for the catalyzed oxidation of iminodiethanol by Ce (IV) both in aqueous and surfactant medium. GRAPHICAL ABSTRACT


Introduction
Electron transfer reactions are intrinsic part of myriad number of metabolic processes to produce energy for sustenance of life. Besides biological, a large number of industrial, environmental and pharmaceutical issues can be addressed through processes that involve redox reactions. Kinetic studies of these electron transfer reactions help in understanding the mechanistic pathways. Oxidation of alcohols to aldehydes or acids is a significant industrial and biological process and hence has been studied extensively. [1][2][3][4][5][6][7][8] Hydrophilic iminodiethanol (a secondary amine) is extensively used in industries for the production of fine chemicals, surfactants, pharmaceuticals [9,10] and hence is the choice as substrate in the present work. In continuation of our earlier work with surfactants and redox reactions, [11][12][13][14][15] we present here the Mn(II) catalyzed oxidation of iminodiethanol by Ce(IV) in acidic conditions in surfactant medium. Although literatures are available [16][17][18][19][20] on the oxidation of iminodiethanol with various oxidizing agents, they present varied kinetic models viz; kinetic expressions, rate determining steps, equilibrium conditions and varied product formation. Also, there is no information on the oxidation process in the microheterogeneous medium that we have executed in the present case. The degradation technique adopted by Yaser et al. [20] used UV radiation process and they have not reported the influence of any parameters besides the effect of pH. While working with similar substrates, Singh et al. [16] has predicted an acid as the reaction product, Shukla et al. and Aswathi et al. [19] has concluded an aldehyde as the final product whereas Puttaswamy et al. [18] has confirmed the formation of a mixture of aldehyde and acid. The selection of Ce(IV), a versatile reagent as oxidant is done because Ce(IV) and Ce(III) form an excellent redox couple with a reduction potential of 1.28-1.70 volts of (Ce 3þ , Ce 4þ ) in different acidic environment. [21][22][23][24][25][26][27] The choice of Mn(II) as the catalyst for the purpose is because of its extensive use in homogeneous catalysis.
Surfactants containing hydrophilic head and hydrophobic tail are extensively used as excipients in drug delivery and as catalyst in numerous biologically and industrially important electron transfer reactions. [28][29][30][31][32][33] At and above certain specific concentration of the surfactant, the hydrophobic domains of the surfactant molecules can associate to form aggregates called "micelles". Micellization depends on the size of hydrophobic domain, the nature and size of polar head groups, temperature, salt concentration, pH etc. The narrow concentration range over which micellization starts is known as critical micelle concentration (cmc). [34,35] Micellar systems can solubilize and compartmentalize various substances modifying the chemical equilibria and reactivity. [36][37][38] Micellar effect on electron transfer reactions involving metal ions has attracted the attention of a large number of workers due to their importance in biological process. However, the reactions mostly investigated were of simple bimolecular type without involving any pre-equilibrium. But electron transfer reactions, involving particularly the metal ion as oxidant, proceed through equilibria prior to the rate determining step. Hence it is considered interesting to study the micellar effect on the oxidation of IME (iminodiethanol) by Ce(IV). In the backdrop of the above discussions our present work with surfactants on the oxidation of iminodiethanol catalyzed by Mn(II) using Ce(IV) as oxidant bears significance. The present study aims to find the following information regarding the oxidation of the iminodiethanol under acidic environment in pure aqueous and surfactant medium: i. catalytic effect of Mn(II) ii. to evaluate the effect of cationic, anionic and nonionic surfactants on the oxidation process iii. to examine the active species involved in the reaction system iv. to suggest a suitable kinetic model both in absence and presence of surfactant v. to evaluate the thermodynamic parameters vi. to establish the stoichiometry and determine the order with respect to each participant in the reaction.
To study the effect of micellar medium on the oxidation of iminodiethanol all the three types viz., anionic (Sodium dodecyl sulfate, SDS), cationic (Cetyltrimethylammonium bromide, CTAB) and nonionic (Triton X-100) surfactants were given a trial. Nonionic surfactant was excluded from the study due to immediate precipitation under the conditions of study. Measurable effects were noticed only in SDS surfactant medium where-as with CTAB, the kinetic changes were marginal.

Materials and methods
AR grade reagents with more than 99.5% purity of iminodiethanol (Sigma-Aldrich), ceric ammonium sulfate, CAS (Qualigen), manganese sulfate tetrahydrate (SD Fine), sulfuric acid (E. Merck) was purchased and used without any further purification. Ce(IV) solution was prepared by dissolving CAS in 1.0 mol dm À3 sulfuric acid and was diluted to desired strength as and when required. Literature method [39] was followed to standardize CAS with Iron(II) ammonium sulfate using ferroin indicator. The stock solution of iminodiethanol was prepared by dissolving requisite quantity in water. The surfactant, sodium dodecyl sulfate (BDH, India) was recrystallized from alcohol and used. All solutions were prepared using doubly distilled conductivity water and stored in stoppered reagent vessels coated with black. Nitrogen gas was bubbled into the solutions prior to kinetic runs.
Shimadzu UV 1700 A double beam spectrophotometer having 1 cm path length quartz cell was used to record the absorbance data. The spectrophotometer was fitted with a Remi 396 LAG model Cryostat to maintain specific temperature. A Systronics l-361 pH meter with temperature probe was used to measure the pH. The cmc values of the surfactant solutions in different concentration were measured using Systronics model 306 conductance bridge with extension conductivity cell, type CD-30 having cell constant of 1 cm À1 .
Kinetics of the oxidation of iminodiethanol was followed under pseudo first order conditions with oxidant concentration tenfold lesser than that of substrate concentration. Thermally equilibrated solutions were mixed in the order; sulfuric acid, substrate (iminodiethanol), catalyst (Mn 2þ ) and ultimately the oxidant (Ce 4þ ) to initiate the oxidation in aqueous medium. Decrease in absorbance of colored Ce(IV) with time at its k max of 380 nm was recorded to monitor the progress of the reaction. It was ensured that interference from other reagents at this wave length is negligible. Beer's law was obeyed with e ¼ 533 ± 20 dm 3 mol À1 cm À1 in the entire concentration range of Ce(IV) used. The surfactant solution previously thermally equilibrated at a temperature as that of the other reagents, was first mixed with the oxidant solution, and this mixture was added to the reaction flask containing other reagents to initiate the reaction in micellar medium. The wave length of maximum absorption, k max of Ce(IV) shows a bathochromic shift of few nm (1-4 nm) upon addition of the surfactant; however, this was ignored and all kinetic measurements were carried out at 380 nm. Constancy of k max in aqueous and surfactant medium also ensures that the product formed is same in both the reaction conditions. It was further verified that in the concentration range studied (10-100 times less than [Ce(IV)]), Mn(II) did not reduce Ce(IV). As such their standard reduction potentials are comparable. [39] To examine the effect of the temperature and obtain the thermodynamic parameters, the oxidation process was carried out in the temperature range of [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46] C both in the pure aqueous and surfactant medium. All the reactions were performed in triplicate and the tabulated rate constants (standard deviation, r is computed and presented in the tables) are the average of these observations. The method of linear regression is followed for the error analysis.

Results and discussion
In pure aqueous medium The kinetics of Mn(II) catalyzed oxidation of iminodiethanol (IME) has been studied spectrophotometrically under pseudo-first order conditions with concentration of IME at least ten times higher than that of Ce(IV). The kinetics is monitored by measuring the absorbance of Ce(IV) at its k max of 380 nm. This is because there is almost no change in the k max value of Ce(IV) is observed when the reactants are mixed ( Figure 1). The oxidation reaction is studied in acidic media and the minimum H þ concentration required is 1 mol dm À3 (sulfuric acid). In the absence of Mn(II), the catalyst, the reaction hardly proceeds as the absorbance at 380 nm stands practically unchanged for hours together.

Stoichiometry
Tenfold excess of Ce(IV) compared to [IME] was equilibrated at 30 C for about eight hours with the experimental conditions. The unreacted oxidant was measured from its absorbance value recorded at its k max of 380 nm. It is worthy to mention here that the protonated form of the iminodiethanol is the reactive species under the acidic conditions. Further, to ascertain the active microspecies of the of IME under the given pH conditions, ChemAxon software was used. Accordingly, the stoichiometry of the oxidation reaction can be represented as

Variation of oxidant
For variation of reaction parameters, (excepting the acid strength), the reactions have been carried out keeping the acid strength at 4.0 mol dm À3 (H 2 SO 4 ). Upon varying the concentration of Ce(IV) from 1 Â 10 À3 to 4 Â 10 À3 mol dm À3 at constant condition of other reaction parameters, the rate of disappearance of Ce(IV) at 380 nm is found to be consistent with the rate Equation (1).
where, k obs is the pseudo-first order rate constant. The first order dependence is evident from good linear plots ( Figure  2, R 2 in the range of 0.9959-0.9979) of log (Absorbance) vs time up to more than three half-lives. Further the k obs values from the gradient of the plots are found to be almost constant with value (9.24360.03) Â 10 À5 s À1 (Table 1) over the entire range of concentration studied.

Substrate variation effect
The oxidation reaction has been studied under varying initial substrate concentration and the rate constant values are presented in Table 1 clearly indicates inverse second order dependence of rate on H þ ion concentration [40,41]

Rate dependence on temperature
The activation parameters that provide much needed useful information about environment in which the reactions take place are determined from Figure 3, the Arrhenius plot (1/T vs logk obs ) for the oxidation of IME at different temperature from 30 to 46 C with R 2 ¼0.9919. The rate constant values increase with rise in temperature ( Table 2). The activation parameters, activation energy (E), enthalpy of activation (DH # ), entropy of activation (DS # ), Gibbs's free energy of activation (DG # ) are presented in Table 3. The activation energy is computed from the slope equation slope ¼ ð À E 2:303ÂR Þ and the pre-exponential constant, A was found from the intercept (¼ log A) of the linear plot in Figure 3.
Eyring's equation, DG ¼ RTln RT Nhk obs is used to calculate the free energy of activation at different temperatures and entropy parameter is obtained from DG ¼ DH À TDS relationship. Highly solvated transition state formation is indicated by the fact that both DH # and DG # are fairly positive.

Influence of solvent composition
The solvent composition is varied by taking mixed solvent of acetonitrile and water in various proportions and its effect on the rate of reaction has been studied. [42] The   Test for free radical and product analysis Formation of free radical as an intermediate has been confirmed by the polymerization test of the reaction mixture with acrylamide. Appearance of silky white precipitate of polyacrylamide when acrylamide solution was added to the reaction mixture ensures free radical formation.
The products of the oxidation reaction are found to be formaldehyde and ammonia. Product analysis is done by carrying out the characteristic tests of aldehydes [43] on the reaction mixture kept overnight at 33 C.

Mechanism and rate equation
The following points have been taken into consideration while proposing the rate mechanism for the oxidation 1. No change in k max value of Ce(IV) is observed when the reactants are added together. 2. The reaction exhibits a clean first order dependence of rate on [IME]. 3. The H þ ion dependence of rate is inverse second order. 4. The entropy of activation is negative pointing to intermediate complex formation. 5. Free radical formation during the course of reaction is ascertained.
On the basis of the above observations, intermediate complex formation involving Ce(IV) is ruled out. However, the variation of rate constant with dielectric constant of the medium suggests formation of intermediate complex involving ions of similar charge. It may be noted that apart from the cationic catalyst and the oxidant, here the substrate also carries positive charge being protonated at the high acid concentration of study. Again, the negative entropy of activation points to intermediate complex formation. Accordingly, the following mechanism is proposed (Scheme 1).
The following rate law derived from the above scheme can be expressed as below.
Ce 4þ þ HSO À 4 K 1 CeSO 4 2þ þH þ , K 1 ¼ 3500 Concentration of different Ce 4þ species in a solution having [Ce(IV)] ¼ 3 Â 10 À3 and [H 2 SO 4 ] ¼ 2.0 mol dm À3 are calculated using equations presenting Ce(IV) complex formation equilibria and Equation (6). The values are approximately [Ce 4þ ] ¼ 4.264 Â 10 À9 , [Ce(SO 4 ) 2þ ] ¼ 1.492 Â 10 À5 and [Ce(SO 4 ) 2 ] ¼ 2.985 Â 10 À3 mol dm À3 . Range of concentration of acid used in the present study and the steep fall in the rate of reaction with increasing concentration of sulfuric acid indicates that [Ce 4þ ] T is most likely present as Ce(SO 4 ) 2 under the experimental conditions although the role of Ce(SO 4 ) 2þ cannot be ruled out. Similar consideration has been reported by other workers also. [40,41] Under such circumstances inverse second order dependence of rate on H þ ion concentration is quite plausible.

In surfactant medium
Oxidation of iminodiethanol in micellar phase has been carried out at different concentrations of sodium dodecyl sulfate starting from below cmc up-to a high concentration at constant conditions of other reaction parameters. Reactions mediated by ionic surfactants are supposed to occur in the Stern layer of the micelle. The nature of the electrostatic and hydrophobic interaction among the substrate and micelle plays a vital role in the admission of the substrate into the micelle. Catalytic effect of micelles on the reaction rate proceeds through attractive electrostatic binding of the substrate molecules to the micelles, subsequent chemical transformation either at the juncture of the aqueous and micellar phase or in the micelle and ultimately phasing out of the products. Prior to the discussion on the mechanism of the electron transfer process in the micellar phase, it is worthy to consider the reactivity pattern in aqueous medium again. The log (absorbance) -time profile was linear and from the slope of this plot, pseudo-first order rate constant (k w ) was computed. The oxidation reaction in the micellar medium retains the same kinetic features as in the aqueous phase, i.e., the reaction is first order with respect to Ce(IV), first order in [iminodiethanol] and shows inverse second order of dependence on [H þ ], etc. Therefore, the mechanism of oxidation of iminodiethanol in micellar phase would broadly remain the same, i.e., a bimolecular transition state of iminodiethanol and Mn (II) would be involved. Further, the temperature effect on the reactions in the microheterogeneous medium was observed by monitoring the reactions at constant [SDS] in the temperature range from 30-46 C. A number of constant concentrations of SDS ranging from 2 Â 10 À3 to 2.5 Â 10 À2 mol dm À3 have been used for the purpose.
Initially, with increasing SDS concentration, the rate constant increases, almost rapidly and this trend continues up to about the designated kinetic cmc. Beyond this kinetic cmc of SDS, there is a rapid fall in rate constant so that the k w -[SDS] profile shows a maximum (Figure 4 and Table  4). The rate constant continues to decrease slowly with increasing [SDS] after the maximum. Increase in temperature increases k w values monotonically and this rise was compounded when there is simultaneous increase in SDS concentration ( Table 4). The maximum value of k w , i.e., the value of k w at around cmc of SDS, also increases systematically with rise in temperature.
Below the cmc of SDS, the k w values increase with [SDS] and this observation is attributed to increasing solubilization of the reactant species which increases with SDS concentration and reaches a limiting value at cmc. The rate enhancement before micellization is regarded as the pre-micellar catalysis. In the post-micellar region, when the number of micelles exceeds that required to bind all of the reactant, there is a dilution of the substrate concentration per micelle with further rise in [SDS] that leads to the rate constant reduction. This results in the formation of a maximum in the plot of k w against [SDS]. Similar observations can be found in literature. [47][48][49][50] The increase in reaction rate at [SDS] < cmc is indicative of the fact that the reactants are incorporated into the micelle. At concentrations below cmc, surfactants existing as monomers can aggregate to form dimers, trimers, tetramers etc. As the [surfactant] reaches to its critical value, i.e., formation of micelle begins, the monomeric form or small aggregates of the surfactant will be in dynamic equilibrium with the micellar aggregates. Small aggregates existing in the pre-micellar stage can interact with the reactant molecules forming active species. [51,52] The rate enhancing catalytic activity in pre-micellar region can be accounted for by the fact that the positively charged iminodiethanol gets associated with the surfactant (ionic) molecules (monomers, dimers, trimers etc.) through electrostatic attraction. The substrate being positioned in the Stern layer of the micelle in association with the SDS head groups facilitate the electron transfer process. The presence of a maximum and subsequent slow fall in the k w -[SDS] profile suggests that most likely here both the iminodiethanol and Ce(IV) are bound to the micelles, but with the preferential hydrophobic or electrostatic forces. So, the phase separation approach of Berezin et.al (Scheme 2), is employed. [53][54][55] Based on the Scheme 2, the rate expression is where k w is the first order rate constant in absence of the micelle and k m ¼ k m /V, V being the molar volume of the surfactant. K S and K O are the binding constants of the substrate (S), iminodiethanol and oxidant (O), Ce (IV) with SDS micelle respectively and C ¼ ([SDS]-cmc). Equation (9) can be rearranged to [10] where the term k m K S K O C has been neglected in comparison to the rate constant in the aqueous medium (k w ). Since the rate enhancement was observed at [SDS] below the literature cmc value (8.1 mM) of SDS (which is evident in presence of electrolytic substances), it was necessary to determine the cmc under the conditions of kinetic runs. The cmc, also regarded as the kinetic cmc [49] in the present case at a particular temperature is the point of intersection of two linear portions in the k w -[SDS] profile below cmc. Figure 4 shows a maximum at around 4 mM and hence the kinetic cmc of SDS was observed to be 4.0 Â 10 À3 mol dm À3 given the conditions of the present case. Again, as C 2 is negligibly small, K S K O C 2 term in Equation (10) can be ignored and is presented as The plot of 1 k W vs C will be a straight line with positive intercept and slope. From the intercepts of such plots ( Figure 5), k w values are computed for the reactions executed at different temperatures and are presented in Table 5 and these values are in close agreement with the k obs values for the reactions in absence of the surfactants. Furthermore, the ratio of slope to intercept ( Figure 5) fetches the binding constant (K S þ K O ). The values of K S þ K O computed at different temperatures (Table 5) are comparable.
The rate retardation in the post cmc region can be attributed to the fact that in the pure micellar medium, partitioning of the iminodiethanol and Ce(IV) takes place effectively to hinder the process of subsequent oxidation reaction. The basic nature of the amino component of the iminodiethanol drives it away from the micellar head, rather it is attracted to the core of the micelle. The anionic micelles here act as a nucleophile or a base to extract H þ ion from the protonated IME. The proton abstraction becomes rather easy given the pK a value of IME (8.9). Once deprotonated, the neutral substrate molecule will be comparatively less attracted by the micellar head. The oxidant, Ce(IV) on the other hand prefers to stay in the Stern layer or at best at the micellar head. This separation between the iminodiethanol and the Ce(IV) on their preference for electrostatic or hydrophobic interaction hinders reaction between them in the micellar phase. Hence the binding constant K S þ K O can be approximated to K S . Further, the K S values are used in Equation (12) to compute the transfer free energy.
The calculated transfer free energy per mole (-Dm 0 ) of the solute from water to the micellar phase [56][57][58] are presented in Table 5. The argument that the substrate is deeply penetrated into the core of the micelle is justified by the high Dm 0 value for the iminodiethanol.

Conclusion
The kinetics of oxidation of iminodiethanol by Ce (IV) is studied both in aqueous and micellar phase. Kinetic data reveals that the reaction involves a bimolecular transition state between the substrate and the oxidant. Fast oxidation of the iminodiethanol in surfactant medium (up to cmc) is the finding in comparison to aqueous medium while cationic and nonionic counter parts are in-effective. However, all the kinetic features like the rate dependence on concentrations of substrate (pseudo first order), acid (inverse second order) and oxidant (first order) has been identical in both the media with comparable thermodynamic parameters. Post cmc, the rate constant in micellar phase shows a steady decreasing trend with rise in surfactant concentration. The basic nature of the amino component of the iminodiethanol and the computed transfer free energy value suggests that the substrate moves to the core whereas the oxidant lies in the stern layer of the micelle. This attributes the mechanistic path-way showing a maximum in k w -[substrate] profile. To sum up the oxidation kinetics for the degradation of the iminodiethanol performed here has been studied extensively in comparison to the previously reported articles [16][17][18][19][20] with similar substrates. Moreover, the work with the surfactant, SDS (can be regarded as a green catalytic material) has not been reported elsewhere.