Application of charged micelle generators on the oxidation of metabisulphite ion by molybdenum-oxime complex: Piszkiewicz kinetic model

The metabisulphite ion is a chemically reactive nucleophile that acts as an inhibitor of microbial development, food spoilage, color enhancement, enzymatic activities, and oxidative processes in biological systems^[1−5]. It has a high probability of affecting the nutritive value of food by interacting with folic acid, thiamine, nicotinamide, and pyridoxal vitamins found in it, hence stimulating its degradation^[6]. Baker et al.^[7] reported that sulfite from the metabisulphite preservative in the 2,6-diisopropyl phenol (propofol) blend generates an oxidative medium when this blending is opened to air, leading to the dimerization and decolorization of the product, which was attributed to the swift development of the reactive sulfite radical. The study of the possible toxic effects of Na₂S₂O₅ on human foreskin cells in vitro revealed that there was a reduction in cell viability on varying the concentration of the antioxidant and an increment in the reactive oxygen species levels in the treated cells. Indicative of significant exertion of toxic effects on human cells via several mechanistic routes such as atom transfer, electron transfer, atom and electron transfer, free radicals facilitated, and intermediates promoters' routes by the sodium metabisulphite^[8].

Zohra et al.^[9] documented the consequence of subchronic incorporation of Na₂S₂O₅ on lipid peroxidation, protein, and enzymatic antioxidants in the gastric tissue and splenic Wister rat. It was observed that the subchronic intake of Na₂S₂O₅ had a hostile consequence on the spleen and stomach by initiating oxidative impairment, leading to an acceleration in lipid peroxidation and an adjustment of enzymatic activity. The effect of a 28-amino acid acylated peptide esterified with octanoic acid on Ser 3 (Ghrelin) given to the Wister rat on sulfite initiated oxidative and apoptotic change in its intestinal mucosa, resulting in a notable decrease in intestinal total oxidant status. The sodium metabisulphite initiated a notable rise in the total oxidant status and number of apoptotic cells. The action of Ghrelin in the process reduced the number of apoptotic cells^[10]. The influence of Na₂S₂O₅ as an antioxidant in overwhelming the drug release change from aged polyox tablets was reported. It was observed that the presence of Na₂S₂O₅ steadied the release of the drug from polyethylene oxide (PEO) matrices, and the absence of it caused a surge in drug release from polyox matrices when stored at 40 °C. Thus, the concentrations of Na₂S₂O₅ investigated were able to restrain the structural alterations of polyox samples and steadied the drug release^[11]. Because food additives are liable to initiate allergic reactions that implicate an immune mechanism, there is a need to have a detailed understanding of their redox behavior and mechanistic routes in a regulated medium.

In addition, a molybdenum framework with bismuth has been explored as a catalyst in the photo-degradation process of methylene blue dye (MBD), as convened by Alahmadi et al.^[12]. The team asserted that the Bi-Mo nano-framework (Bi₂O₃/MoSe₂) enhanced the MBD degradation more than the individual species (Bi₂O₃ and MoSe₂) due to its improved surface area and crystallinity, which is characterized by smaller particle size.

The study of the effect of charged micelle generators on a reaction medium is key to understanding the realities of solvent effect on the rate of reactions. The micelle media are sustainable, recoverable, and have a direct drive on reaction rate, and they position surfactants as a more reliable medium than organic solvents might take part in the reaction by oxidizing or reducing themselves^[13−15]. CTAB and SDS with cationic and anionic polar heads respectively, have demonstrated useful interaction/binding traits with substrates. Quantitatively, their influence on the reaction rate has been determined using Piszkiewicz's, Berezin's, and Pseudophase ion-exchange models. The models presume the distribution of the substrates over the whole bulk of the micelles^[16−18]. However, Srivastava and coworkers^[19] pointed out the non-supportive character of CTAB and SDS in the oxidation of acid-red-13 dye with peroxydisulphate and neutrality of triton X-100 on the catalysis of the dye degradation process was obvious. In the oxidation system of l-phenylalanine with chromic acid, triton X-100, and SDS facilitated the oxidation process, whereas CTAB dragged the reaction rate due to the charge state of the substrates at the slow step^[20]. In the same vein, CTAB elevated the oxidation of l-phenylalanine with Fe(III)-cyano complex, which was attributed to hydrophilic supremacy^[21]. In a system where opposite charge species dominate such as oxidation of l-leucine with Fe(III)-cyano complex, CTAB also adequately accelerated the process and SDS championed an opposite influence^[22]. A similar observation was reported in the oxidation of L-glutamic acid with the complex^[23]. On the other hand, The analogue of CTAB, cetylpyridinium chloride was employed to accelerate the oxidation of l-phenylalanine with Cu(III)-periodate complex and the impact was massive with four-fold increment in reaction rate compared to surfactant-free system^[24]. The same analogue of CTAB replicated the influence on the oxidation of l-glutamic acid with Fe(III)-cyano complex and impediment was noticed at high concentrations of the surfactant due to ion saturation^[25].

The impact of molybdenum, as a constituent of nitrogenase enzymes, on the oxidation of nitrogen to ammonium during nitrogen fixation in plants, and the alteration of ruminant animal skin color when it is in excess holds promise for a better understanding of its redox mechanistic route^[26]. Hence, a need to have detailed information on the mechanistic pathway of their electron transfer reaction and quantitatively determine the micelle-substrates hydrophobic and hydrophilic effects.

Sigma Aldrich, Germany: syn-2-pyridinealdoxime (99% pure), cetyltrimethylammonium bromide (99% pure), molybdenum(IV) oxide (99.9 % pure), and sodium dodecyl sulfate (99% pure) were used. Na₂S₂O₅ (97% pure), HCl (37% pure), NH₄Cl (99.5% pure), HCOONa (98% pure), SnCl₂ (97% pure), K₂Cr₂O₇ (99.5% pure), BaCl₂ (99% pure), H₂SO₄ (98% pure), KSCN (98% pure), NaCl (99.9% pure), ethanol (95% pure), methanol (99.5% pure), acrylamide (98.5% pure), sodium ascorbate (98% pure), sodium nitroprusside (98% pure), and diethyl ether (99% pure) were purchased from Fisher Chemical (Nigeria).

Synthesis of molybdenum-oxime complex

The approach by Konidaris et al.^[27] was engaged in the synthesis of the MOC and the synthetic detail is recorded in our previous work on the redox reaction of allylthiourea with MOC^[18], and it was characterized with an FTIR Spectrometer (Model-8400S, Shimadzu), UV spectrometer (Cary 300 Series), and conductivity meter (HACH Sension5).

Rate measurement

The reactant's mole contribution was learned as described beforehand^[28−30] using photo-titrimetric at a constant electrolyte concentration and temperature. The point of interpolation on the display of absorbance against mole proportionality signaled the reaction stoichiometry.

A 721 PEC Medical ultraviolet-spectrometer model was involved in the kinetic surveys by noting the drop in concentration using change in absorbance of the molybdenum-oxime complex following the pseudo-first-order protocol with [S₂O₅²⁻] in a ten-fold multiple over the [MOC]^[31−38]. The adjustment in the salt concentration (μ), medium polarity (D), and the [H⁺] involvement in the oxidation was probed by varying one at a time while upholding the other parameters at a fixed amount^[39−42]. The observed rate constant (k_ob) was assessed from the slope of the relationship between InA vs t [Eqn (1)], and the rate constant (k₂) was secured from the segmentation of k_ob with [S₂O₅²⁻] [Eqn (2)]. Discovery of free radicals was made possible by the inclusion of acrylamide solution (0.15 cm³) and methanol (5 cm³) in the reaction in progress^[43−46].

Micelle formation and kinetic model

Micelle formation was verified via the critical micelle concentration, CMC, of the surfactants, which was obtained by measuring the specific conductivities of various [surfactant] solutions. The conductivity meter was attuned using 0.02 molar sodium chloride. The interjection point in the conductivity-concentration graph was considered as the CMC^[47,48]. Piszkiewicz’s model was explored following its scientific equation [Eqn (3)].

Thus, K_D is the detachment constant of the micelle back to its free components. k_w and k_m are the reaction rate constants in the absence and presence of surfactant accordingly. n is the number of surfactant molecules (D) and k_ob is the pseudo-first-order rate constant. [D] is the surfactant concentration^[14,49].

The UV/visible spectrophotometer of the MOC produced a maximum wavelength of 560 nm. The FTIR spectra of the oxime-ligand and the MOC are presented in Figs 1 and 2, respectively. The molecular vibrations of the MOC and ligand, and the conductivity measurement of the MOC are presented in Table 1.

Figure 1. 

Oxime-ligand FTIR spectrum.

Figure 2. 

MOC FTIR spectrum.

The observed molar conductivity of 133 S cm²·mol⁻¹ (Table 1), depicts the electrolytic nature of the MOC in solution^[50−52].

The band observed at 3,075.1 cm⁻¹ from the FTIR spectrum of the ligand (Fig. 1) is assigned to C-H stretching and 1,654.9 cm⁻¹ band is assigned to C=N stretching, which shifted to 1,300.7 cm⁻¹ in the MOC as a result of the C-N vibrational mode (Fig. 2). Indicative of complexation of the imine group with the central metal ion. The bands observed at 767.1 and 745.5 cm⁻¹ in the spectrum of MOC are assigned to Mo=O stretching, indicating symmetrical and anti-symmetrical vibrations of MoO₂ with a cis-MoO₂ orientation, as a trans-MoO₂ component would typically show one Mo=O band due to the asymmetry stretch, while that at 584.7 cm⁻¹ is assigned to Mo-N stretching^[53].

Stoichiometry

The resultant mole proportionality describes a two-electron contribution from a mole of the S₂O₅²⁻ ion to the 2 moles of the MOC (Eqn. (4) as shown in Fig. 3). The place of interjection in Fig. 4 establishes the number of moles implicated in the reaction.

Figure 3. 

Stoichiometric equation of the reaction of MOC and metabisulphite ion.

Figure 4. 

Graph of Abs vs mole ratio for the oxidation of S₂O₅²⁻ by MOC. Condition: [MOC] = 1.9 × 10⁻³ mol·dm⁻³, [S₂O₅²⁻] = (0.4.18 – 95.0) × 10⁻⁴ mol·dm⁻³, [H⁺] = 0.02 mol·dm⁻³, T = 301 K, λ_max = 560 nm.

The involvement of an unequal mole of the reactants results in the realization of SO₄²⁻ ion, SO₂, and Mo²⁺ products, which are confirmed classically. The appearance of an insoluble white precipitate on the addition of BaCl₂ solution and in excess HCl confirms the presence of SO₄²⁻ ion^[54]. The change of the damp filter paper dipped in acidified potassium dichromate from orange to green indicates the presence of SO₂^[55]. The formation of a red color on the addition of three drops of KSCN solution (0.3 M) and two drops of acidified SnCl₂ solution (0.14 M) infers the manifestation of the Mo²⁺ ion product [Eqn (4) shown in Fig. 3]^[56].

Kinetic investigations

The probing of the observed rate constant, k_ob, through the graph that relates absorbance with time (Fig. 5), the output discloses a straight path that authenticates an oxidation with a single-order in the MOC concentration. The incline (1.0019) of the curve of log k_ob against log [S₂O₅²⁻] (Fig. 6) also suggests a single-order in the [S₂O₅²⁻]. Protonation of the S₂O₅²⁻ ion [Eqn (5) shhown in Fig. 11] is accommodated in the process, which necessitated an increase in the rate of the reduction-oxidation process as the [H⁺] is amplified (Table 2). The redox-rate is accelerated as the electrolyte concentration in the system is adjusted, originating from the collision of identical-charged species at the slow step [Eqn (6) shhown in Fig. 11], and this is reinforced by the deceleration of the oxidation speed on the change in the reaction system dielectric constant from 77.2–71.2 with ethanol (Table 3). There is a manifestation of the association of similar charged moieties at the slow step when the oxidation of S₂O₅²⁻ ion is catalyzed and inhibited by the inclusion of NH₄⁺ and HCOO⁻ ions, respectively (Table 4), standing as an indicator of an interaction involving an outer sphere mechanism. However, the generated sulfite radicals were essential in the formation of sulfur dioxide and reduction of the MOC [Eqn (7) shown in Fig. 11].

Figure 5. 

Typical pseudo-first order plot for the oxidation of S₂O₅²⁻ by MOC. Condition: [MOC] = 1.9 × 10⁻³ mol·dm⁻³, [S₂O₅²⁻] = 9.5 × 10⁻² mol·dm⁻³, [H⁺] = 0.02 mol·dm⁻³, T = 301 K, λ_max = 560 nm.

Figure 6. 

Graph of logk_ob against log[S₂O₅²⁻] for the oxidation of S₂O₅²⁻ by MOC. Condition: [MOC] = 1.9 × 10⁻³ mol·dm⁻³, [S₂O₅²⁻] = (1.9 − 11.4) × 10⁻² mol·dm⁻³, [H⁺] = 0.02 mol·dm⁻³, T = 301 K, λ_max = 560 nm.

Relatively, the metabisulphite oxidation with hexacyanoferrate(III), HCF, in buffer system convenes inverse [H⁺] impact on the rate constant with the generation of hydrogen sulfite and sulfite radicals that recombined to form dithionate ions in a fast mode. Oxidation was favored by high solvation of active ion species, which made the outer coordination of HCF a center for interaction and implicated an outer-sphere mechanism, as supported by a large entropy of activation (−205 J·K⁻¹·mol⁻¹)^[57].

Kinetically, the intermediate formation is confirmed by using a Lineweaver-Burk plot, a modification of Michaelis-Menten plot (Fig. 7), where lack of intercept is an indication of non-availability of intermediate species and vice versa. Thus, the structural integrity of the complex remains intact due to the stability emanating from the crystal field stabilization energy, the metal ion, and the strong field ligand at an average temperature.

Figure 7. 

Lineweaver-Burk plot of Michealis-Menten’s plot modification for intermediate determination.

Effect of micelle generators

The CTAB and SDS micelles generation in H₂O:ethanol mixture at 301 K is confirmed from the point of interjections (32.0 × 10⁻⁴ and 90.0 × 10⁻⁴ M, respectively) at the conductivity-concentration graphs (Figs 8 and 9), which depict the concentration at which surfactants aggregate to form micelles. Comparative investigation showed that in the presence of 0.1 M KNO₃, CMCs of 4.0 × 10⁻³ and 7.0 × 10⁻⁴ M were reported for SDS and CTAB, respectively. The presence of salt concentration in surfactant solutions was attributed to the decline in the CMCs^[58]. Wei et al.^[59] reported CTAB CMC of 9 × 10⁻⁴ and 0.24 M in water and ethanol, respectively at ambient temperature. Thus, CMC decreases by shrinking the repulsive action between the charged-head groups of the surfactant monomers and it works against micelle formation, making micelles form at a reduced surface-active agent concentration^[36].

Figure 8. 

Specific conductance-concentration curve for CTAB.

Figure 9. 

Specific conductance-concentration curve for SDS.

The influence of cetyltrimethylammonium bromide on the oxidation rate of the S₂O₅²⁻ ion by which the Piszkiewicz model is examined illuminates a substantial binding (K_D⁻¹) and cooperativity (n > 1) prevailing between the substrate and micelle, which results in an increased oxidation rate. This effect can be attributable to the associative mode that is assumed by the CTAB's polar head with the charged MOC on the Stern stratum of the CTAB aggregates, which is well-thought-out to possesses a great ionic concentration of the CTAB and reduced polarity. The catalytic micelle – substrate model of Piszkiewicz's concept based on enzymatic bimolecular oxidation of metabisulphite ion by MOC is presented in Fig. 10, wherein M²⁻ is the MOC, Kn' is the association constant of the extra interactions, and n' is the extra number of CTAB monomers. The undeviating minimum-squares style is employed in estimating the factors of Eqn (3) (Table 5).

Figure 10. 

The catalytic micelle - substrate model of Piszkiewicz's concept.

Furthermore, the result of SDS on the oxidation speed of S₂O₅²⁻ ion expresses weak ionic and non-ionic collaborations between the SDS and the S₂O₅²⁻ - MOC at the Gouy-Chapman district of the micelle. This is perhaps connected to the association of similar charged moieties at the speed-limiting phase [Eqn (6) shown in Fig. 11], indicating that the S₂O₅²⁻ - MOC moieties associated inadequately with the SDS aggregates that occasioned an unfavorable collision between them. The control of the inhibitory effect above the catalytic action of SDS on the oxidation rate is strengthened by a lower associative constant (1.1951) of the micelle with the substrates compared to the associative constant (1.6355) of the CTAB micelle with the substrate. The stratum of this interaction can be assumed to be a district of lower ionic concentration and lower aggregate separation. Thus, CTAB and SDS increase and decrease the rate by 7-fold and 8-fold, respectively, compared to aqueous: ethanol medium and the result recommends that Piszkiewicz’s approach is pertinent in this redox route. The usage of the model gives substantial evaluation coefficients (r²) and the k_m is higher than the corresponding k_w value, which marks the model as effective.

Owing to the above kinetic evidence, a mechanism without an intermediate is proposed [Eqns (5)−(7)] as offered in Fig. 11.

Figure 11. 

Simple redox routes of MOC and S₂O₅²⁻.

inserting Eqn (9) into (8);

where k = Kk₁[H⁺] and the rate law support the first-order actualization in the redox moieties and a single acid-dependent route. The rate law derived from the mechanism underpins first-order kinetics with respect to [MOC] and [S₂O₅²⁻] and proton assistant in the reaction, reinforcing the generated kinetic data.

The application of charged micelle generators on the oxidation of the metabisulphite ion by MOC uncovers a 2:1 stoichiometry and kinetics of first order in [S₂O₅²⁻] and [MOC]. The realization of sulfur dioxide product is moved by the involvement of sulfite radicals. The variation of salt concentration within the system progression incited a positive kinetic-salt-effect that describes a redox reaction stemming from like-charged molecules at the speed-limiting level of the oxidation, and the fall in the system dielectric constant retarded the rate. The catalysis initiated by the counter-ion (NH₄⁺) is possible due to the involvement of negatively-charged moieties at the slow phase. The Piszkiewicz quantitative data generated exposes the great role of cationic micelles in the reaction, with a compelling binding constant that credits the impact observed in the process. The overlap of redox groups with the CTAB and SDS micelles implicated electrostatic and hydrophobic interactions as uncovered by the observed cooperativity-index. Piszkiewicz’s model is appropriate in this study as it aids in informing the underlying forces of the reaction in a medium-dictated system.