Introduction

Oxyanions with sulfur have a rich redox activity in cell metabolism and regeneration. Research have shown that their reactivity is medium sensitive as their antioxidant property is concerned1. The dithionite ion is a good reducing agent with an S–S linkage that has application in the pulp and paper industry. The rich chemistry of dithionite ion could be connected to its sulfur to sulfur linkage that is capable of promoting electron cloud within its host or system2. In the view of the above, some studies have been done to kinetically probe its reactivity. Pal and Gupta3 observed that hydrogen ions had a limiting impact on the rate of the reduction of hexachloroplatinate(IV) with dithionite over the range of pH studied. The change in solution concentration and relative permittivity increased the rate with a negative entropy, and the participation of free radical was positive. Ukoha et al.4 noted nonexistence of catalysis with added cation and anion, an increment in redox rate with a rise in [H+], and an invariant in reaction rate of reduction of adipato bridge iron(III)-salen complex with S2O42− as ion concentration and system medium permittivity varied. Gupta et al.5 recorded free radical generation and intermediate complex formation, and S–S bond cleavage to give 2HSO3− ion for the reduction of tris-(pyridine-2-carboxylato)manganese(III) by dithionite in sodium picolinate-picolinic acid buffer medium.

The chemistry of complexation of molybdenum from higher to lower oxidation states is of great interest due to its role as enzymes in nitrogen metabolism and as a vital micronutrient in lower animals. It can be found in soybeans, chicken liver, cow milk, rabbit liver, hog liver, bovine liver, azotobacter, and coli in different forms of enzymes6. We aim to probe the likely roles of molybdenum in enzymes by building a model system that will give insight into some of the properties of molybdenum in enzymes and generate kinetic data that will explain its redox property. Also, the understanding of its redox reaction in a regulated ion-medium will be a plus to its chemistry.

An anionic surfactant (sodium dodecyl sulfate, SDS) has applications in many cleaning and hygiene products and is frequently used as a constituent for solubilizing cells throughout RNA and/or DNA isolation, as well as for denaturing proteins7. Its application in chemical industries is centred on the catalysis of organic and inorganic molecular reactions due to ionic interaction between substrates or intermediates (cationic species) and the surface of the micelle, resulting in layer contact’s increase among the containing reactant molecules. Its aggregates inhibited the oxidation of DL-aspartic acid slightly8, acetophenone9, dextrose10, and cyclopentanone11. The Piszkiewicz model assumes a catalytic micelle generated by surfactant monomers and its treatment in redox reactions is used to check the dependency of the rate constant on the surfactant concentration and is useful in examining reaction mechanistic routes12. The enhancement of the reaction rate depends on increased local concentration of the reactants at the surface and in the interior of the micelle, stabilisation of the transition state of the reaction, and medium effects (polarity, microviscosity, and charge effects inside the micelle)13. The applicability of the Piszkiewicz model to divers substrate–micelle systems has been reported. The oxidation of thioglycolic acid with salicylidene-manganese(III) complex in the presence of SDS showed that the existence of pre-micellar composites and hydrophobic interaction contributed positively to the redox process14. The fading of nitrophenol violet with hydroxide ions in the presence of cationic and anionic surfactants revealed nitrophenol violet’s responsiveness for investigating nucleophilic attack systems sponsored by electrostatic phenomenon15,16. Also, the oxidation of methyl violet with hydroxide ions in the presence of cationic and anionic surfactants showed that high binding affinity which is provided by electrostatic and hydrophobic interactions assisted the degradation of the dye12.

The resultant SDS’s influence on the reaction kinetics is quantitatively determined with the Piszkiewicz kinetic model that shows the extent of interaction between the micelle and the substrates17. This model will further reveal the cooperativity status and binding affinity of molybedenum and sulfur-containing reductant with the aid of sodium dodecyl sulfate.

Experimental

The synthesis and characterization of the bis-(2-pyridinealdoximato)dioxomolybdate(IV) ion ([MoIVO2(paoH)2]2−) was achieved by engaging the synthetic approach of Konidaris et al.18 and fourier transform infrared spectrophotometer (FTIR-8400S Shimadzu, Double Beam) and ultraviolet–visible (Cary 300 Series UV–Vis Spectrophotometer, Agilent Technologies, USA) correspondingly. The complex ion was synthesized using 2-pyridinealdoxime and molybdenum(IV) oxide gotten from Sigma Aldrich, Germany. Dithionite ion and hydrochloric acid obtained from Merck were used as a reductant and proton (H+) generator respectively. Change in the influence of salt was investigated by the use of NaCl obtained from the British Drug House, Nigeria. Variation in the reaction medium permittivity was carried out with ethanol (BDH, Nigeria). Sodium dodecyl sulfate (Sigma Aldrich, Germany) was used to generate a micelle in the reaction system. The counterion catalysis was studied by using ammonium chloride and sodium formate gotten from British Drug House, Nigeria. Methanol with acrylamide (Merck) was used to check the involvement of unstable molecules with unpaired electron in the redox system. Filter paper, K2Cr2O7 (Merck), H2SO4 (BDH), SnCl2 (BDH), and KSCN (Merck) were used in the product examination.

The spectrophotometric approach of mole ratio analysis was applied to the stoichiometry of the reaction as archived earlier19,20,21 at stable temperature (303 K), salt effect, and 560 nm. The breaking position on the curve of Abs versus mole ratio was gotten from the absorbance recorded within 600 min. The product mixture that gave the breaking point was used to classically examine the compounds/ions formed. The appearance of molybdenum(II) ion at the end of the oxidation–reduction process was determined by adding 0.1 cm3 of concentrated H2SO4, heating to a thick white haze, and cooling. Following that, 0.5 cm3 distilled H2O, 0.5 cm3 KSCN (1.0 mol dm−3), and one drop of acidified 0.25 mol dm−3 SnCl2 solutions were added22,23. The presence of SO2 was also tested by dipping a filter paper into an acidified 0.5 mol dm−3 K2Cr2O7 solution and then transferring it into the solution of the product mixture23.

The order with respect to the concentrations of the redox partners was determined from rate data obtained from the variation of absorbance with time as the concentration of the reaction mixture decreased using an ultraviolet visible spectrophotometer (Model 721 PEC Medical) at a 560 nm absorption maxim24,25,26. The investigation of change in salt effect (µ), concentration of acid, and reaction medium permittivity (D) was done by varying them while retaining the concentration of other parameters27,28,29. The slope of the graph of lnA against time was used to calculate the 1st order kinetic constant, and the arithmetic ratio of kobd with [S2O42−] was used to calculate the 2nd order rate constant (k2). The contributions of unstable molecules in the redox process were studied by introducing 0.4 cm3 acrylamide to the reacting mixture with surplus methanol30,31. Thermodynamic parameters were investigated by varying the temperature of the reaction mixture with Grant JB1 thermostated H2O immersion, and the data generated was evaluated with Eyring approach to evaluating activated parameters32,33,34. The surfactant effect on the reaction rate was explored quantitatively by using the Piszkiewicz equation (Eq. 1).

$$k_{{obd}} = \frac{{k_{M} \left[ D \right]^{n} + ~k_{W} K_{D} }}{{\left[ D \right]^{{n~}} + ~K_{D} }}$$
(1)

where km and kw represent the kinetic constants in the presence and absence of surfactant correspondingly. n represent the amount of micelle (D) and KD denote the detachment constant of the surfactant molecules to their unrestricted state. [D] denote the [surfactant] while [D]n is obtained by subtracting critical micelle concentration (CMC) from [D]. kobd designate the 1st kinetic constant12,15.

Results and discussion

The relative mole ratio of the reacting redox species proceeds with a double-electron exchange from a single mole of the dithionite ion to a double mole of the molybdenum complex. It is supported by the mole ratio (2) breaking point in Fig. 1.

Figure 1
figure 1

Reaction stoichiometry of [MoIVO2(paoH)2]2− and S2O42− at [MoIVO2(paoH)22−] = 13.0 × 10–4 mol dm−3 and T = 303 K.

Full size image

The consumption of one mole of dithionite ion by a double mole of the Mo(IV) complex resulted in the generation of sulfur dioxide and molybdenum(II) products. The change of the damped paper submerged in acidified K2Cr2O7 from orange to green points to the presence of SO2. The identification of molybdenum(II) ion at end of the redox activity is demonstrated by the formation of a red color after the inclusion of 0.5 cm3 KSCN (1.0 mol dm−3) and a single drop of acidified SnCl2 (0.25 mol dm−3), among other reagents (Eq. 2).

(2)

The order assumed by the complex and the dithionite ion concentrations are determined from the 1st order kinetics graph and the logarithm graph of the 1st kinetics constant with the concentration of the dithionite ion as shown in Figs. 2 and 3 respectively. The linearity of the pseudo-first graphs and the slope (0.9888) of the logarithm plot of kobd with [S2O42−] suggested a first order in [complex] and [S2O42−] respectively. The consistency of k2 in Table 1 with respect to the change in the concentration of S2O42− ion reinforced the first order in [S2O42−]. Also, the increase in acid concentration favored the protonation of S2O42− ion that portrayed a single acid dependent pathway. The observed acid catalysis eases the electron transfer by adequately polarizing the medium and providing an efficient link for interaction of the reactants. Thus, the oxygen exchange between dithionite ion and water is faster due to an enhanced reactivity of protonated dithionite ion35.

Figure 2
figure 2

Nonlinear least square graph of InA against time for the reduction of [MoIVO2(paoH)2]2− by S2O42−.

Full size image
Figure 3
figure 3

Logarithm plot of kobd against [S2O42−] for the reduction of [MoIVO2(paoH)2]2− by S2O42−.

Full size image
Table 1 Kinetics constants for the reduction of [MoIVO2(paoH)2]2− by S2O42−.
Full size table

The detected rise in reaction rate with change in salt effect on the reaction medium as shown in Table 1, suggests a positive salt effect that brings about the coming together of the negatively charged-like redox species28 (Eq. 6). The presence of dissolved charged ions in the reaction medium contributed to the electrolytic nature of the system that aided the acceleration of the reaction rate. This observation, is maintained by the reducing of reaction rate on the decrease in the medium polarity (permittivity) as shown in Table 2. The decrease in the medium permittivity decreases the charge to charge ratio interaction by making them less mobile in the system24.

Table 2 Effect of system permittivity on the reduction rate of [MoIVO2(paoH)2]2− by S2O42−.
Full size table

Counterion catalysis emanating from the introduction of ammonium ion into the reaction medium is notable for the reaction rate, suggesting the reactivity of like-charged species at the equilibrium collision state14,30 (Eq. 6). Meanwhile, the addition of formate ion inhibited the kinetic rate due to repulsive electrostatic interaction between the redox species (Table 3). The polymerization observed in the reaction system on the addition of monomeric acrylamide with excess methanol suggests the production of sulfite radical35 which was necessary in the reduction of the complex as displayed in Eq. (8).

Table 3 Effect of counterions on the reaction rate for the reduction of [MoIVO2(paoH)2]2− by S2O42−.
Full size table

The thermodynamic study of the reaction shows that the reaction rate was rapidly accelerated as temperature was elevated (Fig. 4) and it portrays a reaction where there is an associative route due to the mutual ordering of solvated molecules at the rate-determining step as shown by the value of ΔS‡ = − 36.25 \(\pm \hspace{0.17em} 0.08\) JK−1 mol–1. The observed ΔH‡ and ΔG‡ (69.12 \(\pm\) 005 and 80.10 \(\pm \hspace{0.17em}0.\)07 kJ mol−1) suggest a reaction scheme where there is a little or no energy loss during the associative processes that result in the formation of the energized activated complex31 (Eq. 6).

Figure 4
figure 4

Rate dependent on temperature plot for the reduction of [MoIVO2(paoH)2]2− by S2O42−.

Full size image

The SDS influence on the observed kinetic rate was negative as the redox partners are greatly influenced by repulsive electrostatic attraction in the presence of surfactant aggregates. This effect was evaluated by using the Piszkiewicz kinetic model that showed an effective binding (1/KD) and positive cooperativity (n > 1) between the substrate and the surfactant aggregates, leading to a deceleration in the reaction rate (Fig. 5). The binding and cooperativity of the surfactant with substrates was endothermically controlled and this could be attributed to the repulsive state of the negative charged SDS’s head with the negative charged substrates at the micelle's Palisade-Stern layer, which is thought to contain an intense molar [SDS] with little polarity. Hence, the probable location of the reaction is at the Gouy-Chapman layer (where counterions are unbound) with less repulsion effect7,12,15,36. The catalytic micelle—substrate model of Piszkiewicz’s approach for the reaction of molybdenum(IV) complex with dithionite ion is shown in Scheme 1. M2− is the complex ion, Kn’ is the association constant of the additional interactions, and n’ denote the additional number of SDS molecules. The parameters of Eq. (1) were obtained using a nonlinear least-squares concept as presented in Table 4.

Figure 5
figure 5

Plot of kobd against [SDS] fitted into Piszkiewicz’s equation for the reduction of [MoIVO2(paoH)2]2− by S2O42−.

Full size image
Scheme 1
scheme 1

The micelle—substrate model of Piszkiewicz’s approach.

Full size image
Table 4 Parameters of Piszkiewicz model.
Full size table

The model (Scheme 1) depicts the mode of responsiveness of the surfactant with the substrates, wherein the complex comes into contact with the micelle to form a complex-micelle energized molecule in an equilibrium state. On the addition of the dithionite ion into the reaction system, the complex-micelle energized molecule interacts with it through the hydrophobic tail of the micelle, and hence, a redox process is executed among them. This process can proceed further by adding more surfactant and substrates to the reaction system.

Scheme 2 describes the pseudo-phase reaction environment with an interface of micelle and water molecules. The interaction of Mo(IV) complex and S2O42− at the micelle environment is accompanied by km, which is driven by the hydrophobic effect, and the aqua environment is associated with kw, which is ion-dependent (electrostatic effect)7,12,15.

Scheme 2
scheme 2

Piszkiewicz pseudo-phase reaction environment.

Full size image

Comparatively, the investigation of SDS micelle’s effect on the oxidation of L-lysine with AuCl3(OH)− showed a pronounced inhibition on the oxidation rate. It was proposed that lysine molecules were linked to surfactant premicellar aggregates, and electrostatic repulsion occurred between SDS and Au(III), limiting redox species closeness in the Stern layer of the micelle37.The presence of SDS on the reaction of tris-(2,2′-bipyridyl)iron(II) and azidopentacyanocobaltate(III) complexes resulted in retardation of the reaction rate and the Piszkiewicz model reveals a good cooperativity pattern (n = 2.184) and poor binding of the reactant molecules on the micelles36. The influence of SDS on the oxidation of diphenyl sulfide with hydrogen peroxide enhanced the reaction rate, resulting in a sulfoxide product38.

On the basis of above data, the below reaction Scheme 3 is proposed for the reaction (Eqs. 5–8) which is analogue to the outer-sphere mechanistic route according to Taube mechanistic approach:

Scheme 3
scheme 3

Elementary redox route of [MoIVO2(paoH)2]2− and S2O42−.

Full size image

Conclusions

The application of Piszkiewicz model on the electron transfer reaction of dithionite ion and bis-(2-pyridinealdoximato)dioxomolybdate(IV) complex was studied. The reaction featured a stoichiometry of 2:1 (complex: S2O42−), signifying that the dithionite ion is a two-electron donor with a reactive sulfite radical specie. First order molecularity was observed in the concentration of the oxidizing and reducing agents. Acid and counterion catalysis prevailed on the reaction rate. The primary salt effect was in operation with an increment in the reaction rate and the medium permittivity buttressed the vitality of dissolved charged molecules in the reduction system for enhancing the kinetic rate. The activation parameters (ΔS‡, ΔH‡, and ΔG‡) reveal that the reaction is endothermically controlled with the establishment of an ordered activated complex. The anionic surfactant interacted with the substrates, leading to a drop in the kinetic rate. This effect could be attributed to the unbounding of counterions at the Gouy-Chapman region of the SDS aggregates. The binding constant (1.442) and cooperativity index (2.3293) obtained from Piszkiewick’s equation supported the resultant influence of the anionic surfactant on the kinetic rate. The observed reactivity between the negatively charged redox species in the presence of anionic surfactant reveals that the reaction is also electrostatically controlled. The observed electrostatic interactions have a vital significance in the assembling of the redox species despite the medium effect that may be in place. Repulsive electrostatic interaction prevailed over the hydrophobic interaction which led to reactivity in the reaction system and eventually stopping the reaction at a high concentration of the surfactant. The outcome of the study reveals the systematic bimolecular redox processes that take place in biological and chemical medium-sensitive systems. The Piszkiewicz model treatment fits the experimental data in showing that the micellar rate enhancement is due to the concentration of both redox species at the micelle-water interface.