Influence of mixed 2-thiocytosine–ionic surfactants adsorption layers on kinetics and mechanism of Bi(III) ions electro reduction: use of the nanostructured R-AgLAFE

Organic substances can inhibit, accelerate or have no effect on the electrode process (Ikeda et al. 1984; Souto et al. 1986; Dalmata 2005; Nosal-Wiercińska 2010a; Nosal-Wiercińska and Kaliszczak 2021).

Studies of the mixed adsorption layers in terms of their influence on the kinetics of depolarizer electro-reduction (Nieszporek 2011; Nieszporek and Dagci 2014; Nosal-Wiercińska et al. 2021b) have indicated changes in the dynamics of this process in the presence of a catalyst substance and a potential inhibitor. A qualitative estimation of the changing effect of TC and surfactants on the kinetics of the analyzed electrode process was confirmed using DC, SWV and CV voltammetry.

The presence of 2-thiocytosine influences the magnitude of the limiting current (Fig. 1a, b). The slope of the polarographic waves also increases, which indicates an increase in the rate of the Bi(III) electro-reduction process in the presence of the organic substance (Nosal-Wiercińska 2014). On the other hand, the addition of CTAB or SDS surfactants into the basic electrolyte solution containing TC, indicates differences in the DC curves image. The presence of CTAB does not affect the magnitude of the limiting current significantly (Fig. 1), whereas SDS (Fig. 1b) causes definite changes in the magnitude of the limiting current, especially above the concentration of 5·10−5 mol·dm−3. The indicated concentration was assigned a critical micellization concentration (Martyna et al. 2022). The determined values of (\(_})\) using the Ilkovič equation (Kaliszczak and Nosal-Wiercińska 2018) confirm the changes in solution viscosity only in the presence of SDS and above the determined CMC (Koundal et al. 2022; Refay et al. 2022). An increase in the slope of the DC wave in the presence of CTAB (at a constant TC concentration) indicates an increase in the reversibility of the electrode process, while SDS does the opposite (Fig. 1a, b) (Nosal-Wiercińska 2014). The presence of CTAB in a 1 mol·dm−3 chlorate(VII) solution containing 1·10−3 mol·dm−3 Bi(III) increases the reversibility of Bi(III) ions electro-reduction. SDS, on the other hand, causes the opposite effect, the peaks decrease, above the concentration of 5·10−5 mol·dm−3 (critical micellization concentration) they are deformed, due to the blocking of the electrode surface by formed hemimicelles (Nosal-Wiercińska et al. 2018).

Fig. 1figure 1

a DC curves of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and with the influence of CTAB addition; b DC curves of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and with the influence of SDS addition

The same changes in the reversibility of the electro-reduction process of Bi(III) ions resulting from the image of SWV curves, for the solutions containing only surfactants can be observed (Fig. 2a–d). SWV voltamperograms confirm the effect of surfactants on the reversibility of the electrode process. The deformation of SWV peaks above a concentration of 3·10−5 mol·dm−3 for SDS was observed (Fig. 2b). The indicated concentration was attributed to the critical micellization concentration (Martyna et al. 2022) and the inhibition of the irreversible electrode reaction (\(_}=\) 1.97 10−4 cm·s−1 for the electro-reduction of 1·10−3 mol dm−3 Bi(III) in 1 mol·dm−3 chlorate (VII) (Nosal-Wiercińska 2010b) to the blocking of the electrode surface by the formed hemimicelles (Kaliszczak and Nosal-Wiercińska 2018) (Fig. 2a, b). However, the presence of the studied surfactants on the SVW peaks of the electro-reduction of Bi(III) ions in 1 mol·dm−3 chlorate (VII) at a constant concentration of the catalyst substance (1·10−3 mol dm−3) indicates a further increase in the reversibility of the electro-reduction process in the presence of CTAB and a decrease in the reversibility in the presence of SDS (Nosal-Wiercińska 2014). An increase in the CTAB concentration causes an increase in the SWV peak current, with a simultaneous slight destabilization of the peak potential (Fig. 2c). However, considering the picture of the peaks, they are still very well defined (for the TC–CTAB mixture), there is no drastic change in width at half their height. This indicates the enhanced dynamics of Bi(III) ions electro-reduction acceleration due to the presence of the cationic surfactant and the formation of mixed adsorption layers at the R-AgLAFE/chlorate(VII) interface (Martyna et al. 2022). On the other hand, the effect of mixed TC-SDS adsorption layers is the change of electrode process reversibility dynamics which is demonstrated by a better defined SWV peak and a decrease in its height, especially above the CMC (Kaliszczak and Nosal-Wiercińska 2018) (Fig. 2d).

Fig. 2figure 2

a SWV peaks of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction with the influence of CTAB addition; b SWV peaks of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction with the influence of SDS addition; c SWV peaks of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction at the presence of 1·10−3 mol·dm−3 2-thiocytosine and with the influence of CTAB addition; d SWV peaks of 1·10−3 mol·dm−3 Bi(III) ions electro-reduction at the presence of 1·10−3 mol·dm−3 2-thiocytosine and with the influence of SDS addition

From the CV voltammetograms (Fig. 3a, b), the values of the anodic and cathodic peak potential difference \(\Delta E\) were determined, which have simple relationships with the changes in the height and position of the corresponding peaks in the SWV square wave voltammetry. The presence of 1·10−3 mol dm−3 TC definitely increases the reversibility of the proces (Nosal-Wiercińska et al. 2021a). The \(\Delta E\) values decrease very much compared to those obtained for the basic electrolyte (1·10−3 mol dm−3 Bi(III) in 1 mol dm−3 chlorate(VII)).

Fig. 3figure 3

a CV curves of 1·10−3 mol·dm−3 Bi(III) electro-reduction at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and with the influence of CTAB addition; b CV curves of 1·10−3 mol·dm−3 Bi(III) electro-reduction at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and with the influence of SDS addition

The addition of surfactants to such a system affects the changes in \(\Delta E\). CTAB causes a further decrease in the potential difference of the anodic and cathodic peaks (Fig. 3a), while SDS has the opposite effect (Fig. 3b). These observations lead to the conclusion that the presence of mixed adsorption layers influences significantly the changes in the dynamics of acceleration of the electro-reduction process of Bi(III) ions by 2-thiocytosine (Nosal-Wiercińska et al. 2021a).

The studies carried out (lack of linearity of the real rate constants \(_}\) determined by the impedance method, taking into account the influence of the double layer) on the Bi(III) ions electro-reduction as a function of the electrode potential (Nosal-Wiercińska 2014) confirmed the multi-step character of the Bi(III) ions electro-reduction process and in the presence of 2-thiocytosine and 2-thiocythosine–surfactant mixtures (Fig. 4a, b). On the other hand, small changes in the anodic and cathodic peak potential difference \(\Delta E\) along with the change in the polarization rate (Table 1a, b) for all the studied systems indicated that the rate of Bi(III) ion electro-reduction process was controlled by the chemical reaction.

Fig. 4figure 4

a Dependence of 1·10−3 mol·dm−3 Bi(III) electro-reduction rate constants at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and various concentrations of CTAB indicated in the figure as a function of R-AgLAFE electrode potential; b Dependence of 1·10−3 mol·dm−3 Bi(III) electro-reduction rate constants at the presence of 1·10−3 mol·dm−3 of 2-thiocytosine and various concentrations of SDS indicated in the figure as a function of R-AgLAFE electrode potential

Table 1 Changes in ∆E for the 1‧10–3 mol‧dm−3 Bi(III) electro-reduction process at the various concentrations of CTAB additions and in the presence of 1‧10−3 mol‧dm−3 2-thiocytosine at the polarization rate v

The previous studies in the case of 2-thiocytosine indicate a reaction of the formation the Bi–(RS–Hg) active complexes on the electrode surface, which mediate electron transfer (Nosal-Wiercińska et al. 2021a). The Bi(III) ions electro-reduction process in 1 mol dm−3 chlorate(VII) in the presence of 2-thiocytosine occurs in the adsorption layer. No changes were observed in the mechanism of Bi(III) ion electro-reduction process in the presence of 2-thiocytosine-CTAB mixture in the base electrolyte solution. The differences \(\Delta E\) with the change in the polarization rate are small (at these low rates v) as for 2-thiocytosine alone. In the case of the mixed adsorption layer formation (Martyna et al. 2022), the adsorption layer can be unraveled due to the repulsive interaction between the positively charged nitrogen atoms in the surfactant molecule, directed with the hydrophilic end toward the solution (Nieszporek and Dagci 2014).

This will make it easier for the previously formed Bi–(RS–Hg) active complexes to get to the electrode surface, resulting in an increase in the rate of the Bi(III) ion electro-reduction process (Fig. 5). Whereas for the 2-thiocytosine-SDS system, there is observed different dependence, the significant differences \(\Delta E\) with the change of polarization rate especially above CMC (5·10−5 mol·dm−3) (Martyna et al. 2022) indicate the changes in the mechanism of Bi(III) ions electro-reduction process. Most probably, the surfactant molecules block the electrode surface pushing out the previously formed Bi–(RS–Hg) active complexes from the adsorption layer (Fig. 6).

Fig. 5figure 5

Scheme of Bi(III) ions electro-reduction in chlorate(VII) including the mediating role of active complexes mediating electron transfer and in the presence of CTAB

Fig. 6figure 6

Scheme of Bi(III) ions electro-reduction in chlorate(VII) considering active complexes mediating electron transfer and in the presence of SDS

This changes the catalytic dynamics of 2-thiocytosine towards inhibition. A similar effect was observed in the paper (Kaliszczak and Nosal-Wiercińska 2018, 2019, 2020). However, it should be emphasized that in both cases the Bi–(RS–Hg) complex plays a key role, as it is the 2-thiocytosine which dominates in the formation of adsorption equilibria of the studied mixed adsorption layers (Martyna et al. 2022).

Kinetic parameters

The determined cathodic transition coefficients (\(\mathrm\)), standard rate constants (\(_\)) based on the CV cyclic voltammetry curves indicated quantitatively changes in the catalytic effect of 2-thiocytosine in relation to the presence of surfactants in the basic electrolyte solution (Tables 2, 3).

Table 2 Values of: the cathodic transition coefficients α and standard rate constants ks of 1·10−3 Bi(III) electro-reduction at various concentrations of CTAB additions and in the presence of 1‧10−3 mol‧dm−3 2-thiocytosineTable 3 Values of: the cathodic transition coefficients α and standard rate constants ks of 1·10−3 Bi(III) electro-reduction at various concentrations of SDS additions and in the presence of 1‧10−3 mol‧dm−3 2-thiocytosine

The increase in the values of the transition coefficients \(\mathrm\) after the addition of CTAB into the base electrolyte solution containing a constant concentration of 2-thiocytosine indicates an increase in the reversibility of the Bi(III) ion electro-reduction process. This also translates into an increase in the standard rate constants \(_}\), confirming the increased dynamics of TC catalysis in the presence of CTAB.

However, the addition of SDS to the tested system shows a decreasing trend in \(\mathrm\). The values of rate constants decrease as the concentration of SDS in the chlorate(VII) solution containing 1·10−3 mol·dm−3 2-thiocytosine increases. Especially the decreasing tendency is visible for the concentration 5·10−5 mol·dm−3 SDS (CMC value) and above. This confirms changes in the electrode mechanics and more blocking of the electrode surface due to the increasing surfactant concentration in the base electrolyte solution (Kaliszczak and Nosal-Wiercińska 2018).

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