EFFECT OF ADSORPTION OF ORGANIC SURFACTANTS ON REVERSIBLE ANODIC PEAKS

V. E. GORODOVYKH, T. V. TOMZA

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

The effect of surfactants (SAS) on the kinetics of electrode processes is associated with their adsorption at the electrode—electrolyte interface.

There are many works in the literature devoted to studying the effect of SAS of various classes on electrochemical reactions. Of great importance for the development of our understanding in this field are the works of Frumkin [1—3], Stromberg [4—7], Loshkarev [8—10], and other researchers [11—12]. It should be noted, however, that in most cases the mechanism of the effect of SAS on the kinetics of electrode processes remains unclear.

According to A. G. Stromberg, SAS may affect both the stage of delivery of a substance to the electrode and the charge-transfer reaction.

Let us consider the effect of SAS on reversible processes in the APN method. In other words, let us assume that adsorption of organic SAS leads only to a change in the rate of delivery of metal ions to the surface of a stationary mercury dropping electrode (s.m.d.e.) during the accumulation stage, or in the rate of removal of ions from the electrode surface during the oxidation stage, and does not affect the rate of the charge-transfer reaction.

The theory of polarographic anodic-cathodic waves developed by Stromberg [4—7] leads to the following relationships for the limiting kinetic current:

\[ I_{\text{kin}}=\frac{K_{\text{kin}}}{K_{\text{kin}}+K_{\text{diff.}}}\,I_{\text{diff.}}, \tag{1} \]

where \(I_{\text{kin}}\) and \(K_{\text{kin}}\) are the limiting kinetic current and the constant of the kinetic current,

\(I_{\text{diff.}}\) and \(K_{\text{diff.}}\) are the diffusion current and the constant of the diffusion current.

If, as a result of adsorption of SAS, additional kinetic difficulties arise, then at \(K_{\text{kin}} \ll K_{\text{diff.}}\)

\[ I_{\text{kin}}\cong K_{\text{kin}}\cdot C_0, \tag{2} \]

where \(C_0\) is the concentration of metal ions in solution.

The kinetic-current constant can be represented as follows:

\[ K_{\text{kin}}=B_{\text{kin}}\exp\left[-\frac{W_{\text{kin}}}{RT}\right], \tag{3} \]

where \(B_{\text{kin}}\) is a factor whose magnitude depends on the degree of coverage of the electrode surface by SAS \((\theta)\).

\(W_{\mathrm{kin}}\) is the activation energy of the process of delivery of the reacting particle to the electrode surface.

The quantities \(B_{\mathrm{kin}}\) and \(W_{\mathrm{kin}}\), obviously, depend on the degree of coverage. At present the physical reasons for the change in \(B_{\mathrm{kin}}\) and \(W_{\mathrm{kin}}\) with increasing degree of coverage remain largely unexplained. This is due to the fact that the nature and sequence of the processes occurring with the reacting particle in the electric double layer, and the character of its interaction with the molecules of the adsorbate, are unknown.

As a first approximation, the reason for the decrease in the kinetic-current constant in the presence of a surfactant may be considered to be hindered diffusion during the passage of ions through a layer of adsorbed organic molecules, which is equivalent to an increase in the “viscosity” of the medium. This formal model says nothing further about the actual mechanism of the process; however,

Fig. 1. Dependence of the electrolysis current on time in a pure solution and in the presence of β-naphthol (the concentration of β-naphthol is indicated in the figure in fractions of saturation).
Recording conditions: \(r_{\mathrm{e}} = 0.03\ \mathrm{cm}\), \(\varphi_{\mathrm{Zn}} = -1.6\ \mathrm{V}\),
\(C_{\mathrm{Zn}} = 7 \cdot 10^{-3}\ M\)

Fig. 2. Dependence of the area under the peak \(Q_{\mathrm{C}}\) and of the anodic-peak constant on the concentration of β-naphthol in solution (in fractions of saturation).
Recording conditions: \(r_{\mathrm{e}} = 0.03\ \mathrm{cm}\), \(t_{\mathrm{e}} = 1.5\ \mathrm{min}\),
\(\varphi_{\mathrm{Cd}} = -1.2\ \mathrm{V}\), \(C_{\mathrm{Cd}} = 1 \cdot 10^{-3}\ M\)

it can serve as a starting point for solving a number of problems of diffusion kinetics.

The process of hindered diffusion in the near-electrode layer can be characterized by an “effective” diffusion coefficient \(D_{\mathrm{eff}}\), which must be smaller than the diffusion coefficient in aqueous solution.

Let us consider from this point of view the influence of the addition of a surfactant on the height of reversible anodic peaks. A decrease in the rate of passage of ions through the surfactant film should lead to a decrease in the electrolysis constant \(K_{\mathrm{e}}\) and in the accumulation current \(i_{\mathrm{e}}\), since

\[ i_{\mathrm{e}} = K_{\mathrm{e}}(\theta) S C_0 . \tag{4} \]

It follows from this that, in the presence of a surfactant, during electrolysis at a stationary mercury drop electrode, a smaller amount of metal will pass into the electrode than without the surfactant,

\[ m = \frac{1}{zF}\int_{0}^{t_{\mathrm{e}}} i_{\mathrm{e}}\,d\tau, \tag{5} \]

where \(m\) is the amount of metal in the electrode.

A decrease in the electrolysis current upon introducing β-naphthol into the solution was observed experimentally by us in the example of zinc reduction against a background of 0.1 N H\(_2\)SO\(_4\) (Fig. 1).

It follows from (5) that in the presence of a surfactant the concentration of metal in the amalgam, equal to \(m/v\) (\(v\) is the volume of the mercury-drop electrode), must decrease; this, in turn, will lead to a decrease in the peak depth, proportional to the amalgam concentration, and in the area under the peak, proportional to the amount of metal in the electrode. This conclusion is confirmed by the data on the dependence of the area under the peak on the β-naphthol concentration, given in Fig. 2.

An adsorbed surfactant film can also create difficulties at the stage of removal of metal ions from the electrode surface during the electrodissolution of the amalgam, especially when polymolecular layers or micellar films are formed. A decrease in the effective diffusion coefficient in this case should lead to a change in the anodic-peak coefficient and in the steepness of the descending branch of the peak.

From the foregoing it is clear that the experimentally observed decrease in the depth of the reversible peak may be due to two causes: a decrease in the electrolysis constant and a change in the anodic-peak coefficient. This makes it possible to propose an experimental criterion that permits establishing at which stage inhibition of the electrochemical process occurs. A decrease in the electrolysis constant and in the area under the peak corresponds to inhibition at the supply stage. A change in the anodic-peak coefficient and in the steepness of the descending branch of the peak indicates inhibition of the removal stage (provided that, under the influence of the surfactant, no decrease in the exchange current has occurred).

Fig. 3. Dependence of the peak depth of zinc and cadmium on the concentration of β-naphthol in solution (in fractions of saturation). Recording conditions: \(r_{\mathrm{e}} = 0.03\) cm, \(t_{\mathrm{e}} = 1.5\) min, \(\varphi_{\mathrm{Zn}} = 1.6\) V, \(\varphi_{\mathrm{Cd}} = -1.2\) V, \(C_{\mathrm{Zn}} = 7 \cdot 10^{-3}\) M, \(C_{\mathrm{Cd}} = -1 \cdot 10^{-3}\) M.

Experimental verification of the theoretical considerations set forth above was carried out by us using the example of reversible cadmium and zinc peaks against a background of 0.1 N H\(_2\)SO\(_4\) with various additions of β-naphthol. The procedure for recording anodic peaks was the usual one. The conditions of the experiments are indicated in the captions to the figures. As can be seen from these figures, with increasing concentration of β-naphthol in the solution, and hence with increasing degree of coverage, there is a considerable decrease in the depth of the anodic peaks, the accumulation current, and the area under the peak. This indicates the appearance of considerable inhibition at the supply stage. The decrease in the anodic-peak constant (Fig. 2) permits the conclusion that inhibition is also present at the stage of removal of metal ions from the electrode surface.

LITERATURE

1. A. N. Frumkin. DAN SSSR, 85, 377, 1952.
2. A. N. Frumkin. In: Advances in Electrochemistry and Electrochemical Engineering, vol. 1, 94, Interscience, N. Y., 1961.

1. A. N. Frumkin. In: Fundamental Problems of Modern Theoretical Electrochemistry. Moscow, “Mir,” p. 302, 1965.
2. A. G. Stromberg. Journal of Physical Chemistry, 27, 1287, 1953.
3. A. G. Stromberg. Journal of Physical Chemistry, 29, 409, 1955.
4. A. G. Stromberg. Journal of Physical Chemistry, 29, 2152, 1955.
5. A. G. Stromberg, L. S. Zagaynova. Journal of Physical Chemistry, 31, 1042, 1957.
6. M. A. Loshkarev, A. A. Kryukova. Doklady of the Academy of Sciences of the USSR, 72, 919, 1950.
7. M. A. Loshkarev, A. A. Kryukova. Journal of Physical Chemistry, 23, 209, 221, 1949.
8. M. A. Loshkarev, A. A. Kryukova, Yu. M. Loshkarev, T. F. Dyachenko. In: Fundamental Problems of Modern Theoretical Electrochemistry. Moscow, “Mir,” p. 380, 1965.
9. Ya. Geyrovsky, Ya. Kuta. Fundamentals of Polarography. Moscow, “Mir,” 1965.
10. P. Delahay. Double Layer and Electrode Kinetics. Moscow, “Mir,” 1967.