STUDY OF THE MECHANISM OF THE INFLUENCE OF SURFACE-ACTIVE SUBSTANCES ON THE KINETICS OF THE DISCHARGE–IONIZATION OF ZINC

V. E. Gorodovykh, O. S. Stepanova, and A. A. Ovcharenko

(Presented by the scientific-methodological seminar of the Faculty of Chemistry and Technology)

Earlier [1] we pointed out the relevance of investigating the mechanism of the influence of surfactants on the kinetics of metal discharge–ionization. In the present work, experimental data are given on the study of the mechanism of the influence of surfactants on the electrode process involving zinc ions. This topic is of practical importance, since electrolytic deposition of zinc from simple electrolytes has found wide application in electroplating, and, in order to improve coating quality and to obtain deposits with a fine-crystalline structure, surfactant additives are introduced into the electrolyte. Up to now, surfactant additives have been selected empirically.

The aim of our work is to elucidate the mechanism of the influence of such surfactants as camphor and n-amyl alcohol on the kinetics of the discharge–ionization of zinc.

The investigations were carried out by the procedure described in [1], in an electrolyte of composition \(10^{-5} \div 10^{-3}\ \mathrm{mol/l}\ \mathrm{ZnSO_4}\), \(1\ \mathrm{mol/l}\ \mathrm{Na_2SO_4}\), and \(x\ \mathrm{C_{10}H_{16}O}\) or \(\mathrm{H{-}C_5H_{11}OH}\) at \(t = 20^\circ\mathrm{C}\). The additions of camphor and n-amyl alcohol were taken in fractions of saturation.

The mechanism of the influence of surfactants on the kinetics of the discharge–ionization of zinc was studied on a mercury dropping electrode, \(\tau = 2\text{–}3\ \mathrm{sec}\), on a stationary mercury drop, \(\tau = 0.04\ \mathrm{cm}\), on an amalgam zinc dropping electrode, \(\tau = 3\ \mathrm{sec}\), and on a solid zinc electrode. The latter electrode was obtained electrolytically on a platinum microcathode with a surface area of \(2.5\ \mathrm{mm^2}\).

When polarization curves were recorded on the indicated types of electrodes, the following results were obtained (Tables 1, 2, 3, 4).

It was established that, with an increase in the content of surfactant in the electrolyte, the half-wave potential or the peak potential of zinc changes, in such a way that in the cathodic process the value of the potential becomes more negative, while in the anodic process it becomes more positive. The current, however, remains practically unchanged on the mercury dropping and solid zinc electrodes.

The oxidation current of zinc amalgam on a stationary mercury drop decreases with increasing additions of camphor and alcohol. A similar picture is also observed when recording the voltammetric curve on an amalgam zinc dropping electrode (Table 4).

The degree of reversibility of the electrode process of zinc reduction was studied in a solution of \(1M\ \mathrm{Na_2SO_4}\) and in a solution of \(1M\ \mathrm{Na_2SO_4}\) with additions of camphor and n-amyl alcohol, using the equation of a reversible polarographic wave [2].

Table 1

Dependence of the half-wave current and potential on additions of camphor and n-amyl alcohol
at a dropping mercury electrode
\(C_{\mathrm{Zn}^{2+}} = 1 \cdot 10^{-3}\ \mathrm{mol/l}\)

Table 2

Dependence of the anodic-peak current and potential on additions of camphor
and n-amyl alcohol at a stationary mercury drop
\((C_{\mathrm{Zn}^{2+}} = 2 \cdot 10^{-5}\ \mathrm{mol/l},\ \varphi_{\mathrm{e}} = -1{,}65\ \mathrm{V},\ \tau = 3\ \mathrm{min})\)

Table 3

Dependence of the cathodic-peak current and potential on additions of camphor
and n-amyl alcohol at a solid zinc electrode
\(C_{\mathrm{Zn}^{2+}} = 1 \cdot 10^{-3}\ \mathrm{mol/l}\)

Table 4

Dependence of the half-wave current and potential
on the addition of n-amyl alcohol
at an amalgam zinc electrode
\(C_{\mathrm{Zn}^{2+}} = 1 \cdot 10^{-3}\ \mathrm{mol/l}\)

\[ \varphi=\varphi_{1/2}-b\lg\frac{i}{I-i}, \]

where

\[ b=2.3\cdot\frac{RT}{zF}=\frac{0.059}{2}, \]

the slope coefficient of the polarographic wave at \(t=20^\circ\mathrm{C}\).

From the plot in the coordinates \(\lg \frac{i}{I-i} - (-\varphi)\) it is clear that the process of discharge of zinc ions from a pure sodium sulfate solution is irreversible \((\beta=\operatorname{ctg}\theta=0.13)\), and in the presence of surfactants “b” it deviates still more strongly from the theoretical value, i.e., the process of discharge of zinc ions becomes even more irreversible. Knowing the value of the slope coefficient, one can estimate the transfer coefficient \(\alpha\):

\[ \operatorname{ctg}\theta=\frac{0.059}{\alpha z}. \]

Thus \(\alpha\) varies from 0.210 to 0.188 depending on the camphor content in the electrolyte.

To estimate the adsorption region and establish the desorption potentials of surfactants, electrocapillary curves were recorded on mercury from solutions of \(1\ \mathrm{mol/l}\ \mathrm{Na_2SO_4}\) and \(1\ \mathrm{mol/l}\ \mathrm{Na_2SO_4}\) with additions of camphor and amyl alcohol (Fig. 1).

Fig. 1. Electrocapillary curves
1 — \(1\mathrm{M}\ \mathrm{Na_2SO_4}\);
2 — \(1\mathrm{M}\ \mathrm{Na_2SO_4}\) and \(0.6\ \mathrm{ml}\ \mathrm{C_{10}H_{16}O}\);
3 — \(1\mathrm{M}\ \mathrm{N_2SO_4}\) and \(0.6\ \mathrm{ml}\ \mathrm{H{-}C_5H_{11}OH}\)

Thus, on the basis of the experimental data it follows that surfactants affect the electrochemical stage of the electrode process, i.e., the discharge–ionization of zinc, and also the supply and removal of zinc ions (for the case of the ionization process on a stationary mercury electrode and discharge of zinc ions on an amalgam zinc electrode). Apparently, surfactants change the structure of the electrical double layer, as a result of which the value of the psi-prime potential changes and the irreversibility of the electrode process increases [3].

REFERENCES

1. V. E. Gorodovykh, O. S. Stepanova, M. G. Bronnikova. Izvestiya TPI. This collection.
2. Ya. Geirovskii, Ya. Kuta. Fundamentals of Polarography, Moscow, 1965, p. 118.
3. A. G. Stromberg, L. S. Zagainova. Journal of Physical Chemistry, 31, 1042, 1957.