STUDY OF THE ADSORPTION OF β-NAPHTHOL ON MERCURY

Yu. P. Oblivantsev, T. V. Gomza, V. E. Gorodovykh

(Presented by the scientific-methodological seminar
of the Department of Technology of Inorganic Substances
and Electrochemical Production)

In recent years a very large number of works have appeared in the literature containing theoretical and experimental material on the adsorption of organic compounds on the surfaces of both liquid and solid electrodes. Interest in the phenomenon of adsorption is due to the influence of the latter on the course of electrode processes. The objects of study are most often substances that have found practical application in electroplating or hydrometallurgy. The present work considers the adsorption of β-naphthol, which is widely used, for example, as a regulator of the growth of cathodic tin deposits [1, 2].

The adsorption of β-naphthol on mercury was studied by the method of electrocapillary curves and differential capacitance. The procedure for recording the ECC did not differ from that described in [3]. The differential capacitance of the mercury electrode was determined using an alternating-current bridge, \(P = 568\) [4]. The measurement procedure was analogous to that described in [5]. Measurements were carried out on a long-dropping mercury electrode with the characteristic \(m = 0.038\ \text{mg/sec}\). Potentials were measured relative to a saturated calomel electrode. Oxygen was removed from the solution by purging with argon. All measurements were performed against a background of \(0.1\ N\) sulfuric acid. The solutions were prepared with freshly bidistilled water. The sulfuric acid used in the work was distilled twice. β-Naphthol was subjected to double distillation with water vapor.

Results and Discussion

Figure 1 presents electrocapillary curves (ECC) recorded against a background of \(0.1\ N\) sulfuric acid (curve 1) and in the presence of various concentrations of β-naphthol. As can be seen from the figure, desorption potentials are not reached on the positive branch of the ECC. M. A. Gerovich [6, 7] showed that such a course of the curve is the result of interaction between the \(\pi\)-electrons of the aromatic ring and the positively charged mercury surface. The point of zero charge (p.z.c.) of mercury shifts, upon introducing β-naphthol into the solution, in the negative direction with increasing surfactant concentration up to \(2.44 \cdot 10^{-4}\ M\). A further increase in concentration leads to a slight shift of the ECC maximum in the positive direction. From the results of measuring the limiting tension of mercury, we calculated the values

Fig. 1. Electrocapillary curves for mercury against a background of 0.1 N sulfuric acid in the presence of β-naphthol: 1) 0.0; 2) \(1.22 \cdot 10^{-5}\) M; 3) \(6.1 \cdot 10^{-4}\) M; 4) \(9.76 \cdot 10^{-5}\) M; 5) \(2.44 \cdot 10^{-4}\) M; 6) \(3.66 \cdot 10^{-4}\) M; 7) \(6.1 \cdot 10^{-4}\) M; 9) \(1.22 \cdot 10^{-3}\) M; 8) \(8.54 \cdot 10^{-4}\) M. The arrows indicate the ECC maxima.

the adsorption of β-naphthol at various concentrations and potentials (Fig. 2). The calculation was carried out using the approximate Gibbs formula. From Fig. 2 it is seen that, with increasing β-naphthol concentration, the potential of maximum adsorption \((\varphi_m)\) shifts in the negative direction. Moreover, for all concentrations \(\varphi_m\) coincides with the region of potentials of the electrocapillary maximum, which may be associated with a change in the orientation of β-naphthol molecules as the positive charge of the electrode decreases, from flat to inclined [8].

The differential-capacitance curves are presented in Fig. 3, a. At potentials close to the p.z.c., the \(C, \varphi\)-curves show a minimum, which increases with increasing β-naphthol concentration. In the anodic region the \(C, \varphi\)-curves do not merge with the background curve and, at low β-naphthol concentrations, have plateaus. Such a course of the differential-capacitance curves corresponds to adsorption of molecules in two positions: flat and inclined [8].

At sufficiently high surfactant concentrations, cathodic desorption peaks are well expressed; their potentials and heights change regularly with increasing β-naphthol concentration. The linear dependence of the peak height \((C^{\max})\) on the logarithm of concentration (Fig. 3, b) indicates that the obtained \(C, \varphi\)-curves do not differ too greatly from equilibrium ones [9]. In the region of high negative charges, reliable results could not be obtained because of hydrogen evolution.

The two regions of β-naphthol adsorption are most clearly seen from the differential-capacitance curves corrected for the background curve (Fig. 4).

Fig. 2. Dependence of the adsorption of β-naphthol on mercury on the electrode potential. Crosses indicate the p.z.c. Concentrations of β-naphthol, mol/L: 1) \(2.44\cdot10^{-5}\); 2) \(6.1\cdot10^{-5}\); 3) \(1.22\cdot10^{-4}\); 4) \(2.44\cdot10^{-4}\); 5) \(3.66\cdot10^{-4}\); 7) \(6.1\cdot10^{-4}\); 8) \(7.32\cdot10^{-4}\); 9) \(8.54\cdot10^{-4}\); 10) \(9.76\cdot10^{-4}\); 11) \(1.22\cdot10^{-3}\).

From the dependence of \((C-C_0)\) on \(\varphi\), where \(C_0\) is the capacitance in the supporting-electrolyte solution, we found the values of the potentials of maximum adsorption \(\varphi_{m i}\) (the index \(i=1\), here and below, corresponds to inclined orientation, and \(i=2\) to flat orientation). By extrapolating the capacitance values at the minimum of the \((C-C_0), \varphi\)-curves to infinitely large adsorbate concentration in the coordinates \(\frac{1}{C-C_0}\), \(\frac{1}{C}\), the values of the limiting capacitance were found: \(C_2=7.2\ \mu\mathrm{F}/\mathrm{cm}^2\) and \(C_2=14.3\ \mu\mathrm{F}/\mathrm{cm}^2\). Considering each adsorption region from the standpoint of the model of two parallel capacitors [9], according to the formula

\[ \Theta=\frac{C_0-C}{C_0-C_1} \]

the degrees of coverage \(\Theta\) were determined at the potentials of maximum adsorption \(\varphi_{m i}\). It should be noted that, for concentrations greater than \(4.9\cdot10^{-3}\ \mathrm{M}\), the values of \(\Theta\) at \(\varphi_{m2}\) are greater than the corresponding values at \(\varphi_{m1}\), although on the \(\Gamma,\ \varphi\)-curves a decrease in adsorption is observed. The reason for this may be reorientation of the adsorbate molecules, with increasing positive polarization, toward a flatter arrangement on the surface of mercury. The difference in the adsorption behavior of β-naphthol molecules is reflected in the area occupied by one molecule. From the values of the adsorption \(\Gamma_i\) and the degree of coverage \(\Theta_i\) at the potential \(\varphi_{m1}\), we determined the area per molecule \(S_i\) and the value of the limiting adsorption \(\Gamma_{m i}\). In all cases a decrease in \(S_i\) is observed with increasing concentration of β-naphthol. The minimum value is \(S_i=50\ \text{Å}^2\), and the maximum \(S_2=105\ \text{Å}^2\); the corresponding values \(\Gamma_{m1}\) and \(\Gamma_{m2}\) are, respectively, \(3.2\cdot10^{-10}\ \mathrm{mol}/\mathrm{cm}^2\) and \(1.58\cdot10^{-10}\ \mathrm{mol}/\mathrm{cm}^2\). It is interesting to note that for naphthalene \(\Gamma_{m1}>2.37\cdot10^{-10}\ \mathrm{mol}/\mathrm{cm}^2\); \(\Gamma_{m2}=1.5\cdot10^{-10}\ \mathrm{mol}/\mathrm{cm}^2\) [10]. Evidently, in all

Fig. 3. a) Differential capacitance curves of a mercury electrode in \(0.1N\) sulfuric acid (dashed line) and with additions of \(\beta\)-naphthol: 1) \(8.25\cdot10^{-6}\) M; 2) \(1.6\cdot10^{-5}\) M; 3) \(6.6\cdot10^{-5}\) M; 4) \(5.3\cdot10^{-4}\) M; 5) \(1\cdot10^{-3}\) M; 6) \(2.1\cdot10^{-3}\) M. b) Dependence of the height of the cathodic desorption peak on the logarithm of concentration.

at potentials, with increasing concentration, the orientation of the molecules changes from flat to vertical.

We determined that the adsorption of \(\beta\)-naphthol on mercury obeys the Frumkin isotherm

\[ B_{\mathrm{m}1}C=\frac{\Theta}{1-\Theta}\exp(2a\Theta) \]

since the plot of \((d\ln C/d\Theta)_{\varphi_{\mathrm{m}1}}\) passes through a minimum at \(\Theta=0.5\). From comparison of the experimental and calculated isotherm data, the values of the attraction constants \(a\) at \(\varphi_{\mathrm{m}1}\) were determined: \(a_1=0.22,\ a_2=1.2\).

The values of the attraction constant \(a\) in the region of potentials of the cathodic peaks were determined from the slope of the dependence \(C^{\max}-\lg C\). The change of \(a\) with potential has an approximately parabolic character, which is evidently associated with the change in the area occupied by one adsorbed molecule.

In an acidic medium (\(\mathrm{pH}=1\text{--}5\)), naphthols are adsorbed in the form of neutral molecules. Apparently, even with an inclined arrangement of the molecules, the influence of the \(\pi\)-electrons of the aromatic rings is manifested. An increase in the concentration of \(\beta\)-naphthol leads to a change in the orientation of the adsorbed molecules and to a weakening of the \(\pi\)-electron interaction, which may be the reason for the shift of the adsorption jump of potentials in the positive direction. However, at the same time the normal component of the dipole moment of the molecule increases, as a result of which the adsorption jump remains negative.

Fig. 4. Curves of the differential capacitance of the mercury electrode, corrected for the background curve, at the content of β-naphthol in solution: 1) \(3.3 \cdot 10^{-5}\) M; 2) \(6.6 \cdot 10^{-5}\) M; 3) \(2.1 \cdot 10^{-3}\) M.

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