Effect of the metal nature on the n-butanol adsorption in the absence of the metal–water chemisorption interaction

Abstract

The potential dependences of differential capacitance at the Pb–Ga/H 2O, Bi–Ga/H 2O, and Sn–Ga/H 2O interfaces in 0.05 M Na 2SO 4 solutions with n-butanol ( n-C 4H 9OH) additions are obtained by impedance measurements at a frequency of 420 Hz and temperature 32 °C. The n-C 4H 9OH adsorption parameters in terms of the model of two parallel capacitors are found using nonlinear regression analysis of the experimental data. They are compared with the corresponding data for Hg and Tl–Ga electrodes. The comparative analysis of the n-C 4H 9OH adsorption at Hg, Bi–Ga, Pb–Ga, Sn–Ga, and Tl–Ga electrodes revealed the effect of the metal nature on the n-C 4H 9OH adsorption energy, despite the absence of the metal–water chemisorption interaction at these electrodes. The obtained results fall into the general correlation dependence between the free energy of n-C 4H 9OH molecule adsorption ( Δ G A 0 ) and the “electronic” capacitance of the electrodes in the absence of the metal–water chemisorption interaction ( C m) phys. These results can be explained by different metal–water physical interaction caused by the difference in the expansion of the metal electronic density beyond the limits set by the metals’ ionic cores. The data obtained for Hg, Bi–Ga, Pb–Ga, Sn–Ga, and Tl–Ga electrodes in combine confirm the suggestion that the using the quantity Δ G A 0 and the potential of the cathodic adsorption–desorption peak E des (expressed in the rational scale) as criteria of the metal–water chemisorption interaction (of the metal hydrophilicity) is only possible when the “electronic” capacitances of the compared metals coincide at the charges where the metal–water chemisorption interaction is absent. In the general case when the “electronic” capacitances do not coincide, some corrections must be introduced. In particular, when the quantity Δ G A 0 is used as a criterion, the correction that allows for the effect of the metal nature on the n-C 4H 9OH adsorption energy in the absence of the metal–water chemisorption interaction ( Δ M Hg G A 0 ) phys must be introduced. If, then, E des is used as a criterion of the metal–water chemisorption interaction, two corrections must be introduced, the first of which allows for the potential drop associated with the unequal “electronic” capacitances of the metals; the other, for the potential drop which is due to the different adsorbability of the organic substance at different metals in the absence of the metal–water chemisorption interaction.