Adsorption of Quaternary Ammonium Pickling Inhibitors at a Dropping Mercury Electrode

Differential capacitance and interfacial tension measurements have been carried out at a dropping mercury electrode in 0·5 M H2SO4and M KCl solutions of n-alkylquaternary ammonium bromides containing C3, C12and C16chains paired to trimethyl-, triethyl-, tripropyl-, and tributylammonium headgroups.For n-alkyl chain lengths of 8 and 12 carbon atoms, the adsorption of the quaternary ammonium ions on mercury increases with the size of the headgroup, and the calculated surface concentrations indicate that, at optimum potentials, the larger headgroups are always adsorbed in excess of a monolayer. The compounds with alkyl chains of 16 carbon atoms all show multilayer adsorption behaviour which appears to be practically independent of the headgroup.The results are discussed in relation to the performance of the n-alkylquaternary ammonium compounds as inhibitors of the acid pickling of steel.