Synthesis of some cationic gemini surfactants and their inhibitive effect on iron corrosion in hydrochloric acid medium

Some gemini surfactants in the series of 1,2-ethane bis(dimethyl alkyl(CnH2n+1) ammonium bromide) referred as n-2-n have been synthesised, where n = 10, 12 and 14. The purity of surfactants synthesised was checked by rutinary methodologies (H-1-NMR, MS and elemental analysis). Their surface active properties at equilibrium in water at 25 degreesC were determined. The inhibitive effect or these compounds, in the case of iron immersed in 1 M HCl, was investigated through mass-loss, potentiokinetic methods and electrochemical impedance spectroscopy. Polarisation data indicate that these compounds act as very good cathodic inhibitors of iron in 1 M HCl. In the anodic range, the surfactants studied do not show an inhibition effect at iron electrode potential more positive than -100 mV/Saturated Calomel Electrode. Impedance plots of iron in the presence of various concentrations of surfactants studied are similar to semicircles in the capacitive quadrant. The associated values of transfer resistances increase by increasing the additive concentrations. From all measurements performed, the variation of the inhibition efficiency versus concentration shows the same trend. The gemini surfactants appear to be adsorbed in the metal/liquid interface through the general adsorption mode following the Frumkin adsorption isotherm model. The electrochemical impedance spectroscopy shows that the maximum inhibition efficiency is attributed to a formation of an adsorbed layer at the iron surface. From tensiometric measurements, it was found that the ability of gemini surfactants studied to adsorb at the interface air/water is concordant with their order of the inhibition efficiency of iron corrosion. (C) 2000 Elsevier Science Ltd. All rights reserved.