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(CnH2 n+1) ammonium bromide) referred as n-2-n have been synthesised, where n = 1 0, 12 and 14. The purity of surfactants synthesised was checked by rutinary methodologies (H-1-NMR, MS and elemental analysis). Their surface active p roperties at equilibrium in water at 25 degreesC were determined. The inhib itive effect or these compounds, in the case of iron immersed in 1 M HCl, w as investigated through mass-loss, potentiokinetic methods and electrochemi cal impedance spectroscopy. Polarisation data indicate that these compounds act as very good cathodic inhibitors of iron in 1 M HCl. In the anodic ran ge, the surfactants studied do not show an inhibition effect at iron electr ode potential more positive than -100 mV/Saturated Calomel Electrode. Imped ance plots of iron in the presence of various concentrations of surfactants studied are similar to semicircles in the capacitive quadrant. The associa ted values of transfer resistances increase by increasing the additive conc entrations. From all measurements performed, the variation of the inhibitio n efficiency versus concentration shows the same trend. The gemini surfacta nts appear to be adsorbed in the metal/liquid interface through the general adsorption mode following the Frumkin adsorption isotherm model. The elect rochemical impedance spectroscopy shows that the maximum inhibition efficie ncy is attributed to a formation of an adsorbed layer at the iron surface. From tensiometric measurements, it was found that the ability of gemini sur factants studied to adsorb at the interface air/water is concordant with th eir order of the inhibition efficiency of iron corrosion. (C) 2000 Elsevier Science Ltd. All rights reserved.