Understanding the effect of the spacer length on adsorption of gemini surfactants onto steel surface in acid medium

Corrosion inhibition of carbon steel in 1 M hydrochloride acid by gemini surfactants alkanediyl-α,ω-bis(dimethyl dodecylammonium bromide) with different spacer lengths (designated as 12-s-12, s = 2, 3 or 6) was studied using the weight loss method. When monolayer forms on the metal surface below or near critical micelle concentrations (CMCs) of the gemini surfactants, it can be found that gemini surfactant with long spacer (12-6-12) tends to be adsorbed with two hydrophilic ionic groups onto the metal surface site, while gemini surfactants with short spacer (12-2-12 and 12-3-12) tend to be adsorbed with one hydrophilic group onto the meta surface site and the other hydrophilic group is free in solution phase, and further increase in surfactant concentration results in the formation of multilayer. Possible adsorption mechanisms of these gemini surfactants with different spacer lengths above and below their CMCs were discussed in detail, respectively.