Adsorption and viscoelastic behaviour of ionic liquid surfactants on gold surfaces

Adsorption isotherms of five ionic liquid surfactants belonging to different families and diverse structures, namely: 1-dodecyl-3-methylimidazolium chloride, [C ₁₂ C ₁ im]Cl; 1-dodecyl methylpyrrolidinium bromide, [C ₁₂ C ₁ Pyrr]Br; 1-dodecyl methylpyridinium chloride, [C ₁₂ C ₁ Py]Cl; 1-dodecyl methylpiperidiniumm bromide, [C ₁₂ C ₁ Pip]Br; and 1-octyl-3-methylimidazolium octylsulfonate, [C ₈ C ₁ im][C ₈ SO ₃ ] on a gold surface have been studied using a quartz crystal microbalance with dissipation (QCM-D). All surfactants show critical micelles concentrations (CMCs) in 10–20 mM range. Simultaneous adsorption and dissipation measurements were made in pre- and post-micellar concentration regions at 298.15 K. Peculiar results were obtained for all the IL surfactants. Frequency measurements show that the adsorbed amounts in the pre-micellar region are small and, as soon as the aggregates formation takes place, the adsorbed amount increases significantly. Dissipation measurements indicate the formation of rigid layers with very low values in the pre-micellar region. However, in the post-micellar region, with the increase of concentration (up to 75 mM~5xCMC) the dissipation significantly increases demonstrating the presence of loosely bound structures/multilayers on the gold surface.