  The aim of this work is to investigate the corrosion inhibition of the mild
steel in the 1 M HCl solution by the 1-octyl-3-methylimidazolium
hydrogensulphate, 1-butyl-3-methylimidazolium hydrogensulphate, and
1-octyl-3-methylimidazolium chloride, using electrochemical, weight loss, and
surface analysis methods as well as the full quantum-mechanical treatment.
Polarization measurements prove that studied compounds are mixed-type
inhibitors with predominantly anodic reaction. The inhibition efficiency
obtained from the polarization curves is about 80-92% for all of the
1-octyl-3-methylimidazolium salts with the concentration higher than 0.005 mol
dm$^{-3}$, while it is much lower for 1-butyl-3-methylimidazolium
hydrogensulphate. The values measured in the weight loss experiments (after
seven days) are to some extent higher (reaching up to 98% efficiency). We have
shown that the influence of the alkyl chain length on the inhibition efficiency
is much larger than that of the anion type. Furthermore, we obtain a realistic
model of a single molecule on iron surface Fe(110) by applying the Density
Functional Theory calculations. We use the state-of-the-art computational
approach, including the meta-GGA strongly-constrained and appropriately normed
semilocal density functional to model the electronic structure properties of
both free and bounded-to-surface molecules of 1-butyl-, 1-hexyl-, and
1-octyl-3-methylimizadolium bromide, chloride, and hydrogensulphate. From our
calculations we extract the HOMO/LUMO gap, hardness, electronegativity, and
charge transfer of electrons from/to molecules-in-question. It supports the
experimental findings and explains the infuence of the alkyl chain length and
the functional group on the inhibition process.
