  This work aims to investigate the corrosion inhibition of the mild steel in
the 1 M HCl solution by 1-octyl-3-methylimidazolium hydrogen sulphate
1-butyl-3-methylimidazolium hydrogen sulphate, 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 a 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 a
concentration higher than 0.005 mol/l, while it is much lower for
1-butyl-3-methylimidazolium hydrogen sulphate. The values measured in the
weight loss experiments (after seven days) are to some extent higher (reaching
up to 98% efficiency). Furthermore, 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 hydrogen sulphate. From the 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 influence of the alkyl chain length and the functional group on
the inhibition process.
