  The corrosion of materials is an undesirable and costly process affecting
many areas of technology and everyday life. As such, considerable effort has
gone into understanding and preventing it. Organic molecule based coatings can
in certain circumstances act as effective corrosion inhibitors. Although they
have been used to great effect for more than sixty years, how they function at
the atomic-level is still a matter of debate. In this work, computer simulation
approaches based on density functional theory are used to investigate
benzotriazole (BTAH), one of the most widely used and studied corrosion
inhibitors for copper. In particular, the structures formed by protonated and
deprotonated BTAH molecules on Cu(111) have been determined and linked to their
inhibiting properties. It is found that hydrogen bonding, van der Waals
interactions and steric repulsions all contribute in shaping how BTAH molecules
adsorb, with flat-lying structures preferred at low coverage and upright
configurations preferred at high coverage. The interaction of the
dehydrogenated benzotriazole molecule (BTA) with the copper surface is instead
dominated by strong chemisorption via the azole moiety with the aid of copper
adatoms. Structures of dimers or chains are found to be the most stable
structures at all coverages, in good agreement with scanning tunnelling
microscopy results. Benzotriazole thus shows a complex phase behaviour in which
van der Waals forces play an important role and which depends on coverage and
on its protonation state and all of these factors feasibly contribute to its
effectiveness as a corrosion inhibitor.
