I. B. Obot etal Der Pharma Chemica; 2009, 1 (1): 151-166   Experimental and theoretical investigation of clotrimazole as corrosion inhibitor for aluminium in hydrochloric acid and effect of iodide ion addition I.B. Obot a, *, N.O. Obi-Egbedi b S.A. Umoren a a Department of Chemistry, Faculty of Science, University of Uyo, Uyo, Nigeria b Department of Chemistry, University of Ibadan, Ibadan, Nigeria ______________________________________________________________________________ Abstract The corrosion and inhibition behavior of aluminium in hydrochloric acid in the presence of clotrimazole (CTM) and potassium iodide (KI) were investigated using weight loss method at 30 and 50 o C. It was found that the inhibition efficiency increased with CTM concentration but decreased with increase in temperature. A synergistic effect was observed between KI and CTM. The experimental results suggest that the presence of iodide ions in the solutions stabilized the adsorption of CTM molecules on the metal surfaces and, therefore improve the inhibition efficiency of CTM. Phenomenon of physical adsorption is proposed for the inhibition and the process followed the Langmuir adsorption isotherm and kinetic / thermodynamic model of El- Awady et al. The mechanism of adsorption inhibition and type of adsorption isotherm were proposed from the trend of inhibition efficiency with temperature, E a , G ads , and Q ads . To further explain the adsorptive behavior of the molecules on the aluminium surface, a semiempirical approach involving quantum chemical calculations using Spartan ’06 was undertaken. The HOMO and the LUMO electronic density of the molecule was used to explain the inhibiting mechanism. The most probable adsorption centers were found in the vicinity of the imidazole group. Keywords: Clotrimazole, Aluminium, corrosion inhibition, synergism, adsorption isotherm, quantum chemical studies, Austin Model 1 (AM1). ______________________________________________________________________________ Introduction The interaction between a material and its environment that results in a degradation of the physical and mechanical properties of that material is termed corrosion. Over the years, reductions in the rate of metal corrosion have been achieved in a number of ways. It involved controlling of the pH or ion concentration of solution or controlling the metal – solution