Thermodynamics of Adsorption of Imidacloprid at Constant Charge Hydrophobic Surfaces:  Physicochemical Aspects of Bioenvironmental Activity

Adsorption of the insecticide 1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine (Imidacloprid) on the hanging mercury drop electrode (HMDE) surface was studied by temperature-dependent stripping voltammetry (TD-SV). At near physiological pH, under reducing conditions, the Gibbs free energy of adsorption, ΔG_ADS, shows two distinct temperature-dependent regimes. (a) At 0° < T < 10 °C a temperature-independent mechanism occurs with a constant ΔG_ADS = −40.5 kJ/mol, resulting in strong chemisorption at high surface coverage. For T < 10 °C a considerable enthalpy gain is estimated, and this represents the driving force for the adsorption of Imidacloprid onto the electrode surface. (b) At T > 10 °C a temperature-dependent mechanism is operative with ΔG_ADS/ΔT = −91.4 J/K mol, resulting in a rapid weakening of adsorption and low surface coverage. On the basis of the present findings we suggest that the strong chemisorption at T < 10 °C at physiological pH under reducing conditions is related to the high specific insecticide activity of Imidacloprid in cool-blooded insects as contrasted to its low efficiency in warm-blooded organisms.