Thermodynamics of Adsorption of Dithiocarbamates at the Hanging Mercury Drop

Two dimethyldithiocarbamate (DMDTC) pesticides, thiram and ziram, are adsorbed onto a Hg drop via an entropically driven process. The adsorption isotherms are described by the Frumkin equation. For both molecules, the adsorption is characterized by a nonlinear pseudosigmoid temperature dependence of the Gibbs free energy. For the temperature range of 273−313 K, ΔG_ADS varies between −43.4 and −56.71 kJ/mol for thiram and −42.60 and −55.67 kJ/mol for ziram. This variation of ΔG_ADS reveals that the adsorption strength is increased at higher temperatures. During the adsorption of either molecule, strong lateral interactions are developed between neighboring adsorbates, which are severely weakened as the temperature increases. A unified reaction scheme is suggested for both ziram and thiram that predicts the formation and adsorption of a surface complex, (DMDTC)₂Hg. In the case of thiram, two DMDTC molecules are formed by the cleavage of the disulfide S−S bond near the Hg electrode. The thermodynamic and structural parameters reveal that there are two limiting thermodynamic regimes for the adsorbed (DMDTC)₂Hg species that originate from two limiting adsorption conformations of the adsorbates on the Hg surface. A transition occurs between these two conformations at temperatures in the region of 285−295 K. This transition is accompanied by large entropic and enthalpic changes.