Studies about the adsorption of progesterone (P4) at glassy carbon electrodes in aqueous buffer solution by square wave voltammetry

The adsorption of progesterone (P4) on the glassy carbon (GC) electrode surface by mean of square wave stripping voltammetry measurements is studied. A cathodic peak is obtained at −1.65 V (vs. ECS) in pH 8 buffer solution after accumulation onto GC surface at open circuit. Peak current variations with the accumulation time, P4 concentration and temperature are presented. The adsorption is characterized by a nonlinear pseudosigmoid temperature dependence of the Gibbs free energy, which reveals that the adsorption strength is increased at higher temperatures. During the adsorption of P4, strong lateral interactions are developed between neighboring adsorbates, which are severely weakened as the temperature increases. The Frumkin isotherm is the model which best fit to experimental results. Values of the Gibbs free energy, enthalpy and entropy of adsorption are calculated. Also, the apparent heterogeneous rate constant for the electron transfer reaction of adsorbed species is determined.