Adsorption impedance Langmuir isotherm

The Langmuir isotherm represents the simplest possible case, but it is of considerable practical utility. It usually describes observed behaviour qualitatively, and quite often with a reasonable quantitative success. The over-simplification which it involves is essentially the disregard of the mutual interactions of the adsorbed molecules themselves. The presence of molecules on a surface may, according to circumstances, either facilitate or impede the adsorption of others, so that much more complicated relations of p and x result. The appropriate equations to express them must in the nature of things involve more constants than the Langmuir isotherm. They constitute a specialized study. 

The increase in phase angle is the more considerable, the more rapid the adsorption process [29]. These qualitative considerations are at present illustrated in a number of theoretical and experimental papers. Faradaic impedance with allowance for adsorption of electrochemically active substances at the equilibrium potential was frequently [30-35] investigated theoretically. Originally [30, 35] it was assumed that the adsorption of the reaction components can be described by the Langmuir isotherm and that the adsorption and desorption rate constants did not depend on the potential. A more general analysis avoiding these assumptions was made by Senda and Delahay [31] on the supposition that only adsorbed particles were electrolyzed and by Lorenz [32], who took account... 

Electrochemical impedance spectroscopy was used to determine the effect of isomers of 2,5-bis( -pyridyl)-l,3,4-thiadiazole 36 (n 2 or 3) on the corrosion of mild steel in perchloric acid solution <2002MI197>. The inhibition efficiency was structure dependent and the 3-pyridyl gave better inhibition than the 2-pyridyl. X-ray photoelectron spectroscopy helped establish the 3-pyridyl thiadiazoles mode of action toward corrosion. Adsorption of the 3-pyridyl on the mild steel surface in 1M HCIO4 follows the Langmuir adsorption isotherm model and the surface analysis showed corrosion inhibition by the 3-pyridyl derivative is due to the formation of chemisorbed film on the steel surface. 

The kinetic parameter whose evolution governs the impedance is the concentration of the intermediate Bads in the adsorbed phase. If Q is the fraction of the electrode surface covered by this adsorbate and j3 is the superficial concentration for a complete coverage by Bads> by assuming a Langmuir adsorption isotherm, the surface concentration of Bads is cb = j30. 

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