© A.W.Marczewski 2002

A Practical Guide to Isotherms of ADSORPTION on Heterogeneous Surfaces

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Global Heterogeneity, H: local heterogeneity function h(θ): FG, Kis
(see global heterogeneity calc. plot)

Local heterogeneity - model lines for FG and Kiselev Local heterogeneity function, h(θ), for Fowler-Guggenheim (FG; mean-field non-specific lateral interactions) and Kiselev (Kis; specific associative interactions) compared with Langmuir equation (in the absence of heterogeneity).

Similarity of FG and Kiselev equations - with respect to the behaviour of their isotherms) is visible. FG and Kiselev isotherms also behave similarly to Langmuir for low and high coverages:
hint(θ → 0) = hint(θ → 1) = hL(θ) = 1
However - in the absence of surface heterogeneity - we always have:
hint(θ) ≤ hL(θ) = 1.

θ=1 is reached for p→∞ or [p/(1-x)]→∞ (for vapors x=p/ps) or for solute adsorption c→∞ or [c/(1-x)]→∞ (for weakly soluble solutes; x=c/cs).
NOTE. Replace p with c for dilute solute adsorption.

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