Question

difference between freundlich isotherm multilayer

Answer 1

Langmuir isotherm assumes monolayer coverage on a homogeneous surface with identical adsorption sites. But these assumptions are valid for gas adsorption on solid surface. In solution-solid systems, with the hydration forces, mass transport effects etc. the system is much more dynamic and complicated, and obeying  the isotherm does not necessarily reflect the validity of the aforementioned assumptions. In such systems the isotherm adequacy can be seriously affected by the experimental conditions, in particular, the range of concentration of the solute/adsorbate.

From my experience, both of Langmuir and Freundlich isothems might adequately describe the same set of liquid-solid adsorption data at certain concentration ranges, in particular if the concentration is small and the adsorption capacity of the solid is large enough to make both isotherm equations approach a linear form.

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