 Solution

The Langmuir adsorption model assumes equivalent independent sites, a maximum of one adsorbate per site, no lateral adsorbate interactions, reversible adsorption-desorption equilibrium and non-dissociative adsorption here. Surface coverage obeys $\theta =bC/(1+bC)$ for the solution analogue.

BET adsorption allows multiple layers: occupation of one layer creates sites for further adsorption above it, and higher-layer binding is treated differently from first-layer binding, commonly using a condensation-like energy. Langmuir saturation is monolayer saturation; the BET model is not simply Langmuir with a different numerical equilibrium constant.