The DSL isotherm model was introduced in an attempt to describe non-ideal adsorbateadsorbent interactions. In this model, it is assumed that the deviations from ideality are due to surface heterogeneity. The model consists of two sites, 1 and 2, that are independent of each other, where each site follows the Langmuir equation. Hence, the dual site adsorption of a pure component is simply the sum of adsorption on each site: