In college, when deriving the Langmuir isotherm for gas-solid adsorption, the professor proposed a modified version of the Van der Waals state equation, what he called the "pseudo Van der Waals gas state equation". The derivation implied equaling the chemical potential for a certain compound in the gas and interphase, and when dealing with the volume for the interphase:
$$V\cdot (p+q) = RT$$
Where $V$ is the volume, $p$ the pressure, $R$ the gas constant, $T$ the temperature, and $q$ a correction term to account for the adsorbate cohesion. He argued that the interphase was something similar to a very condensed gas, so that equation of state was valid. Indeed, he obtained correctly the Langmuir isotherm, that later was derived using the kinetic approach.
However, I have been reading a lot of books and articles looking for that "pseudo Van der Waals gas", and I haven't found anything similar. I guess the approximation is just to neglect the excluded volume, but I am not sure whether my professor was right or he took some license by himself.