In my opinion, Nitrogen adsorption-desorption isotherms exhibit loops due to a phenomenon called hysteresis. During adsorption, nitrogen gas molecules freely enter pores in the material. As the relative pressure (P/P₀) increases, more nitrogen fills the pores. This initial adsorption often appears as a steep curve on the isotherm.

When pressure starts decreasing (desorption phase), the removal of nitrogen from the pores isn't as straightforward. Nitrogen molecules trapped in smaller pores, like bottlenecks or constricted areas, require a lower pressure to escape compared to the pressure at which they entered. This creates a lag in the desorption curve. Chrisma Rose Babu