Which one of the following is not a property of physical adsorption?

We start by recalling that physical adsorption, also called physisorption, is governed by weak van der Waals forces. Because these forces are non-specific and act at appreciable distances, several characteristic trends follow automatically.

First, according to Le-Chatelier’s principle, an increase in external pressure $$P$$ drives the equilibrium $$\text{Gas (adsorbate)} \rightleftharpoons \text{Adsorbed layer}$$ to the right. Hence, higher $$P$$ results in a larger amount of adsorbed gas. So the statement “Higher the pressure, more the adsorption” is definitely a property of physical adsorption.

Next, adsorption takes place at the surface of the solid adsorbent. The greater the available surface area $$A_s$$, the larger the number of sites on which gas molecules can settle. Therefore, the sentence “Greater the surface area, more the adsorption” is again fully consistent with physisorption.

Further, the process is exothermic. For an exothermic process, lowering the temperature $$T$$ shifts the equilibrium to give more product—in this case, more adsorbed species. Thus “Lower the temperature, more the adsorption” also matches the well-known behaviour of physical adsorption.

Now we analyse the last statement: “Unilayer adsorption occurs.” Because the binding forces are weak, molecules that are already adsorbed do not block further molecules from sticking on top of them; instead, additional layers can pile up one after another. Hence physisorption generally produces multilayer, not monolayer, coverage. Monolayer (unilayer) formation is characteristic of chemisorption, where strong chemical bonds confine adsorption to the first layer only.

Therefore, the sentence about unilayer adsorption does not represent a property of physical adsorption while the other three statements do.

Hence, the correct answer is Option D.