Inert means chemically non-reactive. This is changing air (with oxygen) to argon. Adsorption capability is the process, the rate, or the equilibrium amount adsorbed. When the chemistry of a surface changes, any of these could change.

So, suppose you run air with moisture over pure iron. Then, you run argon with the same amount of moisture over pure iron. You might see a different adsorption capability for the water vapor.

I agree with the previous note. The adsorption capacity depends on the surface properties of the solid and on the relative humidity of the surrounding atmosphere. The latter may be slightly influenced by the presence of other gases (i.e. not oxygen or nitrogen).

Interesting question. Being agreed with both responses, I would even add that it depends on which material you are talking about and what was the atmosphere before being replaced with an inert one. For example, some materials are becoming swollen at the presence of CO2. Therefore, by replacing CO2 with nitrogen, and maintaining the same water vapor pressure in the atmosphere, there is a chance to see different water sorption ability. 

Thanks a lot for your answers!!

I understood the fact that if I replace air with Ar or N2, moisture adsorption can be optimised. But I am trying to understand the fundamentals. Say for example, silica gel, a well known material for adsorbing moisture....can we do something to make it hydrophobic and then change it to hygroscopic, with some simple process?

> I understood the fact that if I replace air with Ar or N2, moisture adsorption can be optimised.

I am puzzled why you think this is true. It is possibly true or possibly false depending on the material and what you really mean by "moisture adsorption" and "optimized".

>  But I am trying to understand the fundamentals.

Two words. Thermodynamics and Kinetics. How much will adsorb, and how fast will it adsorb. Imagine a bath tub. The faucet is the water supply to the surface. The drain is the return to the air. Imagine the rate of draining depends on the depth of water. Kinetics is the difference between how open the faucet is relative to how much goes down the drain. Thermodynamics is the limit reached when the faucet and drain are finally flowing at the same rate.

> Say for example, silica gel, a well known material for adsorbing moisture....can we do something to make it hydrophobic and then change it to hygroscopic, with some simple process?

In this case, you want to change the size of the bath tub. First, it should hold no water at all. Then, it should hold lots of water. Nothing about the words hydrophobic or hygroscopic is changed by changing the faucet or the drain.