Brief Thermodynamic Description of the Gas Adsorption Phenomenon

The amount of adsorbed gas M is a function of pressure P, temperature T, concentration c, and the adsorption interaction potential E between a solid and a gas, which is expressed by  

In adsorption experiments M is generally shown on the vertical axis, and the horizontal axis represents absolute pressure P or relative pressure P/P0 (Po is the saturation vapor pressure of an adsorptive at an experimental temperature), partial pressure in mixed gases and absolute or relative concentration for a solid-liquid system. Here, since we are focusing on the property of gas adsorption, a solid-gas system is discussed. 

The amount adsorbed M can be a function of pressure P at a fixed temperature T. This is called an adsorption isotherm, and is most often used for the qualitative analysis of gas adsorption. In contrast, the amount of adsorption as a function of 

The relationship between P and T for a constant adsorption amount M is called an adsorption isostere, which is used for calculating the isosteric heat of adsorption from the isotherm curves at various temperatures. 

If an adsorptive is exposed to an adsorbent, the amount of adsorption will increase with time till the amount of adsorption reaches an equilibrium with equilibrium pressure. The temporal response of the amount of adsorption is the adsorption rate. The amount of adsorption is expressed as the amount of adsorbed gas per unit mass of adsorbent wa (mol g 1) or w (g g-1). If an adsorbate is a gas, it may also be expressed by the converted gas volume of the adsorbate va (STP cm3 g-1) under a standard condition (1 atm, 0 °C). Generally, the equilibrium amount adsorbed tends to be proportional to the surface area A, of the adsorbent. Moreover, coverage 0(= tf/tf m) is frequently seen (nam amount of monolayer absorption). The equilibrium amount adsorbed na is a function of pressure P (or concentration c) and temperature T. 

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