A thermodynamic method of determining the specific surface of adsorbents

Conclusions

• 1. A thermodynamic method of determining the capacity of a monomolecular layer and, consequently, the specific surface for nonporous adsorbents, based on an analysis of the adsorption isotherms of vapors at different temperatures and a calculation of the basic thermodynamic functions, has been proposed.

• 2. When one argument (value of the adsorption) is replaced by another (Gibbs free energy), all the differential thermodynamic functions (enthalpy, entropy, and free energy) are linearized; moreover, in this case the existence of two regions, corresponding to monomolecular and polymolecular adsorption, is detected. The transition from one region to another is characterized by a change in the slope of the linear portions of these dependences, occurring at the same value of the adsorption a_M for all functions, identified with the capacity of a monomolecular layer. The integral thermodynamic functions (surface pressure, entropy and enthalpy of the surface layer) behave in entirely the same way, being linearized when the value of the adsorption is replaced by the surface pressure.

• 3. The point of intersection of the two linear portions of the Gibbs free energy, corresponding to the mono-and polymolecular regions, permits a determination of a_M and, consequently, makes it possible to find the specific surface of the solid. For the eight adsorption systems considered, the values of a_M obtained by this method are in rather good agreement with the values of the monolayer capacity (a_m), found according to the BET equation.