Excess surface work—A modelless way of getting surface energies and specific surface areas directly from sorption isotherms

Sorption isotherms can be easily transformed into excess surface work (ESW) isotherms, computed as the product of the adsorbed amount and the change in chemical potential. Plotted against the amount adsorbed at least one minimum is yield. Thermodynamically ESW is the sum of the surface free energy and the isobaric isothermal work of sorption. Therefore, ESW is not a model, instead it is just another way of presenting an isotherm. From the amount adsorbed in the first minimum one can obtain a specific surface area similar to the BET surface area. The depth of the ESW in the minimum gives a sorption energy, which corresponds approximately to the loss of degrees of freedom of the sorptive. In this contribution the ESW plots of various sorption isotherms on highly ordered alumina with cylindrical pores of 25 nm width and mesoporous SBA 15 will be presented and discussed.