Experimental method for the determination of adsorption-induced changes of pressure and surface stress in nanopores

The change of surface stress is an important quantity characterising the behaviour of nanoporous systems, however, it is difficult to assess experimentally. In this letter we develop and demonstrate an experimental method for the determination of adsorption-induced changes of the surface stress in nanoporous materials. With the aid of ultrasonic measurements we determine the dependence of the adsorbate's longitudinal modulus beta(Ar), ads on the adsorption-induced normal pressure, Delta p(S), which is exerted by the adsorbate on the porous matrix. From this dependence we deduce the normal pressure at saturation, Delta p(S)(sat), and thereby changes of the surface stress (Delta f)(sat) at the interface between the solid matrix and the liquid adsorbate. For the model system of argon in nanoporous glass (pore radius r(P) = 3.8 nm) the ultrasonic method reveals a value for Delta p(S)(sat) that is in very good agreement with the theoretical value known for the argon-silica interface. The disclosure of this experimental method and its application on other systems will enable a better understanding of the behaviour of adsorbates in nanoporous materials.